Earthquakes result from the sudden movement and release of stored energy along a fault line within the Earth’s crust. This energy discharge sends powerful seismic waves outward, causing the ground to shake. When describing an earthquake, the two most frequently mentioned locations are the focus and the epicenter, which describe the event’s origin deep underground and its projection onto the surface.
The Subterranean Origin: The Focus (Hypocenter)
The Focus, also known as the Hypocenter, represents the three-dimensional point beneath the Earth’s surface where the initial rupture or movement occurs. This is the precise location where the rock first breaks and seismic energy is released from the strained fault. The focus is the true point of origin, defining its latitude, longitude, and depth within the crust or mantle. All seismic waves, including P-waves (primary) and S-waves (secondary), radiate outward from this point. Since the focus is defined by three coordinates, it is the fundamental measurement for seismologists studying the fault rupture.
The Surface Projection: The Epicenter
The Epicenter is the point on the Earth’s surface that lies directly above the focus. Unlike the focus, the epicenter is a two-dimensional location described only by its latitude and longitude. It serves as a geographical marker, making it easier to plot on a map and relate to human settlements. While the epicenter is a projection, it is generally the place where the shaking is felt most strongly. Maximum surface damage often occurs near the epicenter, though this intensity is modified by the distance to the focus below.
The Measure of Separation: Focal Depth
The most significant difference between the focus and the epicenter is the vertical distance separating them, which is known as the Focal Depth. Focal depth is the variable that determines how destructive an earthquake will be at the surface, even if two earthquakes have the same magnitude. This depth is the distance the seismic energy must travel before reaching the surface.
Earthquakes are classified into categories based on this depth measurement. Shallow-focus earthquakes occur at depths less than 70 kilometers and are the most common type, accounting for about 75% of all seismic activity. Because the seismic waves travel a shorter distance, they retain more energy when they reach the surface, resulting in more intense shaking and greater potential for damage near the epicenter.
Intermediate-depth earthquakes range from 70 to 300 kilometers deep, and deep-focus earthquakes occur at depths between 300 and 700 kilometers. As the focal depth increases, the energy dissipates over a greater distance, making the shaking at the epicenter less severe. Intermediate and deep quakes are primarily found in subduction zones where one tectonic plate is forced beneath another, but even these deep events can still be felt at the surface.
Why Location Matters: Reporting and Intensity
The practical application of these two locations differs between public reporting and scientific analysis. The public and media primarily rely on the epicenter because it is an easily mappable surface location that directly relates to populated areas and damage reports. It is the most straightforward way to communicate the approximate location of the shaking.
Seismologists rely on the Focus (hypocenter) location and focal depth to understand the underlying tectonic activity and accurately calculate the earthquake’s magnitude. To pinpoint both the focus and the epicenter, scientists use a method called triangulation, which requires data from at least three seismograph stations. This technique measures the difference in arrival times between the P-waves and the S-waves at each station to calculate the distance to the event. By intersecting the distance circles from multiple stations, seismologists precisely locate the epicenter and then calculate the depth to the focus.