Paper No. 5
Presentation Time: 2:30 PM


FUIS, Gary S.1, HOLE, John A.2, STOCK, Joann M.3, DRISCOLL, Neal W.4, KENT, Graham5, GOLDMAN, Mark R.1, ROSE, Elizabeth J.1, CATCHINGS, Rufus D.6, RYMER, Michael J.7 and LANGENHEIM, Victoria E.8, (1)Earthquake Hazards Team, U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, (2)Geosciences, Virginia Tech, Blacksburg, VA 24061, (3)Seismological Laboratory, California Institute of Technology, 1200 E California Blvd, MC 252-21, Pasadena, CA 91125, (4)Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093, (5)Nevada Seismological Laboratory, University of Nevada Reno, 1664 N. Virginia St. MS 0174, University of Nevada Reno, Reno, NV 89557-0174, (6)Earthquake Science Center, U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025, (7)U.S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025, (8)U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025,

The southernmost San Andreas fault (SAF) system, in the northern Salton Trough (Salton Sea and Coachella Valley), is considered likely to produce a large-magnitude, damaging earthquake in the near future. The geometry of the SAF and the velocity and geometry of adjacent sedimentary basins will strongly influence energy radiation and strong ground shaking during a future rupture. The Salton Seismic Imaging Project (SSIP) was undertaken, in part, to provide more accurate information on the SAF and basins in this region.

We report preliminary results from modeling four seismic profiles (Lines 4-7) that cross the Salton Trough in this region. Lines 4 to 6 terminate on the SW in the Peninsular Ranges, underlain by Mesozoic batholithic rocks, and terminate on the NE in or near the Little San Bernardino or Orocopia Mountains, underlain by Precambrian and Mesozoic igneous and metamorphic rocks. These lines cross the Coachella Valley, which is underlain by Miocene to Holocene sedimentary deposits. Line 7 crosses the Salton Sea and sedimentary basin deposits to the northeast similar to those of the Coachella Valley. On three lines (7, 4, 6) there is evidence from our seismic imaging, potential-field studies, and (or) earthquakes that active strands of the SAF dip moderately NE.

From south to north, on Lines 7, 4, 5, and 6, maximum sedimentary basin depths are approximately 5.5?, 5.5, 3.5, and 3.5 km, respectively, as measured from the surface to the 5.3 km/s velocity contour. (In prior studies of the Imperial Valley, unmetamorphosed sediment is interpreted to lie above this approximate velocity contour.) Basement rocks that can be traced from the Peninsular Ranges to depth beneath the Coachella Valley are characterized by relatively high velocities, averaging 4.9 km/s at the surface and 6.4 km/s at a depth of 4 km. They are also characterized by high velocity gradients, averaging > 0.4/s. In contrast, rocks of the Little San Bernardino and Orocopia Mountains are characterized by relatively low velocities, averaging 3.9 km/s at the surface and 6.1 km/s at a depth of 4 km; and they are characterized by low velocity gradients, averaging < 0.4/s. The rocks of the Peninsular Ranges can be seen on Lines 4 and 6 extending at depth northeastward beyond the active surface trace(s) of the SAF.

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