Constraints from 3D earthquake location for aftershocks of the Mw=7.8 April 16, 2016 Pedernales Earthquake (Ecuador subduction zone)

The Mw7.8 Pedernales earthquake is associated with the subduction of the Nazca Plate beneath the South American Plate. The mainshock caused many casualties and widespread damages across the Manabi province. The 150 km-long coseismic rupture area is found beneath the coastline, near 25 km depth. The rupture propagated southward and involved the successive rupture of two discrete asperities, with a maximum slip (~ 6 m) on the southern patch. The rupture area is consistent with the highly locked regions observed on interseismic coupling models, overlaps the 7.2 Mw rupture zone, and terminates near where the 1906 Mw 8.8 megathrust earthquake rupture zone ends. Two neighboring highly coupled patches remain locked: (A) southern to and updip of the coseismic rupture zone and (B) northern and downdip. In this study, we relocate the aftershocks and compare the seismicity distribution to the interseismic coupling and the rupture area. We use continuous seismic traces recorded on the permanent network partly installed in the frame of the collaboration between l’Institut de Recherche pour le Developpement (IRD-France) and the Instituto Geofisico, Escuela Politecnica Nacional, Quito, Ecuador. Detections are conducted using Seiscomp in play-back mode. Arrival-times are manually picked. To improve earthquake location, we use the MAXI technique and a heterogeneous a priori P-wave velocity model that approximates the large velocity variations of the Ecuadorian subduction system. Aftershocks align along 3 to 4 main clusters that strike perpendicularly to the trench, and mostly updip of the co-seismic rupture. Aftershock seismicity develops indifferently over portions of plate interface that are known to be strongly locked or almost uncoupled. The seismicity pattern is similar to the one observed during a decade of observation during the interseismic period with swarms such as the Galera alignment, Jama and Cabo Pasado, and between Manta and Puerto Lopez.