IMPLICATIONS OF THE OCTOBER 17, 1989 LOMA PRIETA EARTHQUAKE FOR THE EMERGENCE OF MARINE TERRACES ALONG THE SANTA CRUZ COAST, AND FOR LONG TERM EVOLUTION OF THE SANTA CRUZ MOUNTAINS

Abstract

Abstract In this paper we synthesize geomorphic, seismic, and tectonic data in order to present a scenario for the evolution of the Santa Cruz Mountains that is genetically tied to the proximity of the Santa Cruz bend in the San Andreas fault. We use the marine terraces of the Santa Cruz coast to infer a repeat time for the Loma Prieta earthquakes of 700 years. When combined with the 1.3-1.9 m of dextral slip in the Loma Prieta event, this recurrence interval cannot accommodate the long term right lateral motion along the San Andreas fault, and other events must be called upon. We discuss possible candidates for these other events, and use the topographic signature of the Santa Cruz Mountains, the terrace elevations, the pre-1989 seismicity, and the Loma Prieta aftershocks to develop a picture of multiple faults affecting the long term history of the area. In the first section of the paper, we demonstrate that the vertical displacement pattern associated with the October 1989 Loma Prieta earthquake is consistent with long term terrace uplift over 60 km of coastline and time scales of >200 ky (ky=1000 years). The terrace ages and elevations, and the magnitude of the vertical uplift in the Loma Prieta event, constrain long-term average recurrence intervals of Loma Prieta type events to be of order 660-720 years. In the second section of the paper, we consider the northern Santa Cruz Mountains. Due to spatial variations in the vertical strain field dictated by the lateral extent of rupture and the 70°SW dip of the rupture plane, the uplift rates associated with repeated Loma Prieta type events reach their maximum 1-2 km to the west of the trace of the San Andreas fault. Over the million year time scales necessary to build the northern mountains, the evolving topography slips significantly toward the NW relative to the uplift pattern. The topography reaches a maximum altitude just beyond the bend, and then succumbs to erosion, decaying to half of its maximum height over a length scale of 30 km, or about 2-3 My (My= million years) at present slip rates. The topography of the northern Santa Cruz Mountains in the vicinity of the San Andreas fault is consistent with the vertical strain field and the inferred recurrence interval of the 1989 Loma Prieta earthquake, with the long-term slip rate on the San Andreas, and with the rates at which the geomorphic processes modify this topography. In contrast to the northern Santa Cruz Mountains, maximum altitudes of the mountains on the east side of the San Andreas Fault in the southern Santa Cruz Mountains (in particular Loma Prieta Peak itself) coincide with the middle of the restraining bend. This massif experienced slight subsidence during the recent earthquake, strongly suggesting that other seismic events with different uplift patterns are needed to explain the topography of the southern Santa Cruz Mountains. In the third section of this paper, we will address the possibility that repeated events on the Sargent-Berrocal Fault system are responsible for the southern Santa Cruz Mountain topography. We therefore propose a seismic scenario for the generation of the Santa Cruz Mountains that includes repeated Loma Prieta events (which accomplish both uplift of the northern range and dextral slip), repeated 1906 events (which accomplish dextral slip), and repeated events associated with the Sargent-Berrocal fault system (which accomplish both uplift of the southern range and dextral slip).