Slip on the San Andreas fault in central California from alinement array surveys

Abstract

Lines of monuments ranging between 30 and 200 meters (m) in length that extend across the traces of several strike-slip faults in California have been surveyed periodically by the U.S. Geological Survey to determine local slip rates, widths of slip zones, and patterns of near-field deformation. Ten monument lines, or alinement arrays, were established during the years 1966 to 1968 along a 180-km-long segment of the San Andreas fault in central California from Cholame Valley northwestward to Watsonville. Data from subsequent alinement array surveys and from other sources show that this segment of the fault can be partitioned into three distinct sections. A 55-km-long central section northeast of King City shows the highest rates of fault creep observed (∼32 mm/yr), essentially matching the long-term slip rate measured geodetically across the entire fault system. The two sections flanking the central section show only partial strain release and are characterized by regional decreases in creep rates outward toward the limits of surface ruptures associated with the great 1857 and 1906 earthquakes. The data show that creep does not occur at present along the zones of surface rupture associated with the two great historic earthquakes, except for minor amounts at scattered sites along the 1906 break. The fall-off in creep rate from the ends of the central section outward along the flanking sections shown by alinement array data is characterized by distinct asymmetry: the gradient of outward creep rate decline along the northwest section is only about half that of the southeast section, implying a similar contrast in rates of strain accumulation. The northwest section characteristically produces frequent microearthquakes and occasional moderate earthquakes as large as magnitude 5 1 4 , while the southeast section is characterized by comparatively lower background seismicity and less frequent moderate shocks which may, however, be as great as magnitude 5 3 4 .