Continental collisions and the origin of subcrustal continental earthquakes1

The existence of subcrustal continental earthquakes beneath the Alpine–Himalayan Belt was recognised more than 60 years ago. There is general agreement that most of those beneath the western part of the belt in the Mediterranean result from the subduction of oceanic lithosphere. There is less agreement about the origin of those beneath Vrancea in Romania, the Hindu Kush, and the Pamir. Because there is little evidence for the former existence of oceanic lithosphere beneath these regions, many authors have argued that these seismic zones result from the separation of the mantle part of the continental lithosphere from the crust before it sinks into the mantle. However, this model has become steadily less satisfactory. Detailed studies of the depth of earthquakes beneath all stable regions of continents have shown that substantial subcrustal earthquakes, with magnitudes greater than 5.5, are rare. We show that this distribution is controlled by temperature, with material hotter than ∼600 °C being aseismic. This simple rule accounts for the distribution of almost all earthquakes in oceanic and continental lithosphere, including those in subduction zones. We argue that the subcrustal continental earthquakes must also result from the subduction of oceanic lithosphere. This proposal is not new but has generally been dismissed because of the lack of surface geological evidence that suitable pieces of oceanic lithosphere existed. However, the depth distribution of continental earthquakes makes it steadily harder to avoid.