Global Earthquake Depths

Earthquake depth is the measurement in kilometers of the sub-surface depth of the source of the earthquake rupture. The source depth amd planview location is commonly referred to as the earthquake hypocenter. As shown in the illustration below for earthaquakes with magnitudes 5.5 and greater between the years 1973 and 2009 the greatest number of reported events has occured at relatively shallow depths between 10 and 60 km. The maximum depth approaches 700 km.

Earthquakes occur in two settings:

  1. along the fault planes where the rigid lithosphere edges belonging to adjacent plates either slides past each other and deforms under friction, or thrusts one under the other or rips apart from each other. The Earth's lithosphere is the strong outer skin (rind) between 10 to 100 km in thickness that breaks in a mode of brittle failure. Rock that is not very warm retains its strength and its brittle behavior. However, silicate materials that form the earths mantle weaken with increasing temperature to a threshold where the rocks then become ductile and deform by flowage without breaking. Since temperature rises in the earth's interior with depth, the mantle becomes sufficently with increasing depth that earthquakes on plate surfaces or edges can no longer gereated from friction-induced brittle rupture.
  2. In the interior of the lithospheric where lithosphere slabs descend (subduct) into the mantle at convergent-type plate boundaries. Since the slabs insert themselves into the ductile mantle at velocities of 50 to 200 km per million years, they can reach depths of hundreds of kilometers before their interiors warm up sufficiently to inhibit brittle deformation. Thus earthquake hypocenters between 100 and 700 km depth are almost allways located within the interior of the subducting slab and not along its edges or outer surfaces as in the shallower regimes.

There are substantially more earthquakes at shallow depths between 10 at 60 km than at deeper depths down to 700 km.

When the depths of earthquakes are compared to their magnitudes for the global distribution as shown in the map, one observes two apparent regimes of behavior. The decrease in magnitude with depth from 0 to 350 km in the shallower regime reflects the progressive weakening of rock materials with increasing temperature and thus their potential to release seismic energy when the material ruptures. In the deeper regime below 350 km, the increasing pressure transforms the original minerals into new and more compacted (higher density) crystal phases of greater strength. But eventually the material warms enough, even in its new altered and stronger state, that it flows rather than breaks and earthquake activity ceases altogether.

Deep earthquakes below 350 km are if interest because thay are in fact quite common (almost 25% of all evens with magnitude >5.5) and have relatively more impulsive sources than the shallow events.


Further Reading:
  • Bonner, J.L., Blackwell, D.D, Herrin, E.T. Thermal constraints on earthquake depths in California. Bulletin of the Seismological Society of America; December 2003; v. 93; no. 6; p. 2333-2354; DOI: 10.1785/0120030041
  • Frohlich, C. The nature of deep-focus earthquakes. Annual Review of Earth and Planetary Sciences, Vol. 17: 227-254 (Volume publication date May 1989)
  • Griggs, T. The sinking lithosphere and the focal mechanisms of deep earthquakes. In: E.C. Robertson, J.F. Hays and L. Knopoff, Editors, The Nature of the Solid Earth, McGraw Hill, New York, N.Y. (1972), pp. 361–384.
  • Isacks, B., J. Oliver, and L. R. Sykes (1968), Seismology and the New Global Tectonics, J. Geophys. Res., 73(18), 5855–5899, doi:10.1029/JB073i018p05855.
  • Maggi, A., Jackson, J.A., McKenzie, D., Priestly, K. Earthquake focal depths, effective elastic thickness, and the strength of the continental lithosphere.. Geology June 2000 v. 28 no. 6 p. 495-498. doi: 10.1130/0091-7613.
  • Tichelaar, B., and L. Ruff, Depth of seismic coupling along subduction zones, J. Geophys. Res., 98, 2017-2037, 1993.
  • Weidner, D. J., and K. Aki (1973), Focal depth and mechanism of mid-ocean ridge earthquakes, J. Geophys. Res., 78(11), 1818–1831, doi:10.1029/JB078i011p01818.