Surface cracks, if preserved, formed during large subduction zone earthquakes potentially provide records of past large earthquakes. However, these subtle surface features disappear within days in most settings. Due to the aridity of the Atacama Desert in Chile, these cracks are preserved and can be used to estimate deformation associated with large earthquakes along the Chilean subduction zone. The recent magnitude 8.2 earthquake in Chile affords an opportunity to directly test the idea that cracks form during these large events and that the cracks can be used to estimate the amount of slip that occurred during an earthquake and the amount of deformation experienced above the earthquake source region. If successful, the project would add a new method to improve understanding of earthquake hazards associated with large megathrust earthquakes in Chile and elsewhere, such as Cascadia, Alaska, and Japan.
The goal of this RAPID project, carried out in collaboration with Chilean researchers, is to collect scan line data of co-seismic Mode I surface cracks in the Chilean Coastal Cordillera produced by the April 1, 2014 Mw 8.2 Pisagua earthquake. Cracks commonly form in unconsolidated deposits and must be measured within about six months from the time of the earthquake in order to be identified as having been produced during the earthquake. Accurate measurement of crack apertures is likewise important for determination of surface strain for comparison with GPS data that will be forthcoming from this event. As the walls of the cracks degrade over time, accurate measurement of aperture becomes more and more difficult. The objectives are to re-measure scan lines that have already been measured, pre-earthquake, at Punta de Lobos and to measure new scan line data for at least three separate sites in the region between the city of Iquique and north of the pueblo of Pisagua. Initial InSAR data suggest that the highest surface deformation will be encountered between Iquique and Pisagua. Co-seismic surface cracks provide a unique long term record of thousands of cycles of plate boundary seismicity. The Pisagua earthquake provides an outstanding opportunity to document the crack population developed during a single earthquake to help refine models of co-seismic crack generation as well as explore the basic question of how much of the co-seismic geodetic signal (predominantly GPS) reflects recoverable elastic deformation and non-recoverable plastic deformation. These data will help refine models of co-seismic surface deformation and average rupture segments, which are key inputs into seismic hazard assessment.
