Seismic Deployment

Team: Susan Beck, Eric Kiser, Ryan Porter, Steve Roecker, Diana Comte, and Patricia Alvarado

This will be the first deployment to utilize nodal instruments at a large spatial and temporal scale, allowing us to simultaneously leverage station density inherent to the portability of the nodes and the large number of seismic sources provided by the 5-6 month deployment time. Deploying two dense lines with ~2-3 km spacing for ~600 km on the northern transect and ~400 km on the southern transect with BB seismic stations at ~30 km spacing (with ~40 stations off the transects to improve earthquake locations, surface wave tomography etc.) will provide unprecedented seismic data for the Andes.

Map showing proposed broadly spaced broadband seismic stations (red stars), existing permanent stations (blue triangles) and previously temporary deployed seismic stations (black circles). Insets show an enlarged map of the two dense transects of 3-component nodal seismic stations (black bars with broadband station every ~30 km (red stars), following roads, based on logistics considerations) with active volcanoes (magenta triangles). Stations CORD, ZOND and TRQA are permanent. Many previous seismic deployments had small apertures and only recorded a limited time (small black circles). Although these data are useful for some studies, we need to have stations across the Andes recording at the same time for many important studies.

Planned Seismic Methods

Examples of types of seismic analysis proposed. (A) Example of finite frequency teleseismic P-wave tomography using 3D ray tracing from Portner et al., (2020) to image the deep portion of the slab especially at latitudes south of 34°S. (B) Reprocessed CCP receiver functions along the dense REFUCA seismic line with 10 km station spacing (Wölbern et al., 2009) at ~21.5°S (Isaacs, U. Arizona M.S. Thesis, 2016). This image was migrated with variable bin sizes based on a minimum number of crossing rays in each bin. The continental Moho (CM) has a high amplitude conversion under the western Altiplano (AP) and the eastern part of the Eastern Cordillera (EC) and Subandean zone (SAZ) but is gradational beneath the eastern Altiplano. The gradational Moho may indicate ongoing lithospheric removal. We interpret the west dipping detachment (WDD) as the Main Andean Thrust based on the Elger et al. (2006) geologic cross-section. We expect to be able to produce much better images along the two proposed dense seismic lines. (C) Example of a joint receiver function surface wave (Ambient Noise) inversion at 25.2°S across the arc in the southern Puna from Delph et al. (2017) using seismic data with ~20-30 km station spacing. (D) Example of local tomography from the Cascades in Oregon. a) Earthquake source gather from a M2.5 earthquake that occurred at the east end of a 130 km east-west line of 174 3C nodal seismometers in central Oregon. Prominent seismic phases are labeled. b) Final Vp model. c) Final Vp/Vs model. Large gray circles are earthquakes used in the local earthquake tomography study and small gray circles are earthquakes from the PNSN catalog for the past 10 years. White dashed line outlines the change from low Vp to high Vp in the lower crust. (E) Progressive joint inversion of local travel times and Rayleigh wave dispersion in the Chile forearc near ~31°S from Comte et al. (2019). Grey circles are earthquakes.