evidence from the a.d. 2000 izu islands earthquake swarm that stressing rate governs seismicity by...
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![Page 1: Evidence from the A.D. 2000 Izu islands earthquake swarm that stressing rate governs seismicity By Toda, S., Stein, R.S. and Sagiya, T. In Nature(2002),](https://reader036.vdocuments.net/reader036/viewer/2022062515/56649cf55503460f949c38ca/html5/thumbnails/1.jpg)
Evidence from the A.D.
2000 Izu islands earthq
uake swarm that stressi
ng rate governs seismici
ty
ByToda, S., Stein, R.S. and Sagiya, T.
InNature(2002), Vol. 419, pg.58-61
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(K. Yamaoka et al., 2005)
Location and Seismicity
by S. Nakada
Tokyo
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(K. Yamaoka et al., 2005)
Background Seismicity
Seismicity record
Swarm events during A.D. 2000
~7000 M 3 shocks≧5 M 6 shocks≧
Total seismic energy release~1.5 × 104 J 0
-2cm
-4cm
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Swarm evolution (26 Jun ~ 29 July)
Off-dyke appears. Expands substantially after two weeks
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http://sicarius.wr.usgs.gov/animations.html
Dike model
8 km
13 km
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http://sicarius.wr.usgs.gov/animations.html
Dike model
8 km
13 km
~20m dike expansions
1.5 km3 vol. increases
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Dike-model testShear stressing rate Seismicity rate change
(shear stress rate) ~ 150 bar/yr
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Dike-model test
Dike-model can explain the swarm seismicity.
But how about other hypothesis?
Heated ground water effect?Propagation rate is not fast enough
Heat diffusion?The aftershocks duration is temperature-independent.
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GPS observation and number of M 3 earthquakes ≧
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Main shock and after shocks duration
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Aftershocks duration time
Shear stress rate
For the normal stress & duration time:
For the M ≈ 6 earthquake close to dike, ~ 0.3d, Calculated stress rate ~150 bar/yr
For the background M ≈ 6 shock, ~ 1 yr,Background stress rate ~0.1 bar/yr
Aσ~ 0.1 bar
(Constant)
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Methods
State variable for seismicity formulation
Background seismicity rate
Reference stressing rate
Seismicity Rate
For the daily seismicity rate(without sudden stress drop )
Proportion of normal stress
State variable before each time step
Shear stress rate
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Seismicity rate change when shear stress increases
)exp(at
t
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80 100 120
Time
Seism
icity Rate
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MethodsFor the sudden stress change:
Earthquake stress change
Proportion of normal stress
State variable before each time step
State variable for seismicity formulation
And also
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Seismicity rate change when sudden stress drop
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GPS observation and number of M 3 earthquakes ≧
With GPS and seismicity data, this event would be a good case to test the “Dieterich Law”
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Aftershocks decay
(Observed) (Predicted)
( Aσ~ 0.1 bar ~ constant)
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If stressing rate model works in a swarm,the rate of damage earthquake can be forecast…
(times)
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Conclusion
Rate/state stress transfer furnishes a comprehensive explanation for distributed swarm seismicity, triggering and clustering.
It also offers the prospect that near-real-time analysis of seismic and GPS data to forecast during future swarms.
The sudden stress change succeeded by a transient stressing rate change can be simulated by combining the two processes.
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Due to the oscillation of the stressing rate?
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