Seismogenic Necking During Slab Detachment: Evidence From Relocation of Intermediate-Depth Seismicity in the Alboran Slab

With few exceptions, intermediate-depth seismicity is associated with active subduction. In the Alboran slab, which is located just east of the Gibraltar Strait in the westernmost Mediterranean, although evidence suggests subduction is no longer active, tens to hundreds of M < 5.0 earthquakes are observed every year at depths of ~80 km. In this paper, we relocated 58 such events recorded by the PICASSO temporary network using a 3-D velocity model and implementing a grid-search approach to minimize the normalized misfit between observed and predicted P and S wave arrival times. Meanwhile, we relocated 908 events recorded by the Spanish national network using the double-difference method and gave a high-precision picture of seismicity distribution within the slab. Relocation results reveal five clusters in this region separated by small gaps. Jointly considering relocation results and a 3-D tomography model indicates one cluster is at a shallower depth above the northern, E-W oriented arm of the Alboran slab, while the other four are aligned from north to south, parallel to the strike of the slab and near its core. Larger events are generally shallower and more scattered, while the deeper, more clustered events tend to have smaller magnitudes. Clusters further to the north show concentrated seismicity on narrow zones that dip ~45° to the south. We suggest this seismicity is associated with active necking and break-off of the Alboran slab, which is progressing from north to south. Shear instability is likely to be the failure-enabling mechanism for the occurrence of these earthquakes.