Shallow earthquakes are considered to occur when prolonged tectonic stress exceeds the local yield strength of rocks, so that brittle failure occurs suddenly, with associated earthquake and stress drop. Intermediate and deep earthquakes appear to have variable focal-plane mechanisms, although they are predominantly associated with slip along fracture planes. Other mechanisms include sudden phase transitions, resulting in an implosion as the mineral density increases and its volume decreases. See fault-plane solution.
Subjects: Earth Sciences and Geography.