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New geophysical approaches to study neotectonics and associated geohazards

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eScholarship - University of California
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High-resolution measurements of earthquake and submarine landslide geomorphology were collected in three different environments along the actively deforming Pacific - North American plate boundary. These four research projects use high-resolution imaging devices and 3D visualization to present new insights into past and future geohazards. Seismic CHIRP imaging with sub-meter accuracy, together with detailed multi-beam and LIDAR derived bathymetry were used to measure fault offset, earthquake derived colluvial wedges, wave-cut paleo-terraces, and other geomorphic subbottom features in the Lake Tahoe Basin. Analysis of these features provides revised and improved extension rates, as well as new information on Holocene faulting. Such an understanding of the deformation has helped place the Tahoe Basin into the larger context of Walker Lane tectonics. The combined east-west extension rate across Lake Tahoe Basin is 0.53 - 1.15 mm/yr. Empirical fault displacement length scaling relationships suggest the Incline Village Fault (IVF) may have ruptured in conjunction with either the Stateline - North Tahoe Fault (SNTF) to the southwest, or faults farther to the north. Assuming the IVF ruptures in concert with the SNTF with a vertical displacement of 3 m and a rupture length of ̃35 km, it would generate a Mw 6.9 ± 0.7 earthquake. High- resolution CHIRP data acquired in the northern Santa Barbara Basin, California provide new insights into the processes that control shallow submarine landslides. Rills formed on the slope by off shelf gravity flows appear to introduce sediment heterogeneity, which may inhibit the development of high pore pressure and thus, limit the occurrence and size of surficial failures. Furthermore, failures in this region are characterized by short run out distances indicative of low velocities and therefore pose little tsunamigenic hazard. Terrestrial laser scanner data acquired from Mecca Hills, California provides improved constraints on fault offsets and paleoearthquake magnitudes for the southern San Andreas Fault. A comprehensive 3D data set of 22 overlapping terrestrial laser scans allows quantitative geomorphology measurements at an unprecedented resolution along an ̃250 m segment of the fault. Our measured slip rate of 15.6 ± 1.9 mm/yr, based on 5.1 m of surface offset since the most recent earthquake (MRE), falls squarely in the middle of our calculated long-term Holocene slip rate of 14-20 mm/yr. Our measured rates approach those calculated by modern geodetic methods and suggest that discrepancies between geologic and geodetic rates may decrease with increased survey resolution. These slip rates combined with the long modern ̃325 yr quiescent period, implies a future rupture would exhibit 5.3 - 7.0 m of coseismic offset at the surface


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