Download 3.4 Seismic waves in a spherical earth 3.5 Body wave travel time study

3.4 Seismic waves in a spherical earth
• Ray paths and travel times
• Velocity distribution
• Travel time curve inversion
3.5 Body wave travel time study
•
•
•
•
Body wave phases
C
Core
phases
h
Upper mantle structure
Lower mantle structure
3.4.1 Ray paths and travel times
1
2
3.4.2 Velocity distributions
3
3.4.3 Travel time inversion
4
3.5 Body wave travel time studies
J-B (1940)
IASP91 (1995)
5
3.5.1 Body wave phases
6
7
8
3.5.2 Core phases
PKP series
9
Detailed Inner core velocity Structure
A
B
Tae-Gyu Yee, 2009
Real observation of core phases
1. The arrivals do not fall along narrow
lines.
2. The PKP-BC branch continues
beyond its geometrically predicted
limit of 153 deg
3. The PKP travel times show an
additional branch not predicted by
geometrical ray theory
10
11
Inner core anisotropy
Spherical harmonic expansion up to
a degree 4 of the PKIKP-P travel
time residual field. Values are in
tenths of a second. A low number
means a fast velocity in the core.
PKIKP and P travel time
residuals are computed
p
with
respect to JB travel time curve.
• Station residuals of the seismic core phase PKIKP have been computed for 400 seismological
observatories worldwide using 5 yr of the International Seismological Center (ISC) Bulletics.
• PKIKP travel times can be corrected for upper mantle propagation by subtracting P delays
9PKIKP-P residuals
• PKIKP-P exhibit a latitudinal dependence
9Polar stations tend to be faster than equatorial stations.
9This pattern may reflect a departure from spherical symmetry in the P-velocity distribution in
the vicinity of the inner core boundary of the Earth.
Poupinet et al., Nature, 1983
12
Super rotation study using doublets
Zhang, Science, 2003
Super rotation study using doublets
Zhang, Science, 2003
13
Topography of ICB using doublets
Cao el al., PNAS, 2007
PKJKP
Cao et al., Science (2005)
14
3.5.3 Upper mantle structure
•
The sub-crustal lithosphere (seismic lithosphere or lid)
–
–
–
–
•
P- and S- wave velocities of about 8.1 and 4.5 km/s
On global average, the thickness is about 80-100 km
At mid-ocean ridge, its thickness becomes zero
Beneath stable cratons,, lithosphere
p
extends to about 200 km
Low velocity zone (asthenosphere – mechanical term)
– In tectonically active regions, the LVZ is well developed and relatively shallow.
– In stable continental regions, the LVZ is deeper and less pronounced, and may
not even present
•
Transition zone
The Transition Zone (TZ)
•
•
•
•
•
410 – 660 km (the lower part of the
upper mantle)
Velocity is increased by 4 and 8% at 410
and 660 km
Phase transition (olivine - β spinel, γ
spinel
i l – perovskite)
ki )
Topography change due to subducting
slab or hot plume
Whole mantle convection vs. upper and
lower mantle convections ?
15
Comaprison of GCA
GCA: the Gulf of California model
ARC-TR: Arc-trench for the Japan
subductioon zone
T7: Tectonically active western portion of
North America
K8: Stable Eurasia shield
A study of the transition zone using SS or PP precursors
Shearer and Flanagan, Science, 1999
16
A thickness of the transition zone
Gu and Dziwonski, EPSL, 1998
3.5.4 Lower mantle structure
•
•
Velocity increase rapidly with depth for
roughly 100 km beneath the 660 km, but then
increase more slowly
• The mineral transformation continues up
to 100 km
• No
N transformation
f
i at deeper
d
depth.
d h Only
O l
controlled by pressure.
• Weak seismic discontinuities at 900 and
1300 km
Very complex layer at the base of the lower
mantle – D”
• Thermal boundary layer between the
mantle and hotter core. (~1000 deg
difference)
17
Very complex structure in D”
Variability of the D”
•
SdS or PdS phase are observed at many
locations on the CMB, but not
everywhere.
• Even nearby
• The Discontinuity has large
topographic
hi variations
i i
over small
ll
spatial wavelengths the focus and
defocus waves.
