San Andreas Fault Zone In San Gorgonio Pass Southern California

Author : Lisa Wolff
Publisher :
ISBN 13 :
Total Pages : 97 pages
Book Rating : 4.:/5 (123 download)

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Download or read book A 6000-year Record of Large, Infrequent Earthquakes on the San Andreas Fault System in San Gorgonio Pass, Near Cabezon, California written by Lisa Wolff and published by . This book was released on 2019 with total page 97 pages. Available in PDF, EPUB and Kindle. Book excerpt: The 15 km-wide compressive stepover made by the San Andreas Fault (SAF) system in the San Gorgonio Pass (SGP) region raises questions about the frequency and types of San Andreas earthquakes that happen in the SGP. The 1812 ~M7.5 Wrightwood rupture did not reach the SGP and supports a model whereby the structural complexity here inhibits rupture. However, the 2008 ShakeOut Scenario M7.8 SAF earthquake models a larger earthquake that breaks through the pass in a 300+ km rupture that extends from the Salton Sea to the northern Mojave. Paleoseismic records north and south of SGP indicate that the SAF breaks in large events that happen on average in ~100 and ~200 yr intervals, respectively. This study describes evidence for timing and recurrence for large earthquakes within the pass to test whether the SGP region inhibits or permits earthquakes like the ShakeOut Scenario event. My thesis report describes the findings of a 'mega' trench excavation (9 m deep, 45 m long, and 30 m wide) across the San Gorgonio Pass Fault Zone (SGPFZ) in summer 2013 near Cabazon, CA. The megatrench uncovered ~6000 yr-old sedimentary sections and two thrust zones with four total splays (zone 1 = F1-F3, and zone 2 = F4). Logging and retro-deformation of the trench walls them provides strong evidence for paleoseismic events 1, 2, 3, and 5, and weak evidence for event 4. We collected 138 charcoal samples from trench 8 and processed 54 samples using 14C Radiocarbon age dating protocols. Radiocarbon ages range from ~400 to ~5400 yrs BP (before present). Calibrated 14C ages constrain the timing of paleoseismic events using OxCal 4.3.4. The most recent event shows a modeled age of 633-750 yrs BP and the oldest event is >5300 yrs BP. This defines two recurrence intervals (Cab1-Cab2 and Cab2-Cab3) of ~450 years ~1800 yrs for the SGPFZ. It must be noted that conventional-sized trenches from 1997 found three thrust zones at the Cabazon site. The megatrench uncovered just two thrust zones, indicating that a third zone may remain uncovered to the south of the excavation. Fault orientations in the trench, plus the occurrence of bedrock in the footwall of F4, support this possibility. The paleoseismic data reported here are likely incomplete. The results of my study support a model where M7.8 ShakeOut type events happen 18 to 4 times less frequently than more common SAF events in areas to the north or south, respectively.