Abstract

The subduction megathrust in northern Chile (19 degrees-23 degrees S) exhibits a mature seismic gap capable of generating large (M similar to 8) earthquakes in the coming decades. The electrical resistivity structure of this area is monitored since 2007 by an array of 11 long-period magnetotelluric (MT) sites, to infer changes in the deep fluid distribution during the seismic cycle. Here we present the main findings from the interval encompassing the seismic sequences of the 2007 Mw 7.7 Tocopilla and 2014 Mw 8.1 Iquique earthquakes, an interval that includes a dozen of reported slow slip events (SSEs). Analysis of the MT data revealed that fluctuations in external magnetic fields strongly influence temporal variations of MT parameters that are used to monitor the subsurface. By applying an approach that minimizes the influence of external effects, we identified a significant anomaly in MT monitoring parameters from mid-2012 to early-2014 at a site located above an unbroken megathrust segment. Using forward modeling, we can explain this anomaly by a decrease in resistivity within a seismically active fault in the upper plate. No robust anomalies in MT parameters were detected at sites located closer to the slip areas of major earthquakes or reported SSEs. Based on numerical simulations of hypothetical resistivity changes in these slip areas, we discuss the monitoring capabilities of the MT array, including the characteristics of possible undetected fluid flow events. Our results, together with the highly-resistive onshore forearc imaged by previous MT studies, suggest low long-term fluid migration from the deep megathrust to the upper plate.