abstract

Comparisons among five repeated level surveys between Santa Monica and Ventura during the period 1960-73 indicate that the epicentral region of the 1973 Point Mugu earthquake (ML = 6.0) sustained measurable vertical crustal movements that preceded and may have accompanied or even followed the main shock. These crustal movements, which have been referred chiefly to bench mark Tidal 3, Santa Monica, as invariant in elevation, were largely episodic and commonly oscillatory. Interpretation of the data is complicated by the relatively intense compaction-induced subsidence developed within the Oxnard Plain and by the fact that the apparent movements are only marginally above possible survey error.

Elevation changes disclosed through comparisons of 1960 with 1968 surveys show that the upper (northern) plate of the west-trending Santa Monica fault rose by 30 to 40 mm at least as far west as Point Mugu, whereas still farther to the west any such positive movements were apparently overwhelmed by differential subsidence within the Oxnard Plain. The 1960-68 uplift is consistent with continuing thrusting along the north-dipping southern frontal fault system of the Transverse Ranges. Between 1968 and 1971 the 1960-68 vertical movement pattern was crudely reversed in both form and magnitude; down-to-the-west tilting between a point about 20 km west of Santa Monica and Point Mugu was accompanied by the cessation of differential subsidence and the development of up to 35 to 40 mm of localized uplift within the Oxnard Plain. The 1968-71 movements suggest left-lateral reverse creep at depth, the possible onset of dilatancy, or some combination of these. Between 1971 (pre-earthquake) and 1973 (post-earthquake) the upper plate of the Santa Monica fault underwent uplift that nearly restored the 1971 surface to its 1968 configuration within the area east of Point Mugu characterized by the 1968-71 anomalous downwarping. Because the maximum measured rebound of more than 30 mm during the period 1971-73 was approximately an order of magnitude greater than the coseismic uplift calculated from dislocation modeling, this uplift may have been principally preseismic and perhaps indicative of an evolving dilatant volume.

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