Along divergent boundaries like the mid-Atlantic ridge and the East Pacific Rise, earthquakes are common, but restricted to a narrow zone close to the ridge, and consistently at less than 30 km depth. Shallow earthquakes are also common along transform faults, such as the San Andreas Fault.
Explanation:
When the earthquake depth is very shallow, it can be reported as a negative depth. Earthquake depths are calculated relative to the World Geodetic System of 1984 (WGS84) geoid, mean sea-level, or the average elevation of the seismic stations that provided arrival-time data for the earthquake location.
