SURFICIAL OFFSETS ON THE CENTRAL AND EASTERN GARLOCK FAULT ASSOCIATED WITH PREHISTORIC EARTHQUAKES

Geomorphic features offset along the central and eastern Garlock fault record the amount of surface slip associated with prehistoric earthquakes. Along the easternmost 90 km of the fault, the smallest offsets cluster around 2-3 m of left-lateral slip, apparently associated with the most recent earthquake on this portion of the fault. Larger offsets along this part of the fault, especially in Pilot Knob Valley, cluster around values consistent with 2-4 m of slip in each of the past several events. Farther west, south of El Paso Mountains, offset geomorphic features suggest that each of the past two earthquakes on this stretch of the Garlock fault was produced by about 7 m of slip, whereas the third event back was produced by about 4 m of slip. Vertical displacements of geomorphic features range from 0% to 30% of the left-lateral offsets. Within Pilot Knob Valley (along the southern side of the Slate Range), vertical displacements are consistently up on the northern side, whereas within the Avawatz Mountains both north- and south-side-up vertical displacements are present. On the basis of the geomorphic offsets, the geometry of the Garlock fault, and the precedents set by historical strike-slip earthquakes elsewhere, a number of different rupture patterns are plausible. These range from rupture of the entire Garlock fault in a single event with a maximum magnitude of about M(w) = 7.8, to separate rupture of the western segment and of the central and eastern segments combined, with approximate magnitudes M(w) less-than-or-equal-to 7.7 and M(w) = 7.5, respectively, to separate rupture of even shorter segments, producing earthquakes of magnitudes M(w) = 6.6 to M(w) = 7.5. In conjunction with available slip rates for the Garlock fault, the geomorphic offsets suggest that average recurrence intervals are probably within the range of 600-1200 years south of El Paso Mountains, about 200-750 years in Searles Valley, about 200-1300 years in Pilot Knob Valley, and about 200-3000 years near Leach Lake and in the Avawatz Mountains.