Why earthquake depth matters
Earthquakes start anywhere from near the surface to about 700 km down. Depth affects how shaking spreads, tells us about plate boundaries and is the least certain part of a location.
Key concepts
- Hypocentre and epicentre
- Shallow, intermediate and deep earthquakes
- Depth and the spread of shaking
- Depth uncertainty and the reference datum
Where an earthquake starts
An earthquake begins at a point inside the Earth called the hypocentre, or focus. The point on the surface directly above it is the epicentre. Maps usually show epicentres, which can hide an important fact: two earthquakes at the same map position can start at very different depths.
The distance from the surface down to the hypocentre is the focal depth. Catalogs such as the one EQRadar uses report it in kilometres.
Shallow, intermediate and deep
Earthquakes occur between the surface and about 700 km below it. For scientific purposes this range is commonly divided into three zones: shallow earthquakes from 0 to 70 km, intermediate earthquakes from 70 to 300 km, and deep earthquakes from 300 to 700 km.
Because the published ranges share their end points, EQRadar uses an explicit rule so that no earthquake falls into two groups: each group includes its lower limit and excludes its upper limit. An earthquake at exactly 70 km is therefore intermediate, and one at exactly 300 km is deep. The deep group also includes 700 km itself.
| Group | Depth range |
|---|---|
| Shallow | 0 km up to, but not including, 70 km |
| Intermediate | 70 km up to, but not including, 300 km |
| Deep | 300 km to 700 km |
How depth affects shaking
For earthquakes of the same magnitude, a shallow one usually produces stronger shaking close to its epicentre, because the energy has less distance to travel before it reaches people and buildings. A deeper earthquake's waves travel farther before reaching the surface, so shaking near the epicentre tends to be weaker but may be felt over a wider area.
Depth is one factor among several. Magnitude, distance, the direction the rupture travelled and local ground conditions all shape what people feel. A deep earthquake can still be damaging if it is large enough.
Where deep earthquakes happen
Rock deep in the mantle is hot and tends to flow rather than break, so most earthquakes are shallow. Earthquakes deeper than about 70 km occur within great slabs of ocean plate that are sinking into the mantle at subduction zones. When catalog depths are plotted in three dimensions, these earthquakes often form an inclined band that traces the sinking slab.
The 3D Depth Explorer lets you see this in real data, for example beneath Japan or the Aleutian Islands. The points show where earthquakes were located; they are not a picture of the slab itself.
Depth is the least certain part of a location
Depth is usually less well constrained than latitude and longitude, and its error bars can be large, especially for earthquakes far from seismic stations. Different seismic networks also measure depth from different reference levels. USGS uses the WGS84 geoid; other networks may use mean sea level or the average elevation of their stations.
This is why a catalog can list a small negative depth. It means the computed hypocentre lies above the reference level, not that the earthquake happened in the air. EQRadar keeps these values and shows them above the catalog reference level instead of rounding them to zero.
Three takeaways
- The hypocentre is where an earthquake starts underground; the epicentre is the point on the surface above it.
- Shallow (0–70 km), intermediate (70–300 km) and deep (300–700 km) groups are commonly used, and deep earthquakes occur in sinking slabs at subduction zones.
- Depth is the least certain part of a location and is measured from a reference datum, so small negative depths can be valid.
Sources
- Determining the Depth of an Earthquake — U.S. Geological Survey
- What reference datum is used for earthquake depths? — U.S. Geological Survey
- Earthquake Hazards Program glossary — U.S. Geological Survey