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 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 ∞ ∞ Central Honshu Recurrence intervals of great earthquakes Izu Kanto

Nankai Trough ~50% Sagami Trough ・ Vc = 3 cm/yr ・ U = 6 m ~100 % ・ Vc = cm/yr ・ U = m Seismic coupling coefficient ・ T = 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

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 MPa1 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. tremor Freq. great earthquakes Subduction of Normal oceanic crust Island-arc crust subduction No dehydration Dehydration 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) 震源 Low-frequency earthquakes & tremor: Upper plate

Okino et al. (1999) Low-frequency tremor ~ Moho depth (Obara, 2002) Kanto E. 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 phase Serpentine 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) a a 110 km 200 km DDC DDT DDT, i.e., lower plane seismicity in the mantle S. Kyushu (no tremor)

Goto et al. (2001) DDTThrust 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 seismicity Izu Kanto Slab

Ni & Barazangi (1986) Mature collision zones: Zagros and Himalaya

Low freq. tremor Freq. great earthquakes Normal oceanic crust subductionIsland-arc crust or continental crust subduction No dehydration Dehydration from crust No low freq. tremor No great earthquakes No dehydration NankaiTroughN. 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)) Epicenter Asperity Larger events

(a) (b) Vp in northen part of n. Honshu S. Ito et al. (2000) Depth = 40 km WE

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°C Anisotropic permeability

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.