evidence from the a.d. 2000 izu islands earthquake swarm that stressing rate governs seismicity by...
TRANSCRIPT
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
(K. Yamaoka et al., 2005)
Location and Seismicity
by S. Nakada
Tokyo
(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
Swarm evolution (26 Jun ~ 29 July)
Off-dyke appears. Expands substantially after two weeks
http://sicarius.wr.usgs.gov/animations.html
Dike model
8 km
13 km
http://sicarius.wr.usgs.gov/animations.html
Dike model
8 km
13 km
~20m dike expansions
1.5 km3 vol. increases
Dike-model testShear stressing rate Seismicity rate change
(shear stress rate) ~ 150 bar/yr
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.
GPS observation and number of M 3 earthquakes ≧
Main shock and after shocks duration
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)
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
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
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
Seismicity rate change when sudden stress drop
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”
Aftershocks decay
(Observed) (Predicted)
( Aσ~ 0.1 bar ~ constant)
If stressing rate model works in a swarm,the rate of damage earthquake can be forecast…
(times)
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.
Due to the oscillation of the stressing rate?