Paleoseismology and neotectonics of the central and eastern Garlock Fault, California

Citation

McGill, Sara Hanley Fagerson (1992) Paleoseismology and neotectonics of the central and eastern Garlock Fault, California. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/N2KZ-NH42. https://resolver.caltech.edu/CaltechETD:etd-03192007-132452

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The Garlock fault is one of the major active faults in California. Although it has not produced any large earthquakes during historic times, abundant fault scarps in Holocene deposits and offset geomorphic features attest to the occurrence of large, prehistoric earthquakes on this fault. In an effort to better characterize the seismic hazard associated with the Garlock fault, I have measured the slip rate of the fault in southeastern Searles Valley, documented the left-lateral displacement associated with past earthquakes on the central and eastern portions of the fault, estimated the size and frequency of those earthquakes, and constrained the age of the most recent large earthquake on the portion of the fault in Searles Valley. A latest-Pleistocene shoreline at the overflow-level of Searles Lake has been offset 82 to 106 meters (best estimate = 90 m) along the Garlock fault, in southeastern Searles Valley. Radiocarbon dates from both surface and subsurface units indicate that the most recent highstand of Searles Lake ended sometime between 10,000 and 13,800 radiocarbon years ago (Stuiver and Smith, 1979; Benson and others, 1990; Bard and others, 1990). The maximum slip rate of the Garlock fault in southeastern Searles Valley is thus 10.6 mm/[...]-yr. If part of the offset of the shoreline is a remnant from older lakestands, then the slip rate may be somewhat less, but a channel that incised after the most recent highstand is offset 69 ± 2 m, indicating that the minimum slip rate is 5 mm/[...]-yr. Subjective evaluation of the constraints on the offset and on the age of the shoreline suggest that the slip rate is most likely between 6 and 8 mm/[...]-yr at this site. If Bard and others' (1990) calibration of the radiocarbon timescale is correct, then the true slip rate of the Garlock fault is between 4 and 9 mm/yr and most likely between 5 and 7 mm/yr. This slip rate is consistent with the [...] rate determined by Clark and Lajoie (1974) at Koehn Lake. Considering the Quaternary, west-northwestward extension that has occurred north of the Garlock fault, one might expect the slip rate of the Garlock fault to decrease eastward (Davis and Burchfiel, 1973). The slip rate determined in southeastern Searles Valley indicates that no eastward decrease in the Garlock fault slip rate is required between Koehn Lake and Searles Lake, but an eastward decrease of up to 3 mm/yr is plausible. Geomorphic features offset along the central and eastern Garlock fault record the amount of left-lateral surface slip associated with prehistoric earthquakes. Along the easternmost 90 km of the fault, the smallest offsets cluster around 2-3 m, apparently associated with the most recent rupture of this portion of the fault. Larger offsets along this part of the fault, especially in Pilot Knob Valley, cluster around values consistent with 2 to 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 resulted from 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 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=7.8, to separate rupture of the western segment and of the central and eastern segments combined, with approximate magnitudes of M[...]<7.7 and M[...]=7.5, respectively, to separate rupture of even shorter segments, producing earthquakes of magnitudes M[...]=6.6 to M[...]=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 yr south of El Paso Mountains, about 200-750 yr in Searles Valley, about 200-1300 yr in Pilot Knob Valley, and about 200-3000 yr near Leach Lake and in the Avawatz Mountains. Stratigraphic relations exposed in two trenches across the Garlock fault in Searles Valley provide clear evidence for several Late Holocene, prehistoric faulting events. A radiocarbon date on detrital charcoal from one of the trenches indicates that the most recent surface-faulting event on this portion of the Garlock fault occurred no more than 530 years ago. This earthquake probably had a magnitude in the range of M[...] = 7.2 to M[...] <= 7.8. Historical evidence suggests that this event occurred more than about 90 years ago. Consideration of these constraints and of the average recurrence interval for this portion of the fault (200-750 yr) suggests that the next large earthquake on the Garlock fault in Searles Valley will occur within the next 660 yr and could, in fact, be overdue.

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