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?