The 1984 July 19 North Wales earthquake—a lower crustal continental event indicating brittle behaviour at an unusual depth

Summary. Attention has recently been focused on the structure and composition of the lower crust in continental areas. It is generally believed that, except in special circumstances, ductile behaviour below mid‐crustal depths precludes the brittle processes that cause earthquakes. The 1984 July 19 earthquake in North Wales occurred at the unexpected depth of 23 km. We report here the location of the larger aftershocks and the relocation of the main shock with respect to one of them. The lower crustal depths of the events are confirmed by tests with a wide range of models. The occurrence of earthquakes at these depths may be related to low heat flow in the region.