Abstract

We combine analysis of geologic and geomorphic data with observations of deformation during the 1989 Loma Prieta earthquake in northern California to evaluate the contribution of aseismic triggered slip and creep processes to cumulative late Cenozoic deformation along the northeastern Santa Cruz Mountains range front. Deformed late Pleistocene alluvial fans and terraces provide evidence for localized late Quaternary uplift above range-bounding reverse faults within the southwestern Santa Clara Valley adjacent to the range front. On the basis of offset of late Quaternary surfaces, the long-term average slip rate on the primary range-bounding structure (Monte Vista fault) is estimated to be ∼0.2 mm/yr. Northeast of the range front are several discontinuous northwest-trending folds, indicated by alignment of late Pleistocene alluvial-fan apices, anomalous stream-channel convexities, and topographic and vegetation lineaments within a 3–5-km-wide, northwest-trending corridor. Subsurface geologic and geophysical data support the interpretation that the surface folds are a result of blind reverse faulting along the Cascade fault beneath the Santa Clara Valley. From stream incision rates we estimate an average uplift rate of 0.2 ± 0.05 mm/yr for the Cascade fault. Measurements of triggered and postseismic slip following the 1989 Loma Prieta earthquake, combined with estimates of the average return period for Loma Prieta–type events, suggest a long-term average deformation rate along the range front of 0.25–0.4 mm/yr associated with these aseismic processes. This range of values is comparable to rates of late Quaternary deformation on the range-front faults derived from geologic and geomorphic data, and it suggests that growth of the overlying folds is at least partially the result of triggered slip and postseismic creep associated with nearby Loma Prieta–type earthquakes. If this inference is correct, then the return period for independent events on the range-front faults probably is greater than that suggested by the long-term average geologic slip rates.

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