1. 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).

2. 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

3. Feature of slip pattern for tsunami earthquakes
1992 Nicaragua earthquake SE NW
(301,12,85)
shallow ★
deep s
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

4. 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 km
Dmax=2.5m km

5. (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

6. 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

7. 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.

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

9. 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

10. 1982 Ibaragi-oki earthquake (Mj 7.0)
-40
40km
Mo=
5.0x10**19 Nm
( Mw = 7.1 )
Depth = 13km
Dmax = 0.7m

11. 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

12. 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

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

14. 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.

15. 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.

16. Thank you