Dual Megathrust Slip Behaviors of the 2014 Iquique Earthquake Sequence

The transition between seismic rupture and aseismic creep is of central interest to better understand the mechanics of subduction processes. A M 8.2 earthquake occurred on April 1st, 2014 in the Iquique seismic gap of Northern Chile. This event was preceded by a 2-week-long foreshock sequence including a M 6.7 earthquake. Repeating earthquakes are found among the foreshock sequence that migrated towards the mainshock area, suggesting a large scale slow-slip event on the megathrust preceding the mainshock. The variations of the recurrence time of repeating earthquakes highlights the diverse seismic and aseismic slip behaviors on different megathrust segments. The repeaters that were active only before the mainshock recurred more often and were distributed in areas of substantial coseismic slip, while other repeaters occurred both before and after the mainshock in the area complementary to the mainshock rupture. The spatial and temporal distribution of the repeating earthquakes illustrate the essential role of propagating aseismic slip in leading up to the mainshock and aftershock activities. Various finite fault models indicate that the coseismic slip generally occurred down-dip from the foreshock activity and the mainshock hypocenter. Source imaging by teleseismic back-projection indicates an initial down-dip propagation stage followed by a rupture-expansion stage. In the first stage, the finite fault models show slow initiation with low amplitude moment rate at low frequency (< 0.1 Hz), while back-projection shows a steady initiation at high frequency (> 0.5 Hz). This indicates frequency-dependent manifestations of seismic radiation in the low-stress foreshock region. In the second stage, the high-frequency rupture remains within an area of low gravity anomaly, suggesting possible upper-crustal structures that promote high-frequency generation. Back-projection also shows an episode of reverse rupture propagation which suggests a delayed failure of asperities in the foreshock area. Our results highlight the complexity of the interactions between large-scale aseismic slow-slip and dynamic ruptures of megathrust earthquakes.