Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles. Credit: Wikipedia
Earthquake booms have been reported since the beginning of time, but experimental and scientific explanations of this strange phenomenon were lacking.
Now it is done! For the first time ever, scientists were able to decipher in laboratory experiments with real rocks, how earthquakes create sonic booms. Their study was published in ‘Science’ on June 7, 2013 (DOI: 10.1126/science.1235637).
They believe, sonic booms are created during so-called supershear earthquakes. During these strange events, the rupturing fault breaks faster than certain seismic waves can travel. This creates a seismic mach cone that fires out the end of a fault’s rupture zone.
Normal vs Supershear Earthquakes
The waves behind the weird phenomenon are called shear waves. In most earthquakes, these slow seismic waves are felt as rolling and shaking motions after the initial shock, which is another, much faster, kind of wave that behaves more like pressure waves that make sound in the air.
In contrast, supershear earthquakes form in three steps and have two big wave arrivals: 1. The initial shock followed by 2. The formation of the powerful supershear mach cone instead of the rolling shear waves. This cone shakes the ground in a direction parallel to the fault zone that created it. Finally, and soon after, 3. A second shear wave hit with ground motions at right angles to the fault zone. This change in the direction of the dominant ground motions is dramatic for buildings.
The San Andreas in California is a prime candidate for such strange seismic events.
Which earthquakes create supershear quakes?
Supershear quakes are not only produced by massive earthquakes. Results suggest that already a moderately strong quake of a magnitude between 6 and 7 could produce supershear waves. And that is not a very unusual earthquake size along the San Andreas.
But don’t be too worry about supershear quakes. They are very rare.
supershear quakes. They are very rare.
“Supershear earthquake ruptures propagate faster than the shear wave velocity. Although there is evidence that this occurs in nature, it has not been experimentally demonstrated with the use of crustal rocks. We performed stick-slip experiments with Westerly granite under controlled upper-crustal stress conditions. Supershear ruptures systematically occur when the normal stress exceeds 43 megapascals (MPa) with resulting stress drops on the order of 3 to 25 MPa, comparable to the stress drops inferred by seismology for crustal earthquakes. In our experiments, the sub-Rayleigh–to–supershear transition length is a few centimeters at most, suggesting that the rupture of asperities along a fault may propagate locally at supershear velocities. In turn, these sudden accelerations and decelerations could play an important role in the generation of high-frequency radiation and the overall rupture-energy budget.”