Download crustal movements in the inner zone of southwest - J

J. Phys. Earth,
37, 101-131,
1989
CRUSTAL MOVEMENTS IN THE INNER ZONE OF
SOUTHWEST JAPAN ASSOCIATED
WITH
STRESS RELAXATION
AFTER
MAJOR EARTHQUAKES
Takao
TABEI*
Geophysical Institute, Kyoto University, Kyoto, Japan
(Received May 30, 1988; Revised April 7, 1989)
Transient deformation resulting from postseismic stress relaxation in underlying viscoelastic layers is examined. The main purpose of the present study is to
understand spatial and temporal modes of postseismic deformation especially due
to strike-slip faulting with a realistic geometry. The fault size as well as the
viscoelastic structure of the earth proved to have large effects on the amount of
postseismic deformation. Therefore, it should be taken into serious consideration in
modeling the faulting. It is noteworthy that the concentration of shear strain into
the near field continues long after the faulting. This strain concentration is similar
to the preseismic strain accumulation and is contrary to the coseismic strain release.
The viscoelastic structure of the earth, however, is hard to determine solely by the
surface measurements performed at long time intervals. Crustal movements in the
Inner Zone of Southwest Japan, in particular, those associated with the 1927 Tango
earthquake of M=7.5, are then investigated in connection with postseismic stress
relaxation. The discrepancies between the observed and calculated postseismic
deformations can be reduced if the effect of tectonic stress is added to the calculated
deformation field.
1.
Introduction
It is widely accepted that the basic model to explain the physical behavior of
the earth's crust and upper mantle generally consists of three layers with different
rheological properties; namely, the uppermost elastic and brittle lithosphere, the
intervenient ductile asthenosphere with low viscosity, and the substratum with
relatively high viscosity. In addition, it is also recognized that there exist considerable regional heterogeneities in both thicknesses and properties of the layered
structure of the earth. These heterogeneities probably characterize the mode of
accumulation and release of tectonic stress in the region, and this mode is reflected
in the crustal movements and seismic activity peculiar to the region.
* Present address:Department of Physics, Faculty of Science,Kochi University,Kochi,
Japan.
101
102
T. TABEI
With the recent advance in accuracy in the determination of hypocenters of
earthquakes, it has become clear that there is a sharp cut-off depth for seismic
activities in tectonically active inland regions (e.g., KOBAYASHI,
1976; OIKE, 1977;
SIBSON,1982; MEISSNER
and STREHLAU,
1982; CHENand MOLNAR,1983; MIKUMOet
al., 1988; ITO, 1988). It seems that most earthquakes that occur inland sufficiently
far from the subduction zone are restricted to the upper crust, in other words, a
seismic-aseismic transition occurs at the mid-crustal depth. The thermal dependence
of fracture strength of rock has been discussed to interpret the lower limit of focal
depth in Central and Southwest Japan (e.g., KOBAYASHI,
1976; MIKUMOet al., 1988;
ITO, 1988). Based on laboratory data on rock deformations, SIBSON(1982) assumed
frictional failure and quasi-plastic flow in the upper and lower crusts, respectively,
and clarified the depth-dependence of shear resistance of the continental crust in the
Western United States. These studies suggest that an anelastic treatment is
indispensable not only for the upper mantle but for the lower crust when the failure
and deformation of the crust in tectonically active inland regions are discussed.
Transient crustal movements following an elastic rebound at the time of a large
earthquake are interesting phenomena in understanding the anelastic properties of
the earth's crust and upper mantle and also in comprehending the mode of
accumulation and release of tectonic stress. NUR and MAVKO(1974) first obtained
analytical expressions of the postseismic displacement fields due to a dislocation in
an elastic layer overlying a viscoelastic half-space. In recent years, further studies
have been carried out on quasi-static surface deformation resulting from postseismic stress relaxation in underlying viscoelastic layers.
Crustal movements due to major underthrust earthquakes occurring repeatedly
at the subduction zone are characterized by large vertical displacements whose
patterns significantly change with time. They have mostly been modeled in
connection with a steady plate subduction and asthenospheric stress relaxation
(e.g., THATCHERand RUNDLE,1979; THATCHERet al., 1980; MIYASHITA,1983;
THATCHER
and RUNDLE,1984; MIYASHITA,
1987) and a relatively simple two- or
three-layered structure model seems to explain appropriately the transient crustal
movements observed.
Similar modelings have also been made for slow and steady deformation along
a strike-slip fault (RUNDLEand JACKSON,
1977; SAVAGE
and PRESCOTT,
1978; YANG
and TOKS_??_Z,
1981). These modelings, however, have mainly been made for a highly
idealized long fault that exists in an elastic layer overlying a viscoelastic substratum.
Thus, it is questionable whether the model results are immediately applicable to
crustal movements due to moderate-size faulting. For example, an earthquake
with a magnitude larger than 7.0 that occurs in the Inner Zone (the Japan Sea side)
of Southwest Japan is mainly represented by strike-slip faulting with a length of
about 30 to 40 km. However, quantitative consideration has not yet been given for
postseismic deformations caused by stress relaxation after such faulting.
In the present study, the effects of fault size and viscoelastic structure of the
earth's crust and upper mantle on the postseismic deformations are examined
Crustal Movements in Southwest Japan after Major Earthquakes
103
through a three-dimensional numerical simulation in which viscoelastic treatments
are introduced for the lower crust and the upper mantle on the basis of the vertical
distribution of microearthquakes. The spatial pattern, amount, and timedependence of the deformation are then estimated for faulting with a realistic
geometry. The deformation due to postseismic stress relaxation is formulated
according to MATSU'URA
et al. (1981) and IWASAKI(1985). In their formulations,
quasi-static deformations caused by faulting with arbitrary orientation and slipvector in a multi-layered half-space can be treated. Furthermore, crustal movements
observed during several decades after the 1927 Tango earthquake of M=7.5, that
occurred in the Inner Zone of Southwest Japan, are investigated especially in
connection with the relation between the postseismic stress relaxation in underlying
viscoelastic layers and the tectonic stress. These investigations will give us a new
insight into crustal movements related to the mode of accumulation and release of
tectonic stress.
2. Modelingof PostseismicDeformation
2.1 Tectonicsetting of the Inner Zone of SouthwestJapan
Figure 1 shows the distribution of the main active faults and tectonic lines in
and around Southwest Japan (THERESEARCH
GROUPFORACTIVEFAULTS,1980).
The Median Tectonic Line separates Southwest Japan into two tectonic regions,
namely, the Inner Zone (the Japan Sea side) and the Outer Zone (the Pacific side).
Tectonics in the Outer Zone is characterized by an interaction between the
Philippine Sea and Eurasian plates. The former is considered to be subducting
northwestward beneath Southwest Japan from the Nankai trough (Fig. 1). Along
this trough, great earthquakes have repeatedly occurred accompanied by large
crustal movements in the coastal regions (ANDO,1975). Studies of the focal
mechanisms of these earthquakes and their aftershocks show that these are the
earthquakes of low-anglethrust type (ICHIKAWA,
1971;KANAMORI,
1972a; SHIONO,
1977).
The tectonic setting is rather complicated in the Inner Zone. The regular
arrangement of reverse faults and conjugate strike-slip faults shown in Fig. 1
suggests that they have been formed by a horizontal tectonic compression in a
nearly E-W direction (HUZITA
et al., 1973). Similarly, studies of focal mechanisms
of shallow earthquakes in this region have revealed that pressure axes lie in a
horizontal E-W to ESE-WNW direction (ICHIKAWA,
1971;NISHIDA,1973;SHIONO,
1977),some examples of which are shown in Fig. 2. Moreover, long-term crustal
movements detected by geodetic measurements indicate a nearly E-W contraction
(GEOGRAPHICAL
SURVEY
INSTITUTE,
1987). It is therefore considered that the
tectonic stress acting in the Inner Zone is dominantly the horizontal compression in
the almost E-W direction and the movement of the Philippine Sea plate is not a
governing factor (e.g., HASHIMOTO,
1982).The detailed mechanism causing such an
E-W compression, however,has not been clarified yet and is under livelydiscussion.
104
T. TABEI
Fig.
1. A map showing the distribution of the main active faults and tectonic lines
in and around Southwest Japan (compiled from THE RESEARCH
GROUP FOR
ACTIVEFAULTS,1980). M.T.L. and I.S.T.L. represent the Median Tectonic
Line and the Itoigawa-Shizuoka Tectonic Line, respectively.
