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( Summary of Perfettini and Avouac , 2004; 2007 )

Theoretical prediction for frictional afterslip: comparison to geodetic observations and implications for aftershock physics. Yoshihiro Kaneko. ( Summary of Perfettini and Avouac , 2004; 2007 ). Outline. Theoretical model of afterslip based on a velocity-strengthening brittle creep rheology

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( Summary of Perfettini and Avouac , 2004; 2007 )

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  1. Theoretical prediction for frictional afterslip: comparison to geodetic observations and implications for aftershock physics Yoshihiro Kaneko (Summary of Perfettiniand Avouac, 2004; 2007)

  2. Outline • Theoretical model of afterslip • based on a velocity-strengthening brittle creep rheology • capable of explaining the time evolution of afterslip and aftershock decays • one end member of the rate-and-state aftershock model • Model prediction vs. geodetic observations: • 1999 Chi-Chi earthquake (Perfettiniand Avouac, 2004) • 1992 Landers earthquake (Perfettiniand Avouac, 2007) • 2005 Nias-Simeulue earthquake (Hsu et al., 2006) • Implications for aftershock physics In this presentation

  3. Model of afterslip T < 250 C T > 400 C Fault model assume no stress transfer to induce ductile flow Constitutive law for BCFZ Spring-slider model stress perturbation Figure 1, 2004

  4. Model of afterslip Evolution of afterslip on BCFZ with time Slip velocity (or slip rate) on BCFZ background loading rate afterslip duration Figure 2, 2004

  5. Model of aftershock rate Key assumption: seismicity rate is proportional to the stress rate (time derivative of applied stress, or loading) slope of 1/t C = 100 Figure 4, 2004 Note: the decrease in seismicity rates lasts longer than increase in seismicity rate.

  6. Figure 5, 6 and 7, 2004

  7. Summary so far • The BCFZ afterslip model predicts a decay rate of aftershocks of the Chi-Chi earthquake. • However, the study was based on single geodetic data and a spring-slider model (no elastic interactions). • To test the model against spatially varying geodetic data, one needs to consider relatively complex fault geometry and elastic interaction on BCFZ: elastic interactions on each cell quasi-dynamic (i.e.Rice, 1993) radiation damping term

  8. (Svarc and Savage, 1997) Postseismic Displacements following the Mw 7.2 1992 Landers Earthquake CPA analysis show that all GPS stations follow about the same time evolution f(t)

  9. - Fault geometry from Yuri Fialko (JGR, 2004) - Postseismic displacements from USGS and SIGN

  10. Modeling Landers afterslip Depth (km) SFZ BCFZ = region of afterslip DCFF (distance, depth) 0 65 Along strike distance (km) Step 1: From coseismic slip distribution, compute Coulomb stress change within BCFZ. Step 2: Compute initial slip rate on BCFZ: constitutive parameter normal stress Assumption: This is spatially uniform.

  11. Modeling Landers afterslip SFZ BCFZ = region of afterslip background loading rate Step 3: Compute slip rate evolution on BCFZ: shear modulus elastic kernel S-wave speed Assumption: homogeneous frictional properties

  12. Modeling Landers afterslip Step 4: Compute displacement U(r,t) in a bulk:

  13. Modeling Landers afterslip Step 5: Find the best-fitting model parameters to the geodetic data maybe due to dynamic stress change too small: maybe due to high pore-pressure Figure 4, 2007

  14. rms=16mm Observed and predicted Displacements relative to Sanh Cumulative displacements after 6 yr

  15. black: predicted by the best-fitting model red and blue: data from 1992 to 1999 (relative to North America) Figure 4 and 6, 2007

  16. The Coulomb stress change patterns and location of aftershocks Table 1, 2007

  17. DCFF on 340ºE striking fault planes at 15 km depthdue to afterslip 6 yr 6 months Figure 9, 2007 DCFF (bar)

  18. DCFF on 340ºE striking fault planes at 5, 10, and 15 km depthdue to afterslip Figure 10, 2007

  19. Conclusions • Postseismic deformation due to moderate/large earthquakes result mainly from frictional afterslip obeying a rate-strengthening rheology. • Reloading of the SFZ by afterslip can be used to explain both the location and temporal evolution of aftershocks. • This result implies that seismicity rate is proportional to the stressing rate (or loading rate).

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