Upper plate deformation measured by GPS in the Coquimbo Gap, Chile

Since the Mw = 7.3 Punitaqui earthquake in 1997, the area between 30°S and 32°S (Coquimbo-Illapel section) of the Chilean subduction has been the locus of a decennial seismic swarm. A dense network of 30+ benchmarks have been installed in this area and surveyed six times with high precision GPS over the last 3 years. Surface deformation here is compatible with elastic loading due to partial locking on the subduction interface at depth. Here we show that in this area, only 40–45% of the total convergence rate between Nazca and South America plates gives way to accumulation of elastic deformation in the upper plate, the remaining 60–55% being dissipated by free or aseismic slip, the cumulative slip due to the seismic swarm explaining no more than 1/3rd to 1/4th of it. We also find that the accumulation decreases northward, to reach almost zero around 30°S (La Serena – Tongoy). Whether this is a steady state or only a transient pattern (a steady decrease of coupling) is not clear since our measurements span only 3 years and since early measurements 10 years ago were sparse and differ only marginally from ours.