ACCURATE DEPTHS OF CENTRAL ANDEAN CRUSTAL EARTHQUAKES: RELATIONSHIP TO TOPOGRAPHY AND DEFORMATIONAL STRUCTURES ABOVE FLAT SLAB SUBDUCTION

We investigate the depth distribution of earthquakes within the South American continental lithosphere to better understand crustal deformation above the two flat slab segments of the Central Andes. Intracontinental event depths have direct implications on large-scale deformational and rheological properties of the crust and lithosphere. Focal mechanisms for over 120 crustal events above the subducted Nazca plate were assembled from the Harvard CMT catalog and published studies covering over 40 years of seismicity. The most seismically active regions continue to be the thick-skinned foreland thrust belts in the eastern Andes of Peru and the Sierras Pampeanas of Argentina. This seismicity is distributed in the crust from 4 to 36 km, while crustal thickness in these regions is as large as 50 km. Dramatic topographic expression of regional-scale geologic structures in the Andean foreland, where uplifted, tilted blocks and sharp fault scarp features are apparent, is clearly associated with the seismicity. Thrust mechanism events dominate these areas, but a minority of strike-slip orientations also occur. Despite the changes in focal mechanism, the P axes orientations remain consistent, predominantly E to NE trending P axes in Peru and E to SE throughout the Sierras Pampeanas. These directions of maximum compression seem influenced by both the direction of convergence of the Nazca and South American plates and the strike of the foreland-side mountain front. The late Cenozoic Andean orogeny is often thought of as an analogy to the Laramide orogeny of the western United States. The Andean foreland structures can represent the early stages of Laramide-type mountain formation found in the Rocky Mountains of the western US. To expand the literature-based earthquake dataset, synthetic seismograms of P and SH body waveforms on teleseismic seismograms are inverted to obtain accurate strike, dip, rake, focal depth, and source time function. Continued integration of accurate earthquake locations and associated topographic signatures enables us to closely study seismicity's influence on landscape evolution and its relationship to deeper crustal structures.