Three large earthquakes in the Inner Zone, namely, the 1927 Tango (M= 7.5),
the 1943 Tottori (M= 7.4), and the 1948 Fukui (M= 7.3) earthquakes, were
accompanied by remarkable crustal movements showing typical strike-slip faulting
(TSUBOI,1932; NASU,1950; SATO, 1973). Their focal mechanisms are indicated in
Fig. 2 by symbols Ta, To, and Fu, respectively. KANAMORI
(1972 b, 1973) calculated
the synthetic seismograms for the three earthquakes on the basis of the distribution
of P-wave first motions, the aftershock distributions, and the triangulation data. He
determined fault parameters from the best-fit synthetic seismograms. The results
obtained are listed in Table 1. The three earthquakes are very similar in scale and
type of faulting and can be regarded as representing typical patterns of the release of
tectonic stress that has been accumulated in this region.
Shallow microseismicities in the Inner Zone are considerably active along the
active faults, as shown in Fig. 3(a) (OIKE, 1977). The vertical distribution of
Crustal Movements in Southwest Japan after Major Earthquakes
Fig.
105
2. Distribution of P-axes and focal mechanisms of major crustal earthquakes
in Southwest Japan (after SHIONO,1977). All diagrams are projected on the
lower hemisphere. Shaded and open quadrants of the diagrams indicate the
compressional region of P-wave first motion and the dilatational one, respectively. Symbols Ta, To, and Fu represent the 1927 Tango (M= 7.5), the
1943 Tottori (M=7.4), and the 1948 Fukui (M=7.3)
earthquakes,
respectively.
Table 1. Fault parameters of the three large earthquakes in the Inner Zone of Southwest
Japan (after KANAMORI,1972b, 1973).
accurately determined hypocenters of microearthquakes indicates a sharp concentration of focal depths into the upper crust and an abrupt disappearance in and
below the lower crust, as shown in Fig. 3(b). These results as well as the distribution
of active faults give important clues for understanding the nature of the crust of the
tectonically active inland region. For example, we can consider that strain buildup
caused by accumulation of tectonic stress and strain release through brittle fractures
alternately take place in the elastic upper crust, while an acting stress is continuously
consumed by an anelastic flow in or below the lower crust, and consequently, stress
is not accumulated there.
106
T. TABEJ
(a)
(b)
3. Distribution of microearthquakes that occurred during the period from
July 1965 to June 1976 (after DIKE,1977). (a) Epicentral distribution of shallow
microearthquakes whose depths are less than 40 km. (b) Representative crustal
structure and vertical distribution of hypocenters in the same region as that
shown in (a).
Fig.
2.2
Mathematical
Transient
viscoelastic
(1981)
layers
and
Figure
overlying
are
and
formulation
deformation
as
IWASAKI
4(a)
to
shows
be
a Maxwell
The
respectively.
When
by
postseismic
according
a
layered
(the
to
stress
the
relaxation
formulations
Coordinates
source
layer),
(behaving
substance
of
half-space
(n + 1)-th
viscoelastic
viscosities
the
induced
modeled
in
by
underlying
MATSU'URA
et
al.
(1985).
substratum
assumed
elastic.
is
the
and
time
for
stress
n-th
layer
fault
function
that
where
as
the
an
the
geometry
can
and
layer
solid
the
shown
represented
for
n
and
layers
are
denoted
in
Fig.
by
parallel
the
layers
substratum
confining
other
substratum
are
be
of
n-th
elastic
deviation)
and
consists
pressure
are
perfectly
by ā'
and ā,
4(b).
the
Heaviside
step
Crustal Movements in Southwest Japan after Major Earthquakes
(a)
Fig.
4.
of
Definition
layers
and
Fault
the n-th
length;
angle;
the
and
layer,
one;
and
(modified
interfaces. ā
and ā'
respectively.
(b)
W, fault
U0, amount
foot
(b)
of parameters
their
107
width;
of final
D,
Geometry
depth
dislocation; ă,
ur(_??_,z),displacement
from
denote
to the
IWASAKI,
the
1985).
viscosities
(a)
Numbering
of the
substratum
of the
finite-dimensional
lower
margin
slip-angle
components
of
of
the
in
cylindrical
the
hanging
fault.
fault; ƒÂ,
wall
L,.
dip
against
coordinates.
function H(t), total displacement fields at the surface are written in the cylindrical
coordinates as
(1)
where ur(_??_
,z) represents the coseismic elastic displacement and vr(_??_,z)
the postseismic
viscoelastic one that is dependent on the elapsed time after faulting. When a point
dislocation is given in the m-th layer, the radial component vr of the viscoelastic
displacements is expressed in the integral representation as
(2)
with
(3)
108
T. TABEI
where
m
U0 is
the
the
geometrical
and
amount
Lam_??_s
functions
of
expressed
in
the
effect
is
are
MATSU'URA
be
integrations
of
fault
with
integrations
lated.
carried
of
grid
have
degree
non-uniform
of
of
a
dip-angle
are
structure
and
the
fault
(ƒÂ)
vz,
and
using
1982).
with
the
time
gravitational
The
a technique
integrals
to
source
The
MATSU'URA,
referred
of
the
geometry.
points
been
W
grid
kernel
developed
respect
to
to ƒÌ
by
in
Eq.
is,
without
of
a
as
dislocation
of
the
any
a
of
(2)
84
the
at
on
can
the
In
(12
and
near
of
field,
a
the
interpolation
the
fault
source
the
surface
be
the
a
is
by
calcu-
to
study,
length
however,
and
width
more
to
it
the
has
case
the
the
grid
Though
time,
as
two-
bi-cubic
reduced
present
7 in
plane
as
interpolation.
applicable
fault
over
study,
represented
computational
modification,
out
present
on
and
double
carried
point
position
accuracy
long
the
selected
functions.
taken
numerical
be
two-dimensional
integration
calculation
rather
In
interpolated
bi-cubic
enhance
requires
4(b)).
are
at
been
must
location
double
the
fault,
source
of
points
the
are
usually
For
and
method
Taking
analytical
has
distribution
(Fig.
displacement
the
selected
a point
a new
Consequently,
of
finite-dimensional
for
displacements
integration
accuracy
to
First,
valuable,
respectively).
method
(2),
layered
to
semi-infinite
using
sources.
integration
directions,
Eq.
time-dependence
the
width
functions.
point
functions.
number
and
out
discrete
numerical
in
the
functions
obtained
L
calculated
B-spline
the
due
solutions
length
of
Then
both
analytical
so that
fields
the
B-spline
location
dimensional
points
(1981)
Ll ƒÊ
the
v_??_,
and
represent
(IWASAKI
by
the
and
and
components,
Vr,j,
in V_??_,j
the integrals
related
them
Kl
(_??_) and
displacement
order l, ăm
and
(2).
They
not
of
layer,
azimuth
two
Eq.
function
m-th
evaluated.
are
bi-cubic
the
in
Bessel
station
included
by
are
displacement
plane
are
approximated
numerically
For
to
affected
they
et al.
the
other
functions.
incorporated
functions
by
is
However,
on
The
which
the
source-existing
similar
kernel
which
function.
can
Vz,j,
Jl(ƒÌr)
the
fault.
forms
viscoelastic
deformation,
the
the
and
as
dislocation,
of
depending
slip-angle(ă)
are
of
constants
this
a
high
of
the
plane.
2.3 Structure model of the earth's crust and upper mantle
The postseismic viscoelastic deformation at the surface is, dependent on the
layered structure of the earth's crust and upper mantle.
On the basis of the delay in arrival time of long-traveled seismic waves, the
upper mantle beneath the Japanese Islands has been thought to have the characteristics of low velocity and large attenuation (KANAMORI,1970). The threedimensional attenuation structure of Northeast Japan, which was estimated by the
inversion of seismic intensity data, shows that the low-Q zones are dominant in the
crust and upper mantle on the backarc side and are in contrast with the high-Q
zones on the Pacific side (HASHIDAand SHIMAZAKI,
1987). Moreover, a numerical
modeling of tectonic flow induced by a subducted slab has indicated high
temperature and low viscosity beneath island arcs and marginal basins (ANDREWS
and SLEEP,1974). Therefore, the idea that the asthenosphere beneath the Japanese
Crustal Movements in Southwest Japan after Major Earthquakes
Table
Islands
has
structure
et
In
we
the
and
upper
et
present
2
ratios
and
by Ą
has
is
A
Fig.
of
the
the
the
an
with
of
In
the
basis
the
the
elastic
the
with
of
of
upper
and
asthenosphere
modeling
of
present
viscosity,
treatment
a finite-element
on
Therefore,
high
as
viscosity
asthenosphere.
consists
viscoelastic
low
substratum
crust
3(b).
relatively
recognized
with
of
lower
Japan
its
similar
in
the
in
Southwest
with
made
material
constants
thicknesses
and
explosion
densities
mainly
after
from
faulting
the
structure
velocities
the
low
lower
crust
Northeast
Japan
employed
in
SATO
are
et
in
layers
et al.,
(1981).
The
al.
varying
units
represents
the
(YOSHII
executed
is measured
where ƒÊ3
model
of
observations
deformation
mantle, ƒÅ/ƒÊ3,
of
P-wave
seismic
are
time
model
employed
possibly
result
additional
(MIZOUE,
1976)
NAKANISHI
(1980)
Shikoku
of
its
Philippine
time,
aseismic
has
ScS
to
Sea
however,
dead
that
plate
he
are
the
mostly
1974).
Maxwell's
rigidity.
Poisson's
of
viscosity
time-
ratio ā'/ā.
relaxation
The ƒÅ/ƒÊ3
the
derived
The
calculations
the
slab
this
that
had
heterogeneities
of
will
is
subducted
plane
of
time
be
denoted
ScSp
phases
regions
(Fig.
dipping
interface
reached
another
intermittently
2),
is
the
the
were
the
velocity
is
were
interpreted
in
upper
mantle.
the
beneath
possibility
descended
the
boundary
Chugoku
that
this
in
the
structure
of
not
at several
which
upper
stations
He
pointed
subducted
At
is related
subduction
in
converted
has
the
region.
the
uniform.
as
of
interface
past
induce
earthquakes
configuration
subduction
observed
structure.
and
subcrustal
that
and
the
slab
seismic
indicate
complex
of
a long
seismic
Japan
interface
had
indicated
lateral
three-dimensional
that
the
that
has
The
plate
reported
P at
the
presence
Southwest
Chugoku
possibility
the
large-scale
Sea
and
from
the
the
beneath
Philippine
ignores
from
deformation.
and
1981)
subducted
waves
here
mainly
surface
(HIRAHARA,
out
by
observation
here.
These
the
of
which
been
the
of
upper
supported
seismic
effect
that
into
shown
crust
thickness.
the
with
treatment
Zone
lower
postseismic
The
an
Inner
mantle),
The
elapsed
the
the
lists
result
dependent
is also
(1981).
study.
the
idea
explosion
asthenosphere
and
compared
microearthquakes
sufficient
al.
Table
of
for
mantle
SATO
from
of
(upper
and
the
a thick
Japan,
a viscoelastic
viscoelastic
viscosity
This
from
that
Southwest
is negligible
introduce
substratum
founded.
derived
assume
of
viscosity
model
crust,
we
crust
distribution
structure
The
study,
high
addition,
is well
velocity
Japan.
1974).
inland
relatively
model for the Inner Zone of Southwest
developed
P-wave
present
the
vertical
of
al.,
the
underlies
by
highly
model
(YOSHII
its
been
Structure
2.
109
the
same
to the
process
110
T. TABEI
because no subcrustal earthquake occurs beneath the Chugoku region. The absence
of subcrustal seismicity implies that at present the interaction between the
subducted Philippine Sea plate and the continental Eurasian plate is not so strong
beneath the Chugoku region. As a first approximation, therefore, ignoring the
effects of subducted slab may be justified for modeling the deformations in the Inner
Zone of Southwest Japan.
The effect of the superficial layer of the crust on the surface deformation is not
taken into account in the present model because it is exceedingly hard to quantify.
TANAKA(1981)has concluded that some of the postseismic deformations detected
by strainmeters or tiltmeters, which were represented by exponential functions with
time constants of a few years, may be caused by an earthquake-induced viscoelastic
relaxation of stress within rocks around the observation vault. We recognize that
the local crustal movements are sometimes largely affected by heterogeneities within
rocks, transient movement of underground water, instabilities of alluvial layers, and
so forth. However, it is uncertain whether the superficial layer of the crust has a
governing factor in long-term regional crustal movements.
3.
Model
Results
3.1
Effect
In
of
studies
dimensional
size
the
problem
treatment
simplification.
major
fault
of
such
However,
factors
evaluate
effect
deformations.
that
we
governing
the
of
the
of
the
are
fault
coseismic
fault
an
infinite
the
idealized
is often
size
both
in
the
be
Table
2
and
is
for
one
fields.
elastic
two-
adopted
may
deformation
on
presented
an
length
fault
postseismic
length
model
coupling,
that
and
of
structure
has
convinced
variation
The
elastic-viscoelastic
of
the
Here
we
viscoelastic
employed
in
the
evaluation.
Holding
fixed,
different
along
fault
The
30 km
In
general,
in
the
strike-slip
faulting
displacement
5
strike
of
to
of
fault
profiles
30•‹,
as
and
fixed
respectively.
The
of
the
the
amount
at
15 km.
fault,
the
for
in
strike-slip
a dip-slip
on
calculated
changes
center
by
for
the
vertical
the
is
slip-vector
are
shows
is normalized
width
profiles
fault.
The
fault
at
Figure
profiles
the
viscosity
ratio ā'/ā
are
in
taken
surface.
shows
the
width
are
with
horizontal
The
dislocation
6
the
plane
cases
of
of coseismic
where
fault
several
and
on
the
changes
in
both
dip-angle
Figs.
are
5 and
6 is
10.
the
than
postseismic
after
to
and
taken
deformation
dip-angle,
displacements
Figure
amplitude
tentatively
width,
perpendicular
displacement
at
fault
lengths.
line
plane.
vertical
the
viscoelastic
parallel
the
displacement
than
the
and
fault
displacement
fixed
that
elastic
variation
the
faulting,
deformation
is twice
in
elastic
fault
one,
as
the
is
from
Figs.
panels
in
not
time
clearly
of
a
more
left
relaxation
are
has
this
seen
(lower
profiles
length
and
larger
5 and
Figs.
the
effect
remarkable
6.
5 and
At
the
6, where
asthenosphere),
distinguishable
on
the
in
dip-slip
early
viscoelastic
faulting
stage
the
elapsed
differences
because
of
of
the
time
among
their
small
111
Crustal Movements in Southwest Japan after Major Earthquakes
ELASTIC
VISCOELASTIC
Fig.
5.
slip
Changes
fault.
parallel
in
The
to
normalized
plane.
The
pendicularly
mation
the
fault
by
U0
profiles
to
(lower
horizontal
fault
the
two
PART
displacement
is
vertical
and
strike
that
are
center
figures),
is
are
the
taken
profiles
its
plotted,
along
of
the
width
the
fault.
viscosity
is
with
of
length
fixed
where
amount
PART
at
the
scale
coseismic
the
line
In
the
of
a pure
on
postseismic
is
strike-
Displacements
displacement
dislocation
extending
ratio ā'/ā
of
15 km.
on
the
is
the
surface
viscoelastic
assumed
to
fault
perdefor-
be
10.
amplitudes. On the other hand, the effect of variation in fault length becomes more
remarkable gradually with time (lower right panels in Figs. 5 and 6). Therefore, we
conclude that the fault length should be taken into serious consideration in
modeling the long-term postseismic viscoelastic deformation.
3.2 Spatial patterns of displacement and strain fields
To understand the time-dependent deformation due to moderate-size strikeslip faulting, displacement and strain fields are calculated at several elapsed times
after faulting. The model fault is assumed to be vertical and have a length of 34 km
and a width of 13km. The upper margin of the fault must be located underground
because singular points appear if a source is located at the surface. Thus the depth
to the lower margin of the model fault is tentatively taken as 13.13 km; that is 1%
larger than the fault width. The fault geometry is nearly equivalent to those of the
1927 Tango, the 1943 Tottori, and the 1948 Fukui earthquakes.
112
T. TABEI
ELASTIC
PART
VISCOELASTIC
Fig.
6.
Changes
fault.
The
The
profiles
to
and
As
are
to
shown
generally
however,
region
in
are
is
in
in the
The
at
(a)
are
the
same
with
fixed
line
left-lateral
plane.
as
length
at
30
in
Fig.
are
represented
be 10
Fig.
km
5.
of
a
pure
and
30•‹,
The
viscosity
dip-slip
respectively.
ratio
is
by
and
and
at the
of
in
the
Figs.
t=
been
7,
8,
and
9,
viscoelastic
Here,
time
has
displace-
postseismic
respectively.
elapsed
1 m
vertical
shown
and
figure,
those
as
in
time
of
the
horizontal
coseismic
Fig.
in
are
the
the
displaced
where
each
(b),
sense
different
the
calculated
deformations
in
component
displacements,
the
10Ą. The
viscoelastic
viscosity
ratio ā
.
7(a)
same
(b)
strike-slip
Horizontal
elastic
and
displacements
positions
pure
fault
strains
remarkably
displacements
postseismic
a
the
quite
profiles
dip-angle
along
coseismic
shown
deformations
'/ā is assumed
are
on
The
are
taken
horizontal
respectively.
displacement
and
10.
with
given
ments,
ones
be
dislocation
uniformly
vertical
width
are
assumed
A
in
fault
PART
7(a)
order
faulting.
not
particularly
maximum
postseismic
displacement
The
of
ones.
and
to
(b).
It
amounts
seems
compensate
However,
that
for
the
the
cumulative
vectors
displacements,
the
surrounding
relatively
small
amounts
of
large.
vertical
displacement
appears
do
not
Crustal Movements in Southwest Japan after Major Earthquakes
113
(a)
(b)
Fig.
7.
fault
Horizontal
plane.
displacement
The
respectively.
the
given
ments
The
fault
width.
on
at
the
the
due
length
and
depth
of
A
fault
elapsed
uniform
plane.
width
the
strike-slip
fault
margin
Coseismic
t=
pure
the
dislocation
(a)
time
lower
to
of
10-Ą. The
are
of
with
the
faulting
assumed
fault
(b)
ratio
with
a
34
and
be
is taken
a left-lateral
displacements.
viscosity
to
is
1%
vertical
13
larger
km,
than
component
of
Postseismic
displace-
assumed
to
be
1 m
is
10.
coincide in Fig. 8(a) and (b). These figures show the turnover from coseismic
upheavals to postseismic subsidences and the reverse case from coseismic subsidences to postseismic upheavals. These turnovers may be caused by the slow
reaction of the underlying viscoelastic layers against the sudden change in stress
fields and by the compensation for the undulation of the boundary between the
elastic and viscoelastic layers. The gravitational effect has been considered in
calculation of postseismic deformations, although it proves not dominant in this
case.
The most noteworthy result is obtained from the pattern of the strain field. As
shown in Fig. 9(a) and (b), the postseismic strains in the near field of the fault are
generally accumulated in the reverse sense to those of the coseismic strain releases.
In other words, the pattern of postseismic strains is roughly similar to the
distribution of shear strains that were accumulated in the near field before strike-slip
faulting. The postseismic strain accumulation implies a concentration of shear stress
in the vicinity of the main fault plane. This stress concentration is induced by the
postseismic anelastic flow in the underlying viscoelastic layers (YANG and TOKS_??_Z,
1981) and will promote a long-term aftershock activity in the hypocentral region or
creep-like secondary faulting in the near future.
114
T. TABEI
(a)
(b)
Fig.
8. Contour maps of (a) coseismic and (b) postseismic vertical displacements.
See the caption of Fig. 7.
(a)
(b)
Fig.
9.
(a) Coseismic and (b)
Fig. 7.
postseismic
horizontal
strains
. See the
caption
of
Crustal Movements in Southwest Japan after Major Earthquakes
3.3
Effect
To
calculated
at
viscosity
line
this
five
ratios.
figure,
Figure
to
from
the
in
the
lower
at
fault.
different
the
as
addition,
of
in
Fig.
relaxation
example,
t =10Ą
is
in
it is
the
case
has
ratio
times
largely
viscoelastic
10,
we
near
that
the
on
determine
even
of ƒÅ'/ƒÅ=•‡
and
if
a
is
plane.
As
in
on
the
to
that
the
each
certain
shown
of
of
the
10
to
ratios
that
upper
40
km
among
structure
model,
ratio.
surface
deformation
the
displacement
difference
obtained
the
means
about
amplitude
in
along
amplitudes
which
simultaneously
the
that
taken
displacement
field
viscosity
different
of
smaller,
the
the
structure
recognize
effect
changed
by
to
fault
approaches
in
little
cannot
time Ą,
difficult
a large
noteworthy
are
the
are
with
are
amount
becomes
and
affected
models
profiles
the
on
appears
it is
the
and
ratio
displacements
structure
dislocation
decreases
elapsed
effect
shown
the
For
at
crust
itself
fault
viscosity
deformation
In
amplitude
ratio ā'/āand
Fig.
the
remarkable
of
horizontal
three
in which
the
viscosity
the
for
results,
of
amount
As
structure,
times
the
the
profiles.
Though
obtained
center
variation
remarkable
obtained.
shows
the
profiles
while
10
the
is the
of
viscoelastic
elapsed
U0 that
more
away
of
by
viscosity
mantle,
structure
effects
different
displacement
the
the
viscoelastic
the
perpendicular
normalized
the
of
estimate
115
profile
between
at
t=
is
viscosity
6Ą in
the
the
is
profile
case
10. Changes in horizontal displacement profiles with time for three structure
models with different viscosity ratios. The profiles are taken along the line
extending on the surface perpendicularly
to the center of the fault. The scale of
displacement is normalized by U0. Parameters denoting the fault geometry are
fixed at those in Figs. 7, 8, and 9.
of
116
T. TABEI
ā
'/ā =10
. Therefore,
intervals
is
not
viscoelastic
of
long-term
kinds
discussed
of
in
measurement
powerful
structure
mation,
other
the
a
the
monitoring
with
the
and
of
are
deformation
establishing
crust
investigations
connection
postseismic
for
earth's
detailed
field
of
method
a
upper
the
The
value
For
time-dependence
of
is
of
long
model
mantle.
deformation
required.
at
unique
time
for
further
the
infor-
indispensable
the
and
viscosity
deformation
itself
in
the
is
next
subsection.
3.4 Time-dependence of deformation
Temporal variations in postseismic horizontal strains at selected positions are
shown in Fig. 11. In this figure, changes in maximum shear strain due to pure strikeslip faulting with a dislocation of 3 m are plotted against the normalized elapsed
time. The positions used for calculations are located perpendicular to the center of
the fault. Viscosity ratio is assumed to be 10 as a representative value, which gives
the deformation with moderate amplitude as shown in Fig. 10.
Figure 11 shows the continuation of viscoelastic deformation over a long time
after faulting, being driven by the anelastic flow due to stress relaxation in the
underlying viscoelastic layers. The apparent relaxation times at the surface are
much longer than the rheological relaxation times of the underlying viscoelastic
layers. This result is most likely due to a coupling of the elastic and viscoelastic
Fig.
11.
Time
changes
dislocation
of
pendicular
to
are
fixed
at
10.
Two
broken
in
the
Inner
the
maximum
The
center
those
in
Zone
two
lines
figure
are
drawn
Southwest
of
and
of
the
7,
8,
lines
Southwest
the
under
Japan
shear
positions
Figs.
straight
These
beneath
in
3 m.
elapsed
an
strain
used
fault.
and
The
the
Japan
which
times
represented
5•~10
19P.
strike-slip
faulting
are
denoting
viscosity
approximate
that
to
calculations
Parameters
9.
assumption
is
due
for
are
stationary
years
viscosity
fault
is
tectonic
estimated
in
the
the
ratio ā'/ā
by
with
located
a
per-
geometry
assumed
to
strain
NAKANE
be
rates
(1973).
at
the
bottom
of
the
asthenosphere
of
this
Crustal Movements in Southwest Japan after Major Earthquakes
layers
and
The
is
quite
viscoelastic
elapsed
time
displacement
deformation.
the
Of
fault
Two
rates
size
the
by
the
of
(1973).
It
in
strain
viscoelastic
nearly
final
of
not
medium
final
state
4%
depends
11
steady
into
that
only
approximate
Southwest
dominant
postseismic
in
about
value
in Fig.
of
NAKANE
is
strain
the
the
the
of
on
at
the
displacement,
the
the
alone.
state
the
coseismic
elastic
structure
model
but
faulting.
lines
Zone
of
reaches
to
final
mode
straight
relaxation
treatment
the
this
and
Inner
triangulation
stress
the
amounts
course,
broken
in
that
from
deformation
t_??_40Ą. Converting
maximum
on
different
transient
117
field
the
Japan,
can
be
the
early
is
gradually
as
seen
from
stage
of
tectonic
from
this
that
the
taken
stationary
estimated
figure
the
the
postseismic
over
by
strain
first-order
strain
due
to
deformation
the
tectonic
and
strain
with
time.
The
rigidity
relaxation
of
the
deformation
of
have
two
at the
viscosity
occur
been
the
seems
appropriate
earthquake
of
words,
estimate
of
must
of
that
the
the
the
however,
we
Zone
Nankai
however,
viscosity
we
have
from
or
of
no
available
beneath
viscosity
Japan
and
time-dependence
asthenosphere
assume
Southwest
the
the
other
studies
region
the
lies
thickness
in
the
about
of
110
time
considered
If
viscosity
the
For
upper
we
of
of
model
of
viscosity
to
have
tectonically
500
on
km.
adopt
mantle
viscosity
of
asthenosphere
is
about
and
2.44
yr.
of
the
has
(1984).
estimated
asthenowhich
been
lithosphere
Japanese
and
is
made
with
however,
viscosity
at
model,
thickness,
which
the
the
as
regions,
low
a finite-
RUNDLE
the
stable
5•~10 19P,
beneath
in
of
estimation
inland
with
of
recently,
earthquakes
structure
a tectonically
active
More
thickness
This
Kanto
thickness
asthenosphere
the
viscosity
1923
asthenosphere
THATCHER
the
this
underthrust
of
a 30-km-thick
the
1983).
low
periodically
1) if the
10 21 P because
asthenosphere
the
in
after
layered
of
10 19 to
and
(1984)
that
(Fig.
the
by
the
depends
from
100
developed
expected.
of
of
RUNDLE
Moreover,
caused
the
uplifts
range
km.
viscosity
of
and
IWASAKI,
and
the
between
a highly
the
of
(1973),
postglacial
are
the
configuration
those
wide
that
and
for
viscosity
This
faulting
trough
and
the
in
18Pa•Es).
observed
Sagami
deformations
plate
thrust
movements
at the
that
earthquakes
asthenosphere.
represented
(5•~10
THATCHER
modeled
vertical
20P
from
of
They
(MATSU'URA
WALCOTT
in the
regions
are
limit
occurred
is somewhere
of
lithosphere
that
different
is unknown,
thickness
the
The
analysis
and
shield
explain
cyclic
by
times
assumption
underthrust
to
2•~10
an
elapsed
is 5•~10 19P
a study
1).
the
the
under
major
viscoelastic
60 km
and
Islands
from
by
11
drawn
Japanese
derived
used
to
are
the
Fig.
a
trough.
are
sphere
figure
in
(Fig.
of
According
lines
trough
(1987)
modeling
relaxation
Outer
other
to
viscosity
overlying
is
MIYASHITA
lower
of
Thus
caused
M=7.9
lithosphere
element
the
mainly
Nankai
lithosphere
for
of
beneath
deformation
at
from
the
in
evaluated
unfavorable,
value
straight
bottom
elastic
the
for
broken
has
cyclic
the
is
Japan.
made
asthenosphere
the
the
be
Islands.
