Earthquakes occur in the crust or upper mantle of the Earth, which ranges from the earth’s surface to about 700 kilometers deep. Below that depth, rocks bend and flow rather than break, since the area is too hot and ductile.
In seismology, the depth at which an earthquake occurs is called depth of focus or focal depth. Earthquakes are labeled “shallow” if they occur at less than 50 kilometers of depth and “deep” if they occur at 300-700 kilometers of depth. Additionally, we label earthquakes with depths from 50 kilometers to 300 as “intermediate”.
In general, shallow earthquakes are more damaging than deeper ones. This is due to seismic waves from deep earthquakes travel farther to the surface, losing energy along the way. The strength of an earthquake shaking at the surface that occurs at 400 kilometers deep is noticeably less than if an earthquake occurred at 10 kilometers depth. Moreover, shallow-focus earthquakes often produce much more aftershocks than deep-focus earthquakes.
The vast majority of earthquakes are shallow. Of the 56,832 moderate to large earthquakes recorded between 1976 and 2020, only about 18 percent were deeper than 70 kilometers. Even fewer, some four percent, struck below 300 kilometers1.
Methods that seismologists use to determine the depth of an earthquake vary depending on where an earthquake occurred relative to seismic stations.
In places where stands a dense network of seismic stations, like California and Japan, seismologists can determine an earthquake’s location and depth with high accuracy, typically within 1 kilometer or less. However, for areas with few or no seismic stations, like in the middle of the ocean, it is much more complicated to determine the exact location and depth, which can be off by 20 km or more. Sometimes seismologists use several different methods to estimate the depth.
In 2021, scientists detected the deepest earthquake ever, a staggering 751 kilometers below the earth’s surface, where up until that time, seismologists expected earthquakes to be impossible. This discovery reveals that the boundaries within the Earth are fuzzier than they’re often given credit for. This specific earthquake was a minor aftershock to a 7.9-magnitude earthquake that shook the Bonin Islands off mainland Japan in 2015.
Prior to 2021, the deepest earthquake recorded was of a magnitude 4.2 and occurred at a depth of 735.8 kilometers in 2004 in Vanuatu.
The strongest deep-focus earthquake in the seismic record was the magnitude 8.3 Okhotsk Sea earthquake that occurred at a depth of 609 km in 2013.
In short, a 0 km or -1km (above the surface of the earth) depth earthquake is physically impossible, since earthquakes always occur due to two blocks or rocks of crust slipping past one another, which is impossible on the Earth’s surface.
But why do we then sometimes report that an earthquake occurred at a depth of 0 kilometers?
According to the USGS, this “mistake” is related to the uncertainty of the determination of the earthquake epicenter, especially due to its distance from the seismic stations2.
As mentioned before, it is no big deal if an earthquake depth error of +/- 1 or 2 km happens when the earthquake’s depth is more than 10 kilometers. However, if the earthquake depth is relatively shallow, this error of +/- 1 or 2 km can become an issue. It can sometimes even lead to reporting a negative depth, which can sometimes be an artifact of the poor resolution for a shallow event2.
According to the USGS, 10 kilometers is a “fixed earthquake depth”. This depth of an earthquake is assigned when the earthquake data are too poor to compute a reliable depth for an earthquake3.
Why that number?
In many areas around the world, reliable depths average 10 kilometers or close to it. For example, if we made a histogram of the reliable depths in such an area, we’d expect to see a peak around 10 kilometers. So, if we don’t know the depth, 10 kilometers is a reasonable guess.
The USGS used to use 33 km, but increased understanding indicates that 10 km is more likely3.
Accurate determination of earthquake depth is necessary for assessing seismic hazards, discriminating earthquakes from nuclear explosions, interpreting Earth’s structure, and understanding tectonic processes. Even though it has long been believed that earthquakes cannot occur at the depth of more than 700 kilometers, a new proof of it gives us an alternative explanation that those deep earthquakes are caused by some kind of “phase transition”, where the physical structure of the rocks suddenly changes to a different state – a process still waiting to be explored and researched.
Sources:
1) Wei-Haas, Maya. 2021. Deepest Earthquake Ever Detected Struck 467 Miles Beneath Japan. Accessed on 08-Sep-2022. Available at: https://www.nationalgeographic.com/science/article/deepest-earthquake-ever-detected-struck-467-miles-beneath-japan
2) USGS. What Does it Mean That the Earthquake Occurred at a Depth of 0 Km? Accessed on 09-Sep-2022. Available at: https://www.usgs.gov/faqs/what-does-it-mean-earthquake-occurred-depth-0-km-how-can-earthquake-have-negative-depth-would
3) USGS. Why do so Many Earthquakes Occur at a Depth of 10 Km? Accessed on 08-Sep-2022. Available at: https://www.usgs.gov/faqs/why-do-so-many-earthquakes-occur-depth-10km
