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What reference datum is used for earthquake depths?

The USGS currently uses the WGS84 Geoid as the reference datum for earthquake depths. Other seismic networks might or might not use different methods. 

The choice of reference datum depends on the method used to locate the earthquake, which varies by seismic network. The Advanced National Seismic System Comprehensive Earthquake Catalog (ComCat) includes data from many different seismic networks, so the process for determining the depth is different for different events. Earthquake depth can be relative to:

  • The WGS84 geoid
  • Mean sea-level
  • The average elevation of the seismic stations that provided arrival-time data for the earthquake location

Depth is the least-constrained parameter in an earthquake's location; the error bars are generally larger than the variation due to different depth determination methods.

Learn More: NOAA - What is the Geoid?