Dual-System Tectonics of the San Luis Range and Vicinity, Coastal Central California Douglas H. Hamilton, Consulting Geologist 2 Bassett Lane, Atherton, California 94027 Text and illustrations from poster displayed at the Wednesday, December 15 Session of the 2010 American Geophysical Union Fall Meeting San Francisco, California Dual – System Tectonics of the San Luis Range and Vicinity, Coastal Central California Douglas H Hamilton Consulting Geologist, Atherton, CA, United States ABSTRACT The M 6.5 "San Simeon" earthquake of December 22, 2003, occurred beneath the San Lucia Range in coastal central California, and resulted in around $250,000,000 property damage and two deaths from collapse of an historic building in the town of Paso Robles, located 40 km from the epicenter. The earthquake and more than 10,000 aftershocks were well recorded by nearby seismographs, which permitted detailed analysis of the event (eg: McLaren et al., 2008). This analysis facilitated evaluation of the hazard of the occurrence of a similar event in the nearby San Luis Range, located along the coast west of the City of San Luis Obispo some 55 km south of the San Simeon epicenter. The future occurrence of earthquakes analogous to the 2003 event in this area had been proposed in the late 1960's (eg: Benioff and Smith, 1967; Richter, 1969) but the apparent hazard of such occurrences came to be overshadowed by the discovery of the "Hosgri" strike slip fault passing close to the area in the offshore. However data accumulated since the early 1970's clearly demonstrate the hazard as being partitioned between nearby earthquakes of strike slip origin, and underlying earthquakes of thrust origin analogous to that of the 2003 San Simeon earthquake. And for the onshore San Luis Range area, an underlying actively seismogenic thrust wedge appears to provide the maximum potential seismic ground motion; exceeding that potentially resulting from large events on nearby strike slip faults of the San Simeon-Hosgri system, for onshore sites. Understanding and documentation of the geology, geomorphology, tectonics and seismogenesis of the San Luis Range and vicinity has recently experienced a quantum improvement as both new and accumulated data have been analysed. An integrated interpretation of all available data now clearly shows that a dual "side by side" system of active tectonics exists in the region. 1 Essentially the most obvious evidence for this is seen simply in the topography; the rapidly uplifting San Luis Range represents the field of NE-SW compression driving a thrust – backthrust thrust-fault wedge "popup" while the adjacent shear-strike slip faulting associated with the plate boundary San Gregorio-Hosgri splay of the San Andreas fault system results in only minor surface deformation of the sea floor surface of late Quaternary marine planation. Interaction between the two tectonic systems occurs mainly along the SE shoreline of Estero Bay where NNW aligned strike slip faults intersect the uplifting San Luis Range thrust fault "popup" wedge, and along the recently identified Shoreline fault, against which the SSW-vergent leading edge of the San Luis Range thrust impinges at depths of 1-5 km. The latter structural relationship gives rise to locally pronounced west facing sea floor surface scarps along a fault with mostly or entirely horizontal strike slip motion. Overall the San Luis Range and vicinity constitutes an excellent full scale laboratory for observation of evidence of a variety of tectonic processes in action. The opportunity for studies of tectonism here arises not only from the geologically and topographically clearly exhibited effects of the two interacting tectonic fields (NNW shear; NE-SW compression) but also from the extensive baseline studies of the area conducted during the past 40 years. 2 Source of Regional Right Shear in Coastal Central California (Figure 1) At the north the San Gregorio fault splays south from the San Andreas fault and extends south as the only west-side first order branch within the San Andreas fault system. This splay diverts approximately 7 mm/yr right slip from the San Andreas fault onto the San Gregorio fault system. - Continuing south, right slip is progressively diverted from the major segments of the San Gregorio-Hosgri fault system along SE – branching splays into Monterey Bay, the northern Santa Lucia Mountains and the Estero Bay offshore in the San Luis Range region. - Southward from San Luis Obispo Bay, right slip along the Hosgri fault has largely dissipated with the southernmost eastward splay branch, the "Shoreline" fault probably accommodating much of that still occurring along the Hosgri at that latitude. - The southernmost Hosgri evolves to a west-vergent thrust complex and appears to merge into the NNW-SSE aligned fold and thrust belt present in the southern part of the offshore Santa Maria Basin. 