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Seismic coupling, down-dip limit of the
seismogenic zone, and dehydration of
the slab
Tetsuzo Seno
（Earthquake Res Inst, Univ of Tokyo）
Along-arc variation of
Seismic coupling
&
Down-dip limit of the seismogenic zone
near Japan
in relation to
Dehydration from the subducted crust
(Low-frequency tremor)
Seismic coupling along the Nankai - Sagami Troughs
N
Okhotsk plate
13
Japan Sea
Northern
Honshu
Eurasian plate
A
80
C
Kanto
Tokai
35°
Shikoku
Pacific
plate
Pacifiic
Plate
D Sagami
B
Trough
79
Kii
Pen.
Seno et al. (1996)
27
42
Hyuganada
59
49
Seno et al. (1993)
500 km
Philippine
SeaPlate
plate
Philippine Sea
30°
130° E
135°
140°
145°
Fig. 1
Occurrence of smaller earthquakes in Hyuganada
Yagi (2002)
Recurrence intervals of great earthquakes
Central Honshu
1923 Taisho
Tokai, 1707, 1854
Kanto
~1500 ∞
~400
~150 yrs
Izu
1703 Genroku
~1500
∞
Seismic coupling coefficient
(Seismic slip/plate motion)
Nankai Trough
Sagami Trough
~100 %
~50%
Taisho Kanto
・Vc = 4 - 6 cm/yr
・Vc = 3 cm/yr
・U = 4 - 6 m
・U = 6 m
・T = 90 - 150 yrs
・T = ~400 yrs
Down-dip limit of the seismogenic zone
Oleskevich et al. (1999)
Miyagi-oki,
n. Honshu
1978 Miyagi-oki earthquake (M7.5)
50 km
Seno & Pongsawat (1982),
Seno et al. (1980)
Northern Honshu: three types of events
Thrust
Down-dip compression Down-dip tension
Kawakatsu & Seno (1983)
Whole section: three types of events
Thrust
Down-dip compression
Down-dip tension
Kawakatsu & Seno (1983)
N
13
Japan Sea
Northern
Honshu
A
80
C
Kanto
Tokai
35°
Shikoku
Pacifiic Plate
D Sagami
B
Trough
79
Kii
Pen.
27
42
Hyuganada
59
49
500 km
Philippine Sea Plate
30°
130° E
135°
140°
145°
Fig. 1
Philippine Sea slab beneath Kanto
Hori (1997)
S. Kyushu P-axes
●
●
●
●●
●
●
thrust
Goto (2001)
Seismic coupling
Down-dip limit of the seismogenic zone
in relation to
Dehydration from the subducted crust
(Low-frequency tremor)
Basic assumptions
Fault strength
Tectonic stress
100 MPa
1 GPA
Lithostatic
pressure
3 km
30 km
Weakening
Depth
Weakening mechanisms
Interplate earthquakes
Elevated pore fluid pressure
Slab earthquakes (intermediate-depth)
Dehydration instability
Dehydration instability: Serpentinite
Raleigh & Paterson (1965)
Dehydration loci for slab seismicity
(a) Cold slab type
(b) Hot slab type
Dehydration from crust
Dehydration from
crust
Dehydration from
serpentine
Dehydration from serpentine
Yamasaki & Seno (2003)
Low-frequency Tremor in the upper plate wedge
Obara (2002)
Subduction of
Normal oceanic crust
Island-arc crust subd
Hot slab type: Nankai Trough
Freq. great earthquakes
Low freq. tremor
No great earthq
No low freq. tr
Upper plate
Crust
No dehydration
Dehydration from crust
No d
N. of Izu
Tokai district:
easternmost
Nankai Trough
Piosson’s ratio (Kamiya & Kobayashi, in prep.)
Low frequency tremor (Obara, 2002)
350°C
Seismicity: Matsumura (1997)
Dehydration
From crust
Temperature: Seno & Yamasaki , in prep.
