A Study of the 2014 Pisagua, Chile Earthquake Sequence

Abstract:

The 2014 Pisagua, Chile earthquake sequence, which struck the famous northern Chilean seismic gap, started with a Mw 6.7 earthquake on March 16^th. This was followed by a 15 day-long earthquake swarm, including multiple Mw>6 earthquakes migrating northward on the plate interface, eventually resulting in a Mw 8.1 megathrust on April 1^st. This produced an energetic aftershock sequence with one Mw 7.7 aftershock occurring on April 3^rd. We have studied this interesting earthquake sequence by jointly inverting teleseismic data as well as strong motion data from the IPOC/CSN network. The initial efforts focused on the kinematic rupture parameters of the Mw 6.7 foreshock, Mw 8.1 mainshock, and Mw 7.7 aftershock. Our results show that the Mw 6.7 foreshock was a shallow intraplate event with a major asperity at ~12 km depth. Although its rupture was entirely within the overriding plate, this event created a strong static Coulomb stress perturbation on the plate interface, with a maximum value of ~0.2 MPa. We find that the hypocenters of its two largest earthquakes, Mw 6.4 on March 17^th and Mw 6.2 on March 23^rd are both located right on the plate interface where the Coulomb stress was increased. The rupture of the Mw 8.1 mainshock, spanning about 120 km along strike, initially travels down dip, roughly N100°E, with an apparent speed of 2.5 km/s for the first 25 seconds. Between 25 and 40 seconds, the rupture travels mainly southward (~N150°E) with an apparent speed of ~3 km/s, and more than 50% of the total seismic moment release in two major asperities. The third stage of the earthquake took place between 40 and 70 seconds with the rupture propagating slowly (<2 km/s) towards the northern end of the coseismic rupture zone. The slip of the Mw 7.7 aftershock is heterogeneous, with three well-resolved asperities, visually complementary to the rupture of the mainshock. We also notice that the complex rupture patterns associated with this interesting earthquake sequence apparently correlate well with the local gravity anomaly and geological settings, which will be further explored.