Imaging the 2007 Mw 7.7 Tocopilla earthquake from short-period back-projection

Abstract

The 14 November 2007 Tocopilla (MW 7.7) earthquake was the first large rupture (M > 7.5) in the 1877 Northern Chile seismic gap. The event only ruptured the lower seismogenic part of the megathrust, raising questions about the mechanism that prevented the trenchward rupture propagation. Here, we present the short-period rupture process of the Tocopilla earthquake from local strong-motion (2.0–8.0 Hz) and teleseismic (0.5–2.0 Hz) back-projections. P and pP teleseismic back-projections were combined from two arrays formed on broadband networks in North America and Africa. The kinematics of the earthquake rupture was also characterized by back-propagating S-wave envelopes from local accelerometers (≤ 200 km from the mainshock). The results show a complex rupture process, including a sub-event, from the distribution of short-period rupture emissions and the main slip asperities. The sub-event location agrees with published estimates based on S-wave arrivals and kinematic inversion. We also observed rupture emissions around the slip patches and systemically classified them relative to the asperities, i.e., down-dip, up-dip, and inside. The short-period emissions are balanced around the asperities, highlighting up-dip rupture emissions. We interpreted the up-dip emissions as a proxy of the high-stress gradient caused by a kink in the slab interface, proposed in previous literature to be the mechanism that arrested the trenchward propagation of the 2007 rupture. The down-dip limit of the Tocopilla event coincided approximately where the slab interface intersects the continental Moho. Thus, it is a region expected to be extensively serpentinized, i.e., the most seaward part of the mantle wedge, defining a velocity-strengthening margin and a source of short-period radiation. The high-stress gradient around the asperities is proposed to define a third source of short-period radiation that may contribute to rupture encircling emissions.