An earthquake is a sudden, powerful shaking of the Earth’s surface caused by a rapid release of energy in the lithosphere. This occurs when blocks of the Earth’s crust slip past one another along a geological fracture known as a fault plane. Seismologists identify the precise point where the rupture begins, and this specific underground location, the origin point of the seismic waves, is called the hypocenter.
The Hypocenter: Where the Earthquake Starts
The hypocenter, also commonly referred to as the focus, is the three-dimensional point deep within the Earth where the initial seismic rupture first begins. It marks the exact location on the fault plane where the stored strain energy in the rock is first released in the form of seismic waves. This point is defined by its latitude, longitude, and a measurable depth.
The hypocenter is where the fracture initiates before spreading along the fault surface, sending out seismic waves in all directions. The depth of the hypocenter can vary significantly, ranging from just a few kilometers below the surface to hundreds of kilometers deep, depending on the tectonic setting.
Hypocenter Versus Epicenter
The hypocenter is the actual point of rupture within the Earth’s crust, making it a three-dimensional location with depth. In contrast, the epicenter is the point on the Earth’s surface situated directly above the hypocenter.
The epicenter serves as the two-dimensional map location typically reported to the public using only latitude and longitude coordinates. It is essentially a surface projection of the deeper hypocenter. While the epicenter is often the area where shaking is most intensely felt, the true origin of the destructive energy is always the hypocenter below.
Depth and the Impact of an Earthquake
The depth of the hypocenter significantly influences the severity and felt intensity of ground shaking at the surface. Seismic waves naturally attenuate, or weaken, as they travel through the Earth’s layers. Therefore, the farther the waves must travel from the hypocenter to the surface, the less intense the shaking will be.
Earthquakes are generally classified by their focal depth. Shallow-focus earthquakes (0 to 70 kilometers deep) are responsible for the most widespread and destructive surface damage because their energy has little distance to dissipate. Deep-focus earthquakes (300 to 700 kilometers) are far less damaging at the surface, as the extreme distance allows seismic waves to spread out and lose substantial energy.
Identifying the Hypocenter
Seismologists determine the precise location of the hypocenter by analyzing the arrival times of seismic waves recorded at multiple monitoring stations. The Earth generates two main types of body waves from the hypocenter: Primary waves (P-waves) and Secondary waves (S-waves). P-waves are compressional and travel faster through the Earth’s interior, arriving at seismic stations first.
S-waves are shear waves that travel slower, arriving after the P-waves. The time difference between the arrival of the P-wave and the S-wave is directly proportional to the distance between the station and the hypocenter. Scientists use this time difference to calculate a distance, which represents the radius of a circle centered on the station.
By using data from at least three different seismic stations, a process known as trilateration, seismologists can pinpoint the exact intersection point. This establishes the precise three-dimensional location of the hypocenter, including its depth.