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Seismic coupling, down-dip limit of the seismogenic zone, and dehydration of the slab

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

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Seismic coupling, down-dip limit of the seismogenic zone, and dehydration of the slab

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  1. Seismic coupling, down-dip limit of the seismogenic zone, and dehydration of the slab Tetsuzo Seno （Earthquake Res Inst, Univ of Tokyo）

  2. 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)

  3. Seismic coupling along the Nankai - Sagami Troughs Okhotsk plate Eurasian plate Pacific plate Seno et al. (1996) Seno et al. (1993) Philippine Sea plate

  4. Occurrence of smaller earthquakes in Hyuganada Yagi (2002)

  5. Recurrence intervals of great earthquakes Central Honshu 1923 Taisho Tokai, 1707, 1854 Kanto ∞ ~1500 ~400 1703 Genroku ~1500 Izu ~150 yrs ∞

  6. Seismic coupling coefficient (Seismic slip/plate motion) Sagami Trough Nankai Trough Taisho Kanto ~100 % ~50% ・Vc = 4 - 6 cm/yr ・Vc = 3 cm/yr ・U = 4 - 6 m ・U = 6 m ・T = 90 - 150 yrs ・T = ~400 yrs

  7. Down-dip limit of the seismogenic zone Oleskevich et al. (1999)

  8. Miyagi-oki, n. Honshu 1978 Miyagi-oki earthquake (M7.5) 50 km Seno & Pongsawat (1982), Seno et al. (1980)

  9. Northern Honshu: three types of events Thrust Down-dipcompression Down-dip tension Kawakatsu & Seno (1983)

  10. Whole section: three types of events Thrust Down-dip compression Down-dip tension Kawakatsu & Seno (1983)

  11. Philippine Sea slab beneath Kanto Hori (1997)

  12. S. Kyushu P-axes ● ● ● ● ● ● thrust ● Goto (2001)

  13. Seismic coupling Down-dip limit of the seismogenic zone in relation to Dehydration from the subducted crust (Low-frequency tremor) Basic assumptions

  14. Fault strength Tectonic stress 100 MPa 1 GPA Lithostatic pressure 3 km 30 km Weakening Depth

  15. Weakening mechanisms Interplate earthquakes Elevated pore fluid pressure Slab earthquakes (intermediate-depth) Dehydration instability

  16. Dehydration instability: Serpentinite Raleigh & Paterson (1965)

  17. 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)

  18. Low-frequency Tremor in the upper plate wedge Obara (2002)

  19. Subduction of Normal oceanic crust Island-arc crust subduction Hot slab type: Nankai Trough Freq. great earthquakes No great earthquakes Low freq. tremor No low freq. tremor Upper plate Crust No dehydration Dehydration from crust No dehydration N. of Izu

  20. Piosson’s ratio (Kamiya & Kobayashi, in prep.) Tokai district: easternmost Nankai Trough Low frequency tremor (Obara, 2002) Dehydration From crust 350°C Temperature: Seno & Yamasaki , in prep. Seismicity: Matsumura (1997)

  21. Low-frequency earthquakes & tremor: Upper plate 震源 Obara(2002)

  22. Low-frequency tremor ~ Moho depth (Obara, 2002) No tremor region Kanto E. Shikoku S.Kyushu Okino et al. (1999)

  23. Kinan seamount chain Okino et al. (1994)

  24. 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

  25. 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

  26. PHS slab beneath Kanto (no tremor) No later phase (Hori, 1990) Hori (1997)

  27. E. Shikoku (no tremor) Kurashimo et al. (2002)

  28. S. Kyushu (no tremor) DDC DDT 110 km a a 200 km DDT, i.e., lower plane seismicity in the mantle Hayashimoto et al. (2001)

  29. S. Kyushu thrust mantle DDT Thrust Goto et al. (2001)

  30. Areas without earthquakes within the subducted crust Kanto E. Shikoku S. Kyushu（？） Areas without low-frequency tremor

  31. High Vp Kanto Slab Top of slab seismicity Izu Obara (2002) Noguchi & Sekiguchi (2001)

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

  33. Normal oceanic crust subduction Island-arc crust or continental crust subduction Freq. great earthquakes No great earthquakes Low freq. tremor No low freq. tremor Upper plate Crust No dehydration Dehydration from crust No dehydration NankaiTrough N. of Izu, Zagros, Himalaya

  34. 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

  35. Philippine Sea slab beneath Kanto Hori (1997)

  36. Origin of the double seismic zone in Kanto Izu-Bonin Arc Serpentine stable V. F. Trench Axis Shikoku Basin Seno et al. (2001); Seno & Yamasaki, in prep.

  37. Kanto Infreq. great earthquakes & deep thrusts No low freq. tremor Dehydration from serpentine No dehydration from crust

  38. Northern Honshu: three types of events Thrust Down-dipcompression Down-dip tension Kawakatsu & Seno (1983)

  39. Larger events 1994 aftershocks (Hino et al., 2000) Kawakatsu & Seno (1983) Epicenter Yamanaka et al. (2001)) Asperity

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

  41. Cold mantle wedge Fractured zone Ductile shear zone + hydrofracturing Anisotropic permeability Serpentine No serpentine 350°C Dehydration 350°C Dehydration Plastic flow Repeated earthquakes N. Honshu, Kanto, S. Kyushu Iwate-oki, Bonin, Tonga

  42. Hot mantle wedge Ductile shear zone + hydrofracturing Serpentine 350°C Dehydration Plastic flow Nankai, Ryukyu, Cascadia, Mexico

  43. Collision zone No serpentine No serpentine No dehydration No dehydration Stable sliding with very high shear stress or delamination N. Izu, Himalaya, Zagros

  44. 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.

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