Rupture characteristics and tomographic source imaging of ML ∼ 3 earthquakes near Anza, southern California

Abstract

The rupture characteristics of several ML ∼ 3 earthquakes near Anza, California, are determined from time domain analysis of path‐corrected, displacement pulses. These pulses are corrected for propagation effects (apparent attenuation, scattering, site resonance) using the waveforms of adjacent small events (ML ≤ 2.1) as empirical Green's functions. The earthquakes studied occurred during two swarm sequences near Cahuilla, in an area characterized by swarm activity at shallow depth (< 5 km). The data consist of seismograms recorded by the Anza seismic network, a high dynamic range local array with digital telemetry. The waveforms of adjacent small events (ML ≤ 2.1) are deconvolved from those of the ML ∼ 3 events to yield the source time functions of the larger earthquakes. The ML ∼ 3 earthquakes display a variety of rupture modes, similar to those reported for large shocks but over much shorter time scales and distances. Events with single and multiple ruptures are observed. Relative locations of the subevents are obtained from azimuthal differences in the time separation of pulses in the deconvolved displacement waveforms. Some events (as small as ML 2.4) display unilateral rupture propagation, as manifested by azimuthal variations in the pulse widths of the deconvolved displacement pulses. A tomographic back projection technique is applied to the deconvolved displacement pulses to image the slip velocity on the fault as a function of time after nucleation and distance along the fault, using a one‐dimensional fault model. The tomographic inversion for one of the events (ML 2.4) reveals a unilateral rupture with rupture velocity about equal to the shear wave velocity (Vs) and fault length of 300 m. A similar inversion for an ML 3.7 event resolves two subevents separated by about 200 m and yields an average rupture velocity of 0.8Vs. These rupture velocities are similar to those reported for large earthquakes, indicating that rupture velocity is independent of seismic moment down to 3.1 × 1019 dyn cm, the smallest event studied. There are clear variations in pulse width for neighboring events with similar seismic moments, implying significant differences in static stress drops. Factor of five variations in stress drop (11–61 bars) are calculated for events within 200 m of each other. Thus the stress and/or strength along this fault zone varies considerably over distances of hundreds of meters.