Earthquake depth is the reported vertical position of the hypocenter, the point where rupture starts, below a defined reference surface. USGS groups earthquakes broadly as shallow, intermediate or deep. Depth affects how far waves travel to the surface, but it does not by itself determine shaking or consequences.
Quick answer
- Shallow: less than 70 km.
- Intermediate: 70 km to less than 300 km.
- Deep: 300 km to about 700 km.
- Depth is usually less tightly constrained than latitude and longitude.
- A catalog depth can be revised when more stations or identifiable depth phases become available.
- A shallow label is not a warning level, and a deep label does not guarantee weak shaking everywhere.
What earthquake depth means
The hypocenter, also called the focus, is the point inside Earth where the rupture begins. The epicenter is the point on the surface vertically above it. Catalog depth refers to the hypocenter; it is not the total vertical extent of the fault and not the deepest point that later slips.
Depth needs a reference surface. Catalogs commonly use a geodetic reference such as WGS84 or a network-specific convention. Very shallow solutions can occasionally appear at zero or slightly negative values because of reference choices and uncertainty. That does not place a tectonic earthquake in the air.
See the epicenter and hypocenter distinction →
How depth is determined
Seismologists compare observed wave arrival times at several stations with predicted travel times through an Earth-velocity model. A location algorithm adjusts latitude, longitude, origin time and depth to find a solution that fits the observations.
Depth is difficult because changes in depth can resemble changes in origin time or velocity structure. Nearby stations, good geometry around the source and clear phase picks improve the constraint. For suitable distant earthquakes, reflected phases such as pP and sP provide additional depth information because their delay relative to the direct P wave changes with source depth.
An automatic system can assign a standard or fixed depth when the observations do not constrain it well. Analyst review may later free, confirm or replace that value. Earthquake24 displays the source value; it does not infer depth from marker size or map color.
How to interpret depth classes
| Class | Boundary used by Earthquake24 | Geological context |
|---|---|---|
| Shallow | 0 to less than 70 km | Crust and uppermost lithosphere; includes many plate-boundary and intraplate events |
| Intermediate | 70 to less than 300 km | Common within descending lithosphere at subduction zones |
| Deep | 300 to about 700 km | Confined to slabs descending into the mantle |
The boundaries are categorical, not gradual physical walls. A solution at 69.9 km and one at 70.1 km can have similar uncertainty even though the display label changes.
If all else were equal, a deeper source places more rock between the rupture and the surface, often reducing the strongest nearby motion. Real events are not equal: magnitude, fault dimensions, radiation pattern, wave frequency and local geology can outweigh a simple depth comparison. Deep large earthquakes may be felt across broad regions.
Limits and common mistakes
Depth is not distance from the epicenter. Epicentral distance is horizontal along the surface; hypocentral distance includes depth.
Depth is not fault length. A large rupture extends away from its starting point. One coordinate cannot describe the entire source surface.
A round value may be provisional. Values such as exactly 10 km sometimes reflect a constrained or default solution. The source metadata and later revisions provide context.
Depth alone cannot map felt area. A generic circle based only on magnitude and depth is an estimate, not observed intensity. See the intensity scale guide for location-specific shaking.
Common questions
What is the average earthquake depth?
There is no single useful global average for every question. Depth distributions differ strongly between shallow crustal regions and subduction zones. A distribution or regional catalog is more informative than one worldwide mean.
What is considered a deep earthquake?
In the USGS scientific grouping, deep earthquakes begin at 300 km. They occur within descending slabs and extend to roughly 700 km.
Why are some depths exactly 10 km?
It can be a genuine best estimate, but it may also be an assigned or poorly constrained starting depth. Consult the source uncertainty and review status instead of assuming artificial precision.
Can the depth change after publication?
Yes. Additional phases, stations, improved velocity models and analyst review can revise the hypocenter.