Australia: The Land Where Time Began

A biography of the Australian continent 

Earthquake distribution

Most of the earthquake activity of the Earth occurs along plate margins, according to plate tectonics theory. As a result of this, the location of plate boundaries can be determined by plotting earthquake epicentre distribution patterns. (See Engdahl et al., 1998). Earthquakes are classified based n their focal depth.

0-70 km, shallow focus

70-300 km, intermediate focus

>300 km, deep focus

On the ocean ridge system, a band of shallow focus earthquakes follows the crest of the ridge, the focal mechanism solutions indicate tensional events associated with plate accretion and strike-slip events where transform faults offset the ridge. Tensional events of shallow focus that occur on land are associated with rifts, such as the basin and Range Province in the western USA, the East African Rift System and the Baikal Rift System.

Destructive plate margins are the sites of all intermediate and deep events. A belt of earthquakes on the margins of the eastern, northern and western Pacific, the Ring of Fire, on planes, that are offset by transform faults in some places, that dip at angles of about 45^o beneath the neighbouring plates. Benioff, or Benioff Wadati zones are the planes of earthquake foci that are typically associated with surface volcanic activity. About 670 km is the deepest events that have been recorded. In places such as the Alpine-Himalayas chain, collisional mountain belts are also characterised by earthquakes that are of intermediate and deep focus, though the seismic activity occurs in a belt that is relatively broad because of the lack of a Benioff zone in such regions. It has been found that some shallow events lie on arcuate strike-slip fault zones that are associated with the collisional event, based on detailed study of epicentre locations.

On this timescale, the intraplate areas are relatively aseismic, large magnitude earthquakes occurring occasionally. Intraplate earthquakes are important as indicators of the nature and direction of stress within the plate.

See Source 1 for more detailed information on all aspects of plate tectonics

Sources & Further reading

1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.