Focal depths of aftershocks

A review of aftershock studies that feature accurate determinations of focal depths leads to the conclusion that prominent, well-defined aftershock sequences occur only at shallow depths, less than about 20 km. Data currently available from various geographic and tectonic regions throughout the world indicate that the occurrence of aftershocks is confined to the earth's crust, and often to the upper layers of the crust, regardless of the magnitude of the main shock. This observation is important to our understanding of the aftershock process and the physical properties of the material in the earth's crust.