The authors describe their use of off-shore observations to demonstrate the existence of splay faulting in the shallow regions of the Chilean Subduction Zone involved in the Mw 8.8, 2010 Maule, Chile earthquake. A splay fault is a relatively steep fault that connects the plate boundary interface with the seafloor that when activated during a large earthquake, can enhance tsunami excitation. Based on previous work noted in the paper, the most likely location of splay faults is along the boundary between outer and inner wedges, which is where the authors, Liessr and others, observed the seismic activity in the Chile study.
Specifically, the authors deployed a 30-station ocean-bottom seismometer network for three months and analyzed the offshore data in concert with observations from another 33 land-based seismic stations. They used a a 2.5 dimensional velocity model derived from seismic reflections profiles and previous local earthquake studies to locate the aftershocks.
Good data coverage provides a road to good results, which in this case includes several interesting outcomes. The aftershock locations illuminate a 50 km long linear structure extending from the plate boundary interface to the seafloor that coincides with a splay fault outcrop (on top of the wedge). The P-wave speed distribution (estimated from active-source and tomographic results) suggests that the splay fault begun to branch off with an angle of 7⁰-8⁰ from the plate boundary interface at ~20 km depth and ~67 km away from the deformation front. Finally, it’s important to mention that the southern part of the study area it does not appear that the main shock experienced any activity associated with a splay fault.
Please see the papers for details: http://geology.gsapubs.org/content/41/12/e309.full
Thamer