Reading earthquake data
Shallow vs deep earthquakes
Depth is the distance from Earth's surface to the point where rupture begins. It helps explain tectonic setting and how seismic energy reaches the surface.
The USGS groups earthquake depths into shallow (0–70 km), intermediate (70–300 km) and deep (300–700 km). Shallow earthquakes often create stronger shaking near the epicenter, but magnitude, distance and local ground conditions also matter.
Hypocenter, focus and epicenter
The underground point where an earthquake rupture starts is called the hypocenter or focus. The epicenter is the point on Earth's surface directly above it. A catalog's depth value refers to the hypocenter, not the elevation of the epicenter.
Why shallow earthquakes can be especially damaging
Seismic waves from a shallow source travel a shorter distance before reaching the surface, so less energy may be lost along the way. If the rupture is close to a populated area, strong local shaking can result. This does not mean every shallow earthquake is dangerous: a small shallow event may be harmless, while a very large deeper event can be widely felt.
Damage depends on the combination of magnitude, depth, distance from the ruptured fault, local geology, building design and duration of shaking. Depth should therefore be read alongside magnitude—not as a stand-alone severity score.
Where do deep earthquakes occur?
Earthquakes deeper than about 70 km are concentrated in subduction zones, where one tectonic plate descends beneath another. Inclined bands of earthquake locations can trace a sinking slab hundreds of kilometers into the mantle. Deep events may be felt across broad areas because their waves spread through large volumes of rock, even when surface intensity near the epicenter is lower than for a comparable shallow event.
How is earthquake depth calculated?
Seismologists compare the arrival times and waveforms recorded by multiple stations. Depth can be harder to constrain than latitude and longitude, especially when stations are sparse or far away. Early automatic solutions may use a fixed or poorly constrained depth and can change as more observations are analyzed.
A revised depth is normal catalog behavior, not evidence that the earthquake moved. Always check the event's update time when exact depth matters.
How to read depth on SeismoWatch
The SeismoWatch earthquake map uses warm colors for shallow events and progressively cooler colors for deeper ones. Hover a marker to see its reported depth. The map makes subduction-zone patterns visible, but the USGS event page remains the authoritative source for revisions and technical products.
Frequently asked questions
Is a 10 km deep earthquake shallow?
Yes. Under the scientific classification, any depth from the surface to 70 km is shallow.
Can earthquakes happen deeper than 700 km?
Recorded earthquake activity is generally limited to about 700 km. Rock conditions deeper in the mantle do not favor the same brittle failure process.
Why do some catalogs show exactly 10 km depth?
Ten kilometers is sometimes used as a default or constrained starting depth when the available data cannot determine depth reliably. Later solutions may revise it.