Source characteristics of inferred from waveform analysis large earthquakes inferred from waveform analysis Yoshiko YAMANAKA (Nagoya Univ.) Source process analysis inferred from seismic waveform has two important roles. Realtime seismology for disaster mitigation Extracing the feature of source processes from accumulation of analysis results Our analysis is based on Kikuchi and Kanamori (1991).

Examples extracted from accumulation of analysis results (1) Feature of slip pattern for tsunami earthquakes (2) Influence of horst-graven structure (3) Influence of seamounts

Feature of slip pattern for tsunami earthquakes 1992 Nicaragua earthquake SE NW (301,12,85) shallow ★ deep 0 90s 120km Mw=7.5 Dmax=0.7m Rupture duration time =　about ８０s -80 Caribbean plate The feature of tsunami earthquake is that the amount of the maximum slip is remarkably small and a source region is very large. Total moment become large. Cocos plate

Comparison of the source parameter of interplate earthquakes with same magnitude 2003/01/22 Mexico (Mw=7.5) Nicaragua (Mw=7.5) rupture duration time Mexico x 3 = Nicaragua NICARAGUA MEXICO -30 50km Dmax=2.5m -80 120km

(2) Influence of horst-graven structure 1896 ASPERITY MAP in northern Japan Some of these asperities were reruptured for repeated events. *The individual asperity has its own location and extent. The amount of slip appears to have been relatively small and rupture area is large

Horst-Graven structure Velocity model by a multichannel seismic survey in the northern Japan Trench subducting Horst-Graven structure (Tsuru et al., 2000) Fluid carried by Horst-Graven structure in deep region may be participating in the feature of seismic slip pattern

Result of a seismic refraction-reflection experiment (Fujie et al., 2002) distribution of distinct reflectors at the plate boundary. Seismic activity aseismic large amplitude reflected waves were observed They infer the thin layer is affected by aqueous fluid and/or hydrated rocks. →　Fluid may make the rupture pattern change??? hypothesis : thin layer with slow velocity exists along the plate boundary in low seismic region.

(3) Influence of seamounts 1982 Ibaragi-oki earthquake (Mj7.0) Daiichi-Kashima Seamount

Earthquake activities in this region along the latitudinal and longitudinal axes Repeating large earthquakes with about M ~ 7 and a recurrence interval (~20 years) have occurred. high activity associated with M7 events

1982 Ibaragi-oki earthquake (Mj 7.0) -40 40km Mo= 5.0x10**19 Nm ( Mw = 7.1 ) Depth = 13km Dmax = 0.7m

Result of an active-source seismic survey using OBSs (Mochizuki et al., 2008) Source region of the 1982 earthquake convex upward structure = subducted seamount plate interface Vp structure along the trench-normal line

Sandbox experiment of upper plate deformation associated with seamount subduction Dominguez et al., 1998 As subduction progresses, a shadow zone forms in the wake of the seamount. A low cohesive sand wedge was built to simulate an accretionary wedge

Relationship with subducted seamount and slip distribution of the 1982 earthquake epicenter of the 1982 event subducted seamount founded by Mochizuki et al.

The model of subducting seamount When seamount begins to subduct …… The base of the overriding plate is eroded. In front of the seamount, pore pressure elevates. fluid Elevated pore pressure may reduce the effective normal stress. Local interplate coupling is weak over the seamount. →　The 1982 type earthquake occurs.

Conclusion The significance of waveform analysis for large earthquakes is Realtime seismology for disaster mitigation Extracing the feature of source processes from accumulation of analysis results We found * slip pattern of tsunami earthquakes * influence of horst-graven structure * influence of seamount Fluid may have caused such a characteristic slip pattern.

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