Today: Basic regional framework Western U.S. setting Eastern California Shear Zone (ECSZ) 1992 Landers EQ 1999 Hector Mine EQ Fault structure 1
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Mojave and Southern Basin and Range - distribution of strike-slip faults (bold lines), vertical axis rotation data (curved arrows), and extensional offsets (straight arrows). McQuarrie N, and Wernicke B P Geosphere 2005;1:147-172 2005 by Geological Society of America 3
TRANSVERSE RANGES Mountains, valleys, and geologic structures lie east-west or transverse to prevailingly northwest-trending grain of So Cal. Collectively referred to as the Transverse Ranges, it consists of several discrete mountain ranges and intervening valleys, including: - The Santa Ynez and Topatopa Mountains - Oak Ridge and the Santa Susanna Mountains - Santa Clara and Simi Valleys - Santa Monica Mountains - San Gabriel Mountains - San Bernardino Mountains - Little San Bernardino Mountains - Pinto Mountains - Eagle and Cottonwood Mountains Causes of uplift (internally consistent) - Change in trend of the SAF (the big bend) - Clockwise rotation - Incipient subduction of continental lithosphere (a drip)? http://geomaps.wr.usgs.gov/archive/socal/geology/transverse_ranges/index.html FROM GEOLOGIC TO GEODETIC TIME SCALES 4
Interseismic displacement across our plate boundary Inferred slip rate, Meade & Hager Note: Garlock Fault is not cut, just bent Diffuse deformation Brian Wernicke & colleagues Pacific-N. America relative motion not just the San Andreas Fault! SAF ~ 60% of 5.6 cm/yr PA-NAM motion 5
Tape et al., GJI, 2009 Mojave Block ECSZ geodetically inferred slip rate: ~ 1cm/yr 6
Landers Aftershocks" Hector Aftershocks" Landers EQ Surface deformation from InSAR. A,B: LOS component CD: Horizontal component E: Vertical component F: Horizontal magnitude and direction 7
Leprince et al., 2007 8
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Landers: Inferred subsurface distribution of slip: Note max slip not at surface An apparent coseismic shallow slip deficit. Model artifact/error? Real? Simons et al., 2002 Fialko et al., 2005 Role of diffuse damage at shallow depths? 10
Landers Aftershocks" Hector Aftershocks" Hector Mine EQ 11
Hector Mine EQ A nearly complete reconstruction of the 3D coseismic displacement field Fialko et al., 2001 Faults of the eastern California shear zone in the Mojave desert of southern California (from Jennings, 1975). The 1992 Landers earthquake rupture is indicated in thick, grayed line. Trench sites are shown as black dots Rockwell et al. (2000) 12
Moment release over time for western Mojave faults. Moment scales to the area under the curve, which are probability density functions that represent the error in age assessment of each event. The uppermost black curve (Sum) is the sum of moments for all events. L&OWSF, Lenwood and Old Woman Springs fault; HF, Helendale fault; njvf, northern Johnson Valley fault; sjvf & KF, southern Johnson Valley and Kickapoo faults; HVF, Homestead Valley fault; nemerson F and semerson F, the northern and southern Emerson faults, respectively; and CRF, Camp Rock Fault. Rockwell et al., BSSA, 2000 After removal of model of surface displacements due to main rupture, find small residual strains on multiple subparallel faults in the Mojave implying strong strength contrasts Fialko (2004) 13
Li et al., 2000 14
Some Potential Questions For Us Can we detect the fault at depth where it is visible on the surface? Can we detect the fault near the surface where no clear coseismic offsets? How deep can we detect it / should we be sensitive to it? Can we detect splay faults? Can we detect fault zones of finite width? Can we detect subsurface offset features by comparing fault parallel structure? Homework Read, Summarize & Synthesize (4 pages or less, double spaced, 12 pt font, 1 inch margins) Due Tuesday, January 21 at the beginning of class Sieh et al., Near-Field Investigations of the Landers Earthquake Sequence, April to July 1992, Science, 260, No. 5105. (Apr. 9, 1993), pp. 171-176. Fialko, Probing the mechanical properties of seismically active crust with space geodesy: Study of the co-seismic deformation due to the 1992 Mw7.3 Landers (southern California) earthquake, JGR, 109, B03307, doi: 10.1029/2003JB002756, 2004 McGill, S. F., and C. M. Rubin (1999), Surficial slip distribution on the central Emerson fault during the June 28, 1992, Landers earthquake, California, JGR., 104, 4811 4833, doi:10.1029/98jb01556. Rockwell et al., 2000, Paleoseismology of the Johnson Valley, Kickapoo, and Homestead Valley faults: Clustering of earthquakes in the Eastern California shear zone: BSSA, v. 90, p. 1200 1236, doi: 10.1785/0119990023. 15