The
years
asthenosphere,
It
Southwest
been
Japanese
the
the
must
concerning
Zone
that
of
quantitatively.
information
Inner
time
asthenosphere,
a
a thin
sufficient
possibly
the
Islands,
the
118
T. TABEI
On
mation
the
cumulative
the
be
fault;
this
noted
and
in the
4.
that
strain.
vicinity
continuously
postseismic
time
strain
because
Observations
these
be
the
after
the
postseismic
of the
fault
the
the
by
plane
about
geodetic
strain
same
strain
similarities
of
field
the
two
same
become
kinds
of
as that
the
of
it should
as that
of
expected
before
distinction
gradually
strain
sense
be
the
vicinity
Here,
can
stress
However,
of
the
measurements.
in the
defor-
maximum
15•~10-6 in
tectonic
will
viscoelastic
the
concentration
question.
whole
and Model
to
the
and
is accumulated
if the
in
that
faulting
surface
strain
region
conclude
amounts
a remarkable
the
from
years
state
verified
Therefore,
of
we
100
final
easily
on
assumptions,
about
until
can
again
tectonic
with
of
for
strain
the
acts
basis
continues
more
on
faulting
of
the
difficult
accumulation.
Calculations
The model of postseismic deformation due to stress relaxation is applied to the
interpretation of crustal movements associated with a major earthquake in the
Inner Zone of Southwest Japan.
Fig.
12. Horizontal displacements at the triangulation points associated with the
1927 Tango earthquake of M=7.5. The data observed during the period from
1884-1889 to 1927, which were compiled by TSUBOI(1932), are employed.
Numerals attached to the leveling route indicate the identity numbers of bench
marks.
Crustal Movements in Southwest Japan after Major Earthquakes
119
4.1 The 1927 Tango earthquake
The 1927 Tango earthquake of. M= 7.5 is probably one of the best geodetically
documented earthquakes in the Inner Zone and most appropriate for examining the
deformation due to postseismic stress relaxation. The epicenter and mechanism
diagram are shown in Fig. 2.
The 1943 Tottori and 1948 Fukui earthquakes seem less appropriate in
examining the postseismic deformation than the Tango earthquake. The revision
survey for the Tottori earthquake was carried out about 14 years after the
earthquake (SATO, 1973). This seems too long to distinguish coseismic and
postseismic deformations from the measurements. Moreover, it is uncertain how
much the thick alluvial layer in the Fukui region (NASU, 1950) affected the surface
deformation. And more unfortunately, the resurveyed regions after these two
earthquakes are smaller than in the case of the Tango earthquake.
Two remarkable seismic faults (the Gomura and Yamada faults), which are
conjugate to each other, appeared at the time of the Tango earthquake. Detailed
investigations of crustal movements associated with this earthquake were made
by TSUBOI(1930 a, b, 1931, 1932). Figure 12 shows the horizontal displacements
at the 137 triangulation points during the period from 1884-1889 to 1927. The
pattern of the displacements reveals large left-lateral strike-slip movements along
the Gomura fault and slightly right-lateral ones across the Yamada fault. Vertical
displacements during the period from 1888 to 1927 along the first-order leveling
route, which is shown in Fig. 12, are plotted in Fig. 13. A large dip-slip movement of
over 1 m is seen across the Yamada fault while the movement across the Gomura
Fig.
13. Changes in relative heights of bench marks along the first-order leveling
route. The solid and broken lines indicate the observed and calculated vertical
displacements, respectively. Bench mark 1366 is assumed to be a datum point.
The observed data were compiled by TSUBOI(1930 a). Fault models adopted
for the calculation are given in Table 3 and the locations of bench marks are
shown in Fig. 12.
120
T. TABEI
fault
is
about
The
of
the
and
0.7
(or
Studies
at
to
interpret
the
WALSH,
1969;
and
the
observed
the
Gomura
parameters):
km,
geodetic
and
are
generally
as
30-35
though
discuss
is
the
temporal
9-20
90•‹.
the
the
20 km
Gomura
Yamada
fault
for
been
1957;
to
CHINNERY,
1982).
by
the
made
On
varying
Gomura
1961;
the
other
fault
fault
para-
that
best
fits
of
km,
of
studies,
(see
Fig.
U0 = 3.0-3.7
strike-slip.
m,
The
discrepancies
the
the
4(b)
of
the
fault
and ă
m, ƒÉ_??_120•‹
takes
plane
the
models
are
of
a small
is
generally
(reverse
parameters
definitions
fault
among
Yamada
fault
for
value
considered
about
as
faulting
the
follows:
with
dip
L=
a
right-
and ƒÂ=40-60•‹.
a
simple
fault
the
of
postseismic
model
main
fault
in
into
the
present
have
the
but
study
the
been
fault
listed
in
dip-angle
obtained
of
after
on
study
is
comprehend
The
are
dislocation
present
to
deformation.
consideration,
parameters
a uniform
of
parameters
transient
adopted
with
purpose
spatial
Table
some
3.
to
and
parameters
the
the
not
of
Taking
the
the
Gomura
fault
modification
of
is
the
models.
Coseismic
Table
displacements
2 and
in
Fig.
observed
profile.
observed
ones,
sufficiently
As
Most
than
have
IWASAKI,
seismological
a few
variations
Other
pre-existing
shown
data
seismograms
follows
10-19
because
distribution
at
and
model
U0 = 1.3-1.8
study,
earthquake
fixed
km,
adopted
spatial
aftershock
are
component),
modes
Tango
(1935).
that
while
(KASAHARA,
left-lateral
parameters
present
plane
W=
there
fault
W=
the
km,
predominantly
strike-slip
In
concluded
geodetic
synthetic
these
The
fault
from
a seismological
to
vertical
10-15
NASU
shallower
50•‹.
observed
fault
L=
a
lateral
by
are
seismogram.
indicating
direction.
shock
He
MATSU'URA
calculated
established
main
traces.
models
1977;
one.
in detail
southwestward)
about
movements
(1973)
has
According
nearly
fault
MATSU'URA,
the
fault
of
strike-slip
investigated
inclined
angle
establish
KANAMORI
meters
the
steeply
an
crustal
the
was
along
vertical
northwestward
than
following
distributed
is nearly
hand,
is smaller
aftershocks
immediately
generally
dips
that
of
aftershocks
fault
in
m
distribution
the
14
Table
3.
models
and
vertical
The
in
have
been
in
Table
ones
calculated
indicating
effective
shown
fault
as
Figs.
first
12
Fault parameters
are
crustal
that
a
calculated
a
simple
approximation
and
14,
using
3. Horizontal
small
plotted
in
movements
structure
Fig.
13
generally
model
to
the
model
displacements
with
interpret
discrepancies
of the 1927 Tango earthquake
a
superposed
agree
well
uniform
the
between
adopted
listed
calculated
are
with
the
with
the
dislocation
mode
of
the
is
faulting.
observed
for the calculations.
and
Crustal Movements in Southwest Japan after Major Earthquakes
Fig.
14.
Calculated
lations
coseismic
coincide
are the same
horizontal
displacements.
with the triangulation
as those
employed
points
in Fig.
Points
shown
121
selected
in Fig.
for calcu-
12. Fault
models
13.
calculated crustal movements are recognized in the southwestern part of the region
and also in the head region of the Tango Peninsula. In the former case, the effects of
the 1925 Tajima earthquake of M= 7.0, which occurred at about 25 km west of and
about 21 months before the Tango earthquake, may be present. In the latter case,
however, the cause of discrepancy is not known. There might be systematic errors
due to movements of reference points which are not considered here. Nevertheless,
we consider that these discrepancies are small and negligible compared with the
general agreement of the calculated crustal movements with the observed ones.
4.2
Postseismic
In
reality,
laborious
is
too
tasks,
long
to
and
grasp
strike-slip
faulting.
following
the
The
has
probably
to
progress
Tango
15
to
that
the
with
been
caused
crustal
the
earthquake
generally
direction
of
shows
stress
surveys
it is unfortunate
Figure
is
due
geodetic
the
1927
deformation
ESE-WNW
deformation
repeated
not
horizontal
interval
between
movements
horizontal
strains
(GEOGRAPHICAL
characterized
a strain
relaxation
detect
by
rate
only
of
by
about
the
in
deformations
consecutive
detail
in
produced
in
cases
the
52
INSTITUTE,
an
0.2-0.3•~10-6/yr.
postseismic
surveys
most
in
SURVEY
a contraction
are
viscoelastic
almost
This
of
years
1987).
E-W
strain
deformation
to
field
122
T. TABEI
1979
15. Horizontal strains in Tango district produced in the 52 years following
the 1927 Tango earthquake (compiled from GEOGRAPHICAL
SURVEY
INSTITUTE,
1987).
Fig.
associated
with
which
is
to
positions
of
with
discrepancies
Tango
the
19P
derived
for
from
3.
The
Therefore,
about
a
the
of
the
the
the
the
is
and
of
the
ratio ā'/ā.
the
the
noteworthy
regional
comparable
calculated
stress,
The
because
latter
those
are
coseismic
also
at
in
is somewhat
there
is
strains
to
Peninsula
because
tectonic
direction.
former
nearly
Tango
upper
viscoelastic
mantle.
the
out
of
quantitative
displacements
in
deformation
as
from
derived
survey
earthquake.
We
shown
On
tentatively
deformation,
This
postseismic
latest
the
the
postseismic
time Ą,
of
are
in
and
the
the
by
same
the
14).
of
of
also
the
than
faults
field
observed
12
viscosity
21Ą after
viscosity
contribution
This
calculations
time
in
strain
others.
studies
but
from
The
(Figs.
the
earthquake
compression
larger
relaxation
the
about
a
apart
fault.
between
For
Tango
by
make
Peninsula
5•~10
time
1927
sufficiently
field
harmony
Sec.
the
characterized
considered
near
-1927
in
the
value
of
in
the
the
Japan,
above
we
viscosity
as
Fig.
15 corresponds
there
is little
= 10,
a value
assume ā'/ā>
been
discussed
viscosity,
contrary,
assume
has
to
in
is
2.44
an
elapsed
yr.
information
which
gives
Crustal Movements in Southwest Japan after Major Earthquakes
123
the deformation with a moderate amplitude, as shown in Fig. 10, but can be
modified later. Here, it should be noted that varying the viscosity ratio affects the
amplitude of deformation but has little effect on the deformation pattern.
The postseismic viscoelastic deformation during the period corresponding to
that shown in Fig. 15 is calculated for 132 mesh points at intervals of 4 km. The
contributions from the Gomura and Yamada faults are superposed, though,
because of its size, the effect of the Yamada fault is comparatively small. The
horizontal displacement field is quite similar to the model result shown in Fig. 7(b),
that is considered to directly reflect the pattern of tectonic flow induced in the
underlying viscoelastic layers. In contrast, the vertical displacement field, not shown
here, indicates a pattern which is quite different from that shown in Fig. 8(b). This is
chiefly because of the dip-slip component of the Gomura fault and partly because of
the existence of the Yamada fault.
Though the general features of the calculated horizontal strain field shown in
Fig. 16 are in harmony with the observed ones shown in Fig. 15, there are a few
discrepancies. The calculated strains are smaller than those observed. The distribution of extension is comparable with that of contraction in the calculated strain
field, while extension is hardly observed except in the Tango Peninsula. Moreover, a
Fig.
16.
Calculated
earthquake.
the
time
=10.
postseismic
Calculation
of
the
latest
horizontal
has
survey
been
shown
strains
made
at
in
Fig.
associated
t=21Ą,
15,
that
with
almost
assuming ƒÅ'/ƒÅ=5•~10
the
1927
Tango
corresponds
to
19P
and
ā'/ā
T. TABEI
124
strain concentration near the faults is seen in the calculated strain field but not in the
observed one. Quantitative discrepancies between the observed and calculated
strain fields can be reduced to some extent if we decrease the viscosity of the lower
crust and make it approach to that of the upper mantle, or if we adopt a smaller
value for the viscosity of the upper mantle keeping the viscosity ratio fixed.
However, the discrepancy between deformation patterns remains mostly unchanged. Thus it is supposed that the observed strain field shown in Fig. 15 is most
likely due to the effect of superposition of the postseismic viscoelastic deformation
associated with the 1927 Tango earthquake and the tectonic stress acting on the
region after the earthquake.
5.
Discussion
Figure 17 shows coseismic and postseismic changes in relative height between
two adjacent bench marks with a distance of about 2 km across the Gomura and
Yamada faults (KANAMORI,1973). The movement across the Gomura fault is
relatively small and its rate decayed rapidly within a few years after the earthquake.
In contrast, the movement across the Yamada fault has shown a gradual progress
that can be fitted by an exponential function with a time constant of about 3.0 yr.
As seen in Fig. 8(b), postseismic vertical displacement is hardly detectable by
geodetic measurements in the short distance across the vertical strike-slip fault. In
the case of the Gomura fault, which has a coseismic dip-slip component of about
0.8 m, calculated postseismic relative displacement between the bench marks above
is, at most, smaller than a few millimeters. Moreover, it is in the reverse sense to that
of the observed movement shown in Fig. 17 and has a much longer time constant.
On the other hand, the pattern of postseismic deformation around the inclined dipslip fault is largely dependent on the dip angle and width of the fault plane
Fig.
17. Coseismic and postseismic changes in relative height between two
adjacent bench marks with a distance of about 2 km across the Gomura and
Yamada faults (after KANAMORI,
1973). The postseismic change until 1930 was
first pointed out by TSUBOI(1931).
Crustal Movements in Southwest Japan after Major Earthquakes
(THATCHER
shows
et
al.,
a better
the
relative
this
creep-like
caused
is noticed
in
the
under
that
centration
the
around
in
the
after
the
such
creep-like
region
probably
in
an
of
is
part
in
the
same
that
a
still
new
remains
has
crustal
as
caused
a
stress
stress
regional
sense
were
there
tectonic
of
and
final
con-
continued
movements
creep
fault
plane
amount
even
frictional
Fig.
17
motions
on
motion
were
strengths
(1979).
plane.
underwent
of
While
functions,
fault
in
the
of
exponential
the
shown
the
parts
MIYATAKE
by
motion
Both
enough
the
same
on
the
there
There
the
long-term
is
no
infor-
is a possibility
consist
of
the
Andreas
San
a series
as
to
that
from
concentration
the
that
of
episodic
fault
(SMITH
reverse
that
of
of
the
the
fault
will
the
of
and
the
be
information
the
coseismic
of
Fig.
somewhat
9(a)
with
Fig.
elastic
taken
part
of
In
the
deformation
into
the
at
in
model
and
that
be
other
of
location
is noticed
will
strain
the
accumu-
words,
in
accumulation:
9(b).
by
the
size
it
Therefore,
the
a
the
time-dependence.
release;
reduced
and
though
the
deformation
strain
plane
case,
elastic
However,
viscoelastic
be
fault
of
the
probably
deformation
this
about
additional
postseismic
the
patterns
those
occurred.
of
on
in
cannot
no
though
additional
slip
The
to
amount
is
part
comparison
near
the
additional,
This
gradual
neglected.
motion
elastic
an
fault.
time-dependent
similar
there
creep-like
sense
sense
the
deformation
since
the
be
in
the
are
quite
except
the
to
to
are
induce
of
parts
additional
in
will
responding
small
where
strain
reverse
the
This
the
additional
strain
strain
in
sense.
Considering
strains
the
contribution
into
the
0.16•~10-6/yr
in
0.04•~10-6/yr
on
the
rate
of
tectonic
to
part.
this
plane
regional
and
surroundings
deformation,
horizontal
the
the
infer
deformation
movements
Though
the
the
on
curves
the
elastic
study,
inferred
after
we
viscoelastic
these
distribution
behavior
earthquake,
postseismic
is
fault
However,
decades
caused
that
duration
represented
deformation
time
in
the
earthquake
which
the
creep-like
in
part
nearly
three
Therefore,
stress.
dominating
MIKUMO
similar
viscoelastic
additional
lated
Yamada
fault.
1968).
of
the
the
non-uniform
by
are
manner
viscoelastic
of
the
displacement
the
transient
present
tectonic
determine
short-term
deformation
the
not
that
expected
and
out
postseismic
consists
the
on
the
WYSS,
slight,
is
However,
pointed
cumulative
The
in
progressed
implies
of
it
concerned
movements
about
and
were
movements
regional
unfortunately,
as
creep-like
creeps
the
faults,
motions.
plane,
the
even
the
Gomura
movement.