Low-frequency earthquakes & tremor:
Upper plate
震源
Obara(2002)
Low-frequency tremor
~ Moho depth (Obara, 2002)
No tremor region
Kanto
E. Shikoku
S.Kyushu
Okino et al. (1999)
Kinan seamount chain
Okino et al. (1994)
Hypothesis:
Subducted continental or
island-arc crust is mainly
composed of granite, then
does not involve dehydration.
No low-frequency tremor
No earthquake within the subducted crust
Discrimination of crustal events
b) Event in the mantle
a) Event in the oceanic crust
Serpentine dehydration loci
Later crustal phase
Serpenine dehydration loci
No later crustal phase
Generally along the Nankai Trough
PHS slab beneath Kanto
(no tremor)
No later phase
(Hori, 1990)
Hori (1997)
E. Shikoku (no tremor)
Kurashimo et al. (2002)
S. Kyushu (no tremor)
DDC
DDT
110 km
aa
200 km
DDT, i.e., lower plane seismicity in the mantle
Hayashimoto et al. (2001)
S. Kyushu
thrust
mantle
Goto et al. (2001)
DDT
Thrust
Areas without earthquakes within
the subducted crust
Kanto
E. Shikoku
S. Kyushu（？）
Areas without low-frequency tremor
High Vp
Kanto
Slab
Izu
Obara (2002)
Top of slab
seismicity
Noguchi & Sekiguchi (2001)
Mature collision zones: Zagros and Himalaya
Ni & Barazangi (1986)
Normal oceanic crust subduction
Freq. great earthquakes
Low freq. tremor
Island-arc crust or continental
crust subduction
No great earthquakes
No low freq. tremor
Upper plate
Crust
No dehydration
Dehydration from crust
NankaiTrough
No dehydration
N. of Izu, Zagros, Himalaya
Areas with island-arc or continental crust subducted
Kanto, E. Shikoku, S. Kyushu
Infrequent large
interplate earthquakes
vs.
N. Izu, Zagros, Himalaya
No or very rare large
interplate earthquakes
Philippine Sea slab beneath Kanto
Hori (1997)
Origin of the double seismic zone in Kanto
Izu-Bonin Arc
Shikoku Basin
V. F.
Serpentine stable
Trench Axis
Seno et al. (2001); Seno & Yamasaki, in prep.
Kanto
Infreq. great earthquakes
& deep thrusts
No low freq. tremor
Dehydration
from serpentine
No dehydration
from crust
Northern Honshu: three types of events
Thrust
Down-dip compression Down-dip tension
Kawakatsu & Seno (1983)
Larger events
1994 aftershocks (Hino et al.,
2000)
Kawakatsu & Seno (1983)
Epicenter
Asperity
Yamanaka et al. (2001))
Vp in northen part of n. Honshu
(a)
W
E
Depth = 40 km
(a)
(b)
(b)
S. Ito et al. (2000)
Cold mantle wedge
Fractured zone
Ductile shear zone +
hydrofracturing
Anisotropic
permeability
Serpentine
No serpentine
350°C
350°C
Repeated earthquakes
N. Honshu, Kanto, S. Kyushu
Plastic flow
Iwate-oki, Bonin, Tonga
Hot mantle wedge
Ductile shear zone +
hydrofracturing
Serpentine
350°C
Plastic flow
Nankai, Ryukyu, Cascadia, Mexico
Collision zone
Noserpentine
serpentine
No
Stable sliding with very high shear stress
or delamination
N. Izu, Himalaya, Zagros
Conclusions
・Sudbduction of island-arc or continental crust does not involve
dehydration of subducted crust, then does not induce lowfrequency tremor in the wedge, or large interplate earthquake.
・If sudbduction of island-arc crust accompanies dehydration
of the serpentinized mantle, it produces large interplate
earthquakes infrequently.
・Thrust zone in the manle part may evolve into either of the
two branches: earthquake-no serpentinized wedge or no
earthquake-serpentinized wedge. This determines the
down-dip limit of the seismogenic zone.
・Earthquake occurrence in Kanto may be a result of
no earthquake occurrence in Bonin; if Bonin generated great
earthquakes, Kanto would have turned into a mountain belt.