This study will to examine the causes and temporal signature of changes in permeability induced by moderate to major scale earthquakes. Almost all previous work on examining earthquake related changes in hydrology has focused on the time period immediately following the earthquake. This work attempts to obtain a more definitive understanding of the coupling which exists between earthquakes and hydrology by examining the pre-seismic, co- seismic and post-seismic behavior of surface and ground water flow. We will examine the response of the San Lorenzo drainage basin to the Loma Prieta earthquake (10/17/89, M 7.1) and its major foreshocks. Because of the density of stream gaging stations in the basin and the number of water wells in the region, this study will provide an opportunity to examine earthquake induced hydrologic changes in detail. We propose to examine the historical relation between stream flow and rainfall in the region, the groundwater hydrology of the basin in detail (by chemically sampling and hydraulically testing existing wells), and the state of stress in the near surface sandstones and shales in the northern portion of the basin.
