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Abstract:
Splay faults, large thrust faults emerging from the plate boundary to the seafloor in subduction
zones, are considered to enhance tsunami generation by transferring slip from the
very shallow dip of the megathrust onto steeper faults, thus increasing vertical displacement
of the seafloor. These structures are predominantly found offshore, and are therefore difficult
to detect in seismicity studies, as most seismometer stations are located onshore. The Mw
(moment magnitude) 8.8 Maule earthquake on 27 February 2010 affected ~500 km of the central
Chilean margin. In response to this event, a network of 30 ocean-bottom seismometers was
deployed for a 3 month period north of the main shock where the highest coseismic slip rates
were detected, and combined with land station data providing onshore as well as offshore coverage
of the northern part of the rupture area. The aftershock seismicity in the northern part
of the survey area reveals, for the first time, a well-resolved seismically active splay fault in the
submarine forearc. Application of critical taper theory analysis suggests that in the northernmost
part of the rupture zone, coseismic slip likely propagated along the splay fault and not the
subduction thrust fault, while in the southern part it propagated along the subduction thrust
fault and not the splay fault. The possibility of splay faults being activated in some segments of
the rupture zone but not others should be considered when modeling slip distributions.
