Depths and focal mechanisms of crustal earthquakes in the central Andes determined from teleseismic waveform analysis and InSAR

Abstract

We investigate the depths of crustal earthquakes (<80 km depth) of the central Andes (5°S to 35°S) to constrain the relationship between earthquake locations and inferred faults. We assemble parameters from 138 moderate‐sized (7.0 > Mw ≳ 5.5) earthquakes from the Global CMT catalog and previous work spanning 1944–2007. For 38 well‐recorded events, we use teleseismic P and SH waveforms to model the strike, dip, rake, focal depth, and source time function. We use InSAR observations of surface deformation from 9 earthquakes to compare inferred fault parameters with the waveform inversions and global catalogs to assess their accuracies. While the depths from the InSAR and waveform analyses generally agree within error, horizontal and depth errors in global catalogs are 10 to 50 km, as found elsewhere. As noted in previous work, the majority of crustal earthquakes occur in the Eastern Cordillera and foreland regions of the central Andes, although a few normal and strike‐slip earthquakes occur beneath the Altiplano plateau and in the forearc in southern Peru and northernmost Chile. We propose a new interpretation of one of the basement thrusts (Shira Mountain, Peru) as a pop up block on the basis of our new earthquake depths. We confirm that earthquakes in the flat slab areas of Peru and Argentina are within the sometimes aseismic lower crust. Lower crustal earthquakes are globally found in all types of tectonic settings only when the thermal lithosphere is more than 80 km thick and the amount of recent shortening/extension is <30%.