Source characteristics of

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Source characteristics of 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).
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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
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Feature of slip pattern for tsunami earthquakes
1992 Nicaragua earthquake
NW
SE
(301,12,85)
shallow
?
deep
0 90s
120km
-80
Mw7.5 Dmax0.7m Rupture duration time about
80s
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
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Comparison of the source parameter of
interplate earthquakes with same magnitude
2003/01/22 Mexico (Mw7.5) Nicaragua (Mw7.5)
rupture duration time Mexico x 3 Nicaragua
NICARAGUA
MEXICO
-30 50km
Dmax2.5m
-80 120km
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(2) Influence of horst-graven structure
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
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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
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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.
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(3) Influence of seamounts
1982 Ibaragi-oki earthquake (Mj7.0)
Daiichi-Kashima Seamount
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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
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1982 Ibaragi-oki earthquake (Mj 7.0)
Mo 5.0x1019 Nm ( Mw 7.1 ) Depth
13km Dmax 0.7m
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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
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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
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Relationship with subducted seamount and slip
distribution of the 1982 earthquake
epicenter of the 1982 event
subducted seamount founded by Mochizuki et al.
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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.
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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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Thank you