1. Seismicity shadows: observations and modelling
D. Marsan 1, G. Daniel 2, and M. Bouchon 2
1Laboratoire de Géophysique Interne et Tectonophysique,
Universite de Savoie, Le Bourget du Lac, France.
2 Laboratoire de Géophysique Interne et Tectonophysique,
Observatoire de Grenoble, France.
Corresponding Author: univ-savoie.fr

2. Shadows
Parsons (2002)

3. First 100 days after the Landers earthquake
Shadows
Marsan (2003)
First 100 days after the Landers earthquake

4. Shadows
Ogata et al. (2003)

5. Felzer and Brodsky (2005)  causality? Marsan and Nalbant (2005)
Shadows
Also:
Toda and Stein (2002)
Mallman and Zoback (2003)
Felzer and Brodsky (2005)  causality?
Marsan and Nalbant (2005)
Ma et al. (2005)
Instances of clear immediate quiescences:
Dieterich et al. (2000)
Toda and Stein (2003)
Woessner et al. (2004)

6. They can develop late after the mainshock
Shadows
So:
Quiescences are rare
They can develop late after the mainshock
They could be triggered by something else than the mainshock…
Or simply happen by chance
Why?
They are difficult to detect (easier to see increases than decreases in seismicity rates)
They could be masked by dynamic triggering
They could be delayed by stress heterogeneity

7. Modelling
Rate and state friction (Dieterich 1979, Ruina 1983) with slowness law (Dieterich 1986)
Total # of earthquakes triggered by a set of stress steps {ti} is proportional to the mean t = E{t} 1
For a fault experiencing a Gaussian distribution N (t,st) of stress changes at time t=0 with t = E{t} < 0 and large st: 2

8. lfault(t) / m initial triggering 1 t total number < 0 rate increase
Modelling
lfault(t) / m
initial triggering
rate increase 1 t
rate decrease
total number < 0

9. 10 km x 10 km self-similar slip distribution down to 40 m
Modelling
10 km x 10 km self-similar slip distribution down to 40 m
Hurst exponent H = 0.7 (Mai and Beroza 2002)

10. Modelling
Change in shear stress for parallel strike-slip faults at various distances

11. Modelling

12. Stress variability can be caused by: slip heterogeneity
Modelling
Stress variability can be caused by:
slip heterogeneity
roughness of the main fault plane
crustal heterogeneity
roughness of the target fault
averaging stress over (large) volumes

13. We analyse two sequences: The 1999 Chi-Chi earthquake (Taiwan)
Analyses
We analyse two sequences:
The 1999 Chi-Chi earthquake (Taiwan)
The 1999 Izmit – Düzce earthquakes (Turkey)

14. Non-stationary Poisson seismicity (cost function)
Analyses
Non-stationary Poisson seismicity (cost function)
N triggers occurring at specified times {ti}
Each creating a normal N(t,st) distribution of stress steps
Numerically invert for the 2xN parameters {ti} and {sti}

15. Analyses
synthetics
inversion

16. 100 trials of forward model + inversion
Analyses
100 trials of forward model + inversion

17. Analyses
Ma et al. (2005)

18. Analyses
due to Chi-Chi?

19. Analyses

20. Analyses

21. Analyses

22. Analyses

23. Analyses

24. late (starts > 1.5 years after Chi-Chi)
Analyses
Quiescence but:
late (starts > 1.5 years after Chi-Chi)
more likely due to a trigger (aseismic?) rather than stress varibility
also: strong dynamical triggering

25. Analyses

26. Analyses

27. Analyses

28. Analyses

29. Analyses

30. Daniel et al. (in revision)
Analyses
Daniel et al. (in revision)
Düzce

31. Analyses

32. Analyses
Quiescence
Düzce

33. For Chi-Chi, there are 2 (+1?) cases of shadows, but:
Conclusion
For Chi-Chi, there are 2 (+1?) cases of shadows, but:
Both develop after initial (dynamic) triggering that lasts several days
One starts >1.5 years after mainshock, possibly triggered by something else than Chi-Chi.
One develops late (>2.4 years) at depth
For Düzce, no clear observation of shadows, except at Yalova, but:
Activity was most likely dynamically triggered…
… and sustained itself for months.
Shadow develops late (> 1 month after Düzce, > 4 months after Izmit)

34. Off-fault shadows are more likely to occur At shallow depths,
Conclusion
Off-fault shadows are more likely to occur
At shallow depths,
After several weeks / months of delay
And could be directly caused by (aseismic?) triggers rather than by co-seismic stress heterogeneities.