Crustal Structure of the Ecuadorian Forearc from the Joint Inversion of Receiver Functions and Ambient Noise Dispersion Data

Following the M_w 7.8 megathrust Pedernales earthquake in Ecuador on April 16, 2016, an international collaboration arranged by the IG-EPN mobilized a rapid deployment of 60 broadband and short-period seismometers in order to capture the aftershock sequence. In addition to understanding the aftershock sequence, this data provides an opportunity to examine how the crustal structure in the forearc region of Ecuador may relate to seismic hazards in the region. We combine this new dataset with a sub-set of data from the permanent stations of the Ecuadorian national network (RENSIG). Here, we present receiver function (RF) common conversion point stacks and a shear-wave velocity model created from the joint inversion of RFs and Rayleigh-wave dispersion data obtained through ambient noise cross-correlations. The geology of the Ecuadorian forearc comprises Miocene basins thought to be underlying by accreted oceanic terrains in the Late Paleocene. These structures lead to complicated RF arrivals that obscures clear arrivals associated with the Moho. Our results show a strong shallow conversion in the Ecuadorian forearc that indicates an increase in velocity at a depth of 15-18 km depth which coincides with significant north-south variations in shear-wave seismic velocity. This north-south change from slow to fast velocities may mark the boundary between accreted oceanic terrains. This change also correlates geographically with the southern rupture limit of multiple high-magnitude earthquakes, including the 1942 and 2016 earthquakes and possibly the 1906 M_w 8.8 earthquake. The earthquake history along the Ecuadorian trench shows that no M_w >7.5 event has ruptured south of 0.5° S in southern Ecuador and northern Peru, which suggests that variations in the forearc crustal structure may, in part, control the occurrence and spatial distribution of high magnitude seismicity in the region.