• There is no actual discontinuity, but
that complex three dimensional
velocity heterogeneities give the
appearance of discontinuity
18
D″ Discontinuity
•Is it intermittent?
•Relation to slabs?
•Due to anisotropy?
•Due to phase change?
•Due to CBL?
Correlation between regions of fast velocities in D” and the projected locations
of fossil slabs from ancient subduction zones
19
A Study of CMB usign SPdKS phase (ULVZ)
Thorne and Garnero, JGR, 2004
A Detailed Seismic Structure of D”
Ni et al., Science (2002)
20
Medical Tomography
CT (Computerized tomography) Scan
Seismic
Tomography
data
Travel time tomography
T = ∫ 1 / v( s )ds = ∫ u ( s )ds
δT = ∫ δu ( s )ds
ΔTi = ∑ Gij Δu j
j =1
21
Travel time tomography
van der Hilst, Nature, 1999
22
3.6 Anisotropic earth structure
SPO (shape-preferred orientation) or LPO (lattice-preferred orientation)
23
“Azimuthal” anisotropy: like transverse isotropy on its side
24
25
3.6.5 Anisotropy in the lithosphere and the asthenosphere
Variations in Pn wave velocities. Maximum
is in the direction of spreading when the
plate formed
Anisotropy in the crust – the presence of the crack
26
How to study anisotropy within and beneath the continental lithosphere?
27
The Appalachian
orogenic belt
SH faster
28
3.6.6 Anisotropy in the mantle and the core
D″ Anisotropy
• Lattice-Preferred
Lattice Preferred
Orientation (LPO)?
• Shape-Preferred
Orientation (SPO)?
• Result of a Chemical
Boundary Layer?
• Related to Slabs?
McNamara, van Keken, and Karato [2002]
29
D″ Anisotropy
• Often most intense at
the top of D″.
• Most always with SH
faster than SV
(Æ transverse isotropy)
• Shows large lateral
variation.
• Often associated with
y
the D″ discontinuity.
Fouch et al. [2001]
30
3.7 Attenuation and anelasticity
•
•
•
•
Geometric spreading
Multipathing
Scattering
Intrinsic attenuation
31
3.7 Attenuation and anelasticity
Earthquake in Texas.
MNV : 15° (Nevada)
MM18 : 14° (Missouri)
Seismic velocity variations
between these areas are
less than 10%
Attenuation vs. Temperature
32
Melt-filled magma chamber
3.7.2 Geometric spreading
“Energy per unit wave front varies as a
wave front expands or contracts”
For body wave, the amp. decreases
as 1/r
33
3.7.3 Multipathing
34
3.7.4 Scattering
a = size of heterogeneity
L = distance wave travels
lambda = wavelength
35
36
3.7.5 Intrinsic attenuation
37
38
39
40
3.7.7 Spectral resonance peaks
41
Three damped harmonic oscillator systems in Seismology
1. The attenuation of the normal modes
2 The behavior of a seismometer
2.
3. The response of a building
42
3.7.8 Physical dispersion due to anelasticity
43
EUS : Eastern US
B&R : Basin and Range
44
45
46
47
48
3.8.1 Density within the earth
49
50
51
52
3.8.2 Temperature in the earth
53
3.8.3 Composition of the mantle
Mantle
Dunite : 92% Olivine
90% Forsterite (Mg2SiO4)
< 50% Fayalite (Fe2SiO4)
Core
Iron + lighter element
e.g., Fe2Si
54
Oxygen (white)
Silicon (Black)
Magnesium/Iron (Grey)
55
410 km
520 km
56
3.8.4 Composition of D”
General thermal convection
The interaction of subducting slabs
with a chemical boundary layer
consisting of dense mantle dregs
A chemical boundary layer formed
from delaminated post-eclogite
g down with the
ocean crust brought
slabs
A mineralogical phase chage
3.8.5 Composition of the core
Inner core is hotter than outer core.
Temperature increases by 3 % but pressure increases by 11 %
57
3.8.6 Seismology and planetary evolution
Whole core molten
The core cooled
Th
l d
The geotherm lowered
A frozen inner core
58
A cause of seismicity : Meteoroid impacts or moonquakes by tidal forces
• The existence of the core is inconclusive
•The moment of inertia ratio of 0.39 allows
at most a small core
Interior of other planets
Europa
Io
59