P‐wave fault‐plane solutions and the generation of surface waves by earthquakes in the western United States

Abstract

Surface waves recorded at regional distances are used to study the source mechanisms of seven earthquakes in the western United States with magnitudes between 4.3 and 5.5. The source mechanisms of events in or on the margins of the Basin and Range show T‐axis with an azimuth of N85°W +/− 16° and a plunge of 12° +/− 16°. Of the seven events, four have P‐wave solutions that are inconsistent with surface‐wave observations. Azimuths of the T‐axis obtained from the surface‐wave mechanisms and from the P‐wave solutions differ by up to 45°. These events have dip‐slip or oblique‐slip mechanisms, and the source depths for three of the events are 5 km or less. Their source mechanisms and small magnitudes make identification of the P‐wave first motion difficult due to poor signal‐to‐noise ratio of the initial P‐wave and close arrivals of pP or sP with significant amplitude. We suggest that mis‐identification of the P‐wave first motion and distortion of the body‐wave ray paths due to non‐planar structure were sources of error in determining the nodal planes for these events.