Earthquakes are a sudden release of energy deep within the Earth’s crust, sending seismic waves radiating outward. Understanding the precise location where this event begins is a primary task for seismologists, providing fundamental data for hazard assessment and emergency planning. Scientists use specific terminology to distinguish between the underground starting point and its corresponding surface location. Pinpointing these locations helps researchers map fault structures and predict the potential impact of future seismic activity.
The Epicenter: Definition and Location
The surface point directly above an earthquake is called the epicenter. This term refers to the location on the Earth’s surface that lies vertically above the point where the earthquake rupture began beneath the ground.
Seismologists determine the epicenter’s precise location using triangulation, which requires data from at least three seismic stations. By measuring the time difference between the arrival of the faster P-waves and the slower S-waves, scientists calculate the distance to the earthquake source. The point where the distance circles from all three stations intersect marks the epicenter. While the epicenter often experiences the most intense ground shaking, this is not always true for very large earthquakes where the fault rupture extends over a great distance.
The Hypocenter: Where the Rupture Begins
The actual starting point of the earthquake within the Earth is called the hypocenter, or focus. This is the exact subsurface location where stored strain energy is first released, initiating the seismic waves. The hypocenter is defined by its three-dimensional coordinates: latitude, longitude, and depth.
The hypocenter directly contrasts with the epicenter, which is the two-dimensional projection of the hypocenter onto the surface. The depth of this point is a significant factor in classification, with events categorized as shallow (less than 70 kilometers deep), intermediate, or deep.
Why the Depth and Distance Matter
The vertical distance between the hypocenter and the epicenter is known as the focal depth, which profoundly influences the intensity of ground shaking felt at the surface. Seismic waves attenuate as they travel through the Earth’s layers. Therefore, a hypocenter closer to the surface results in more concentrated and severe shaking upon reaching structures.
Shallow-focus earthquakes (less than 70 kilometers deep) pose a greater hazard than deep-focus earthquakes of the same magnitude. For instance, a magnitude 6.7 event with a shallow hypocenter causes far more intense surface shaking and wider damage than a similar event occurring deeper. Determining focal depth is a primary factor in predicting localized damage severity and understanding the region’s tectonic setting.