LANDFORMS AND TECTONICS OF THE CENTRAL COAST REGION OF CALIFORNIA

Understanding of the tectonics of the central coastal region of the Southern Coast Ranges of California has been significantly advanced by recent development of two general categories of geoscience knowledge. These are 1.) new developments in landform imaging at regional to detailed local scale, and 2.) imaging of the structure of the underlying crust by seismologic analysis of increasingly well recorded seismicity (eg. Hardebeck 2010) together with acquisition and modeling of gravity and magnetic potential field data (eg. Langenheim et. al. 2012) plus characterization by deep drilling and by geophysical techniques.

The general pattern of contemporary tectonism in the central Coast Ranges and adjacent near coastal offshore is clearly shown by the distribution and form of the principal mountain ranges in the region. Between Monterey Bay and the Transverse Ranges these local ranges mostly have discrete fault-controlled topographic boundaries and overlie zones of concentrated microseismicity. This indicates a characteristic structural form of uplifting "pop-up" wedge blocks for the mountain ranges of the region (as was determined by McLaren et.al., 2008 for the Santa Lucia Range source region of the 2003 San Simeon earthquake). The uplift of the wedge-block ranges represents accommodation to east-west, rotating southward to nearly north-south, regional compression. Earthquakes of M7.0-<7.5 appear possible on the reverse faults along which these ranges are being uplifted.

This general pattern of compressional tectonism onshore is bounded on the west by the 400 km long zone of active strike slip tectonism of the San Gregorio-Hosgri fault system. This system has produced no large earthquake during the 250 years of recorded history but exhibits local paleoseismic evidence of generating earthquakes of M>7.5.

It is suggested here that the evidence from the south central Coast Ranges seen in the terrain as well as in its tectonic underpinning, indicates a level of tectonism and consequent seismic potential that exceeds that now accepted for design and safety analysis of major infrastructure features in the region.