Holocene and Latest Pleistocene Activity on the Mesquite Lake Fault near Twentynine Palms, Eastern California Shear Zone: Implications for Fault Interaction

摘要

Paleoseismic data from the Mesquite Lake fault reveal evidence for up to four prehistoric earthquakes since the latest Pleistocene, with three large surface- rupturing events in about the past 10.2 ka. Three events were recorded in trenches excavated for this investigation on Mesquite Lake playa. A buried fault scarp bracketed by accelerator mass spectrometer radiocarbon ages of detrital charcoal indicate that a mid to late Holocene event occurred between about 2.7 and 7.4 ka, most likely between about 3.9 and 4.6 ka. An early Holocene rupture defined by a subtle buried scarp and upward-terminating fissures is bracketed between about 7.7 and 10.2 ka. Evidence for an inferred late Pleistocene event prior to about 10.2 ka includes a subtle scarp, upward termination of faults, folding, and in-filled craters and fissures. A previous paleoseismic investigation just south (2 km) of the playa identified at least one event in the past 1.3 ka, although this event was not identified in this study at the playa site. Apparent vertical offsets of about 1.0 m and 1.2 m for the last two events at the playa site suggest a sequence of similar serial ruptures. These separations are comparable to the height of surficial scarps just south of the playa. Because the vertical separations produced by each event are similar, the Mesquite Lake fault, at least locally, experiences similar slip-per-event. Since the initiation of Holocene playa development just over 10.2 ka ago, we estimate a horizontal slip rate of 0.7– 0.9 mm yr–1 that is consistent with slip rates for other faults in the eastern California shear zone. Despite large uncertainties, event stratigraphy at the playa site shows that periods of quiescence up to thousands of years separated large ruptures, similar to other faults in the eastern California shear zone. Comparison of paleoseismic studies in the eastern California shear zone suggest that the Mesquite Lake, and possibly the Bullion and Lavic Lake faults in the eastern part of the shear zone may have failed during periods of seismic quiescence in the western part of the zone, at about 1– 5 ka. This suggests that seismic strain release may vary spatially across the shear zone and that temporal clusters of earthquakes are not necessarily spatially limited to specific regions.