Aftershock distribution as a constraint for the 2004 Parkfield earthquake coseismic slip model
Abstract
We present preliminary results for a coseismic slip model for the 2004 Parkfield earthquake where geodetic observations from the event were used to obtain the slip model with the constraint that edges of the slip patches align with aftershocks. This constraint is based on the hypothesis that coseismic slip on a patch may increase the stress on an adjacent patch where no slip has occurred. This could in turn cause aftershocks to occur at the edges of coseismic slip patches. Adapting the method of Gallardo and Meju (2004, 2007), our technique uses the cross gradient (the cross product of the gradients of slip and aftershock density) as a weighted penalty function to encourage the edges of the slip model to align with the aftershock distribution. The aftershock distribution and slip model are considered structurally identical when the cross gradient value in each cell is minimized. The gradient values associated with the aftershocks are non-zero only within and along the edges of the aftershock clusters, and they are largest at the edges of the clusters. Thus, minimization of the cross-gradient constraint should produce a preferred solution where the edges of coseismic slip patches are encouraged to align with the aftershocks clusters. More over, the coseismic slip model is allowed to change freely in locations devoid of aftershocks. Cross validation is used to select optimal weights for smoothing and the cross gradient penalty with the optimal weights chosen as those which minimize the average residual. Preliminary results indicate that the unconstrained and constrained models have similar distributions of coseismic slip and moment magnitudes, and agree well with previously published coseismic slip models. Overall, the addition of imposing the cross gradient constraint slightly improves the fit to observed geodetic data compared to the unconstrained inversion. However, the overall effect of encouraging the edges of slip to line up along areas of dense seismicity is not completely achieved. Instead, the cross gradient constraint's effect on the slip model locates high slip in cells with the highest density of aftershocks. Future work will focus on applying a new constraint instead of the cross gradient. This new constraint will encourage the gradient of slip and seismicity to be anti-parallel in cells with or adjacent to seismicity. This may discourage an individual cell from having a dense number of aftershocks and a high value of coseismic slip while still encouraging the edges of slip patches to align with aftershock clusters.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.S51D1786B
- Keywords:
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- 7205 Continental crust (1219);
- 7209 Earthquake dynamics (1242);
- 7212 Earthquake ground motions and engineering seismology;
- 7215 Earthquake source observations (1240)