The exfoliation of cratonic Australia in earthquakes

Abstract

The cratonic shield system of central and western Australia, with its lithosphere up to 200 km thick, is geologically similar to other ancient, stable continental interiors. But since 1968 it has experienced a number of moderate-sized (Mw 5.0–6.6) earthquakes characterised by the extreme shallowness of their sources (the deepest is 8 km and most are shallower than 4 km). At least 11 of these have produced co-seismic faulting, often very long compared to their depth, with typically no evidence of previous movement on those faults in either the local geomorphology or paleoseismological trenching. Other earthquakes show that cratonic Australia, like other shield regions, has a seismogenic layer about 30–40 km thick, but the intense very shallow seismicity in the region of thickest lithosphere stands out and is unusual. A clue to the origin of these shallow earthquakes lies in their association with some of the largest continental gravity anomalies outside the forelands of young orogenic belts, yet in essentially flat topography. The wavelength of the gravity anomalies (∼240 km) is large compared with the seismogenic thickness (∼30 km) of the lithosphere, and their amplitude is ∼50 mGals. These anomalies need stresses to support them, which can be estimated by a simple model of a flexed elastic plate that reproduces the essential features of the earthquakes, including their focal mechanisms and shallow depth limit. The model shows that the maxima of the compressive stress occur beneath the maxima and minima of the gravity, on the upper and lower boundaries of the layer respectively. Perhaps surprisingly, the magnitude of such stresses is considerably greater than most estimates of the regional stress within plates. The maxima of the shear stress occur on planes with dips of 45°. The locations and mechanisms of the earthquakes show the same features. We conclude that the earthquakes release stored elastic stresses in an exfoliation process, perhaps activated by a reduction in strength through weathering, erosion or some other process.