The world-wide network of seismic recording stations was originally set up partly to improve detection of underground nuclear weapons tests. It is the source for the mapping of variations in seismic-wave speeds in the mantle by seismic tomography that is revolutionising ideas about the Earth’s internal dynamics. Nowadays nuclear explosions have been miniaturised so that detecting them and their locations and distinguishing them from small natural earthquakes has become difficult. The growing concerns about nuclear weapons proliferation have spurred an upgrade and expansion of seismic monitoring, and other means of verifying that seismic signals have indeed been produced by underground nuclear explosions, such as sensitive analysis of air sample for isotopes leaking from tests (Clery, D. 2009. Test ban monitoring: no place to hide. Science, v. 325, p. 382-385). If this enhanced source of seismic data is routinely made available to tomography researchers, it should boost resolution of seismic speed anomalies and sharpen up ideas about deep tectonics.

Supershear earthquakes

In an analogous fashion to the sonic booms made by aircraft travelling faster than sound, it seems possible that the rupture of a fault may travel faster than the seismic waves that it generates. Evidence is accumulating that such faults produce the equivalent of a sonic boom (Fisher, R. 2009. Seismic boom. New Scientist, v. 203 (1 August 2009), p. 32-35) despite mathematical suggestions that faults cannot propagate so fast. Experiments show that there is a seismic equivalent of the Mach fronts associated with sonic booms, and they amplify the shock of earthquakes that produce them. High amplitude at the Mach front causes it to travel further away from a fault line than normal seismic surface waves – those that cause most damage, and it also gives rise to ground motions different from those normally linked with earthquakes: more like a hammer blow than shaking. The net conclusion is that these ‘supershear’ earthquakes may pose hazards beyond those involved in risk assessment near active fault zones. Field evidence for supershear events are signs of disturbance by recent earthquakes that are further from an active fault zone than existing theory predicts. So far such evidence has only turned up along active strike-slip faults on continents, such as the Kun Lun Fault in Tibet and the North Anatolian Fault in Turkey. Yet, these form the longest seismically active zones, including the infamous San Andreas Fault in California.