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
Karen Felzer1 and Emily Brodsky2
1. U.S. Geological Survey 2. University of California, Los Angeles
Previous work on spatial aftershock decay include:
What’s different about our work?
We make composite data sets from aftershocks of the M 2-3 & M 3-4 mainshocks
Spatial stack, M 3-4 mainshocks
Mainshocks = gray star
Mainshocks are shifted to the origin in time and space
Spatial aftershock decay follows a pure power law with an exponent slightly < -1
Aftershocks > M 2.
The aftershocks may extend out to100 km
Aftershock from the first 5 minutes of each sequence
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
3. Robustness of observations
Is our decay pattern from actual aftershock physics, or just from background fault structure?
Random earthquakes have a different spatial pattern: Our results are from aftershock physics
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
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
Linear density ===cr-1.4
Felzer & Brodsky
Kagan & Knopoff, (1980)
Helmstetter et al. (2005)
Max. pos. for r>10 km
Solutions consistent with observations
Static stress triggering not consistent with observations
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.
Mainshocks are moved to the origin in time and space to obtain a composite data set
Aftershocks from Northern Cal and Japan also follow power law decay
Another way to observe distant triggering: Time series peaks at the time of the mainshocks in different distance annuli
Peak at time of mainshocks