Depth: the most overlooked earthquake parameter
When an earthquake makes the news, the headline almost always leads with magnitude. Depth, if mentioned at all, is buried in the third paragraph. This misses a critical point: depth fundamentally controls how much shaking reaches the surface and how wide an area it affects.
A magnitude 6.5 earthquake at 5 kilometers depth will produce violent shaking in a concentrated area directly above the fault. The same magnitude at 300 kilometers depth may produce weak, rolling motion spread over a much larger area but cause almost no structural damage. The energy released is identical — what changes is how much of it reaches the human-occupied surface.
How seismologists classify depth
Earthquakes are classified into three depth categories. Shallow earthquakes occur at 0–70 kilometers and account for about 72 percent of global seismic energy release. Intermediate earthquakes occur at 70–300 kilometers. Deep earthquakes occur at 300–700 kilometers, the deepest point at which rocks can still undergo brittle failure or phase-transition faulting.
The majority of destructive earthquakes are shallow. The 2010 Haiti earthquake (13 km depth), the 2023 Turkey-Syria earthquakes (10–18 km depth), and the 2015 Nepal earthquake (15 km depth) all caused catastrophic damage in part because their hypocenters were close to the surface beneath populated areas.
Shallow quakes: intense but localized
Shallow earthquakes concentrate their energy over a smaller area. Seismic waves traveling a short distance from the source lose less energy to attenuation, so the shaking at the surface directly above is much more intense. However, the area of strong shaking is relatively compact.
This means shallow earthquakes are disproportionately dangerous for cities located near their epicenters but may not be felt at all a few hundred kilometers away. Ground acceleration values from shallow events can exceed 1g near the epicenter — enough to throw unsecured objects into the air and cause complete structural failure in poorly built buildings.
Deep quakes: mild but widespread
Deep earthquakes produce shaking over a much wider geographic area because the seismic waves travel a longer path through the mantle before reaching the surface. The energy is more dispersed, so any single point on the surface receives less intense shaking.
Deep events above magnitude 7 can be felt across entire countries or even continents. A magnitude 8.2 earthquake at 610 km depth beneath Bolivia in 1994 was felt as far away as Canada, but caused minimal damage because the shaking intensity at the surface was low everywhere.
Depth and tsunami generation
Tsunami risk is almost entirely a shallow-earthquake phenomenon. Tsunamis are generated when a large fault displacement near the seafloor vertically displaces the water column above it. This requires a shallow rupture — typically less than 50 km depth — on a fault that has a significant dip-slip (vertical) component.
The devastating 2004 Indian Ocean tsunami was generated by a magnitude 9.1 earthquake at approximately 30 km depth on the Sunda megathrust. If the same event had occurred at 300 km depth, it would have produced no tsunami at all despite releasing the same total energy.
Why depth estimates change after initial reports
Initial earthquake reports often use a default or poorly constrained depth value. The USGS commonly assigns a preliminary depth of 10 km when the true depth is not yet resolved — a convention that can mislead early damage expectations.
Depth is harder to determine than location because it depends on the vertical distribution of seismic stations and the velocity model of the earth's crust. As more seismograms arrive and analysts refine the solution, the depth estimate can shift by tens of kilometers, sometimes dramatically changing the expected impact.
Soil amplification: when shallow gets worse
Depth interacts with local geology to compound damage. When seismic waves from a shallow earthquake reach soft sedimentary basins, the waves slow down and their amplitude increases — a phenomenon called soil amplification. Mexico City, built on an ancient lakebed, is the textbook example: the 1985 earthquake's waves were amplified by factors of 50 in certain frequency bands by the soft clay beneath the city.
This means a shallow earthquake beneath or near a sedimentary basin can produce unexpectedly severe shaking even at moderate magnitudes. Conversely, a city built on hard bedrock may experience relatively mild shaking from the same event at the same distance.
How PlanetSentry uses depth data
PlanetSentry displays depth alongside magnitude and location for every seismic event. The USGS feed provides depth values for all cataloged earthquakes, and the detail panel shows this information so users can immediately assess whether a reported event is shallow and potentially destructive or deep and likely to produce mild widespread shaking.
Understanding depth helps users avoid the common mistake of treating every large-magnitude earthquake as equally dangerous. A magnitude 7 at 10 km depth near a city demands immediate attention. A magnitude 7 at 500 km depth beneath an ocean basin is scientifically interesting but unlikely to cause harm.