Deep focus earthquake

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A deep focus earthquake is an earthquake that occurs at depths of 600 to 700 km beneath the Earth's surface. Their cause is still not entirely understood since subducted lithosphere at that pressure and temperature regime should not exhibit brittle behavior. They are believed to be associated with mineralogical phase changes within the slab, such as olivine transforming into a more compact spinel structure. Deep focus earthquakes usually occur within subduction zones at oceanic-continental convergent boundaries.