Graduate Thesis Or Dissertation
 

A method for determining the source mechanism in small earthquakes with application to the Pacific Northwest region

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  • A technique was developed in the present study to determine fault-plane solutions for small earthquakes. The method uses the direction and amplitude of initial P-wave motions recorded at a small number of seismic stations for epicentral distances less than 2000 km. Seismic arrivals recorded on short-period seismograms were identified as p, P or Pn waves for crustal shocks and P waves for subcrustal shocks. Source amplitudes were converted from station amplitudes using known theoretical methods, based on determining angles of incidence at the surface of the earth and straight ray paths in experimenta1 crustal models. Source amplitudes were calculated for three stations and were then projected back to the earthquake source. The source amplitudes were compared to amplitudes that correspond to more than 6000 theoretical amplitude patterns. The pattern which most nearly fitted the first motions was taken as the fault-plane solution. P-wave amplitudes, velocity structures, focal depth and wave attenuation were varied to show the relative deviations of the dip and strike in a fault-plane solution. When the S-wave was identified, it was found that polarization could be determined for epicentral distances less than 20°. Thirty-three earthquakes in the Pacific Northwest region were analyzed, and twenty-two fault-plane solutions were determined by the method described in this paper. Seven additional fault-plane solutions were determined using the well-known Byerly method. The fault-plane solutions generally showed large dip-slip components. This was particularly evident in fault-plane solutions for earthquakes occurring off the coast of Oregon and northern California, and west of the Cascade Mountains. The solutions for earthquakes east of the Cascade Range and off the coast of British Columbia have either dip-slip or strike-slip components. The solutions obtained by the present technique were compared with solutions for generally larger earthquakes in western North America as previously determined by other investigators, using the Byerly method. Satisfactory agreement was found between the two methods. Two general tectonic hypotheses are proposed from the study of earthquake stresses in the Pacific Northwest region.
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