There is a lot of terminology that can confuse geographic enthusiast. Focus, Hypocenter, Epicenter? What does that exactly mean? When we spoke about seismic waves we mentioned that they originate from different areas. The point of which the wave originates is what we call the Earthquake focus (foci plural). It is also known as the Hypocenter. These foci of natural earthquakes are found at different depths below the ground surface. There are three levels, Shallow at 10-100 km below, Intermediate 70-300 km and Deep foci at 300 km or below.
Shallow earthquakes create the most damage and 3/4 of total energy released of earthquakes in the world. They unlike intermediate and deep focus earthquakes, shallow earthquakes occur at all type of plate boundaries like ocean trenches, subduction zones, etc.
So what is the epicenter of an earthquake, we always hear that. “The epicenter of the quake.” The point on the ground surface directly above the focus is known as the epicenter [Bolt].
There is also another amazing geographical phenomenon that occurs. It is a major tectonic feature related to foci and it is called the Wadati Benioff (dipping seismic) zone. This is a narrow zone, that dips under the Earth’s crust to depths of up to 700 km. It is a deep foci that generally occurs in Island arcs and deep ocean trenches.
How do foci and epicenters relate to earthquakes? Why do we study them? What are some of the biggest quakes and are they from shallow foci or deep?
This is a photo of the destruction created after a great earthquake occured in Loma Prieta, CA in October 1989.
“The epicenter was located at 37.04° N. latitude, 121.88° W. longitude near Loma Prieta peak in the Santa Cruz Mountains, approximately 14 km (9 mi) northeast of Santa Cruz and 96 km (60 mi) south-southeast of San Francisco.”