Geologic Hazards and Disasters Research Group

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Abstracts: Earthquake geology at the Van Matre Ranch Site

To understand the spatial and temporal variation in fault slip it is important to improve the spatial coverage of slip and slip rate measurements along major active faults. A set of well-preserved channels are offset across the San Andreas Fault at the Van Matre Ranch (VMR) site (35.154N, 119.700W) in the Elkhorn Hills area of the Carrizo Plain. The fault zone and offset channels at VMR were exposed by excavation in 1993 and 2004. This study include one fault-perpendicular and 5 fault-parallel trenches that exposed the burried thalwegs of several offset channels. Seventeen samples were collected from channel margin deposits for C14 dating and survey data was taken for accurate offset measurement of the burried thalwegs and geomorphic channels. The geomorphic history of the site is well manifested in the excavations with clear evidence for initial incision of the channels into Plio-Pleistocene fan units that were typically heavily bioturbated. The channels then back filled and the stratified channel sediments grade laterally into clayey silts. the burried thalweg of the currently active channel is offset 24.8 m, while the geomorphic offset is 27.6 m (qualitatively defined conservative uncertainties on offsets are ±1m). The thalweg of the first beheaded channel is offset48.8 m with a geomorphic offset of 51.8 m. The geomorphic offset of the second beheaded channel ranges from 71.9 to 79.0 m. there are no ages associated with these channels. The median dates of samples from the clayey silts in the currently active channel margin range between A.d. 1221 and 1108, implying a 34.7 mm/yr slip rate. The significance of the samples ages is dependent upon interpretation of the sediments in which they were collected. They were collected from clayey silts which are either colluvium, washed down from adjacent hillslopes, or autochtonous alteration of the channel deposits by pedogenic processes (largely burrowing). If the samples were derived from colluvial processes , the ages of the samples would provide a maximum slip rate. However, if the samples were derived from older channel sediments, then they would indicate a slip rate minimum. This preliminary slip rate is consistent with the measured slip rate at Wallace Creek, approximately 18 km to the northwest where Sieh and Jahns documented a late Holocene slip rate of approx. 33.9±2.9 mm/yr, and with the regionally assumed 35 mm/yr rate derived from decadal time-scale geodetic measurements.

Well-preserved channels offset across the San Andreas fault (SAF) at the Van Matre Ranch (VMR) site in the northwestern Elkhorn Hills area of the Carrizo Plain offer the opportunity to measure slip rate and examine geomorphic development of the channels. The fault zone and offset channels were exposed by excavation in one fault-perpendicular and five fault-parallel trenches. The geomorphology and stratigraphy in the channels reveal a record of filling by fluvial sedimentation, lateral colluviation, and pedogenesis. The buried thalweg of the currently active channel is offset 24.8 +/- 1 m, while the geomorphic channel is offset approximately 27.6 +/- 1 m. Seventeen samples were collected from channel margin deposits for 14C dating. An OxCal model of the radiocarbon dates with stratigraphic control suggests that the oldest date for channel incision was ~1160 AD. Minimum and maximum slip rates ranging from 29.5 to 35.8 mm/y result from different assumptions about the timing of channel incision and offset. Minimum slip rate results from using the maximum time interval from the earliest channel incision date to the present. Maximum slip rate is derived from a shorter time interval culminating in the most recent 1857 co-seismic offset. The range of slip rates at VMR agree well with the late Holocene slip rate of 33.9 +/- 2.9 mm/yr at Wallace Creek, approximately 18 km to the northwest, and imply that within measurement uncertainty the 30-37 mm/yr velocity gradient across the SAF from decadal time-scale geodetic measurements is accommodated across the several meter wide SAF zone at VMR over the last millenium.

Sets of well-preserved channels offset across the San Andreas fault (SAF) at the Van Matre Ranch (VMR) site in the northwestern Elkhorn Hills area of the Carrizo Plain offer the opportunity to measure slip rate and examine geomorphic development of the channels. The fault zone and offset channels were exposed by excavation in one fault-perpendicular and five fault-parallel trenches. The geomorphology and stratigraphy in the channels reveal a record of filling by fluvial sedimentation, lateral colluviation, and pedogenesis. The buried thalweg of the currently active channel is offset 24.8 ± 1 m, while the geomorphic channel is offset approximately 27.6 ± 1 m. Seventeen samples were collected from channel margin deposits for 14C dating. An OxCal model of the radiocarbon dates with stratigraphic control suggests that the oldest date for channel incision was A.D. 1160. Minimum and maximum slip rates ranging from 29.3 to 35.6 mm/yr are derived from different assumptions about the timing of channel incision and offset. The resulting slip rates at VMR agree well with the late-Holocene slip rate of 33.9 ± 2.9 mm/yr at Wallace Creek, approximately 18 km to the northwest, and imply that within measurement uncertainty the 30–37 mm/yr velocity gradient across the SAF from decadal time- scale geodetic measurements is accommodated across the several-meter-wide SAF zone at VMR over the last millennium.