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Surface fracturing occurred along the southern San Andreas, Supersti-

Multiple‐filter analysis is applied to surface waves with their paths almost entirely inside the Philippine Sea plate. The resulting dispersion data are used to study the crustal and upper mantle structures of the plate. Effort is made to examine the intraplate structural variations. Part of the Philippine Sea plate appears to have a thin plate thickness of about 30 km and is overlying a very low velocity mantle. Lateral structural variations in the plate are indicated by the dispersion data. The plate can be divided into several elongated regions, all of which are parallel to the eastern plate boundary and each having a rather distinct crustal and upper mantle structure. The active marginal basin region, located immediately to the west of the Mariana trench, probably represents an area of lithospheric generation. This is supported by the very low shear wave velocity, high heat flow, and magmatism. The Kyushu‐Palau ridge along the central portion of the plate seems to have a thicker lithosphere, in contrast to its two flank regions, the Philippine basin and the Parece Vela basin.

We invert peak ground velocity and acceleration (PGV and PGA) to estimate rupture direction and rupture velocity for 47 moderate earthquakes (3:5 ≥ M ≥ 5:4) in northern California. We correct sets of PGAs and PGVs recorded at stations less than 55-125 km, depending on source depth, for site amplification and sourcereceiver distance, then fit the residual peak motions to the unilateral directivity function of Ben-Menahem (1961). We independently invert PGA and PGV. The rupture direction can be determined using as few as seven peak motions if the station distribution is sufficient. The rupture velocity is unstable, however, if there are no takeoff angles within 30°of the rupture direction. Rupture velocities are generally subsonic (0:5β-0:9β); for stability, we limit the rupture velocity at v 0:92β, the Rayleigh wave speed. For 73 of 94 inversions, the rupture direction clearly identifies one of the nodal planes as the fault plane. The 35 strike-slip earthquakes have rupture directions that range from nearly horizontal (6 events) to directly updip (5 events); the other 24 rupture partly along strike and partly updip. Two strike-slip earthquakes rupture updip in one inversion and downdip in the other. All but 1 of the 11 thrust earthquakes rupture predominantly updip. We compare the rupture directions for 10 M ≥ 4:0 earthquakes to the relative location of the mainshock and the first two weeks of aftershocks. Spatial distributions of 8 of 10 aftershock sequences agree well with the rupture directivity calculated for the mainshock.

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Aftershocks of the Oroville, California, earthquake of 1 August 1975 define a 16- by 12-kilometer fault plane striking north-south and dipping 60 degrees to the west to a depth of 10 kilometers. Focal mechanisms from P-wave first motions indicate normal faulting with the western, Great Valley side downdropped relative to the Sierra Nevada block. The northward projection of the fault plane passes beneath Oroville Dam and crops out under the reservoir.

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