Abstract

In subducting slabs, a high seismicity rate in a concentrated volume (an earthquake nest) is often associated with geometric complexities such as slab detachment, tearing, or contortions. In Colombia, the Cauca cluster has a high rate of intermediate‐depth earthquakes between 3.5°–5.5° N and 77.0°–75.3° W. From January 2010 to March 2014, the Colombian National Seismic Network reports 433 earthquakes in the cluster at depths of 50–200 km with local magnitudes ranging from ML 2.0 to 4.7. We determine precise relative locations of the intermediate‐depth earthquakes in the cluster and investigate the cause of the cluster by estimating its geometry from earthquake relocations. Earthquake relocations show a continuous 20‐km‐thick intraslab seismic zone dipping at 33°–43°, with the dip angle increasing to the south. In addition, earthquakes locate in two isolated fingers that extend 30–40 km normal to and above the slab. The depth and vertical separation of the earthquakes in the two fingers indicate that the two fingers do not belong to the intraslab seismic zone or the overriding crustal seismic zone and instead are in the mantle wedge. Changes in the velocity model, starting earthquake locations, or the precision of the arrival picks do not lead to significant changes in the relative locations. The Cauca cluster, with earthquakes located in and above a continuous slab, appears to have a different mechanism than previously studied earthquake nests. The high seismicity rate in the cluster may, instead, correspond to high volumes of dehydrated fluid.

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