Surficial offsets on the Central and Eastern Garlock Fault associated with prehistoric earthquakes

Abstract

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 Mw=7.8, to separate rupture of the western segment and of the central and eastern segments combined, with approximate magnitudes Mw≤1.1 and Mw=7.5, respectively, to separate rupture of even shorter segments, producing earthquakes of magnitudes Mw=6.6 to Mw=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.

Surficial slip distribution on the central Emerson fault during the June 28, 1992, Landers earthquake, California

“…One simple observation may help to guide the interpretation of some offset geomorphic features: in cases where there are multiple downstream channel or gully segments offset from a single source, the channel segments offset a larger amount must have been offset in more than one earthquake. Although this configuration is somewhat rare, it has been found along several strike-slip faults [Sieh, 1978;McGill and Sieh, 1991 Figure 10a has been offset in more than one earthquake. This is not a trivial observation, since one could argue that the Garlock fault is long enough to produce an earthquake with 8.8 m of displacement [Wells' and Coppersmith, 1994].…”

Lateral Offsets on Surveyed Cultural Features Resulting from the 1999 Izmit and Duzce Earthquakes, Turkey

“…Some workers have concluded that measurements of a factor of 20%-100% in slip variability over distances of tens to hundreds of meters are not uncommon, after studying the lateral variability of slip based on offset geomorphic features after the Landers earthquake (McGill and Rubin, 1999). This has implications for discriminating between one or multiple discrete events or estimating the size of past earthquakes (Sieh, 1978;Rockwell and Pinault, 1986;Lindvall et al, 1989;McGill and Sieh, 1991;Weldon et al, 1996;McGill and Rubin, 1999) using displaced deposits in trenches or offset geomorphic features. After the M 7.4 İzmit earthquake of 17 August 1999 and the M 7.1 Düzce earthquake of 12 November 1999, teams of geologists mapped the surface ruptures and measured hundreds of offset geomorphic and cultural features in order to resolve slip distribution (Figs.…”

Preliminary Surficial Geology of the Dove Spring Off-Highway Vehicle Open Area, Mojave Desert, California

“…This suggests that there may be considerable strain accumulated that could be released in large earthquakes. Rupture of short segments of the Garlock Fault could produce M w 6.6 to 7.5 earthquakes (McGill and Sieh, 1991). The largest potential earthquake (M w ~7.8) could result from the (unlikely) rupture of the entire length of the fault.…”