A discrepancy between long‐ and short‐period mechanisms of earthquakes near the Long Valley Caldera

The largest events in the 1980 Mammoth Lakes earthquake sequence show a discrepancy between fault mechanisms which are determined on the basis of the local short‐period first motions and those determined by modeling of long‐period regional and teleseismic waveforms. The short‐period solutions are left‐lateral strike‐slip on north‐striking, near vertical planes. The long‐periods invariably require a much more moderately dipping fault plane with a significant dip‐slip (normal) component. Persistence of disagreements between short‐ and long‐period polarities to teleseismic distances suggests that the source‐time functions are complicated and may be responsible for at least part of the discrepancy. In addition, there seems to be a systematic difference between local short‐period polarities and teleseismic long‐period polarities that is related to travel paths across portions of Long Valley Caldera. It is possible that a low velocity zone related to recent magmatic activity is causing the deflection of local seismic rays, thus distorting the fault plane projection.