Earthquakes are natural events involving the sudden shaking of the ground. They arise from the release of accumulated energy within the Earth’s crust. Understanding the precise origin point of an earthquake beneath the surface is a fundamental aspect of seismology. This specific point of origin is known as the focus.
Understanding the Earthquake Focus
The focus of an earthquake, also referred to as the hypocenter, is the exact location within the Earth where rock first ruptures and stored strain energy is released. This release generates seismic waves that travel outward from this subsurface point. The focus can be situated at varying depths beneath the Earth’s surface.
The focus is distinct from the epicenter, which is the point on the Earth’s surface directly above the focus. While the focus represents the true origin of the earthquake deep underground, the epicenter is its corresponding geographical position on a map. Scientists often use the epicenter to describe the surface location most directly affected by the earthquake. All seismic waves, including those that cause surface shaking, originate from the focus.
The Significance of Focus Depth
The depth of an earthquake’s focus significantly influences the shaking and potential damage experienced at the Earth’s surface. Earthquakes are categorized into three types based on their focal depth: shallow, intermediate, and deep.
Shallow-focus Earthquakes
These occur at depths between 0 and 70 kilometers. They are the most common and often the most impactful, as seismic waves have less distance to travel before reaching the surface, resulting in less energy dissipation.
Intermediate-focus Earthquakes
These are situated at depths ranging from 70 to 300 kilometers. They can still cause considerable damage.
Deep-focus Earthquakes
These originate at depths greater than 300 kilometers, extending up to about 700 kilometers below the surface. Although these deep events can be felt at the surface, their seismic waves travel a greater distance, leading to a significant loss of energy and generally less intense shaking upon arrival.
How Focus Location is Determined
Seismologists determine an earthquake’s focus and epicenter by analyzing the arrival times of seismic waves recorded at multiple seismograph stations. Earthquakes generate P-waves (primary waves) and S-waves (secondary waves), which travel at different speeds. P-waves are faster and arrive first at a seismograph station, followed by the slower S-waves. The time difference between the arrival of these two wave types at a single station indicates the distance from that station to the earthquake’s origin.
To pinpoint the exact location of the focus, data from at least three seismograph stations are required. For each station, scientists calculate the distance to the earthquake based on the P-wave and S-wave arrival time difference. A circle is then drawn around each station on a map, with its radius representing this calculated distance. The point where these three circles intersect indicates the earthquake’s epicenter on the surface. From this surface location, the depth of the focus is determined by analyzing seismic wave behavior and travel times through the Earth’s interior.