The terms shallow focus and deep focus (and a third term: intermediate focus) earthquakes relates to the depth below the earth’s surface at which the earthquake occurs.
An earthquake originates at a point known as the focus or hypocentre, and most occur less than 70 km below the surface. (The epicentre is the point on the surface directly above the focus.) Earthquakes occurring near the surface are known as shallow focus earthquakes. Those with a focus 70 300 km deep are called intermediate focus earthquakes, and those occurring at depths greater than 300 km are deep focus earthquakes. (Note that some geologists use the figures 60 and 150 km to classify shallow and deep focus earthquakes.)
The earth’s surface appears to us to be a continuous surface, but it is actually composed of sections called tectonic plates, which are moving slowly in relation to each other. Some are moving apart, some closer, and some are moving under another plate (a process called subduction). Movements of one plate against another can produce enormous stresses within the earth’s crust at the boundaries between the plates, and these stresses cause deformation of the crust. When the deformations and stress pass a critical point part of the crust can give way suddenly, rather like a rubber band snapping, releasing the energy suddenly in the form of seismic vibrations and violent shaking on the surface.
Shallow focus earthquakes are much more common than deep focus earthquakes, and unfortunately they cause most damage on the surface because they are closer to the surface and therefore produce stronger shaking on the surface. For example, most earthquakes around the San Andreas Fault in California usually have a focus less than 20 km deep. Shallow focus earthquakes occur around a fault line such as the San Andreas Fault, and they are generally associated with mountain ranges or with mid-ocean ridges or trenches.
There are many examples of shallow-focus earthquakes, but among those best known were the 1906 San Francisco earthquake, the 1994 earthquake in Los Angeles, and the most recent earthquake in Italy.
Deep focus earthquakes have a focus greater than 300 km deep. (Below this depth any disruptions tend to be by flowing rather than breaking and faulting because of the extreme heat and pressure at such depths.) Deep focus (and also intermediate focus) earthquakes rarely cause damage on the surface. They are generally associated with deep water trenches in the ocean and with coastal mountain ranges such as those of South America and Japan.
An example of a deep focus earthquake was the 6.8 magnitude quake that occurred 349 km under the Sea of Japan on July 2007, 140 km off the coast of Honshu. This quake was probably caused by a release of stress as the Pacific Plate subducts beneath the Okhotsk Plate. There were no casualties.
Earthquakes are among the most frightening of natural disasters, and of these the shallow-focus earthquakes are the most deadly of all.