Earthquake focus; Focal point
A point beneath the Earth’s surface where the vibrations of an earthquake originates and which corresponds to the location where the motion on a fault surface starts. The focal depth of earthquake hypocenters may vary from the surface down to nearly 700 km in subduction zones.
Earthquake magnitude, and more precisely the Moment Magnitude, is function of the amount of slip and area of the fault surface that ruptures during an earthquake. For small earthquakes, such as magnitude 4 or less, or for earthquakes displayed on maps and cross-sections at a regional scale, the hypocenter is essentially a point with three coordinates (latitude, longitude, and focal depth). As the magnitude increases, the earthquake rupture is no longer a point but is rather a surface. In this latter case, the hypocenter is defined as the point where the rupture started.
The traditional method used to calculate the earthquake hypocenter is similar to...