Over the past decade, the Isla Santa María (south-central Chile) has provided one of the most exquisitely detailed records of deformation and tectonically controlled sea-level change anywhere along the Andean margin (Bookhagen et al., 2006; Melnick et al., 2006, 2009, 2012). This work has been combined with extensive offshore suites of data, especially seismic reflection profiles and local and regional geophysical studies, to provide an unparalleled view of the offshore portion of the southern Chilean forearc. Melnick et al. (2012) recently postulated the activity of a splay thrust fault, the Santa María Fault System (SMFS), during the 2010 Maule earthquake. This work is timely given current interest in the role of splay faults in contributing to total moment release during great subduction zone earthquakes, local tsunami hazard, and rupture segmentation. Given the importance, one must ask whether their evidence is so compelling as to require the activity of a splay thrust, especially considering that their field observations show only normal fault activity.
As described in their paper, Melnick et al. (2012) document exclusively normal fault ruptures and tilting of the island. No direct observational evidence of reverse fault activity of the SMFS, either on- or offshore, during or after the Maule event, is provided: there are no well-documented crustal reverse fault focal mechanisms and no surface features indicative of reverse faulting. The inference that the SMFS was active during the Maule event is based on a previous hypothesis that surface extensional features on the Isla Santa María represent stretching in a broad anticline located at the tip of the SMFS (Melnick et al., 2006). Additionally, they show shallow seismicity with reverse focal mechanisms on the SMFS during the interseismic period preceding the Maule event (Bohm et al., 2002).
One of the notable characteristics of the Maule earthquake was that the largest aftershocks were produced by crustal normal faults in both the upper and lower plate. The best known of these, the Pichilemu sequence, which began 12 days after the main event, cut the entire crust of the forearc as shown by local and regional seismic network data and modeling of geodetic data (Farías et al., 2011; Ryder et al., 2012; Aron et al., 2013). Curiously, Melnick et al. (2012) show the Pichilemu structure as a fault of “unconstrained kinematics” in their figure 1A.
In the northern Chile forearc, where there is substantially better exposure than in the onshore portion overlying the Maule rupture, we have documented numerous cases of reverse fault reactivation of crustal normal faults (Allmendinger and González, 2010). Melnick et al. (2006) also show numerous examples of reactivated faults interpreted on their seismic profiles. Given the distribution of slip during the Maule rupture, the northeast strike and relatively steep dip of the SMFS make it almost ideally oriented for co/post-seismic reactivation as a normal fault (Aron et al., 2013). Thus, it would appear to us that a simpler and more straightforward interpretation of the Isla Santa María normal fault ruptures associated with the Maule event is that they are the surface expression of extensional reactivation of SMFS, which is active as a reverse fault during interseismic periods but, like most of the rest of the forearc, is stretched during coseismic rebound.
We freely admit that we cannot disprove the splay fault hypothesis of Melnick et al. (2012), and furthermore, we readily recognize that splay faulting during great subduction earthquakes can be an important process, as with the reverse faults described by Plafker (1967) on Montague Island following the 1964 Alaska earthquake. But, given the documentation of normal fault surface ruptures, indisputable crustal normal faulting during the Maule event elsewhere, and the complete lack of documentation of co- or post-seismic reverse faulting on the SMFS, we suggest that coseismic extensional reactivation of the SMFS is at least an equally viable hypothesis. Thus, one would have to conclude that the evidence for splay thrusting beneath the Isla Santa María during the Maule earthquake is equivocal at best.