6.2: Earthquake Focus and Epicenter

Earthquake Focus and Epicenter

Earthquakes originate at a point called the focus Links to an external site. (plural foci) or hypocenter. From this point, energy travels outward in different types of waves. The place on the Earth’s surface directly above the focus is called the epicenter Links to an external site. (Figure 6.4). Earthquake foci may be shallow (less than 45 miles from Earth’s surface) to deep (greater than 185 miles deep), though shallow to intermediate depths are much more common. Earthquake frequency and depth are related to plate boundaries. The vast majority (95%) of earthquakes occur along a plate boundary, with shallow focus earthquakes tending to occur at divergent and transform plate boundaries, and shallow, intermediate, and deep focus earthquakes occurring at convergent boundaries (along the subducting plate). The earthquakes associated with convergent boundaries occur along Wadati-Benioff zones Links to an external site., or simply Benioff zones, areas of dipping seismicity along the subducting plate (Figure 6.5).