Geomechanical modeling of the nucleation process of Australia's 1989 M5.6 Newcastle earthquake

Inherent to black-coal mining in New South Wales (Australia) since 1801, the discharge of ground water may have triggered the M5.6 Newcastle earthquake in 1989. 4-dimensional geomechanical model simulations reveal that widespread water removal and coal as deep as a 500 m depth resulted in an unload of the Earth's crust. This unload caused a destabilization process of the pre-existing Newcastle fault in the interior of the crust beneath the Newcastle coal field. In tandem, an increase in shear stress and a decrease in normal stress may have reactivated this reverse fault. Over the course of the last fifty years, elevated levels of lithostatic stress alterations have accelerated. In 1991, based on the modeling of the crust's elastostatic response to the unload, there has been the minimal critical shear stress changes of 0.01 Mega Pascal (0.1 bar) that reached the Newcastle fault at a depth where the 1989 mainshock nucleated. Hence, it can be anticipated that other faults might also be critically stressed in that region for a couple of reasons. First, the size of the area (volume) that is affected by the induced stress changes is larger than the ruptured area of the Newcastle fault. Second, the seismic moment magnitude of the 1989 M5.6 Newcastle earthquake is associated with only a fraction of mass removal (1 of 55), following McGarr's mass-moment relationship. Lastly, these findings confirm ongoing seismicity in the Newcastle region since the beginning of the 19th century after a dormant period of 10,000 years of no seismicity.