The Seismic Sequence of the 2016 Mw 7.8 Pedernales, Ecuador Sarthquake

On the 16th April 2016, a Mw 7.8 mega-thrust earthquake occurred in Northern Ecuador, close to the city of Pedernales. The event ruptured an area of 120 x 60 km and was preceded by a Mw 5.0 foreshock, located only 15 km south of the epicentre, and registered 10 minutes before the main event.A few weeks after the main event a large array of instruments was deployed by a collaborative project between the Geophysical Institute of Ecuador (IGEPN), IRIS (USA), Géoazur (France) and the University of Liverpool (UK). This dense seismic network, with more than 70 stations, includes broadband, short period and strong motion instruments and is currently recording the aftershock activity of the earthquake. It is hoped that this data set will give further insights into the structure of the subduction zone mega thrust beneath Ecuador.Using data recorded both on the permanent and the recently deployed network we located and calculated the moment tensor solutions for the foreshock event, and the large aftershocks (M > 5). We analyse the spatial distribution of the seismicity and its relation with the co-seismic slip, estimated by inverting radar satellite interferometry data, and with previous models of inter-seismic coupling (e.g. Chlieh et al., 2014). It is possible to identify two lineations in the aftershock activity located to the north and south of the rupture. Moreover, the geodetic slip model shows that the boundaries of the maximum coseismic slip coincides with the observed lineaments in the aftershocks and with the rupture area of a previous Mw 7.8 event in 1942. This suggests that the features to the north and south may impose a barrier to rupture propagation, creating different segments in the subduction zone beneath Ecuador. In addition, we model the Coulomb stress change caused by the foreshock and mainshock in order to investigate whether this could explain the aftershock distribution and potential earthquake interactions. Previous activity has presented a northward-propagating series of ruptures greater than Mw 7 spaced approximately 20 years apart. An open question is therefore whether the present event is the start of a further series of large magnitude events in northern Ecuador, and/or whether slow slip events/creep south of the rupture have partly accommodated the strain due to subduction.