Distance to the earthquake epicenter is the horizontal surface distance, typically measured on the Earth's surface, from a specific observation point, such as a seismograph station, a population center, or any given location to the epicenter of an earthquake. The epicenter itself is defined as the point on the Earth's surface that lies directly above the hypocenter (also called the focus), which is the underground location where the earthquake rupture begins along a fault. This distinction is standard in seismology: the hypocenter is the three-dimensional origin point within the Earth, while the epicenter is its vertical projection onto the surface.
Distance to Earthquake Epicenter Formula |
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| \( D \;=\; \dfrac{ \Delta t }{ \dfrac{ 1 }{ V_s } - \dfrac{ 1 }{ V_p } } \) (Distance to Earthquake Epicenter) | ||
| Symbol | English | Metric |
| \( D \) = Distance to Epicenter | \(ft\) | \(m\) |
| \( \Delta t \) = Arival Time Difference Between S-waves and P-waves | \(s\) | \(s\) |
| \( V_s \) = S-wave Velocity | \(ft \;/\; sec\) | \(m \;/\; s\) |
| \( V_p \) = P-wave Velocity | \(ft \;/\; sec\) | \(m \;/\; s\) |
Seismologists determine the distance to the epicenter (often called the epicentral distance) primarily by analyzing the arrival times of different seismic waves recorded at a station. Primary (P) waves travel faster than secondary (S) waves; the time lag between their arrivals allows calculation of the distance traveled, using established travel time curves or tables calibrated from known seismic velocities in the Earth. It forms the basis for triangulating the epicenter's location when data from multiple stations are combined.