3 Source of Sub-Regional NE-SW Compression and Resultant Crustal Shortening (Figure 1) The region of the broad zone of the southern Coast Ranges bounded generally by the Salinas Valley on the NE and the Pacific Ocean coastline on the SW, is characterized by mountain chains that rise abruptly from intervening lowlands. The three principal lowlands of this region are from north to south, the Los Osos, Santa Maria, and Santa Ynez River valleys. These valleys are separated by the San Luis Range and the Casmalia – Solomon - Purisima Hills. Although the present shoreline of these valleys is deeply scalloped by marine erosion, the onshore structural trends continue offshore to their termination at the Hosgri basin boundary fault. The ranges in this SW sector of the Southern Coast Ranges appear to all be developed by Quaternary compressional deformation of long-lived Tertiary synclinal troughs in which thousands of meters of clastic sediments accumulated. The compressional deformation is active at the present. It is manifest most obviously by the terrain aspect, but also by geologically young reverse and thrust faults along the range margins and by compressional mechanism earthquakes. The source of the NE-SW compression that drives this deformation has been attributed variously either to active or residual effects of clockwise rotation of the western Transverse Ranges (eg:, Luyenduyk, 1980, McLaren and Savage, 2001) or to "escape tectonics" acting along the north margin of the Western Transverse Ranges (eg: Wells et. al. 1998, Hardebeck, 2010). We see no compelling argument for preferring either the WTR clockwise rotation or the "escape tectonics" hypotheses but note that geometrically the "clockwise rotation" hypothesis appears to work well for the observed onshore deformation while the "escape tectonics" hypothesis could explain the E-W compressional deformation in the southernmost offshore Santa Maria Basin, west of the Hosgri fault. 4 Seismicity of the South Central Coastal Region The seismicity of the south central coastal region of California as related to terrain is shown on the two maps of earthquake hypocenters, with depth indicated by color code plotted on a digital terrain base (Figures 2, 3). The focal mechanisms of sufficiently well recorded events from this region are shown on separate plots (Figure 4, 5). Seismicity data is shown for the period prior to the occurrence of the San Simeon earthquake in 2003 (Figure 4) and for the period extending through November 2010 (Figure 5). This allows observation of the general pattern of seismicity (the pre 2003 plot) compared with that overprinted by the dense cloud of aftershocks of the San Simeon event, which obscure the general pattern of events that characterize the seismicity in the Santa Lucia Range. Focal mechanisms of both the pre 2003 seismicity in this region and of the main shock and aftershocks of the San Simeon earthquake show the dominant role of compressional seismic faulting in the San Lucia Range. The inset to the post 2003 seismicity map is a cross section through the hypocenters of the main shock and aftershocks of the 2003 San Simeon earthquake (Figure 6). Note that the east-dipping principal source fault projects toward the surface near the offshore trace of the San Simeon fault, passing c. 3 km below the surface trace of the Oceanic fault. Compare this cross section with the cross section plotted at the same scale, of the seismicity beneath the San Luis Range (Figure 9). The pattern of seismicity in the south central coastal onshore and near shore region is noteworthy for the close association of seismic activity with compressionally uplifted ranges of hills and mountains (Santa Lucia, San Luis, Casmalia-Solomon-Purisima) north of the Western Transverse Ranges and east of the San Simeon-Hosgri fault system. Linear patterns of seismicity are generally discernable only along the Hosgri and Shoreline faults opposite Estero Bay and the Irish Hills and less distinctly, within Estero Bay. As shown by the focal mechanism plots, these are all areas with predominantly strike slip faulting. 5 Right-Slip Faulting and Seismicity in the San Simeon - Estero Bay – Offshore Irish Hills Region The uplift–exposed landward part of the compressional Piedras Blancas antiform is transected by the active right slip San Simeon fault, as well as by older related faults. These faults have prominent erosionally produced geomorphic expression, and less obvious tectonic expression marked by linear low scarps. The more active-appearing traces have been explored by trenching (eg: Asquith, 1977, PG&E, 1988). Both the visible landform and the trenching results demonstrate beyond doubt that the San Simeon and related faults represent major loci of right slip faulting. This zone of faulting clearly extends offshore both north and south of the Piedras Blancas onshore exposure (Figure 1). South of San Simeon Bay this zone of faulting continues along strike, paralleling and lying directly offshore from the shoreline between Cambria and Point Estero. In addition to the linear shoreline this reach of the San Simeon fault is marked by a few clusters of epicenters of small earthquakes recorded since 1987.