Abstract

The coast-parallel elevation pattern of marine terraces along 50 km of coast centered about Santa Cruz, California, provides an opportunity to assess the relative roles of candidate uplift mechanisms. We assemble 130 existing latitude-longitude-elevation inner-edge locations from three of these marine terraces to constrain elastic dislocation models of coseismic uplift. Uplift resulting from repeated oblique slip seismic events similar to the Loma Prieta earthquake (Ms = 6.9) of October 17, 1989, appears to explain well the portion of the terrace uplift pattern south of Santa Cruz. Depending on whether the lowest terrace is assumed to correspond to isotope stage 5c or 5e, the calculated mean repeat time for Loma Prieta events is either 490 or 820 yr, corresponding to either 4.1- or 2.4mm/yr slip rates on the Loma Prieta fault. Residual uplift not explained by such events increases monotonically to the north toward Año Nuevo, where the San Gregorio fault zone intersects the coastline. We assess whether this residual uplift may result from repeated events on the San Gregorio fault. Given only the strike of the San Gregorio fault, we use an extensive parameter search to determine the geometry of, and the style of motion on, the implied rupture plane that best explains this residual uplift. The fault parameters that minimize the misfit between data and model imply activity of a moderately steep (70°) north-east-dipping San Gregorio fault that ruptures in oblique thrust slip from ∼15 km to the surface. The thrust slip component dominates the uplift pattern, resulting in uplift of the coastline from Santa Cruz northward. These parameters match available geological, geodetic, and seismic data on the dip and sense of motion on strictures within the San Gregorio fault zone. The San Gregorio fault has therefore been an active late Pleistocene participant in North American-Pacific plate boundary motion. We conclude that the complex pattern of uplift along this 50 km reach of coastline can be explained by coseismic mechanisms tied to San Andmas bend-related fault structures onshore, and to the San Gregorio fault zone offshore. As both of these mechanisms result locally in considerable shore-normal uplift gradients, caution is warranted in applying common correlation techniques based solely on uplift rates in the assignment of terrace ages in this and other such settings. In addition, while it is tempting to use terrace culmination offsets as a means of estimating San Andreas fault dextral slip rates, we provide a formalism to determine under what conditions such calculations are meaningful. We conclude that the terrace offsets along the Santa Cruz coastline result not from dextral slip on the San Andreas, but from differential indentation of the Loma Prieta uplift pattern by the various sea- level highstand coastlines.

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