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

Abstract

[1] 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%.

Geodetic Constraints on the 2014 M 6.0 South Napa Earthquake

“…To derive a source model and corresponding uncertainties for the South Napa earthquake, we begin by jointly inverting the GPS and downsampled InSAR observations for the location (longitude, latitude, and depth), orientation (strike, dip, and rake), and dimensions (along-strike length and down-dip width) of a single-fault patch with homogeneous slip embedded in a homogeneous elastic half-space using the neighborhood algorithm (Okada, 1992;Sambridge, 1999). We do not use seismic event location information (i.e., hypocenter) to precondition the inversion as earthquakes in other locations globally may be mislocated by tens of kilometers (e.g., Elliott et al, 2010;Devlin et al, 2012;Barnhart et al, 2013). Event moment tensors variably indicate the ruptured fault dipped steeply to the west or east (http://earthquake.usgs.gov/earthquakes/ eventpage/nc72282711#scientific_moment-tensor; last accessed December 2014), so we allow the neighborhood algorithm to sample both dip domains, searching over a broad range of strike, dip, and rake within each region of model space.…”

Tectonic and climatic controls on the Chuquibamba landslide (western Andes, southern Peru)

“…Schildgen et al (2009) proposed that a Tertiary episode of strike-slip and normal faulting occurred between 14 and 2.2 Ma. However, relocated microseismicity (Grange et al, 1984) and teleseismic data (Devlin et al, 2012) demonstrate present-day reverse and strike-slip components on the Lluta fault segment and more regionally for the Incapuquio fault system in the Arequipa region (Fig. 2).…”

“…2b) and focal mechanisms ( Fig. 2c) (M w < 5; < 20 km, Grange et al, 1984;Devlin et al, 2012). Local reverse and normal apparent surface movements are known to occur along major strike-slip faults in the forearcs of northern Chile (Victor et al, 2004) and southern Peru (Hall et al, 2012).…”

“…During the Eocene to Neogene, volcano-sedimentary deposits of the Moquegua group were overlain onto the Western Cordillera. This group is partly covered by the Huaylillas ignimbrite, produced Grange et al (1984) and Devlin et al (2012); the kinematics of the Incapuquio fault system are indicated by black arrows.…”

“…In contrast, because of the potential seismotectonic trigger (Keefer, 1984(Keefer, , 2002McPhillips et al, 2014), landslide triggering along subduction active margins has been studied for a number of years, but most previous studies focused on humid climatic settings (Taiwan, New Zealand, Papua New Guinea, Japan; Meunier et al, 2008;Hovius et al, 2011). In southern Peru, the topographic gradient (average slope of 4 % between the coast and the Western Cordillera), the crustal seismotectonic activity and the aridity of the forearc region has been directly linked to Andean uplift and subduction of the Nazca Plate for the last 25 Myr at least (Devlin et al, 2012;Alpers and Brimhall, 1988;Dunai et al, 2005). However, Quaternary tectonic crustal activity and its role in the localization of landslides along the Western Andean Escarpment has never been explored in southern Peru and northern Chile.…”