17
This
occurrence
dependent
mation
for
the
earthquake.
e cannot,
W
fault
Fig.
creep-like
to
the
1 cm
observed
in
for
relaxation.
these
similar
than
exceed
the
movements.
state
possibility
not
shown
that
calculated
observation
than
stress
coseismic
stress
acting
smaller
postseismic
It
displacement
the
does
is far
movements
by
that
Relative
with
displacement
earthquake;
the
1980).
agreement
125
strain
the
of
postseismic
about
of
tectonic
strain
ESE-WNW
basis
regional
field.
direction
the
0.2•~10-6/yr
three
Here,
and
following
in
maximum
stress,
we
we
assume
introduce
contraction
of
extension
of
perpendicular
observations:
shear
uniform
the
(NAKANE,
stationary
1973),
the
T. TABEI
1 26
Fig.
18.
Calculated
strain
superposed.
a rate
of
of
postseismic
The
0.16•~10-6/yr
0.04•~10-6/yr
horizontal
regional
in
in
the
the
strain
ESE-WNW
perpendicular
strains
is assumed
with
to
direction
direction
the
consist
and
to
that
uniform
regional
of contraction
extension
of
the
with
with
a rate
contraction.
strain observed at the positions sufficiently apart from the fault as shown in Fig. 15,
and the strikes of the Gomura and Yamada faults. Figure 18 shows the calculated
postseismic strain field, where the uniform regional strains are superposed on the
postseismic viscoelastic ones shown in Fig. 16. The additional deformation due to a
postseismic creep-like motion is not considered. This figure shows a better
agreement with the observed results shown in Fig. 15, though the discrepancy in the
Tango Peninsula remains little improved.
Figure 18 also indicates the difficulties in understanding and distinguishing the
effects of the deformation caused by postseismic stress relaxation from the strain
fields observed. This is because the pattern of the postseismic viscoelastic strain
fields is quite similar to that of the regional tectonic strain. To remove the ambiguity
resulting from the similarities and to distinguish between these two types of
deformations, a long-term detailed monitoring of postseismic crustal movements is
indispensable in the region extending to about 100km from the fault. This is
because remarkable postseismic displacement will appear in the near field about 15
to 40 km away from the fault, as shown in Fig. 10. Unfortunately, such monitoring
has not yet been accomplished, mainly because of the shortage of measurements
Crustal Movements in Southwest Japan after Major Earthquakes
127
resulting from the laborious tasks involved in repeated geodetic surveys and partly
because of a lack of accuracy. However, we are convinced that these problems will
be overcome in the near future by employing a new and more accurate method with
a simpler management; for example, by three-dimensional relative positionings
using the Global Positioning System (GPS) engaged in advanced space techniques.
According to YAMAUCHI
(1975), the apparent relaxation times of the postseismic deformations detected by continuous observations of crustal movements or
geodetic measurements range from about 20 min to about 8 yr, though not all of the
deformations accompanied earthquakes of strike-slip fault type. The remarkable
postseismic deformation with its small relaxation time, that is often detected by
continuous observations with strainmeters or tiltmeters, cannot be explained by
stress relaxation in the underlying viscoelastic layers. Therefore, another deformation mechanism should be considered. TANAKA(1981) has concluded that some
of the deformation with a time constant of a few years may be caused by viscoelastic
stress relaxation within rocks around the observation vault. Moreover, it is
supposed that the earthquake-induced transient movements of underground water
sometimes have large effects on local surface deformations during the period
immediately after an earthquake (TABEI,1987). The mechanism of the postseismic
deformation that originates from the superficial layer of the crust, however, has not
been considered in the present study. To understand the nature of the crust, more
details of the mechanism of postseismic deformation should be investigated.
6.
Concluding
Remarks
Transient deformation caused by postseismic stress relaxation in underlying
viscoelastic layers is examined through a three-dimensional
numerical simulation
mainly for strike-slip faulting in the Inner Zone of Southwest Japan. In the present
model, viscoelastic treatments are introduced to the upper mantle and also to the
lower crust on the basis of the depth distribution of hypocenters
of microearthquakes in the region concerned.
It is made clear that the fault size as well as the viscoelastic structure of the
earth's crust and upper mantle has a large effect on the amount of postseismic
deformation.
Thus the fault size should be taken into serious consideration
in
modeling the faulting.
The most noteworthy result in the model calculations
is the distribution
patterns of the postseismic strain field. A large shear strain, which is similar to the
preseismic strain accumulation and is contrary to the coseismic strain release, is
induced for a long time after faulting in the near field of the fault zone. The
viscoelastic structure of the earth's crust and upper mantle, however, is hard to
determine solely on the basis of geodetic measurements
executed at long time
intervals. Therefore, a long-term detailed monitoring
of the deformation
is indispensable and other kinds of field investigations are required to obtain further
information about the viscoelastic structure.
128
T. TABEI
Crustal movements associated with the 1927 Tango earthquake of M= 7.5 are
examined for both the coseismic elastic rebound and the postseismic viscoelastic
transient deformation caused by stress relaxation after faulting. The coseismic
movements can be fitted by a simple conjugate fault system in which a uniform
dislocation on the fault plane is assumed. The discrepancies between the observed
and calculated postseismic movements are most likely due to the effect of tectonic
stress. The tectonic stress is considered to have continued acting and dominating the
regional crustal movements even after the earthquake.
The ambiguity resulting from the similarities between the deformation due to
postseismic stress relaxation and the regional tectonic movements remains unsolved. But we are convinced that it will be overcome by long-term detailed
monitoring of the movements employing a new technique with a higher accuracy
and a simpler management; for example, by three-dimensional relative positionings
using the Global Positioning System, in the near future.
I would like to thank Professor Ichiro Nakagawa for giving helpful advice throughout
the course of this study and critically reading the manuscript. I wish also to acknowledge the
valuable discussions with Professor Norihiko Sumitomo, Mr. Yutaka Tanaka, Mr. Kunio
Fujimori, and the members of our laboratory. My thanks are also due to Professors
Yoshimichi Kishimoto, Takeshi Mikumo, Torao Tanaka, and Kazuo Oike, who kindly
offered helpful comments for the revision of the manuscript.
The numerical calculations were executed with the computer system at the Data
Processing Center of Kyoto University.
REFERENCES
ANDO, M., Source mechanisms and tectonic significance of historical earthquakes along the
Nankai trough, Japan, Tectonophysics, 27, 119-140, 1975.
ANDREWS, D. J. and N. H. SLEEP, Numerical modelling of tectonic flow behind island arcs,
Geophys. J. R. Astron. Soc., 38, 237-251, 1974.
CHEN, W. and P. MOLNAR, Focal depths of intracontinental
and intraplate earthquakes and
their implications for the thermal and mechanical properties of the lithosphere, J.
Geophys. Res., 88, 4183-4214, 1983.
CHINNERY,M. A., The deformation of the ground around surface faults, Bull. Seismol. Soc.
Am., 51, 355-372, 1961.
GEOGRAPHICALSURVEYINSTITUTE,Horizontal strain in Japan 1985-1883, Technical Data F.
1-No. 6 in Geographical Survey Institute, 133 pp., 1987.
HASHIDA, T. and K. SHIMAZAKI,Determination of seismic attenuation structure and source
strength by inversion of seismic intensity data: Tohoku district, Northeastern Japan, J.
Phys. Earth, 35, 67-92, 1987.
HASHIMOTO,M., Numerical modelling of the three-dimensional
stress field in southwestern
Japan, Tectonophysics, 84, 247-266, 1982.
HIRAHARA,K., Three-dimensional
seismic structure beneath Southwest Japan: The subducting Philippine Sea plate, Tectonophysics, 79, 1-44, 1981.
HUZITA, K., Y. KISHIMOTO,and K. SHIONO, Neotectonics and seismicity in the Kinki area,
Crustal Movements in Southwest Japan after Major Earthquakes
129
Southwest Japan, J. Geosci. Osaka City Univ., 16, 93-124, 1973.
ICHIKAWA,
M., Reanalyses of mechanism of earthquakes which occurred in and near Japan,
and statistical studies on the nodal plane solutions obtained, 1926-1968, Geophys. Mag.
Jpn. Meteorol. Agency, 35, 207-274, 1971.
ITO, K., Depth of seismicity and its relation to thermal structure and large intra-plate
earthquakes in northern Kinki district, Japan, Rep. Meet. Stud. Earthq. Predict. Kyoto
Univ., 17, 6-18, 1988 (in Japanese).
IWASAKI,T., Quasi-static deformation due to a dislocation source in a Maxwellian
viscoelastic earth model, J. Phys. Earth, 33, 21-43, 1985.
IWASAKI,T. and M. MATSU'URA,Quasi-static crustal deformations due to a surface load:
Rheological structure of the earth's crust and upper mantle, J. Phys. Earth, 30, 469-508,
1982.
KANAMORI,
H., Mantle beneath the Japanese arc, Phys. Earth Planet. Inter., 3, 475-483, 1970.
KANAMORI,H., Tectonic implications of the 1944 Tonankai and the 1946 Nankaido
earthquakes, Phys. Earth Planet. Inter., 5, 129-139, 1972 a.
KANAMORI,
H., Determination of effective tectonic stress associated with earthquake faulting,
The Tottori earthquake of 1943, Phys. Earth Planet. Inter., 5, 426-434, 1972 b.
KANAMORI,
H., Mode of strain release associated with major earthquakes in Japan, Ann. Rev.
Earth Planet. Sci., 1, 213-239, 1973.
KASAHARA,
K., The nature of seismic origins as inferred from seismological and geodetic
observations (1), Bull. Earthq. Res. Inst., Univ. Tokyo, 35, 473-532, 1957.
KOBAYASHI,
Y., A relationship between the distribution of focal depth of microearthquakes
and surface heat flow in the southwestern Japan and central Japan, Proc. Earthq.
Predict. Res. Symp., 184-193, 1976 (in Japanese).
MATSU'URA,
M., Inversion of geodetic data. Part II. Optimal model of conjugate fault system
for the 1927 Tango earthquake, J. Phys. Earth, 25, 233-255, 1977.
MATSU'URA,M. and T. IWASAKI,Analysis of geodetic data by using a new inversion
technique. Redetermination of fault model of the 1927 Kita-Tango earthquake, Prog.
Abstr. Seismol. Soc. Jpn., No. 1, 1, 1982 (in Japanese).
MATSU'URA,M. and T. IWASAKI,Study on coseismic and postseismic crustal movements
associated with the 1923 Kanto earthquake, Tectonophysics, 97, 201-215, 1983.
MATSU'URA,M., T. TANIMOTO,
and T. IWASAKI,Quasi-static displacements due to faulting in
a layered half-space with an intervenient viscoelastic layer, J. Phys. Earth, 29, 23-54,
1981.
MEISSNER,
R. and J. STREHLAU,
Limits of stresses in continental crust and their relation to the
depth-frequency distribution of shallow earthquakes, Tectonics, 1, 73-89, 1982.
MIKUMO,T. and T. MIYATAKE,
Earthquake sequences on a frictional fault model with nonuniform strengths and relaxation times, Geophys. J. R. Astron. Soc., 59, 497-522, 1979.
MIKUMO,T., H. WADA, and M. KoizuM!, Seismotectonics of the Hida region, central
Honshu, Japan, Tectonophysics, 147, 95-119, 1988.
MIYASHITA,K., Surface displacement patterns associated with asthenospheric stress relaxation following major underthrust earthquakes, Zisin (J. Seismol. Soc. Jpn.), 36, 541550, 1983 (in Japanese).
MIYASHITA,
K., A model of plate convergence in Southwest Japan, inferred from leveling
data associated with the 1946 Nankaido Earthquake, J. Phys. Earth, 35, 449-467, 1987.
MIZOUE,M., Some remarks on the characteristics of subcrustal earthquake activities, Proc.
130
T. TABEI
Earthq. Predict. Res. Symp., 97-105, 1976 (in Japanese).
NAKANE,K., Horizontal tectonic strain in Japan (II), J. Geod. Soc. Jpn., 19, 200-208, 1973 (in
Japanese).
NAKANISHI,
I., Precursors to ScS phases and dipping interface in the upper mantle beneath
Southwestern Japan, Tectonophysics, 69, 1-35, 1980.
NASU,N., Supplementary study on the stereometrical distribution of the after-shocks of the
great Tango earthquake of 1927, Bull. Earthq. Res. Inst., Univ. Tokyo, 13, 335-399,
1935,
NASU,N., Crustal deformation, in The Fukui Earthquake of June 28, 1948, ed. Committee for
the Study of Fukui Earthquake, pp. 93-130, Tokyo, 1950.
NISHIDA,R., Earthquake generating stress in eastern Chugoku and northern Kinki districts,
Southwest Japan, Bull. Disas. Prey. Res. Inst., Kyoto Univ., 22, 197-233, 1973.
NUR, A. and G. MAVKO,Postseismic viscoelastic rebound, Science, 183, 204-206, 1974.
OIKE,K., Seismic activities and crustal movements at the Yamasaki fault and surrounding
regions in the Southwest Japan, J. Phys. Earth, 25, Suppl., S31-S41, 1977.
RUNDLE,J. B. and D. D. JACKSON,
A three-dimensional viscoelastic model of a strike slip
fault, Geophys. J. R. Astron. Soc., 49, 575-591, 1977.
SATO,H., A study of horizontal movement of the earth crust associated with destructive
earthquakes in Japan, Bull. Geograph. Surv. Inst., 19, 89-130, 1973.
SATO,K., H. Tam, and A. TAKAGI,Characteristics of crustal stress and crustal movements in
the northeastern Japan arc. I: Based on the computation considering the crustal
structure, Zisin (J. Seismol. Soc. Jpn.), 34, 551-563, 1981 (in Japanese).
SAVAGE,
J. C. and W. H. PRESCOTT,
Asthenospheric readjustment and the earthquake cycle,
J. Geophys. Res., 83, 3369-3376, 1978.
SHIONO,K., Focal mechanisms of major earthquakes in Southwest Japan and their tectonic
significance, J. Phys. Earth, 25, 1-26, 1977.
SIBSON,R. H., Fault zone models, heat flow, and the depth distribution of earthquakes in the
continental crust of the United States, Bull. Seismol. Soc. Am., 72, 151-163, 1982.
TABEI,T., On the postseismic deformation at Ikuno immediately after the 1984 Yamasaki
rthquake, J. Phys. Earth, 35, 225-239, 1987.
ea
TANAKA,
T., On the viscoelastic changes appearing on tiltmetric and extensometric records of
the ground, J. Geod. Soc. Jpn., 27, 225-238, 1981.
THATCHER,
W. and J. B. RUNDLE,A model for the earthquake cycle in underthrust zones, J.
Geophys. Res., 84, 5540-5556, 1979.
THATCHER,
W. and J. B. RUNDLE,A viscoelastic coupling model for the cyclic deformation
due to periodically repeated earthquakes at subduction zones, J. Geophys. Res., 89,
7631-7640, 1984.
THATCHER,
W., T. MATSUDA,
T. KATO,and J. B. RUNDLE,Lithospheric loading by the 1896
Riku-u earthquake, Northern Japan: Implications for plate flexure and asthenospheric
rheology, J. Geophys. Res., 85, 6429-6435, 1980.
THE RESEARCH
GROUPFORACTIVEFAULTS,Active Faults in Japan, the University of Tokyo
Press, Tokyo, 239 pp., 1980 (in Japanese).
TSUBOI,C., Investigation on the deformation of the earth's crust in the Tango district
connected with the Tango earthquake of 1927 (Part I), Bull. Earthq. Res. Inst., Univ.
okyo, 8, 153-221, 1930a.
T
Crustal Movements in Southwest Japan after Major Earthquakes
131
TSUBOI,C., Investigation on the deformation of the earth's crust in the Tango district
connected with the Tango earthquake of 1927 (Part II), Bull. Earthq. Res. Inst., Univ.
Tokyo, 8, 338-345, 1930b.
TSUBOI,C., Investigation on the deformation of the earth's crust in the Tango district
connected with the Tango earthquake of 1927 (Part III), Bull. Earthq. Res. Inst., Univ.
Tokyo, 9, 423-434, 1931.
TSUBOI,C., Investigation on the deformation of the earth's crust in the Tango district
connected with the Tango earthquake of 1927 (Part IV), Bull. Earthq. Res. Inst., Univ.
Tokyo, 10, 411-434, 1932.
WALCOTT,R. I., Structure of the earth from glacio-isostatic rebound, Ann. Rev. Earth Planet.
Sci., 1, 15-37, 1973.
WALSH,J. B., Dip angle of faults as calculated from surface deformation, J. Geophys. Res.,
74, 2070-2080, 1969.
YAMAUCHI,
T., On the elastic after-effect associated with earthquake, J. Geod. Soc. Jpn., 21,
75-80, 1975 (in Japanese).
YANG,M. and M. N. TOKS_??_Z,
Time-dependent deformation and stress relaxation after strike
slip earthquakes, J. Geophys. Res., 86, 2889-2901, 1981.
YOSHII,T., Y. SASAKI,
T. TADA,H. OKADA,S. ASANO,I. MURAMATU,
M. HASHIZUME,
and T.
MORIYA,The third Kurayosi explosion and the crustal structure in the western part of
Japan, J. Phys. Earth, 22, 109-121, 1974.