Spatio-temporal evolution of early afterslip following the 2016 Mw 7.8 Pedernales earthquake
2018
Many studies of postseismic deformation following large earthquakes focus on time scales ranging from weeks to years following the main earthquake. By contrast, fewer studies have investigated the spatio-temporal distribution of postseismic deformation in the hours to days following the mainshock. Focusing on this early postseismic period is important for better understanding how stresses are redistributed on the fault immediately following the mainshock, and the relationship between the mainshock, aftershocks and postseismic slip. In this study, we estimate the spatio-temporal distribution of early afterslip following the 2016 Mw 7.8 Pedernales earthquake offshore Ecuador, with the aim of studying how its spatio-temporal distribution relates to those of the coseismic and longer-term postseismic phases. We base our analysis on time series of 30-second high-rate GPS data spanning 2 minutes to 3 days after the mainshock at 27 stations of the IGEPN and IRD joint network operating in Ecuador. We invert these time series with the Principal Component Analysis-based Inversion Method (PCAIM) [Kositsky and Avouac, 2010], incorporating a sensitivity-regulated regularization scheme. Our results suggest afterslip concentrated in regions of the fault updip of two areas that experienced peak coseismic slip, which is similar to that of afterslip estimated by inverting daily GPS time series spanning 30 days after the earthquake. However, we find a noticeable improvement in the fit of the data when a moderate amount of afterslip is also located within and downdip of the mainshock rupture area, suggesting that the coseismic and early postseismic slip episodes might partially overlap. Preliminary comparisons between the early afterslip and relocated aftershocks show good spatial agreement, consistent with the idea that the aftershocks are driven by afterslip. Finally, the equivalent moment magnitude of our models ranges from Mw 7.1 to 7.2, represents 35-50% of the geodetic moment of the 30-day afterslip model of Rolandone et al. [2017], and 9-13% of the coseismic moment released during the Mw 7.8 earthquake. These inversion results will guide our exploration of frictional properties on the megathrust, which potentially allow us to probe the physical processes that occur during the early postseismic period.
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