Revealing the Secrets of the Western Mediterranean: A Deep Earthquake and the Overturned Slab

The Rif–Betics–Alboran region has been vital in the tectonic evolution of the western Mediterranean. Seismic images support the idea of continuous slab rollback being a prominent force in this region. However, the detailed slab structure and the physical mechanisms generating local deep (>600 km) earthquakes remain unclear. Here, we analyze waveforms recorded from dense seismic networks above the deep earthquake beneath Granada in 2010 to study the slab structure. We discover a thin low-velocity layer (LVL) at the base of the slab to explain both the long codas observed in Morocco and the secondary arrivals observed in Spain. This LVL indicates the presence of hydrous magnesium silicates extending to ∼600 km depth, which suggests that dehydration embrittlement promotes the occurrence of deep-focus earthquakes. Our findings contradict the traditional slab model with the LVL sitting on the top of the slab, suggesting that the Alboran slab has been overturned.