HR: 0830h
AN: S41D-0124    [PDF]
TI: Deep Crustal Earthquakes in the Sierras Pampeanas Region of the South Central Andean Backarc
AU: * Alvarado, P M
EM: alvarado@geo.arizona.edu
AF: University of Arizona, Gould-Simpson Bldg. 1040 E Fourth St., Tucson, AZ 85721 United States
AU: * Alvarado, P M
EM: alvarado@geo.arizona.edu
AF: Universidad Nacional de San Juan, Meglioli 1160 S, San Juan, 5400 Argentina
AU: Beck, S
EM: beck@geo.arizona.edu
AF: University of Arizona, Gould-Simpson Bldg. 1040 E Fourth St., Tucson, AZ 85721 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: University of Arizona, Gould-Simpson Bldg. 1040 E Fourth St., Tucson, AZ 85721 United States
AU: Fromm, R
EM: rfr@geo.arizona.edu
AF: University of Arizona, Gould-Simpson Bldg. 1040 E Fourth St., Tucson, AZ 85721 United States
AU: Araujo, M
EM: mmaraujo@inpres.gov.ar
AF: Universidad Nacional de San Juan, Meglioli 1160 S, San Juan, 5400 Argentina
AU: Araujo, M
EM: mmaraujo@inpres.gov.ar
AF: Instituto Nacional de Prevencion Sismica INPRES, Roger Balet 40 N, San Juan, 5400 Argentina
AU: Bufaliza, M
EM: bufaliza@inpres.gov.ar
AF: Universidad Nacional de San Juan, Meglioli 1160 S, San Juan, 5400 Argentina
AU: Bufaliza, M
EM: bufaliza@inpres.gov.ar
AF: Instituto Nacional de Prevencion Sismica INPRES, Roger Balet 40 N, San Juan, 5400 Argentina
AU: Triep, E
EM: triep@andes.edu.gov.ar
AF: Universidad Nacional de San Juan, Meglioli 1160 S, San Juan, 5400 Argentina
AB: In the last 20 years almost 500 earthquakes with depths less than 50 km and magnitudes over 4.0 have occurred in the Andean cordillera and backarc region between 29-34S as a consequence of the compression between the Nazca and South American plates. Many of these earthquakes as well as large (M$>$7) damaging shallow earthquakes in 1861, 1894, 1944, 1952 and 1977 are associated with the Precordillera and Sierras Pampeanas, characterized by a thinn-skinned fold and thrust belt that contrast with the thick-skinned basement cored uplifts deformation, respectively. This high level of crustal seismicity corresponds to a segment of flat slab subduction beneath Argentina. Very few crustal earthquakes occur in the high cordillera in the volcanic arc region that shut off approximately 10 Ma ago when the subducting slab became flat. We have used regional broadband seismic waveforms recorded on the portable CHilean Argentinean Geophysical Experiment network to determine focal mechanisms and depths for 22 crustal earthquakes (M $>$ 3.6) during 2001 and 2002. We first located the earthquakes and then used a least squares moment tensor inversion technique to match 3-component data to determine the best focal mechanism and depth. A simple crust and upper mantle velocity model was used in the inversions (crustal thickness 45 km, crustal Vp=6.2 km/sec, Vp/Vs=1.70). Our results indicate that 20 of the events have predominately thrust fault focal mechanisms with an average P-axis azimuth orientation of 97$\deg$. More than 85% of the earthquakes had locations in the Pampean Ranges and focal depths between 18 and ~40 km, corresponding to mid to lower crust. The furthest east crustal earthquakes occur inland nearly 800 km from the trench. Five of the events are concentrated near the oldest part of the Pampean Ranges (Sierra Pie de Palo), which was also the site of the most recent large damaging crustal earthquake (1977 San Juan earthquake). We identified only one shallow strike-slip event (depth=7 km) near the Sierra de Cordoba (Eastern Pampean Ranges). The depths of the crustal earthquakes in the mid to lower crust implies that brittle faulting associated with the basement cored uplifts in the Sierras Pampeanas extends to deep crustal levels. This implies the Pampean crust is very cold, perhaps a result of the refrigeration effect of the flat subduction of the Nazca slab that insulates the South American lithosphere from warm asthenosphere.
DE: 7205 Continental crust (1242)
DE: 7215 Earthquake parameters
DE: 7218 Lithosphere and upper mantle
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting