Imaging rapid early afterslip of the 2016 Pedernales earthquake, Ecuador

Abstract

High-Rate (HR) GPS time series following the 2016 M w 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 captured in 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 spatiotemporal distribution of early afterslip with HR-GPS time series that span ~ 2.5 minutes to 72 hours 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. Our inversion technique enables us to image the nucleation 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 nucleated updip of and adjacent to peak coseismic slip asperities, in two localized 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. More interestingly, we find that postseismic slip starts immediately after the earthquake at a rapid rate. Indeed, we find that early afterslip represents a significant contribution to the postseismic geodetic moment-afterslip in the first 72 hours is ~60 % greater than that estimated with daily GPS time series that span the first three post-earthquake daily GPS positions (i.e. covering the same time window). 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 3 | P a g e budgets, and provides additional insights into the postseismic slip behaviour of faults.

Repeating Earthquakes at the Edge of the Afterslip of the 2016 Ecuadorian MW7.8 Pedernales Earthquake

“…This is consistent with suggestions that the Pedernales aftershocks are driven in large part by afterslip (Agurto‐Detzel et al., 2019), similarly to repeaters. In fact, repeaters primarily occur updip of the mainshock rupture, where most of the afterslip occurs (Rolandone et al., 2018; Tsang et al., 2019). With a few exceptions north‐east of the coseismic rupture, both repeaters and aftershocks are contained within three trench‐perpendicular seismicity streaks that stretch between the coseismic rupture zone and the trench.…”

Coevolving early afterslip and aftershock signatures of a San Andreas fault rupture

“…The concurrent evolution of afterslip and aftershocks have been observed in some subduction zones and crustal faults, typically over time scales longer than hours or spatial scales larger than tens of kilometers (1,41,48,49). Questions remain on whether the two processes reflect similar or common statistical fault properties, physical mechanisms, or both.…”

GNSS Constraints to Active Tectonic Deformations of the South American Continental Margin in Ecuador

“…Moreover, the CASA GPS data evidence a partitioning of continental deformation that is highly controlled by the diverging motion of two continental slivers: the Inca Sliver in northern Perú and southern Ecuador and the North Andean Sliver in Ecuador and Colombia [15]. These relative motions are linked to the sequence of great earthquakes during the last century [10,11,15,18,53,54]. The GPS studies evidence in addition the creep character of active faults in the Andes [55], accommodating deformation and decreasing their related seismicity.…”