New evidence doubles the seismic potential of the Hayward Fault

The Hayward Fault produced its last major (M7) earthquake in 1868 (Figure 1). It is widely considered to be the most hazardous fault in the San Francisco Bay region. In large part, this is because it lies so close to a densely populated urban corridor that has an abundance of old structures highly vulnerable to seismic hazard. The probability of a major earthquake on this fault was estimated to be 45% in thirty years (Working Group on California Earthquake Probabilities, 1990). In 1991, Lienkaemper et al. (hereinafter referred to as L91) estimated the potential for seismic slip along the entire length and depth of the fault zone. Recent studies have provided new information relevant to the seismic potential of the fault, which results in markedly larger estimates of its present potential for producing major earthquakes. Here we define the seismic potential as the total seismic moment on all parts of the fault accumulated since the time of the last major earthquake that has not been released by fault creep. Principal reasons for the change in potential are (1) a deeper locking zone and better characterization of the creeping zone, and (2) better information on the extent of the 'southern' 1868 earthquake and the timing of the most recent 'northern' earthquake.