The inner workings of bizarre and potentially dangerous earthquakes that break the seismic sound barrier have now for the first time been confirmed in laboratory experiments with real rocks, report scientists in today’s issue of the journal Science.
What are called 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,” explained Eric Dunham, an assistant professor and seismological researcher at Stanford University who has done modeling work on supershear waves.
The waves behind the weird phenomenon are called shear waves, which normally are 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, explained 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,” Passelègue told Discovery News. 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.”
Luckily, supershear quakes appear to be very rare, said 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,” said Dunham. The success of Passelègue’s experiment, on a scale of centimeters, suggests otherwise. A moderately strong quake of a magnitude between 6 and 7 ought to be able to produce supershear waves, he said. And that is not a very unusual earthquake size along the San Andreas.