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Imaging rapid early afterslip of the 2016 Pedernales earthquake, Ecuador

Abstract:

High-Rate (HR) GPS time series following the 2016 Mw 7.8 Pedernales earthquake suggest significant postseismic deformation occurring in the early postseismic period (i.e. first few hours after the earthquake) that is not resolved with daily GPS time series. To understand the characteristics of early postseismic deformation, and its relationship with the mainshock rupture area, aftershocks and longer-term postseismic deformation, we estimate the spatio-temporal distribution of early afterslip with HR-GPS time series that span ∼2.5 min to 72 hr after the earthquake, and compare with afterslip models estimated with daily GPS time series spanning a similar postseismic time period and up to 30 days after the earthquake. Inversion of the HR-GPS time series enables us to image the initiation of afterslip in the initial hours after the earthquake, bringing us closer to the transition between the coseismic and postseismic phases. The spatial signature of early afterslip in the region updip of the mainshock rupture area is consistent with longer-term afterslip that occurs in the 30-day postseismic period, indicating that afterslip initiated updip of and adjacent to peak coseismic slip asperities, in two localised areas, and subsequently continued to grow in amplitude with time in these specific areas. A striking difference, however, is that inversion of the 72-hour HR-GPS time series suggests early afterslip within the mainshock rupture area, but which may have been short-lived. Finally, using the first daily GPS position as the origin of the postseismic displacement (here at 12 hr after the earthquake) biases the postseismic geodetic moment, with ∼60% missing over the first 72 hr, that corresponds to ∼10% over the first 30 days. The results of our study demonstrate that imaging the spatio-temporal evolution of afterslip using subdaily GPS time series is important for evaluating postseismic slip budgets, and provides additional insights into the postseismic slip behaviour of faults.