What does it mean that the earthquake occurred at a depth of 0 km?  How can an earthquake have a negative depth; that would mean it’s in the air.  What is the geoid, and what does it have to do with earthquake depth?

An earthquake cannot physically occur at a depth of 0 km or -1km (above the surface of the earth). In order for an earthquake to occur, two blocks of crust must slip past one another, and it is impossible for this to happen at or above the surface of the earth. So why do we report that the earthquake occurred at a depth of 0 km or event as a negative depth sometimes?

First of all, the depth of an earthquake is usually the most difficult part of its location to nail down with great accuracy.  Since most earthquakes are deep within the crust, an error of +/- 1 or 2 km is irrelevant; in other words, it is a small error when the depth is something like 13 km.  If the earthquake depth is relatively shallow, however, it becomes more of an issue.  A negative depth can sometimes be an artifact of the poor resolution for a shallow event.

For quarry blasts that are recorded by the seismic network, the depth is fixed at 0 km since we can never determine a precise depth for these, but we know they are very close to the surface.

Sometimes because of the density of the seismic network and the close proximity of the seismic stations to an earthquake epicenter, we are able to determine a very precise depth.  When the earthquake depth is very shallow, it can be reported as a negative depth. Earthquake depths are calculated relative to the WGS84 geoid, mean sea-level, or the average elevation of the seismic stations which provided arrival-time data for the earthquake location. The choice of reference depth is dependent on the method used to locate the earthquake, which varies by seismic network. NEIC currently uses the reference geoid called WGS84, but since ComCat includes data from many different seismic networks, the process for determining the depth is different for different events. The depth is the least-constrained parameter in the earthquake location, and the error bars are generally larger than the variation due to different depth determination methods.

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