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National Information Service for Earthquake Engineering
University of California, Berkeley |
Geologic Assessment of the Relative Contribution of Strike-Slip Faulting, Reverse-Slip Faulting and Bulk Squeezing in the Creation of the Central Santa Cruz Mountains, California
Gianluca Valensise, Istituto Nazionale de Geofisica, Via de Vill Ricotti 42, Roma, Italy.
To deduce the tectonic history of the central Santa Cruz Mountains in relation to the activity of the San Andreas fault we investigated the state of deformation of three geologic datums: the First (125ka) and Third (320 ka) Santa Cruz terraces, and the base of the Upper Miocene and Pliocene Purisima Formation. Terrace data were take from published work, whereas the Purisima was investigated by reconstructing its structural contours based on elevations of the contact with the underlying formations. We compared the shape and elevation of these three datums to explore the relative importance of two main tectonic processes: uplift of the Santa Cruz region by repetitions of a 1989 Loma Prieta-type earthquake, and progressive northwestward advection of the Pacific plate. The main conclusions are that (1) the source of the Loma Prieta-type earthquake is stationary relative to the North America plate and in relative motion with respect to the Pacific plate over time scales of approx. 1m.yr.; (2) the central Santa Cruz Mountains have uplifted at an average rate <0.5 mm/yr following repeated oblique-reverse slippage at a rate of approx. 3mm/yr on the Loma Prieta earthquake fault and northwestward motion of the Pacific plate at geologic rate of 5-20 mm/yr; (3) only a fraction of the topography of the Santa Cruz Mountains can be explained by motion along the Loma Prieta earthquake fault; (4) geologic evidence shows that much of the observed topography is generated by a process which dissipates plate boundary-normal compression in the upper 5 km of the crust.
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