HR: 16:30h
AN: S52I-03    [PDF]
TI: Seismotectonics of the San Fernando Basin
AU: * Pujol, J
EM: pujol@ceri.memphis.edu
AF: Center for Earthquake Research and Information, The University of Memphis, Memphis, TN 38152 United States
AU: Mueller, K
EM: Karl.Mueller@colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309 United States
AU: Shen, P
EM: pshen@rice.edu
AF: Department of Geology and Geophysics, Rice University, Houston, TX 77005 United States
AU: Chitupolu, V
EM: vanivamsipriya@hotmail.com
AF: Department of Mathematical Sciences, The University of Memphis, Memphis, TN 38152 United States
AB: We summarize the results of a tomographic inversion of aftershocks of the Northridge earthquake (recorded during 1994) and San Fernando earthquake carried out using a high resolution inversion package (Benz, et al., JGR, 1966). The study area covers an 80 by 80 km region and the velocity model is parameterized by 2 km wide cubic blocks. The dependence of the inversion model on the initial velocity model (including the SCEC 3-D model) is relatively minor. Moreover, the inversion of realistic synthetic arrival times suggest that our results are reliable. A narrow and well-defined 10 km wide zone of Northridge aftershocks is located in basement rocks between about 10 and 20 km and defines the fault that slipped during the mainshock. The fault plane inferred from seismicity is in good qualitative agreement with the geodetic fault plane determined by Hudnut et al. (BSSA, 1996). Horizontal velocity slices show that the Northridge earthquake occurred within a narrow basement wedge, which may have controlled the size of the earthquake. A dense cluster of aftershocks located immediately above the fault plane is coincident with the deepest portion of the San Fernando Basin where the mainshock rupture offsets sedimentary rocks. Most of the aftershocks of the Northridge earthquake occurred within the basin with a large concentration of events located to the east of the eastern boundary of the mainshock fault plane. The latter events also dip to the southwest, most of them are shallower than 12-13 km and form a band of seismicity 3 to 4 km wide within and near the edge of the basin that we interpret as a major transfer fault that limited the extent of the Northridge rupture. These results are confirmed by the inversion of a larger data set of arrival times from events in an area that is to the east of, and partially overlaps, the Northridge aftershock zone. These events do not lie on the mainshock fault plane, which we interpret as suggesting the San Fernando and Northridge mainshocks are formed as a pair of en-echelon conjugate faults, and that the San Fernando fault does not truncate the Northridge fault, as stated by previous workers. Alternatively, an 8 km wide, almost N-S cross section of the Northridge aftershocks east of the mainshock and aftershocks of the San Fernando earthquake shows that seismicity occurs in a zone that trends across a high velocity basement high (e.g. an extensional horst block). We interpret this as suggesting that older Miocene fault block architecture in this region was subsequently truncated by the Northridge blind thrust, which is a new structure forming in response to shortening across the Transverse Ranges.
DE: 7205 Continental crust (1242)
DE: 8180 Tomography
SC: Seismology [S]
MN: 2003 Fall Meeting