Earthquake processes in the Long Valley Caldera Area, California

At least three large earthquakes in 1978 and 1980 near Long Valley caldera have unusual non‐double‐couple mechanisms, unlike those appropriate for shear faulting. This conclusion is supported by short‐period P first motions, long‐period P and SH first motions, surface wave spectral amplitudes and initial phases, and long‐period P and SH waveforms. Two explanations for these anomalous mechanisms have been proposed: (1) simultaneous strike slip and normal motion on separate faults and (2) tensile failure under high fluid pressure. To evaluate these conflicting hypotheses, we have inverted P and SH waveforms recorded by the Global Digital Seismograph Network (GDSN) to obtain moment tensors for the earthquakes and have also generated broadband seismograms by deconvolving short‐ and long‐period GDSN data. The largest event (1633:44 UTC, May 25, 1980) has a duration of about 20 s and can be resolved into three events, separated by intervals of about 7 and 10 s, each having a non‐double‐couple mechanism. This contrasts with previously reported inversion results for other complex earthquakes, all of which have double‐couple subevents. Furthermore, the earthquake of 1450:56 UTC May 27, 1980, appears to be simple and not composed of smaller events. Thus the non‐double‐couple mechanisms are probably intrinsic to the source process and not artifacts caused by misinterpretation of multiple ruptures as single events. Rapid opening of cracks under high fluid pressure is a likely possibility for this process, although its dynamics are complex and not yet fully understood.