Seismicity, tectonism, and surface faulting in the western United States during historic time

Maps of tectonic flux are presented for the conterminous United States west of longitude 109°W, for periods of time before and after 1932, and for the entire historic period through 1961. The most active continuous seismic zone in this region during historic time extended over 750 km, from a point off the California coast near Ventura to Winnemucca in north-central Nevada. Although this zone is characterized by a discontinuous line of historic surface faulting, it is neither sharply defined by, nor closely related to structures along its path that are generally considered to be the major tectonic elements of the region. The broad areal extent of this, and five other active zones, suggests that the tectonic processes causing earthquakes and surface faulting in the western United States are distributed over broad regions, and are not confined to geologic or physiographic provinces. Seismicity maps for different periods indicate that seismic activity in some areas has shifted with time. Within major seismic zones, gaps in the seismicity pattern are filled in by successive large earthquakes. Recurrence curves support a high level of activity for the Ventura-Winnemucca zone, and they indicate a lower rate of activity for the San Andreas fault zone than for other areas in the region studied. Recurrence curves for the central California area indicate that the rate of activity in a given region may remain practically constant over periods at least as long as a century, whether or not large earthquakes occur in the region during those periods. There appears to be a general correlation between observed shear or slippage, and the seismicity of any given region. Based on these results, it is proposed that, where historic faulting has occurred in areas with little or no historic seismic activity, such faulting is due to the propagation of fractures into the inactive areas from adjoining seismic zones.