Rupture Directions for Selected Northern California Earthquakes

Fault rupture tends to focus wave energy along the fault, a phenomena called directivity. The table below links to results from a simple inversion of peak ground motions to estimate rupture direction and rupture velocity. We correct the horizontal peak accelerations (PGA) and peak ground velocities (PGV) from Northern California ShakeMap data for site amplification and source-receiver distance to obtain a set of residual peak S-wave motions. We fit these residual peak motions using the directivity function for a unilateral line source (see detailed explanation). The rupture direction is not constrained to lie along a nodal plane: good solutions yield rupture directions close to a nodal plane. Events with slow rupture velocities are ambiguous: they could be either complex or bilateral ruptures, or slow unilateral ruptures. We have created seismicity maps and cross sections for earthquakes with M ≥ 4.0 and more than 7 aftershocks.

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