Results from Rupture Directivity of Moderate Earthquakes in Northern California

by Linda Seekins and John Boatwright

We analyzed the directivity for 47 moderate 3.5≤M≤5.4 earthquakes that occurred between 1998 and 2008. View the solutions for these earthquakes.

The map shows the locations of 36 of the earthquakes analyzed: one event near Mammoth Lakes, three events in Humboldt county, and four events near Paso Robles are located outside of the map. The earthquakes discussed in the text are identified. The small dots represent Shakemap recording stations.

Distribution of Rupture Direction and Velocity

The earthquakes we have analyzed for directivity comprise 35 strike-slip earthquakes, 11 thrust earthquakes, and 1 normal-faulting earthquake.

figure 9a — Rupture directions for the 33 strike-slip earthquakes analyzed, these directions are plotted as they would be seen from the southwest for northwest striking faults and from the southeast for northeast striking faults.

figure 9b — Rupture directions for the 9 thrust and 2 normal faulting earthquakes analyzed: these directions are plotted as they would be viewed looking down on the faults.

figure 10a — Rupture velocity distribution for the PGA inversions. To obtain these rupture velocities, we relaxed the limit of ν ≤ **0.92**β : four inversions obtained transonic rupture velocities β < ν ≤ α. The blue area represents a smoothed average.

figure 10b — Rupture velocity distribution for the PGV inversions. Three inversions obtained transonic rupture velocities β < ν ≤ α. The yellow area represents a smoothed average.

The most striking result of the study is the prevalence of strong directivity in the peak ground motions. The second striking result of this study is the tendency for strike-slip earthquakes to rupture updip.