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The western boundary of South America is an active subduction zone, where the Nazca plate is being thrust beneath the South American plate at a rate of approximately 7 cm/yr. Earthquakes occur along this boundary frequently, resulting in a long record of activity that led to the identification of several “seismic gaps,” sections of the plate boundary where a large earthquake is overdue. One of these gaps ruptured in 2010 with the M8.8 Maule earthquake. Another gap, in northern Chile and Peru, has been monitored by the Integrated Plate boundary Observatory – Chile (IPOC) program since 2007. Consequently the onshore region was well instrumented for monitoring earthquake activity and strain accumulation when a M8.1 earthquake struck on April 1, 2014. This earthquake, however, only partially filled the gap, leaving a long stretch of un-ruptured plate boundary to the south. Two features of this earthquake caught our attention: an extended and well-characterized sequence of seismic events in the months and weeks prior to the mainshock, and a striking correlation between this sequence and anomalies in the earth’s gravity field. Tiny perturbations in gravity are indicative of geologic differences in the composition (and therefore, the density) of the earth’s crust. PICTURES is designed to image this region using techniques that are analogous to those used for non-invasive imaging of the human body (a combination of a CAT scan and an ultrasound) in order to understand the geologic origin of the gravity anomalies and their impact on how earthquakes nucleate and propagate.

This map shows the earthquake history of northern Chile and southern Peru since 1868 on the left. The vertical lines show the estimated north-south extent of the rupture zone in different earthquakes, with the largest events highlighted in red. The map on the right shows the topography of the seafloor, with dark blue being as deep as 7000 m and the pink and grey showing shallow seafloor on the Iquique Ridge. The black line with triangles shows the plate boundary where the Nazca plate plunges beneath South America and the arrow shows the direction of motion. The colored ovals just offshore Iquique and Pisagua show a model for the slip during the April 1, 2014 earthquake, with contours at 1 m intervals. Light green shaded areas are the rupture areas of other recent earthquakes. The dashed white line shows the segment of the plate boundary that ruptured during the 2014 earthquake, and the solid white line shows the remaining gap. Had the entire gap ruptured at once, it could have generated an earthquake as great as ~M9. (adapted from Schurr et al., Nature, 2014)