Coseismic slip and early afterslip of the 2015 Illapel, Chile, earthquake: Implications for frictional heterogeneity and coastal uplift

Abstract

Great subduction earthquakes are thought to rupture portions of the megathrust, where interseismic coupling is high and velocity‐weakening frictional behavior is dominant, releasing elastic deformation accrued over a seismic cycle. Conversely, postseismic afterslip is assumed to occur primarily in regions of velocity‐strengthening frictional characteristics that may correlate with lower interseismic coupling. However, it remains unclear if fixed frictional properties of the subduction interface, coseismic or aftershock‐induced stress redistribution, or other factors control the spatial distribution of afterslip. Here we use interferometric synthetic aperture radar and Global Position System observations to map the distribution of coseismic slip of the 2015 Mw 8.3 Illapel, Chile, earthquake and afterslip within the first 38 days following the earthquake. We find that afterslip overlaps the coseismic slip area and propagates along‐strike into regions of both high and moderate interseismic coupling. The significance of these observations, however, is tempered by the limited resolution of geodetic inversions for both slip and coupling. Additional afterslip imaged deeper on the fault surface bounds a discrete region of deep coseismic slip, and both contribute to net uplift of the Chilean Coastal Cordillera. A simple partitioning of the subduction interface into regions of fixed frictional properties cannot reconcile our geodetic observations. Instead, stress heterogeneities, either preexisting or induced by the earthquake, likely provide the primary control on the afterslip distribution for this subduction zone earthquake. We also explore the occurrence of coseismic and postseismic coastal uplift in this sequence and its implications for recent hypotheses concerning the source of permanent coastal uplift along subduction zones.

Piecemeal Rupture of the Mentawai Patch, Sumatra: The 2008 Mw 7.2 North Pagai Earthquake Sequence

“…We show that the estimated afterslip following the 2008 event partially overlaps the updip region of its coseismic rupture area. Similar overlapping cases were also inferred for afterslip following the 2011 Tohoku‐Oki earthquake (Johnson et al, , ), the 2010 Maule earthquake (Bedford et al, ), the 2015 Illapel earthquake (Barnhart et al, ; Shrivastava et al, ), the 2007 Bengkulu earthquake (Tsang et al, ), and the 2010 Mentawai earthquake (Feng et al, ). In the Mentawai patch, afterslip following the recent earthquakes does seem to have a preference to overlap.…”

Integrated geophysical characteristics of the 2015 Illapel, Chile, earthquake

“…The selected data sets, processing steps, model setup, and inversion approach are identical to those used in Barnhart et al . []; however, the fault geometry in this study differs so that it matches the seismic FFM geometry (section 2.2), allowing us to directly compare the solutions.…”

“…We note that there could be a small number of M 4.5+ events that are not in the catalog because their signal is obscured by the waveforms generated by other aftershocks [e.g., Herman et al ., ]. The 844 aftershocks have a total seismic moment of 1.6 × 10 20 Nm, equivalent to a M w 7.4 earthquake and comparable to estimates of afterslip moment [ Barnhart et al ., ]. Their mechanisms are dominantly thrust faulting with shallowly eastward dipping nodal planes (Figure a), and in cross section it is apparent that they occur dominantly on the plate interface [ Hayes et al ., ] over a depth range of 10–50 km, highly concentrated near the rupture zone of the mainshock, especially near the mainshock hypocenter (Figure ).…”

Machine Learning Can Predict the Timing and Size of Analog Earthquakes

“…Therefore, the pattern of ISC may serve as a proxy for constraining the size of future earthquakes. In contrast, recent investigations reveal that high ISC zones may span the whole range of coseismic slip magnitudes (Barnhart et al, 2016;Métois et al, 2016) and that earthquakes may leave large portions of highly coupled megathrust unruptured (e.g., Konca et al, 2008). In contrast, recent investigations reveal that high ISC zones may span the whole range of coseismic slip magnitudes (Barnhart et al, 2016;Métois et al, 2016) and that earthquakes may leave large portions of highly coupled megathrust unruptured (e.g., Konca et al, 2008).…”

“…Moreover, some earthquakes (e.g., M w 8.4 Sumatra 2007) propagated or even nucleated into areas of low/no coupling (Konca et al, 2008), highlighting the potential for temporal variations in the ISC pattern as well as uncertainties associated to the ISC inversion method. This situation is partly due to uncertainties associated with inversion methods, inconsistent data availability before and after the event, and missing spatial coverage of the offshore seismogenic region, meaning that improving the robustness of spatial correlation between slip and locking models is an ongoing area of research (e.g., Barnhart et al, 2016;Loveless & Meade, 2011). The 2010 M w 8.8 Maule earthquake (Chile) demonstrates the challenges of setting up and interpreting ISC models.…”