INVERSION OF REGIONAL SURFACE-WAVE SPECTRA FOR SOURCE PARAMETERS OF AFTERSHOCKS FROM THE 1992 PETROLIA EARTHQUAKE SEQUENCE

Regional distance surface waves are used to study the source parameter s for moderate-size aftershocks of the 25 April 1992 Petrolia earthqua ke sequence. The Cascadia subduction zone had been relatively seismica lly inactive until the onset of the mainshock (M(s) = 7.1). This under thrusting event establishes that the southern end of the North America -Gorda plate boundary is seismogenic. It was followed by two separate and distinct large aftershocks (M(s) = 6.6 for both) occurring at 07:4 1 and 11:41 on 26 April, as well as thousands of other small aftershoc ks. Many of the aftershocks following the second large aftershock had magnitudes in the range of 4.0 to 5.5. Using intermediate-period surfa ce-wave spectra, we estimate focal mechanisms and depths for one fores hock and six of the larger aftershocks (M(d) = 4.0 to 5.5). These seve n events can be separated into two groups based on temporal, spatial, and principal stress orientation characteristics. Within two days of t he mainshock, four aftershocks (M(d) = 4 to 5) occurred within 4 hr of each other that were located offshore and along the Mendocino fault. These four aftershocks comprise one group. They are shallow, thrust ev ents with northeast-trending P axes. We interpret these aftershocks to represent internal compression within the North American accretionary prism as a result of Gorda plate subduction. The other three events c ompose the second group. The shallow, strike-slip mechanism determined for the 8 March foreshock (M(d) = 5.3) may reflect the right-lateral strike-slip motion associated with the interaction between the norther n terminus of the San Andreas fault system and the eastern terminus of the Mendocino fault. The 10 May aftershock (M(d) = 4.1), located on t he coast and north of the Mendocino triple junction, has a thrust faul t focal mechanism. This event is shallow and probably occurred within the accretionary wedge on an imbricate thrust. A normal fault focal me chanism is obtained for the 5 June aftershock (M(d) = 4.8), located of fshore and just north of the Mendocino fault. This event exhibits a la rge component of normal motion, representing internal failure within a rebounding accretionary wedge. These two aftershocks and the foreshoc k have dissimilar locations in space and time, but they do share a nor th-northwest oriented P axis.