The 2007 M7.7 Tocopilla northern Chile earthquake sequence - along and across strike rupture segmentation

In November 2007 a M7.7 earthquake occurred near the coastal town of Tocopilla in the southern part of a presumed seismic gap extending some 500 km along the northern Chile subduction zone. This major segment last broke in a magnitude ≧8.5 earthquake in 1877. Assuming a complete lock of the interface, it has accumulated more than 8 m of slip deficit. The contiguous segments to the north and south broke in M≧8 earthquakes in 2001 and 1995. Teams from Chile (Universidad Católica del Norte and Universidad de Chile), France (IPGP) and Germany (GFZ) started in 2006 to install semi-permanent multi-parameter observatories within the Integrated Plate Boundary Observatory Chile (IPOC) Initiative to monitor deformation at a variety of spatial and temporal scales in the final stage of the seismic cycle. At the time of the Tocopilla earthquake, 12 sites were equipped with seismic broadband and strong-motion sensors recording both the mainshock and its aftershock series. The earthquake rupture extended for about 160 km from the centre of the Mejillones peninsula (MP) to about 20 km north of the town of Tocopilla. Slip was confined to the depth range 30-55 km and concentrated in two patches in the north and south with a maximum of about 2.6 m. Hence the earthquake released only a fraction of the slip deficit and broke only the down-dip part of the plate interface, with the up-dip limit of the rupture approximately following the coastline. This poses the important question why rupture did not extend offshore, where the interface is presumably locked based on models of long-term interseismic deformation. We relocated more than 1000 aftershocks occurring in the week following the mainshock using hand-picked arrival times, cross-correlation based differential travel times and the double-difference algorithm. Despite the sparseness of the network, the aftershocks sharply define the plate interface. Seismicity in the first 24h is congruent to the slip distribution with the area of highest slip being relatively aseismic. In the following days aftershocks spread off-shore west and north-west of MP. Juxtaposing the Tocopilla with the 1995 M8 Antofagasta aftershocks to the south produces a striking symmetry across an E-W axis in the center of MP. For both sequences seismicity is concentrated along the coastline even following the subtle feature around MP. The Antofagasta earthquake broke the off-shore part of the seismogenic zone, hence the aftershocks are demarcating the up-dip and own-dip limits of the ruptures respectively. The northern Chilean coastline seems to separate the seismogenic zone in an on- and off-shore segment and serve as a stress concentrator at depth. We identify a similar behavior for other earthquakes in Chile and Peru, where the off-shore segments break in great M≧8 earthquakes, and the on-shore segments in large, but not quite great M≈7 earthquakes. This finding seems to reflect an abrupt change of frictional behavior from an unstable/locked part to a partly stable/partly locked part across strike of the seismogenic zone.