Time-Dependent Stress Drop Variations After Large Earthquakes Along the North Chilean Megathrust

ABSTRACT We observe a time dependence of the median stress drop in spatiotemporal proximity of large earthquakes. The median stress drop of the early aftershock seismicity is elevated for only a few days after the mainshocks and then rapidly falls back to the long-term average. This short-term variation has remained largely unnoticed by previous studies, presumably due to their usually low temporal resolution. Our study uses a recent extensive stress drop catalog, which contains more than 51,000 events from northern Chile. It includes observations from three Mw>7 megathrust earthquakes, namely the 2007 Mw 7.7 Tocopilla earthquake, the 2014 Mw 8.1 Iquique earthquake, and its largest Mw 7.6 aftershock, as well as another eleven Mw>6 earthquakes. A detailed analysis reveals that the elevated stress drop is not primarily linked to an increase of seismic moment during the early aftershock phase but is rather attributable to higher measured corner frequencies in the corresponding time interval. We propose two possible explanations: (1) The high stress changes induced by the mainshock allow failure of strong structures at or adjacent to the main rupture area, which produces higher stress drop events. After a few days, afterslip activity has reduced the in situ stresses and thereby the failure potential of the stronger regions. (2) The mainshock disturbs the fluid-sealing plate interface, allowing overpressured fluids to escape, exhibiting the so-called fault-valve behavior. The effect appears to persist only for several days until the sealing effect is restored and average stress drop levels are recovered.