RAPID: Collaborative: April 16, 2016 Mw 7.8 Pedernales Earthquake, Ecuador: The Role of Asperities in Rupture Propagation, Aftershock Sequences, and Post-Seismic Deformation

A Mw 7.8 earthquake occurred along the Ecuadorian subduction zone on April 16, 2016. This earthquake was devastating to the country, causing >660 fatalities >12,000 injuries, and >$3B in property damage. This personal and economic toll is amplified because Ecuador is a developing country, with a troubled economy. Preliminary seismic data from international networks indicate that the April 16 earthquake ruptured from north to south along the interface between the South American and Nazca plates. The ridge formed by the trace of the Galapagos hotspot lies just to the south of the end of the rupture and may have limited the extent of the earthquake. This is one of the principal hypotheses driving this proposal, which will also help determine the relationship between the 2016 earthquake and the Mw 7.8 earthquake that occurred nearby in 1942. Twenty broadband seismometers and 5 continuous GPS instruments will be deployed in the region around the April 16th earthquake, in addition to surveying existing GPS monuments. These seismic and GPS deployments are designed to complement the existing national Ecuadorian network, to map the aftershock sequence in great detail, and evaluate stress transfer between segments of the subduction zone. The area to the south of the Galapagos hotspot track has not ruptured in recent times, but geodetic data indicate that this section is locked. Because it is closer to a large population center, Guayaquil, this locked segment poses an increased seismic hazard.

On April 16th, 2016 a Mw7.8 earthquake struck along the Colombia-Ecuador trench near Pedernales, Ecuador. This earthquake represents the release of accumulated strain along the Nazca-South American plate margin since its last ruptures in 1942 and 1958. Initial data suggest that the Mw7.8 earthquake ruptured the same part of the plate interface as the Mw7.8 1942 earthquake. The Pedernales earthquake offers an opportunity to use state of the art technologies to measure the history of slip on the plate interface over the phases of the earthquake cycle: interseismic, coseismic, and postseismic and to study the role of asperities in focusing stress, segmenting ruptures, promoting and/or inhibiting rupture propagation, and the role of slow slip events in the earthquake cycle. This collaboration with the Escuela Politecnica Nacional (Instituto Geofisico, IG) and Instituto Geografico Military (IGM) shall investigate the co- and post-seismic deformation associated with this destructive event through: 1) deployment of a broadband seismic array to record aftershocks; 2) observation of existing eGPS sites; 3) the installation of five new cGPS stations in regions with low site density, but high potential for post-seismic signals. The Nazca plate subducts beneath the South American plate and/or the North Andes block at ~6 cm yr-1, along the Colombia-Ecuador trench, generating M>7.5 earthquakes. The 1906 Mw 8.8 earthquake ruptured a large portion of the plate boundary from central Ecuador to central Colombia. The April 16th, 2016 Mw7.8 earthquake falls within the rupture area of the 1906 event and appears to overlap with the 1942 event. The Pedernales earthquake, offers the opportunity to test the hypothesis that the asperities for large to great earthquakes are persistent over multiple cycles, and to investigate the role of asperities in arresting slip along strike and the relationship between locked and creeping parts of the interface. The combined GPS and seismic data will provide: 1) high precision aftershock locations and focal mechanisms and their relation to the interplate rupture zone and adjacent segments; 2) detailed structure of the Pedernales rupture region; 3) characterization of the postseismic deformation field, including afterslip and viscoelastic relaxation to quantify coseismic and postseismic stress transfer; 4) identification of low frequency and/or slow slip aftershocks and/or non-volcanic tremor, and the role of slow slip events in the earthquake cycle; 5) identification of time varying changes that might reflect rupture zone healing or other time-varying material properties in the rupture zone; 6) information to characterize site amplification effects and consequent variable hazard.