A quake supposedly has a limited speed
"Mathematical equations show that ruptures cannot propagate at speeds in a so-called "forbidden zone", between around 3 and 3.5 kilometres per second. At these speeds, the fault's frictional sliding would have to convert heat into mechanical energy - something that is thermodynamically impossible. Since a rupture can never accelerate through this zone, the possibility of quakes faster than 3.5 kilometres per second was ruled out."
Now they've tossed that theory out and they've found that earthquakes can travel faster than this, reaching up to 5 kilometers per second.
What's the difference?
If a quake goes "supershear", it can travel much faster, but also farther, including places that we thought were safe(er)
"it could mean that regions previously thought to be outside of the worst effects of an earthquake, and maybe even beyond its reach altogether, could be caught unawares by a supershear quake. The Mach front's high amplitude means that it travels further through the ground than normal shear waves, putting millions more people at risk."
"The most recent building rules in the US, established in the late 1990s, place tight restrictions on the design of structures within 5 kilometres of an active fault. That's because these regions are considered vulnerable to the so-called "near source pulse" of an earthquake."
Fortunately, the researcher believes these supershear quakes should only occur along a "strike-slip" fault if it happens to have long, straight sections "that do not deviate by 5 degrees or more over a distance of 100 kilometres", dubbed "superhighways".
Links:
http://www.newscientist.com/article/mg20327191.400-seismic-boom-breaking-the-quake-barrier.html?full=true
http://blogs.laweekly.com/ladaily/city-news/superfast-earthquake-could-hit/
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