CONSTRUCTING FOREARC ARCHITECTURE WITH MEGATHRUSTS: GEOLOGICAL AND GEOPHYSICAL EXAMPLES FROM THE 2010 MAULE EARTHQUAKE REGION

We present new structural data of the Chilean Coastal Cordillera, near the northern and central parts of the area ruptured by the 2010 M_w 8.8 Maule earthquake. The structural style of the region is dominated by kilometer-scale normal faults which have been active at least throughout the Quaternary. The strikes of these structures define three structural systems: (1) a NE and (2) a NW sets of margin-oblique faults, and (3) a ~NS-NNE, margin-parallel set. The Pichilemu normal fault, an intraplate forearc structure reactivated after the Maule event with earthquakes up to M_w7.0, belongs to the second group. From north to south, two domains can be identified: (a) the northernmost segment displays a bimodal orientation of groups (1) and (2); and (b) at the southern end of the regions studied, faults of set (3) are the most common. Reverse faults coexist with the normal structures but are scarce and alternate between NW- and EW-striking.

In the Maule earthquake region as well as in others subduction zones, both geodetic and seismic data show an alternation of the forearc deformation style across the seimic cycle. Large coseismic normal faulting and extension parallel to the subduction rebound follows great subduction earthquakes. Generally minor reverse kinematics and convergence-parallel shortening are observed interseismically. Although smaller than the Pichilemu sequence, an interseismic M_w6.3 crustal reverse earthquake was recorded in the forearc 20 years prior to the Maule megathrust. Paleoseismological evidence in Japan and northern Chile indicates that the long-lived, forearc normal faults may have recurrence times an order of magnitude longer than the subduction cycle.

The shallow crustal normal faults reactivated by the 2010 Maule earthquake show a consistent kinematic history throughout thousands of subduction seismic cycles. We propose that the normal fault-dominated architecture of the forearc may have been constructed by repetition of megathrusts. Evidence for Quaternary reverse faulting suggests that the interseismic deformation signature may also be preserved in the structural grain. Analogous observations along the 2011 Tōhoku earthquake rupture in Japan imply that such link between the short- and long-term deformation patterns of the forearc are not exclusive of the Maule earthquake region.