Hypocenter

The hypocenter of an earthquake is the place where the rupture of the tectonic fault begins, and it is generally found at a certain depth below the surface of the ground, in the inner part of the earth. The moment an earthquake occurs, the hypocenter radiates a series of seismic waves that are directed in all directions. The expanding seismic waves travel at the speed of several kilometers per second and allow scientists to determine the location of the hypocenter.

The speed variations that are generated due to the materials with which the earthquake waves meet , before reaching each monitoring station , are taken into account to give an initial estimate of the location of the hypocenter.

It is also known as the seismic focus  and is located in the inner part of the earth’s crust in the same place where the earthquakes originate and is located below the epicenter .

The hypocenter of an earthquake is considered as the position where the tension energy that is stored in the rock is released for the first time, marking the point where the fault begins to break. This occurs directly below the epicenter, at a distance known as the hypocentral or focal depth .

Etymology

The word etymology is derived from the words of Greek origin ” ὑπο ” which has the meaning below and the word κέντρον pronounced as kéntron , and which has the meaning center . As the Greek prefixes also indicate, it is a point located inside the lithosphere . So, in other words, it is the exact or precise point (kéntron) that is located below (hypo) the epicenter of the earthquake.

The term was coined in 1859 by the Irish-born geophysicist Robert Mallet , and he used it to determine the point deep in the earth where the force responsible for producing an earthquake originates.

How the hypocenter differs from the epicenter

The hypocenter of an earthquake is the actual location by estimating the rupture , and the depth of the rupture along the fault or subduction zone. The epicenter of an earthquake is the point on the surface directly above the hypocenter, this allows us the correct location on a surface map and serves to show the affected areas. In other words, the hypocenter is the point within the surface where the earthquake happens, it is measured in kilometersdeep and the epicenter is the place where this hypocenter is located. On some occasions, the epicenter of the earthquake is the place where the most damage occurs, although this is not a general rule, since depending on the intensity of the movement, the damage can spread in places other than the epicenter.

Examples of hypocenter

• Earthquake in Japan : This earthquake occurred in March 2011 and had a hypocenter of 24 kilometers deep. It was caused by a reverse fault of the Pacific and North American interplates . It had a magnitude of 9 degrees that gave rise to a tidal wave with waves up to 10 meters high. It lasted approximately 6 minutes and was preceded by another earthquake two days earlier with an intensity of 7.2 and a hypocenter of 14.1 kilometers. NASA experts consider that the earthquake managed to move the Japanese island 2.4 meters and altered the axis of theearth in 10 centimeters.
• Coquimbo earthquake : it happened on Wednesday, September 16, 2015 and reached a magnitude of 8.4 degrees on the Richter scale. Its hypocenter was 37 kilometers deep and was located southwest of Canela Baja in Coquimbo. It could be felt even in Argentina , Uruguay and Brazil. It originated from the Nazca plate when it entered under the South American plate . This earthquake had aftershocks for 30 days with magnitudes greater than 3 degrees.
• Mexico earthquake : in 1985, on Thursday, September 19, 1985, a huge earthquake with a magnitude of 8.1 degrees occurred. Its hypocenter was only 15 kilometers deep in the earth’s crust. One of the most affected cities was Mexico City and is considered the most damaging in history. The official death toll was never known and it affected the country economically, 250,000 people lost their homes and more than 900,000 were forced to leave their homes due to the instability of the structures.