HR: 11:50h
AN: GP42A-07    [Abstracts]
TI: Paleomagnetic Definition of Crustal Segmentation, Quaternary Block Rotations and Limits on Earthquake Magnitudes in Northwestern Metropolitan Los Angeles
AU: * LEVI, S
EM: slevi@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AU: YEATS, R S
EM: yeatsr@geo.oregonstate.edu
AF: Department of Geosciences, Oregon State University, Corvallis, OR 97331 United States
AU: NABELEK, J
EM: nabelek@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331 United States
AB: Paleomagnetic studies of the Pliocene-Quaternary Saugus Formation, in the San Fernando Valley and east Ventura Basin, show that the crust is segmented into small domains, 10-20 km in linear dimension, identified by rotation of reverse-fault blocks. Two domains, southwest and adjacent to the San Gabriel fault, are rotated clockwise: 1) The Magic Mountain domain, 30 +/- 5 degrees, and 2) the Merrick syncline domain, 34 +/- 6 degrees. The Magic Mountain domain has rotated since 1 Ma. Both rotated sections occur in hangingwalls of active reverse faults: the Santa Susana and San Fernando faults, respectively. Two additional domains are unrotated: 1) The Van Norman Lake domain, directly south of the Santa Susana fault, and 2) the Soledad Canyon domain in the San Gabriel block immediately across the San Gabriel fault from Magic Mountain, suggesting that the San Gabriel fault might be a domain boundary. Plio-Pleistocene fragmentation and clockwise rotations continue at present, based on geodetic data, and represent crustal response to diffuse, oblique dextral shearing within the San Andreas fault system. The horizontal dimensions of the blocks are similar to the thickness of the seismogenic layer. The maximum magnitude of an earthquake based on this size of blocks is Mw = 6.7, comparable to the 1971 San Fernando and 1994 Northridge earthquakes and consistent with paleoseismic trenching and surface ruptures of the 1971 earthquake. The paleomagnetic results suggest that the blocks have retained their configuration for the past \~ 0.8 million years. It is unlikely that multiple blocks in the study area combined to trigger much larger shocks during this period, in contrast to adjacent regions where events with magnitudes greater than 7 have been postulated based on paleoseismic excavations.
DE: 8107 Continental neotectonics
DE: 8150 Plate boundary--general (3040)
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting