Deep earthquakes may be caused by mineral changes

Ars Technica : The earthquakes that most people are familiar with generally occur within 15 km of Earth’s surface. But 90 years ago, evidence was found suggesting some earthquakes occur at depths of several hundreds of kilometers. How they occur is unclear. At those depths, the pressures and temperatures cause rock to flow, so it doesn’t experience the sorts of shocks that cause normal earthquakes. Thorne Lay of the University of California at Santa Cruz examined a recent quake more than 600 km below the Okhotsk plate in the Pacific Ocean. He found that the quake had enough energy to cause 180-km-long fractures in the rock near the epicenter. Under the pressures at that depth, the fractures spread at nearly 14 000 kph. Alexandre Schubnel of Ecole Normale Supérieure in France suggests that the fractures are similar to those that accompany the transformation of the mineral olivine into the mineral spinel , which occurs under extreme pressures and temperatures. Schubnel has mimicked the occurrence of such fractures on a laboratory scale and believes that a similar transformation could be happening to minerals in Earth’s crust to cause the deep earthquakes.