Upper plate reverse fault reactivation and the unclamping of the megathrust during the 2014 northern Chile earthquake sequence

Abstract

After 137 years without a great earthquake, the M w 8.1 Pisagua event of 1 April 2014 occurred in the central portion of the southern Peru-northern Chile subduction zone. This megathrust earthquake was preceded by more than 2 weeks of foreshock activity migrating ~3.5 km/ day toward the mainshock hypocenter. This foreshock sequence was triggered by an M w 6.7 earthquake on a reverse fault in the upper plate that strikes at a high angle to the trench, similar to well-documented reverse faults onshore. These margin-oblique reverse faults accommodate north-south shortening resulting from subduction across a plate boundary that is curved in map view. Reverse slip on the crustal fault unclamped the subduction interface, precipitating the subsequent megathrust foreshock activity that culminated in the great Pisagua earthquake. The combination of crustal reverse faults and a curved subduction margin also occurs in Cascadia and northeastern Japan, indicating that there are two additional localities where great megathrust earthquakes may be triggered by upper plate fault activity.

Characterizing Afterslip and Ground Displacement Rate Increase Following the 2014 Iquique‐Pisagua Mw 8.1 Earthquake, Northern Chile

“…The largest aftershock is located at the southern edge of the negative ΔCFS region. This may be associated to premainshock stress conditions and potentially reflects interseismic stress buildup preceding the mainshock, for example, due to the foreshock on 16 March 2014 (e.g., González et al, ).…”

“…Another cause for local (point‐wise) misfit of the GPS vectors as, for example, observed for stations VIRI or COLC in Period 3 could be upper crustal faulting, as has been observed in many parts of Northern Chile and Bolivia (e.g., Allmendinger & González, ; Lamb, ). González et al () even argued that the Iquique‐Pisagua mainshock could have been triggered by a reactivated trench‐oblique upper plate reverse fault (the M w 6.7 foreshock on 16 March 2014). However, we do not observe a spatially coherent, long‐wavelength misfit pattern in the far field as potentially caused by viscous relaxation (Wang et al, ).…”

Influence of the megathrust earthquake cycle on upper-plate deformation in the Cascadia forearc of Washington State, USA

“…Forearc structures may accommodate margin-normal and/or parallel slip related to the orientation of plate convergence (e.g., McCaffrey, 1993) and may be sensitive to stress induced by great subduction earthquakes (Aron et al, 2013;Loveless and Pritchard, 2008). Accordingly, megathrust earthquakes may trigger slip on forearc faults (Sherrod and Gomberg, 2014), or conversely, ruptures on upper-plate structures may initiate subduction earthquakes by decoupling the megathrust (González et al, 2015). Forearc structures may also serve as boundaries that control the length and location of subduction zone ruptures over multiple earthquake cycles (Melnick et al, 2009).…”

Crustal faults in the Chilean Andes: geological constraints and seismic potential

“…The seismogenic potential of a fault -that is, the size and recurrence rate of the earthquakes it produces-may be clearer and more tectonically significant when that fault is viewed in the geodynamic context of the subduction earthquake cycle. For example, inactive crustal faults which are optimally oriented with respect to inter-or co-seismic stress fields may be reactivated and produce shallow earthquakes with magnitudes of up to Mw 7 (e.g., Farías et al, 2011;Aron et al, 2013;González et al, 2015). This scenario is particularly relevant for optimally-oriented basement (inherited) crustal faults undergoing sudden high strain rates, a condition which can potentially be achieved co-seismically with large 'megathrust' subduction earthquakes (Mw>8, King et al, 1994;Stein et al, 1994;Stein, 1999;Kilb et al, 2000;Lin and Stein, 2004;Loveless et al, 2010;Seeber and Armbruster, 2000).…”