A reexamination of the moment tensor solutions of the 1980 Mammoth Lakes Earthquakes

Abstract

The moment tensor solutions determined from both long‐period body and surface waves for most of the large earthquakes in the 1980 Mammoth Lakes sequence have substantial deviations from a pure double‐couple mechanism. Julian (1983) and Aki (1984) argue that these deviations require nonshear faulting seismic sources such as a compensated linear vector dipole (CLVD) which could represent magmatic dike injection. Here we reexamine the moment tensor solutions and investigate the effects of the non‐double‐couple source on body wave synthetics. An error function which is based on the cross correlation of a synthetic and an observed waveform was used to assess the difference in fit between the complete moment tensor solution and one constrained to be a double couple. For the main shock (May 25, 1980; 1633) the sum of the errors differed by less than 5% for the two models. Similarly, the errors differed by only 3% for the aftershock on May 27, 1980. The conclusion is that the CLVD mechanism is not resolved in the long‐period data. In addition, the complete moment tensor solution has an extra parameter; a χ2 test was applied to normalize the number of parameters, and it was found that the complete solution was slightly less significant than the double‐couple solution. Finally, the complete solution was compared to a pure CLVD solution; for all the events examined, the total moment tensor is as far removed from a CLVD as it is a double couple. Although this does not rule out the possibility of dike injection, it strongly suggests that faulting can account for the long‐period seismic observations in the 1980 swarm.