Stress drop variations of small aftershocks after the 2016 Pedernales earthquake (Ecuador)

Subduction zones are highly heterogeneous regions capable of both aseismic slip and destructive earthquakes. These different behaviors are controlled by the geometry, structural heterogeneity and stress state of the megathrust. Thus to determine whether large earthquakes can nucleate on a portion of the fault, it is necessary to characterize the fault properties and the processes that act upon it in detail. Seismology offers several tools to help with this endeavor. In particular, earthquake source size, magnitude and stress drop can help us better constrain the state of stress and friction on a fault.

We recover the seismic moments, stress drops, and P and S corner frequencies of 597 aftershocks of the April 16th, 2016 Mw 7.8 Pedernales earthquake (Ecuador) using spectral ratios. In addition to regular aftershocks, we also recover the source properties of 187 events belonging to families of repeating earthquakes, which allows us to examine the evolution of source properties on a single seismic asperity over time. We find that stress drops are anomalously low near the trench, which is best explained either by a high pore fluid pressure or by different frictional properties in that region. In addition, stress drops of repeaters systematically decrease over in the region, which hints at a potential increase in pore fluid pressure near the trench over the postseismic period.