Graduate Thesis Or Dissertation

 

Late Holocene Earthquake History Recorded in Alluvial Fan Sequences, Panamint Valley, Eastern California Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9019s8742

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  • The Panamint Valley fault zone (PVFZ) is an active, dextral-oblique normal fault that partially accommodates dextral shear across the Eastern California Shear Zone (ECSZ). The fault system has a complex geometry, characterized by a relatively high-angle dextral oblique normal fault in the south and a low-angle detachment system that accommodates active slip along the central and northern segments of the fault system. Previous studies suggest that these different segments have different degrees of obliquity of the net slip vector, but it is not known whether paleoseismic ruptures are confined to individual segments or whether they rupture simultaneously. Previous studies of the paleoseismic history along the southern fault segment revealed evidence for 3-4 earthquakes during the Late Holocene (< 4 ka); the most recent earthquake (MRE) occurred ~500 cal yr B.P. This study seeks to reconstruct the surface rupture of the MRE—its length, surface displacement, and displacement kinematics—evaluates the event chronology of previous surface ruptures as preserved in the geomorphic record, and determines slip rates associated with earthquakes during the Late Holocene (~5 ka). Four distinct Late Holocene alluvial units are recognized based on post-depositional modification of original bar-and-swale morphology, the degree of desert pavement and varnish accumulation, and the relative degree of soil development. A calibrated soil chronosequence that relates soil development to age of known surfaces is used to provide chronologic estimates of the timing of alluvial surfaces. These results suggest that all four of the alluvial fan sequences mapped along the PVFZ were deposited within the past 4-5 ka. The kinematics and magnitude of displacement associated with the MRE was determined by reconstructing displaced geomorphic markers. These analyses utilized both airborne lidar data collected as part of the NSF Earthscope program as well as targeted high resolution data sets generated from stereo photogrammetry and drone surveys. Results suggest that the average vertical displacement associated with the MRE ranges from 2.5-5 m along a minimum strike length of 45-50 km. Horizontal displacements associated with this rupture along strike-slip segments, characterized from offset channel markers, ranges from 2-3 m. Reconstruction of the MRE displacement vectors suggests 2-3 m of extension along different sections of the complex fault geometry, indicating that the MRE involved both the normal-slip and strike-slip motion. Comparison to empirical scaling of past earthquake ruptures suggest that the MRE was likely at least a moment magnitude 6.9–7.2 event. Notably, this event appears to have ruptured across the boundary between a high-angle fault system in the south and the detachment system in the north. Reconstruction of cumulative vertical offsets of previous surface ruptures appear to be ~7-8 m in surfaces ranging in age from 1.5-3 ka surfaces and ~10-14 m in surfaces of 3-5 ka. These data suggest that the PVFZ experienced Late Holocene throw rates of 2-5 mm/yr, consistent with longer-term (Late Pleistocene-Holocene) slip rates of the PVFZ. Collectively, these results provide insight into the mechanics of fault behavior along geometrically complex fault systems, a re-assessment of the seismic hazard associated with the Panamint Valley fault zone, and the active tectonics of the ECSZ.
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