Repeating earthquakes on the Chile subduction zone following the Maule 2010 M 8.8 earthquake

We investigate repeating earthquakes (REs) on the Chile subduction zone, in the first 9 months following the Maule 2010 M 8.8 earthquake. Using the aftershock catalogue of approximately 30,000 events (Rietbrock et al., 2012) and the data from the International Maule Aftershock Dataset (IMAD), we identify 1550 clusters of small magnitude (Mw ~1.5-3) events showing similar waveforms (cross-correlation coefficients>0.9). Clusters are found from the surface to depths of ~60km, indicating the generation of RE on pre-existing crustal faults and slab interface. A particularly dense band of clustered seismicity runs NE-SW along the length of Chile at 37-47km depth on the slab/continent interface, apparently defining the limit of plate coupling (Rietbrock et al, 2012; Lange et al., 2012). Relocation of deep clusters, via the double difference method (hypoDD), reveal that they lie within a region of increased fluid content (interpreted from high Vp/Vs ratio (Hicks et al., 2012)), and define streaks of seismicity orientated down-dip. Moment tensor analysis of selected aftershocks shows that larger events M 4-5 are located at the interface or deeper in the slab (5-8km beneath the slab interface) and show thrust motion along the direction of the plate interface. REs, in contrast, show predominantly strike-slip motion and are located close to the interface. Temporal analysis also shows non-constant recurrence times of events within clusters, which we interpret as an indication that the seismicity in the deep clusters are driven by pulses of after-slip from the Maule 2010 event together with episodic fluid migration. We introduce a model of Mixed Mode Fault Slip (MMFS) to explain our observations, where aseismic sliding of trench sediments in the subduction channel build up stress on fragments of ocean crust, causing them to repeatedly fracture and generate repeating earthquakes.