Earthquake Depth Explained: Shallow, Intermediate and Deep Quakes
USGS classifies earthquakes by the depth where the rupture started. Shallow earthquakes occur 0 to 70 km (0 to 43 miles) deep, intermediate earthquakes 70 to 300 km (43 to 186 miles) deep and deep earthquakes 300 to 700 km (186 to 435 miles) deep. Shallow earthquakes usually cause stronger shaking at the surface because the seismic waves travel a shorter distance to reach people and buildings.
Depth is also one of the hardest numbers to pin down. USGS calls it the least-constrained part of an earthquake location, which is why early reports often show a round value such as 10 km.
What do shallow, intermediate and deep mean?
USGS uses three depth classes, listed on its page about determining the depth of an earthquake.
| Class | Depth (km) | Depth (miles) | Where they occur, per USGS |
|---|---|---|---|
| Shallow | 0 to 70 | 0 to 43 | Includes continental faults such as the San Andreas, which USGS says are active only in the shallow crust, perhaps to about 20 km (12 miles) |
| Intermediate | 70 to 300 | 43 to 186 | Inside slabs of lithosphere sinking into the mantle |
| Deep | 300 to 700 | 186 to 435 | In the core of subducting slabs, which stay brittle enough to break to depths of about 700 km |
USGS notes that all earthquakes deeper than 70 km are located within great slabs of lithosphere that are sinking into the Earth’s mantle. Those slabs are the subducting plates at subduction zones, so intermediate and deep earthquakes are found near them.
Why are shallow earthquakes usually felt more strongly?
Seismic waves lose strength as they travel. USGS explains that shaking at the surface from an earthquake 500 km (311 miles) deep is considerably weaker than if the same earthquake had happened 20 km (12 miles) deep.
Depth is one factor among several. A USGS general-interest publication lists magnitude, local geology, focal depth, distance from the epicenter and building design as the factors behind an earthquake’s destructiveness. Our guide to magnitude vs intensity explains how these combine into the shaking level at a given town.
Depth also affects what follows. USGS says deep earthquakes, deeper than 30 km (19 miles), are much less likely to be followed by aftershocks than shallow ones. See our guide to aftershocks for how USGS forecasts them.
How deep were some well-known US earthquakes?
Two damaging earthquakes in Washington and Alaska started inside a subducting plate rather than near the surface.
- 2001 Nisqually, Washington: magnitude 6.8, depth about 52 km (32 miles) in the USGS catalog, within the subducting Juan de Fuca plate.
- 2018 Anchorage, Alaska: magnitude 7.1, depth about 47 km (29 miles) in the USGS catalog, within the subducting Pacific plate.
Labels vary by context. USGS research papers call the Anchorage earthquake an intermediate-depth event, while the general USGS depth classes draw the line between shallow and intermediate at 70 km. You can see current depths in recent earthquakes in Alaska and recent earthquakes in Washington.
How does USGS measure earthquake depth?
Seismologists locate an earthquake from the arrival times of seismic waves at many stations. Depth is harder to fix than latitude and longitude. A USGS FAQ gives a rule of thumb: a reliable depth requires the nearest station to be closer to the epicenter than the earthquake is deep. USGS adds that modern methods can now produce reliable depths at greater distances, so the rule does not always apply.
For distant earthquakes, USGS describes the use of depth phases. The pP phase is a P wave that reflects off the Earth’s surface near the source. At distant stations it arrives after the direct P wave, and the gap between the two changes slowly with distance but quickly with depth. Reading that time difference against travel-time tables gives the depth. The sP phase can be used the same way.
Depth is measured from a reference surface, not from the ground under a given house. USGS currently uses the WGS84 geoid, while some networks use mean sea level or the average elevation of their stations. USGS has a separate FAQ on why some very shallow earthquakes are listed at 0 km or even at a negative depth.
Why is the depth often exactly 10 km?
A depth of exactly 10 km (6.2 miles) is often a fixed value, not a measurement. USGS says that when data are too poor to compute a reliable depth, the depth is set to 10 km. The most common reason is that the earthquake was too far from the nearest seismic station.
USGS used 33 km (20.5 miles) as its fixed depth in the past and changed to 10 km because increased understanding shows it is more likely. The USGS catalog documentation adds that default depths of 5 or 10 km are often used in mid-continental areas and on mid-ocean ridges. In subduction zones, where many earthquakes are much deeper than 10 km, USGS says a deeper fixed depth would probably be appropriate. Details are in the USGS FAQ Why do so many earthquakes occur at a depth of 10 km?
Why does the depth change in early reports?
The first depth comes from automatic processing with the data available in the first minutes. As more stations report and analysts review the event, USGS recalculates the location. A fixed 10 km depth can be replaced by a computed value at that point. USGS also notes that depth error bars are generally larger than the differences between depth methods.
To check how settled a depth is:
- Open the event on the USGS Latest Earthquakes map.
- Look at the depth and the review status in the origin details. “Automatic” means no person has checked it yet.
- Treat exactly 10.0 km on an automatic solution as a likely fixed depth.
- Check again after a few hours, when most real-time data have arrived.
Our guide on how to read a USGS earthquake report walks through the other fields on the same page.
Bottom line
Depth tells you where under the surface the rupture began. USGS calls 0 to 70 km shallow, 70 to 300 km intermediate and 300 to 700 km deep. Shallower usually means stronger surface shaking for the same magnitude, and a round 10 km figure on a new event is often a placeholder until better data arrive. The USGS FAQ on earthquake depth has more background.
FAQ
Does a deeper earthquake always mean less damage?
No. Depth reduces surface shaking, but USGS lists magnitude, distance, local geology and building design as other factors. The Nisqually and Anchorage earthquakes caused damage from depths of roughly 47 to 52 km (29 to 32 miles).
What is the deepest an earthquake can be?
USGS says earthquakes occur from the surface down to about 700 km (435 miles), inside the core of subducting slabs.
Is depth the same as distance from the epicenter?
No. The epicenter is the point on the surface directly above where the rupture started. Depth is how far below that point the rupture began.
Are deep earthquakes followed by aftershocks?
Less often. USGS says earthquakes deeper than 30 km (19 miles) are much less likely to be followed by aftershocks than shallow earthquakes.