The hypocenter, or focus, is the estimated point inside Earth where an earthquake rupture starts. The epicenter is the point on the surface vertically above that hypocenter. They are linked coordinates, but neither one describes the full area of a large fault rupture.
Quick answer
| Term | Location | What it represents |
|---|---|---|
| Hypocenter or focus | Inside Earth | Rupture starting point |
| Epicenter | Surface | Point directly above the hypocenter |
| Fault rupture | Two- or three-dimensional surface | Area that slipped during the earthquake |
| Intensity observation | A measured or reported location | Shaking or effects at that place |
Epicenter, hypocenter and rupture
Rupture begins at the hypocenter and can propagate along a fault for seconds or minutes. For a small earthquake, the rupture may be compact. For a great earthquake, the slipped area can extend hundreds of kilometres. The epicenter remains one point above the rupture start, not the geometric center of everything that moved.
Depth is the vertical coordinate of the hypocenter relative to the network’s reference surface. The earthquake depth guide explains why it can be more uncertain than the horizontal coordinates.
How the location is calculated
Seismic stations record different wave arrivals. A locator compares observed P and S arrival times with predictions from a velocity model, then adjusts origin time, latitude, longitude and depth. Nearby stations and good coverage around the source usually reduce uncertainty.
The epicenter is derived from the horizontal hypocenter coordinates; it is not independently detected by looking for the strongest surface motion. A reviewed catalog can move the point when phase picks, station timing or the Earth model improves.
How to interpret the map point
A marker is a compact representation of a catalog coordinate. It does not show fault length, rupture direction, affected area or a safety boundary. Its label may name the nearest recognizable place rather than a jurisdiction containing the source.
The strongest shaking need not occur exactly at the epicenter. It can be closer to another part of a long rupture, enhanced by rupture direction or amplified by local sediments. ShakeMap and intensity observations are better tools for describing spatial shaking when available.
Limits and common mistakes
“Epicenter” does not mean the place with the most damage. “Hypocenter” does not mean the center of the whole fault. Distance to the epicenter is not the same as distance to the nearest ruptured fault segment.
Coordinates with several decimals still have uncertainty. Offshore and poorly instrumented regions may be less constrained. Early automatic solutions can shift substantially during review.
Common questions
Are focus and hypocenter the same?
Yes in earthquake seismology: both name the rupture starting point inside Earth.
Is the epicenter always on a fault?
It is vertically above the estimated start. Its surface point need not lie on a mapped surface trace, especially for dipping or buried faults.
Why did an epicenter move on the map?
More observations, corrected phases, a different velocity model or analyst review changed the best-fitting solution.