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Super seismo-sonic earthquakes are real

Suped-up shake The inner workings of bizarre and potentially dangerous earthquakes that break the seismic sound barrier creating a sonic boom have been confirmed in laboratory experiments.

And these events -- known as supershear earthquakes -- may be more common than previously thought, report scientists in today's issue of the journal Science .

While scientists have previously investigated supershear earthquakes using plastics, the lack of experiments in real rocks limits our understanding of how these events happen in nature, they write.

In the latest study, the scientists used acoustic sensors to analyse 200 'micro-earthquakes' in granite rocks in the laboratory to emulate the conditions under which real supershear earthquakes occur.

Supershear earthquakes are strange events in which the rupturing fault breaks faster than certain seismic waves can travel, creating a sort of seismic mach cone that fires out the end of a fault's rupture zone -- the part of the fault that breaks loose allowing two rock surfaces to jerk past each other. That cone and the waves that follow can cause inordinately severe shaking, out of proportion to the earthquake's magnitude.

"It's like the [seismic] waves are propagating along and all of a sudden it steps on the accelerator," explains Eric Dunham, an assistant professor and seismological researcher at Stanford University, who was not involved in the latest study but has done modeling work on supershear waves.

Shear waves are normally relatively slow seismic waves that move over the surface in a manner similar to ocean waves. These are the waves that are felt as rolling and shaking motions after the initial shock of normal earthquakes. The initial shock is another, much faster, kind of wave that behaves more like pressure waves that make sound in the air.

In a supershear earthquake, however, the shear waves are created very quickly when a long fault, like the San Andreas, breaks loose faster than the speed shear waves normally travel.

When this happens the shear waves mach cone is created that can reach the same speed as the pressure waves, explains the paper's lead author, François Passelègue of the Geology Laboratory at École Normale Supérieure in Paris, France.

"The main additional hazard due to supershear earthquakes is that there are two big wave arrivals," says Passelègue.

To someone riding out such a quake, the first thing to arrive would be the sharp pressure wave, but instead of the rolling shear waves following it, the powerful supershear mach cone would arrive and shake the ground in a direction parallel to the fault zone that created it. Then, soon after, a second shear wave would hit with ground motions at right angles to the fault zone.

"This sudden change in the direction of the dominant ground motions is dramatic for buildings."

Rare but ...

Luckily, supershear quakes appear to be very rare, says Passelègue, with only a few ever recorded. And faults like the San Andreas in the very populous California are prime candidates for this sort of suped-up seismic event.

"The other issue is whether this happens only at very large earthquakes," says Dunham.

The success of Passelègue's experiments suggests otherwise, with slips of just a few centimetres producing supershear velocities.

A moderately strong quake of a magnitude between 6 and 7 ought to be able to produce supershear waves, says Passelègue.

And that is not a very unusual earthquake size along the San Andreas.