Abrupt change in the dip of the subducting plate beneath north Chile

No large tsunamigenic earthquake has occurred in north Chile since 1877 and the region has been largely recognized as a mature seismic gap(1-9). At the southern end of the seismic gap, the 2007 M-w 7.7 Tocopilla earthquake ruptured the deeper seismogenic interface, whereas the coupled upper interface remained unbroken(4,6,7). Seismological studies onshore show a gently varying dip of 20 degrees to 30 degrees of the downgoing Nazca plate(3,6), which extends from the trench down to depths of 40-50 km. Here, we study the lithospheric structure of the subduction zone of north Chile at about 22 degrees S, using wide-angle seismic refraction and reflection data from land and sea, complemented by hypocentre data recorded during the 2007 Tocopilla aftershocks(7). Our data document an abrupt increase in the dip of the subducting plate, from less than 10 degrees to about 22 degrees, at a depth of approximately 20 km. The distribution of the 2007 aftershocks indicates that the change in dip acted as a barrier for the propagation of the 2007 earthquake towards the trench, which, in turn, indicates that the subduction megathrust is not only segmented along the trench, but also in the direction of the dip. We propose that large-magnitude tsunamigenic earthquakes must cross the barrier and rupture the entire seismogenic zone.