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The rate of aftershock density decay with distance

The rate of aftershock density decay with distance. Mainshocks. Karen Felzer 1 and Emily Brodsky 2. 1. U.S. Geological Survey 2. University of California, Los Angeles. Outline. Methods Observations Robustness of observations Physical Implications. 1. Methods.

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The rate of aftershock density decay with distance

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  1. The rate of aftershock density decay with distance Mainshocks Karen Felzer1 and Emily Brodsky2 1. U.S. Geological Survey 2. University of California, Los Angeles

  2. Outline • Methods • Observations • Robustness of observations • Physical Implications

  3. 1. Methods

  4. Previous work on spatial aftershock decay include: • Ichinose et al. (1997), Ogata(1998), Huc and Main(2003) Ogata Main What’s different about our work? • Relocated catalog (Shearer et al. (2003)) • Small mainshocks (& lots of ‘em!) • Only the first 30 minutes of each aftershock sequence used

  5. We make composite data sets from aftershocks of the M 2-3 & M 3-4 mainshocks Temporal stack Spatial stack, M 3-4 mainshocks Mainshocks = gray star Mainshocks are shifted to the origin in time and space

  6. 2. Observations

  7. Spatial aftershock decay follows a pure power law with an exponent slightly < -1 Aftershocks > M 2.

  8. The aftershocks may extend out to100 km Aftershock from the first 5 minutes of each sequence

  9. The distribution of aftershocks with distance is independent of mainshock magnitude Data from 200 aftershocks of M 2-3 mainshocks and from 200 aftershocks of M 3-4 mainshocks are plotted together

  10. 3. Robustness of observations

  11. Is our decay pattern from actual aftershock physics, or just from background fault structure? A) Random earthquakes have a different spatial pattern: Our results are from aftershock physics

  12. B) Does the result hold at longer times than 30 minutes? Aftershocks from 30 minutes to 25 days Yes: the power law decay is maintained at longer times but is lost in the background at r > two fault lengths

  13. C) Do we have power law decay in the near field? Distances tomainshock fault plane calc. from focal mechs. of Hardebeck & Shearer (2002) Yes -- the same power law holds until within 50 m of the fault plane

  14. 4) Physical Implications

  15. Linear density ===cr-1.4 Fault Geometry Physics Felzer & Brodsky Kagan & Knopoff, (1980) Helmstetter et al. (2005) Max. pos. for r>10 km = r = c rDrcr-1.4

  16. Solutions consistent with observations Static stress triggering not consistent with observations Joan Gomberg r -1.4 using D=1 from Felzer and Brodsky. This agrees with max. shaking amplitudes (based on our work with Joan Gomberg & known attenuation relationships) r -2.4using D=2 from Helmstetter et al. (2005). Static stress triggering plus rate and state friction predicts exp(r-3) at short times (Dieterich 1994). This is not consistent with the observations. Solutions for

  17. Conclusions • The fraction of aftershocks at a distance, r, goes as cr -1.4. • Aftershocks of M 2-4 mainshocks may extend out to 100 km. • Our results are consistent with probabilityof having an aftershock  amplitude of shaking. • Our results are inconsistent with triggering by static stress change + rate and state friction

  18. Supplementary Slides

  19. Mainshocks are moved to the origin in time and space to obtain a composite data set

  20. Aftershocks from Northern Cal and Japan also follow power law decay

  21. Another way to observe distant triggering: Time series peaks at the time of the mainshocks in different distance annuli Peak at time of mainshocks

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