Earthquakes, quaternary faults, and seismic hazard in California

Data describing the locations, slip rates, and lengths of Quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in California. The expected seismic moment M_0^e and the strength of ground shaking resulting from the entire rupture of each mapped fault (or fault segment) are estimated using empirical relations between seismic moment M_0, rupture length, source to site distance, and strong ground motions. Assuming a fault model whereby the repeat time T of earthquakes on each fault equals M_0^e/M_0^g (where the moment rate M_0^g is proportional to fault slip rate), it is observed that the moment‐frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150‐year historical record. The agreement is consistent with the argument that the geologic record of Quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in California. The estimates of T for each fault are the foundation for constructing maps that depict the average return period of ≳ 0.1g peak horizontal ground accelerations, and the horizontal components of peak acceleration, peak velocity, and the pseudovelocity response (at 1‐period and 5% damping) expected to occur at the level of 0.1 probability during a 50‐year period of time. A map is also formulated to show the probability that ≳ 0.1g horizontal ground accelerations will occur during the next 50 years. The maps serve to illustrate the potential value of Quaternary fault studies for assessing seismic hazard. Interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the San Andreas, San Jacinto, Calaveras, Hayward, and Ventura Basin fault zones. Other regions of similarly high hazard may yet remain unrecognized. This inadequacy results primarily from an incomplete data set. Numerous faults, for example, are mapped along the coastal region of northern California and within the Modoc Plateau, but relatively few studies relating to fault slip rate are reported. A similar problem exists for other stretches of coastal California where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate. Geological and geophysical field studies can work to remove these deficiencies. A concerted effort to locate and define rates of activity on all faults in California is the most promising means to further quantify present levels of seismic hazard in California