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Geophysical studies along the southern portion of the Elsinore fault
Allison, Merle Lee
McEuen, Robert B.
Rehfuss, DonaldGastil, Gordon
A vertical magnetic intensity map based on 481 stations spaced an average of one-quarter mile is presented for the southern portion of the Elsinore Fault from Banner to Vallecito Valley. A very-low-frequency electromagnetic (VLF-EM) survey encompassing 917 stations in 35 miles of line also covers the area from Banner to Vallecito Valley. The results confirm the placement of the Elsinore Fault through Ranchita and Rodriguez Canyons but to the south it is further to the east in the Vallecito Mountains rather than in Mason Valley, as generally mapped. The fault is apparently continuous through the study area with numerous short segments paralleling it to the southwest. At Campbell Grade to the south the Elsinore Fault is apparently cut by two northeast trending faults. Vertical throws of up to 1800 feet have been calculated on the Elsinore Fault, with the northeast side of the fault upthrown. Discovery of the Chariot Canyon Fault was made using both geophysical methods. This fault joins (?) the Elsinore Fault just north of Banner and was traced geophysically along the entire length of Chariot Canyon. Reconnaissance geologic mapping traces it south to Campo and it probably extends across the border into Mexico. Eight kilometers of right-lateral separation is postulated based on distribution of Julian Schist. A vertical throw of 600 feet was calculated on the fault. Mason Valley may be underlain by gabbro apophyses which tore apart the older Triassic (?) and Jurassic rocks, elevating them on the valley flanks where they now shed sedimentary rocks into the valley, covering the gabbro. Thirty-six aftershocks of the September 13, 1973, magnitude 4.8, Agua Caliente earthquake, were studied and show that the southern branch of the Elsinore Fault is active. The focal mechanisms are consistent with right-lateral strike-slip along the southern branch with northeast dip in the main aftershock area and southwest dip just south of the main area. Hypocentral locations support these conclusions. Based on the length of the Elsinore Fault the maximum probable event is a magnitude 7.5-7.6. A magnitude 7.5-7.6 event can be expected once every 100 years and would produce a ground acceleration in San Diego of 0.17-0.18 g. The tectonic style of the Elsinore Fault changes from strike-slip near Lake Elsinore to dip-slip in Vallecito Valley. This change may be attributed to binding up of the fault at Campbell Grade. The eastward bulge in the northern branch may be due to deformation of crustal blocks from continuing stress or to left-lateral offsets on northeast trending faults. Right-lateral motion which had not taken place on the northern branch because of its being bound, is now occurring on the southern branch which appears younger and follows the general trend of the fault system. Thirty-two kilometers of right-lateral offset documented on the Elsinore Fault in the Los Angeles basin area, which occurred mostly during the Miocene, may have been taken up by the Chariot Canyon Fault and other faults associated with the Elsinore Fault system.
San Diego State University
Master of Science (M.S.) San Diego State University, 1974
Peninsular RangesColorado DesertJulian SchistStonewall granodioriteSan Marcos GabbroGreen Valley TonaliteBonsall TonaliteWoodson Mountain GranodioriteRattle Snake graniteTierra Blanca brecciaCanebrake ConglomeratePalm Spring FormationChariot Canyon faultElsinore fault
North America -- United States -- California -- San Diego County
Northlimit = 33.083333 Westlimit = -116.583333 Eastlimit = -116.333333 Southlimit = 32.916667
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