Long-period mechanism of the 8 November 1980 Eureka, California, earthquake

The seismic moment and source orientation of the 8 November 1980 Eureka, California, earthquake (M_s = 7.2) are determined using long-period surface and body wave data obtained from the SRO, ASRO, and IDA networks. The favorable azimuthal distribution of the recording stations allows a well-constrained mechanism to be determined by a simultaneous moment tensor inversion of the Love and Rayleigh wave observations. The shallow depth of the event precludes determination of the full moment tensor, but constraining M_(zx) = M_(zy) = 0 and using a point source at 16-km depth gives a major double couple for period T = 256 sec with scalar moment M_(0) = 1.1 · 10^(27) dyne-cm and a left-lateral vertical strike-slip orientation trending N48.2°E. The choice of fault planes is made on the basis of the aftershock distribution. This solution is insensitive to the depth of the point source for depths less than 33 km. Using the moment tensor solution as a starting model, the Rayleigh and Love wave amplitude data alone are inverted in order to fine-tune the solution. This results in a slightly larger scalar moment of 1.28 · 10^(27) dyne-cm, but insignificant (