A continent that still moves

Stand on the Adelaide plain on a clear morning and look east. The Mount Lofty Ranges do not roll up gently the way a worn-down old hill range should; they stand up in one long, abrupt wall, as if the country had been folded and someone had forgotten to sand the crease. That crease is real, and it has a name: a fault scarp, and the country east of it is still being lifted.

Australia is supposed to be the boring continent, tectonically speaking — sitting mid-raft, far from the grinding plate edges where mountains are built and earthquakes are made. But a raft is rigid, and you cannot lean on its edge without the whole thing feeling it. Forces applied thousands of kilometres away — at the Himalayan collision, the crumple zone against New Guinea, the grind past New Zealand’s Southern Alps — travel inward through the stiff crust and squeeze south-central Australia roughly east to west, a slow vice that switched on only in the last ten million years or so. Old basement faults along the range front take up that load the way a plate under pressure always does: by cracking and sliding, one block shoved up over another. Among them are the Eden-Burnside and the Willunga. Near Meadows, marine rock that once lay on a shallow sea floor now sits close to 290 metres above it, and the arithmetic of that says the ranges have risen something like 250 metres in the last five million years — an average of about fifty metres every million years (Sandiford 2003).

That is glacially slow by the standards of a real plate boundary, and also entirely capable of violence. The Willunga Fault, the best studied of the lot, slips at only about four-hundredths of a millimetre a year, yet has produced four or five great earthquakes in the last few hundred thousand years and is long enough to deliver a magnitude 7.2 one day (Clark et al. 2022). On the first of March 1954, a smaller rupture near Darlington cracked the cathedral and the GPO clock tower and damaged some three thousand buildings — Australia’s most damaging recorded earthquake for thirty-five years. More faults run within reach of Adelaide’s suburbs than of any other Australian capital, which is a tentative claim about exposure to shaking, not a boast about how often the ground moves. Either way, the wall east of the city is not scenery. It is a machine, mid-stroke.