Focal Mechanism of the August 1, 1975 Oroville Earthquake by Charles

Long-period teleseismic P and S waves from the WWSS and Canadian networks are modeled to determine the focal parameters for the main shock in the Oroville earthquake series. Using the techniques of P first motions, wave-form synthesis, and phase identification, the focal parameters are determined as follows: dip 65°; rake 70°; strike 180°; depth 5.5 + 1.5 km; moment 5.7 _+ 2.0 x 1024 dyne-cm; and a symmetric triangular time function 3 sec in duration. This is a north-south c striking, westward dipping, normal fault with a small component of left-lateral motion. The time separation between the small foreshock and mainshock appears to be 6.5 sec at teleseismic distances, rather than 8.1 sec as observed at short distances. INTRODUCTION Although the mainshock in the Oroville earthquake sequence was relatively small by seismological standards (M L = 5.7), the interest in this event is enhanced by the possible relation it has with the nearby Oroville Reservoir. The obvious scientific and legal ramifications of induced seismicity has made this particular field currently one of the fastest growing and most exciting areas of geophysics. It is not the object of this paper, however, to speculate on any causal relationship of the earthquakes to the reservoir. Instead, we will pre~ent results for the focal parameters of the mainshock of August 1, 1975, in light of some recently presented techniques of wave-form analysis. The main Oroville shock had some peculiarities which effectively thwarted standard location and focal mechanism techniques using nearby stations. As reported by Morrison et al. (1975), a magnitude 4.5 foreshock preceded the mainshock by 8.1sec. Consequentially, the location of the mainshock, although inferred to be at the same place as the foreshock, was hard to pin down. The foreshock was also large enough to obscure local P first motions, so that the faulting mechanism was also unknown until sufficient aftershock data were processed to get an indirect look at the fault plane (Bufe et al., 1976; Ryall and VanWormer, 1975). Because of these reasons, and also since the event was well recorded at teleseismic ranges, a body wave-form analysis was carried out to determine estimates of the orientation, depth, time function, and seismic moment parameters for the earthquake. DATA AND ANALYSIS Immediately after the earthquake, requests for the longand short-period vertical components from each station in the WWSS and Canadian networks were sent out with excellent response from most. This particular component was requested primarily because of a travel-time study being conducted for the region. Fortunately, in view of the clear long-period P and S waves observed, some stations sent the horizontal components also. These turned out to be very helpful in constraining the focal mechanism. Before any wave-form interpretation could be done, however, the extent of interference of the foreshock with the mainshock had to be determined. Figure 1 shows the shortperiod vertical component for the station MSO. This is one of the few teleseismic stations l l l l 1112 CHARLES A. LANGSTON AND RHETT BUTLER where the foreshock is clearly recorded, and only because MSO is relatively close to the epicenter. The long-period component shows little interference between the shocks at this particular distance. For more distant stations, the foreshock is virtually always in the noise, even on the short-period components. Figure 1 also indicates a possible discrepancy in the time between the foreshock and mainshock. Where the foreshock and the P wave for the mainshock can be read on short-period seismograms, the time difference observed is nearer 6.5 sec than the 8.1 sec reported by Morrison et al. (1975) from local stations. An explanation for this possible discrepancy based on the focal mechanism will be given in the discussion. MSO ~=9 .5 °, A z = 5 4 . 7 ° P I