Source characteristics of large earthquakes

inferred from waveform analysis

Yoshiko YAMANAKA (Nagoya Univ.)

Source process analysis inferred from seismic waveform has two important roles.

1.Realtime seismology for disaster mitigation

2.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

1992 Nicaragua earthquakeMw=7.5Dmax=0.7m

Rupture duration time= about ８０ s

★

NWSE

-80 120km

(301,12,85)

0 90s

Feature of slip pattern for tsunami earthquakes

deep

shallow

Cocos 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.

Caribbean plate

Comparison of the source parameter of interplate earthquakes with same magnitude2003/01/22 Mexico (Mw=7.5) Nicaragua (Mw=7.5)

MEXICO NICARAGUA

-30 50km

-80 120kmDmax=2.5m

rupture duration timeMexico x 3 = Nicaragua

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

(2) Influence of horst-graven structure

(Tsuru et al., 2000)

Velocity model by a multichannel seismic survey in the northern

Japan Trench

subducting Horst-Graven structure

Fluid carried by Horst-Graven structure in deep region may be participating in the feature of seismic slip pattern

(Fujie et al., 2002)distribution of distinct reflectors at the plate boundary.

Result of a seismic refraction-reflection experiment

Seismic activity

hypothesis : thin layer with slow velocity exists along the plate boundary in low seismic region.

aseismic

large amplitude reflected waves were observedThey infer the thin layer is affected by

aqueous fluid and/or hydrated rocks. → Fluid may make the rupture pattern change???

1982 Ibaragi-oki earthquake (Mj7.0)

Daiichi-Kashima Seamount

(3) Influence of seamounts

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 = 13kmDmax = 0.7m

Vp structure along the trench-normal line

Source region of the 1982 earthquake

Result of an active-source seismic survey using OBSs (Mochizuki et al., 2008)

plate

inte

rface

convex upward structure = subducted seamount

A low cohesive sand wedge was built to simulate an accretionary wedge

Dominguez et al., 1998

Sandbox experiment of upper plate deformation associated with seamount subductionAs subduction progresses, a shadow zone forms in the wake of the seamount.

subducted seamount founded by Mochizuki et al.

Relationship with subducted seamount and slip distribution of the

1982 earthquake

epicenter of the 1982 event

The model of subducting seamount

The base of the overriding plate is eroded.

In front of the seamount, pore pressure elevates.Elevated pore pressure may reduce the effective normal stress.

Local interplate coupling is weak over the seamount.

When seamount begins to subduct ……

→ The 1982 type earthquake occurs.

fluid

Conclusion

The significance of waveform analysis for large earthquakes is1.Realtime seismology for disaster mitigation2.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 seamountFluid may have caused such a characteristic slip pattern.

Thank you