The focus of an earthquake is the location where this major earth-moving event occurs within the outer layer, called the lithosphere, of our planet Earth. The damage it inflicts, from our human perception living on the surface as we do, is partly dependent on the depth below the Earth’s surface of its location, called the hypocenter. It is also determined by the magnitude (strength) of the earthquake and the composition of the intervening crust material, the rock types and forms between the hypocenter and the surface. The shock wave tremors resulting from the massive earth disturbance occurring at the focus dissipate with distance. Therefore, the deeper the hypocenter, the less damaging the effects on the surface will be, in comparison to earthquakes of a similar magnitude occurring closer to the land surface in the same type of rock strata.
Our news programs, when describing an earthquake, typically tell us two things: the epicenter and the magnitude. The epicenter is the location on the Earth’s surface directly above where the earthquake occurred, no matter how deep that hypocenter was. The magnitude is typically given using the Richter scale, which is a logarithmic, mathematical scale where each higher digit represents a 32-fold increase in the energy released at the focus or focal point. That is, an earthquake registering five on the Richter scale is 32 times more powerful at its hypocenter than one registering four.
This information can be confusing because it is natural for us to presume that earthquakes of similar magnitude should be equally damaging, and yet obviously are not, as shown by the various video reports on our news programs. This variation is due to the depth of the hypocenter, the strength of the shock and the geologic composition of the interposing rock strata.
Scientists divide earthquakes into three categories depending on the depth of the hypocenter. Shallow-focus earthquakes have a hypocenter within 60 kilometers of the Earth’s surface. The focal point of an intermediate-focus earthquake is between 60 and 150 kilometers below the Earth’s surface and deep-focus earthquakes are centered at least 150 kilometers down into the Earth’s lithosphere. The lithosphere is up to 200 kilometers thick beneath the continents, but may be as little as 55 kilometers thick beneath the seas in some parts of the globe, so deep-focus earthquake’s epicenters always occur on land or close to shore.
The tremors from even a comparatively weak shallow-focus earthquake can cause considerable damage because of the short distance they have to travel to the surface. The energy waves generated have less crustal mass above them to vibrate, so are still strong. The deeper the focal point (hypocenter), the more mass needs to be vibrated by the initially generated energy of the earthquake, reducing the effect on the Earth’s surface. The mass of the interceding material is also significant. If the majority of crust material between the hypocenter and the Earth’s surface is granite, for example, the impact will be considerably milder than if it is a lighter material, such as limestone.