HR: 1340h
AN: S43A-1062    [Abstracts]
TI: Paleoseismology and Geomorphology of the Northern Calico Fault: Testing for Temporally Variable Slip Rate and Earthquake Clustering
AU: * Ganev, P N
EM: ganev@usc.edu
AF: University of Southern California, Earth Science Department, Zumberge Hall, 3651 Trousdale Parkway, Los Angeles, CA 90089, Los Angeles, CA 90089, United States
AU: Le, K
EM: kle@email.unc.edu>
AF: University of North Carolina at Chapel Hill, Department of Geological Sciences, 228 Mitchell Hall, Chapel Hill, NC 27599-3315, Chapel Hill, NC 27599, United States
AU: Dolan, J
EM: dolan@usc.edu
AF: University of Southern California, Earth Science Department, Zumberge Hall, 3651 Trousdale Parkway, Los Angeles, CA 90089, Los Angeles, CA 90089, United States
AU: Oskin, M E
EM: oskin@email.unc.edu
AF: University of North Carolina at Chapel Hill, Department of Geological Sciences, 228 Mitchell Hall, Chapel Hill, NC 27599-3315, Chapel Hill, NC 27599, United States
AU: Elliott, A
EM: ajelliott@usc.edu
AF: University of Southern California, Earth Science Department, Zumberge Hall, 3651 Trousdale Parkway, Los Angeles, CA 90089, Los Angeles, CA 90089, United States
AB: The Calico-Blackwater is the longest fault system in the Mojave section of the Eastern California shear zone (ECSZ), and recent studies (Oskin et al., 2006a) indicate a long-term slip rate of 1.8±0.3 mm/yr for the Calico fault. Although this is the fastest slip rate among the six major faults that comprise the Mojave section of the ECSZ, the rupture history of the Calico fault remains unknown. In order to better understand the spatial and temporal pattern of seismic activity on the Calico fault, we excavated paleoseismic trenches across a desert playa in Newberry Springs, California (~30 km east of Barstow). We also measured small-scale offsets of geomorphic features along a 10 km stretch of the fault extending southward from the trench site. The trenches exposed fluvial, lacustrine, and playa sediments, consisting of pebble gravel and coarse-grained sands, silts, and clays, respectively. Several of these strata also displayed well-developed carbonate soils which help us establish rough constraints on the age of deposition. Preliminary field analysis of soil development suggests that the section exposed in the trenches encompasses latest Pleistocene through Holocene sediments (dating from ~15 or 20 ka). We see evidence for four surface ruptures during this time interval. Two of the ruptures appear to be latest Pleistocene in age; the penultimate event is latest Pleistocene or early Holocene in age and the most recent event (MRE) occurred during mid- or latest-Holocene time. In order to determine the absolute timing of all four ruptures, we have collected over two dozen samples from stratigraphic units throughout the trenches for optically stimulated luminescence dating. Our paleoseismic observations are consistent with geomorphic evidence from displaced alluvial surfaces cut by the Calico fault. Field observations of alluvial surfaces interpreted to be latest Pleistocene to early Holocene in age reveal only ~5.5 m of right-lateral offset of incised channels. Smaller offsets of bar-and-swale topography suggest that offset in the MRE along the northern Calico fault was only ~1.5-2 m. The occurrence of only two Holocene surface ruptures, coupled with the geomorphic evidence for only ~5.5 m of total slip during Holocene time, is in apparent conflict with the relatively rapid (1.8±0.3 mm/yr) long-term slip rate measured by Oskin et al. (2006a) over the past ~60,000 years. This discrepancy may result from: (1) temporal variability in slip rate across faults within the Mojave section of the ECSZ with slip rates on the Calico fault decreasing during the Holocene; or (2) the absence of a latest Holocene rupture event on the Calico fault, with the possibility of occurrence of a near-future earthquake as part of the ongoing ECSZ seismic cluster. We note, however, that even if a large-displacement event (e.g., 5 m of slip) were to occur tomorrow, the Holocene slip rate of the Calico fault would still only be ~1 mm/yr.
DE: 8036 Paleoseismology (7221)
SC: Seismology [S]
MN: 2007 Fall Meeting