For example, a magnitude 7.5 Benioff Zone quake beneath Oaxaca, Mexico occurred on Sept. 30, 1999. Benioff Zone earthquakes usually have dip-slip, or normal faulting , and no large aftershocks. These earthquakes are caused by mineral changes as the plate goes to deeper depths and is exposed to increased temperature and pressure. The mineral changes cause the plate to shrink and become more dense. Stresses build up which pull the plate apart.
Plate is known from several lines of evidence.
Earthquakes within the JDF plate are shown in different colors, according to their depths. Earthquake depths range from 30 km in the western part of the Olympic Peninsula to more than 70 km beneath the Cascades. The depth and dip are given below each point. Leave us your comments or questions about our web material.