Long dormancy, low slip rate and similar slip-per-event for the Emerson fault, eastern California shear zone

Department of Geological Sciences Research

Charles M. Rubin, Department of Geological Sciences, Central Washington University, Ellensburg, Washington and Kerry Sieh, Seismological Laboratory, California Institute of Technology, Pasadena, California

Abstract

Excavations in a playa along the 1992 rupture of the Emerson fault reveal evidence of two paleoseismic events, with only one large prehistoric rupture in the past 15 millennia. Accelerator mass spectrometer radiocarbon ages of charcoal from playa sediments and from fault-scarp colluvium directly beneath the playa beds indicate that the last large prehistoric slip event occurred about 9,000 years ago. Trench-wall exposures revealed clear evidence of at least one pre-9 ka rupture at the playa site. The event horizon of this earthquake is between two pedogenic carbonate layers that have radiocarbon ages of 14,800 and 24,100 years, implying that the earthquake occurred about 20,000 years B.P. The actual bracketing ages for this rupture are likely a few thousand years older because of the mean residence time for the pedogenic carbonate and calibration of the 14C ages by 230Th dating.

Despite the large uncertainties, a dormant period of at least 6,000 years to as much as 13,000 years separates the older event from the 9,000-yr-old event. Because the scarp formed by the penultimate event is similar in height to the scarp formed by the 1992 MW 7.3 Landers earthquake, the penultimate rupture was, at least locally, similar in size to the most recent rupture. This similarity supports the concept of characteristic slip for the Emerson fault. Preliminary results from paleoseismic studies at other sites on the 1992 rupture suggest that large ruptures occurred on other nearby faults within a few hundred years of this penultimate event on the Emerson fault. The interseismic period that preceded the 1992 earthquake on the Emerson fault was about 40 times longer than the average interval between large events on the nearby San Andreas fault. Therefore, in comparison to events on the San Andreas fault, the 1992 Landers earthquake was an exceedingly rare event.

An oblique photograph taken at the playa site on the Emerson fault. Photograph taken from a hill to the west of the playa; view is to the east.

Selected Images(jpg) from the JGR paper

Overview of active faults in the Mojave Desert.

Map of the 1992 Landers earthquake surface rupture and faults of the central portion of the Eastern California shear zone (Mojave Desert) (modified from Sieh et al. [1993] and Dokka and Travis [1990]. Inset shows North American and Pacific plate boundary and major active faults. Fault map modified from Dokka and Travis [1990] and Jennings [1975]. Plate vector direction and rate from DeMets et al. [1994].

Schematic map of surface faults along the Emerson fault near the playa site, including measurements of 1992 offset [Rubin and McGill, 1992]. Solid circles, east-side-up (esu); solid circles, west-side-up (wsu); south-side-up, ssu; rl, right-lateral. A, soil site A; B, soil site B.

Detailed topographic map of the playa lake floor. Elevations of the pre-1992 shoreline, surveyed by an electronic EDM/theodolite indicate coseismic vertical deformation; arbitrary datum used. Southwest of the fault, shoreline has been vertically warped as much as 274 mm.

Schematic diagram of soil profiles at sites A and B from the area near the playa site (W. B. Bull, written communication, 1994). Sites are shown in Figure 2.

(a) Generalized cross-section across the southeastern wall of the trench, which was approximately per-pendicular to the fault zone. Surface offset from the 1992 earthquake is 71 ± 2 cm. (b) Topographic profile and geologic cross section across the Emerson fault at the playa showing relations between various units and the alluvial fan surface. Because the dendro-corrected ages are not available for samples older than 10,000 years, samples in units 20 and 10 are reported as 14C years ago before 1950.