Subduction earthquakes release vast amounts of energy to crust and mantle lithosphere. The products of such drastic events are rarely observed in the field because they are mostly lost by subduction. We present new observations of deformation products formed by a few very large and numerous small intermediate-depth Alpine subduction earthquakes that are preserved along the exhumed gabbro–mantle peridotite contact of the Piemont-Liguria oceanic basin in Corsica. The abrupt release of energy resulted in shear heating events that completely melted both gabbro and peridotite. The large volumes of melt that were generated can be studied in the fault and injection vein breccia complex along the fault zone. The energy required for wholesale melting of a large volume of peridotite along the fault combined with previous estimates of stress drops show that very large earthquakes took place along the Moho of the subducting plate. Because these fault rocks formed by intraplate seismicity, we suggest, by analogy with present-day subduction, that they represent a proxy for the lower seismogenic zone.