Abstract:
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.
