Long - Period Mechanism of the Eureka

The seismic moment and source orientation of the 8 November 1980 Eureka, California, earthquake (Ms = 7.2) are determined using long-period surface and body wave data obtained from the SRO, ASRO, and IDA networks. The favorable azimuthal distr ibution 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 Mzx = May = 0 and using a point source at 16-km depth gives a major double couple for period T = 256 sec with scalar moment Mo = 1.1 • 1027 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 f ine-tune the solution. This results in a sl ightly larger scalar moment of 1.28o 1027 dyne-cm, but insignif icant (<5 °) changes in strike and dip. The rake is not well enough resolved to indicate signif icant variation from the pure strike-slip solution. Addit ional amplitude inversions of the surface waves at periods ranging from 75 to 512 sec yield a moment estimate of 1.3 __. 0.2° 1027 dyne-cm, and a similar strike-slip fault orientation. The long-period P and SH waves recorded at SRO and ASRO stations are util ized to determine the seismic moment for 15to 30-sec periods. A deconvolution algorithm developed by Kikuchi and Kanamori (1982) is used to determine the time function for the first 180 sec of the P and SH signals. The SH data are more stable and indicate a complex bilateral rupture with at least four subevents. The dominant first subevent has a moment of 6.4.1026 dyne-cm. Summing the moment of this and the next three subevents, all of which occur in the first 80 sec of rupture, yields a moment of 1.3 • 1027 dyne-cm. Thus, when the multiple source character of the body waves is taken into account, the seismic moment for the Eureka event throughout the period range 15 to 500 sec is 1.3 _+ 0.2 • 1027 dyne-cm.