2016 M=7.8 Muisne, Ecuador, Earthquake Overlapped or Re-Ruptured the 1942 M=7.8 Patch, and May Have Left a Large Gap to the North That Has Not Slipped since 1906

The April 16, 2016 Mw=7.8 Ecuador earthquake killed more than 650 people and left several thousand homeless. The 2016 event likely ruptured much of the same area as a 1942 Mw 7.8 earthquake that caused strong shaking along the Ecuadorian coast. Both earthquakes occurred within the southern end of the rupture zone of the great M 8.6 Ecuador-Columbia megathrust event, which triggered a tsunami that was recorded throughout the Pacific. Chlieh et al. (2014) had pointed to a possible seismic gap between the 1942 earthquake and the 1958 Mw 7.7 shock that also re-ruptured part of the 1906 rupture zone. It is difficult to be certain that such a seismic gap exists; it could simply be an area of higher slip in 1906, or an artifact of location uncertainties in the earthquakes since 1906. If, indeed, there is a site of high accumulated stress, then the 2016 quake likely filled only part of it. The aftershocks of the 2016 earthquake were located in a halo around the site of highest mainshock slip as calculated by Gavin Hayes of the USGS. We calculate that the majority of aftershocks struck in areas where Coulomb or shear stress increased following the mainshock by more than 2 bar. The mean stress increase in the possible seismic gap to the north of the 2016 rupture is variable and doesn't indicate that a significant earthquake is more likely there than before. The right-lateral Jama-Quininde Fault, associated with the southernmost part of the 1906 rupture, extends from the Pacific Ocean into mainland Ecuador. The Coulomb stress imparted by the mainshock to this fault is variable along strike, with no large patches of increased failure stress. Further, thus far there have been no right-lateral aftershocks on or near the Jama-Quininde Fault since the mainshock.