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)

A

B

C

D

Kanto

NorthernHonshu

Tokai

KiiPen.

Shikoku

Japan Sea

Pacifiic Plate

Philippine Sea Plate

SagamiTrough

Fig. 1

Hyuganada

80

79

59

27

42

49

13

130° E 135° 140° 145°

30°

35°

N

500 km

Seismic coupling along the Nankai - Sagami Troughs

Eurasian plate

Okhotsk plate

Pacific plate

Philippine Sea plate

Seno et al. (1996)

Seno et al. (1993)

Yagi (2002)

Occurrence of smaller earthquakes in Hyuganada

1703 Genroku

1923 Taisho Tokai, 1707, 1854

~150 yrs

~400

~1500

~1500 ∞

∞

Central Honshu

Recurrence intervals of great earthquakes

Izu

Kanto

Nankai Trough

~50%

Sagami Trough

・ Vc = 3 cm/yr

・ U = 6 m

~100 %

・ Vc = 4 - 6 cm/yr

・ U = 4 - 6 m

Seismic coupling coefficient

・ T = 90 - 150 yrs ・ T = ~400 yrs

Taisho Kanto

(Seismic slip/plate motion)

Oleskevich et al. (1999)

Down-dip limit of the seismogenic zone

Seno & Pongsawat (1982),Seno et al. (1980)

1978 Miyagi-oki earthquake (M7.5)

50 km

Miyagi-oki,n. Honshu

Kawakatsu & Seno (1983)

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

A

B

C

D

Kanto

NorthernHonshu

Tokai

KiiPen.

Shikoku

Japan Sea

Pacifiic Plate

Philippine Sea Plate

SagamiTrough

Fig. 1

Hyuganada

80

79

59

27

42

49

13

130° E 135° 140° 145°

30°

35°

N

500 km

Hori (1997)

Philippine Sea slab beneath Kanto

●●●

●●

● thrust

Goto (2001)

S. Kyushu P-axes

●

Seismic coupling　　　　 Down-dip limit of the seismogenic zone

in relation to

Dehydration from the subducted crust (Low-frequency tremor)

Basic assumptions

100 MPa 1 GPA

3 km

30 km

Lithostatic pressure

Depth

Tectonic stress

Weakening

Fault strength

Interplate earthquakes

Elevated pore fluid pressure

Slab earthquakes (intermediate-depth)

Dehydration instability

Weakening mechanisms

Dehydration instability: Serpentinite

Raleigh & Paterson (1965)

(a) Cold slab type (b) Hot slab type

Dehydration from crust

Dehydration from crust

Dehydration from serpentine

Dehydration from serpentine

Dehydration loci for slab seismicity

Yamasaki & Seno (2003)

Obara (2002)

Low-frequency Tremor in the upper plate wedge

Low freq. tremorFreq. great earthquakes

Subduction ofNormal oceanic crust Island-arc crust subduction

No dehydrationDehydration from crust

No low freq. tremor

No great earthquakes

No dehydration

Hot slab type: Nankai Trough

N. of Izu

Crust Upper plate

Tokai district:easternmost Nankai Trough

Seismicity: Matsumura (1997)

350°C

Low frequency tremor (Obara, 2002)

Dehydration From crust

Piosson’s ratio (Kamiya & Kobayashi, in prep.)

Temperature: Seno & Yamasaki , in prep.

Obara(2002)

震震

Low-frequency earthquakes & tremor: Upper plate

Okino et al. (1999)

Low-frequency tremor ~ Moho depth (Obara, 2002)

KantoE. Shikoku

S.Kyushu

No tremor region

Okino et al. (1994)

Kinan seamount chain

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

Hypothesis:

Later crustal phaseSerpentine dehydration loci

Serpenine dehydration loci

a) Event in the oceanic crust b) Event in the mantle

Generally along the Nankai Trough

Discrimination of crustal events

No later crustal phase

Hori (1997)

PHS slab beneath Kanto(no tremor)

No later phase(Hori, 1990)

Kurashimo et al. (2002)

E. Shikoku (no tremor)

Hayashimoto et al. (2001)

110 km

200 km

DDC

DDT

DDT, i.e., lower plane seismicity in the mantle

S. Kyushu (no tremor)

Goto et al. (2001) DDT Thrust

mantle

thrust

S. Kyushu

Areas without earthquakes within the subducted crust

Kanto

E. Shikoku

S. Kyushu （？）

Areas without low-frequency tremor

Noguchi & Sekiguchi (2001)Obara (2002)

High Vp

Top of slab seismicityIzu

KantoSlab

Ni & Barazangi (1986)

Mature collision zones: Zagros and Himalaya

Low freq. tremorFreq. great earthquakes

Normal oceanic crust subduction Island-arc crust or continental crust subduction

No dehydrationDehydration from crust

No low freq. tremor

No great earthquakes

No dehydration

NankaiTrough N. of Izu, Zagros, Himalaya

Crust Upper plate

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

Hori (1997)

Philippine Sea slab beneath Kanto

Shikoku Basin

Izu-Bonin Arc

Trench Axis

Serpentine stable

V. F.

Origin of the double seismic zone in Kanto

Seno et al. (2001); Seno & Yamasaki, in prep.

No dehydration from crust

No low freq. tremor

Infreq. great earthquakes& deep thrusts

Kanto

Dehydration from serpentine

Kawakatsu & Seno (1983)

Northern Honshu: three types of events

Thrust

Down-dip compression Down-dip tension

Kawakatsu & Seno (1983)

1994 aftershocks (Hino et al., 2000)

Yamanaka et al. (2001))EpicenterAsperity

Larger events

(a)

(a)

(b)(b)

Vp in northen part of n. Honshu

S. Ito et al. (2000)

Depth = 40 km

W E

No serpentine

Fractured zone

Dehydration

Ductile shear zone + hydrofracturing

Repeated earthquakes Plastic flow

Dehydration

Cold mantle wedge

N. Honshu, Kanto, S. Kyushu Iwate-oki, Bonin, Tonga

Serpentine

350°C350°C

Anisotropicpermeability

Plastic flow

Dehydration

Hot mantle wedge

Nankai, Ryukyu, Cascadia, Mexico

Ductile shear zone + hydrofracturing

Serpentine 350°C

No serpentine

Stable sliding with very high shear stress

No dehydration

Collision zone

N. Izu, Himalaya, Zagros

No serpentine

No dehydration

or delamination

Conclusions

・ Sudbduction of island-arc or continental crust does not involve dehydration of subducted crust, then does not induce low- frequency 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.