A “back-propagating rupture” sounds like something that will require a trip to the emergency room and some serious physical therapy, but it may be worse than that … far worse, especially If you live in an earthquake fault zone. These so-called “boomerang” earthquakes have long been theorized but never confirmed. That changed recently when seismologists identified a strange equatorial earthquake in 2016 as the first “back-propagating supershear rupture” ever. Are you worried yet? When you find out what “supershear” means in an earthquake, you will be.
“We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the transform fault intersects the mid-ocean ridge, and then (2) an unusual back-propagation westwards at a supershear speed towards the centre of the fault.”
A research team led by scientists from the University of Southampton and Imperial College London was studying a strange and powerful earthquake which occurred in 2016 in the Romanche Trench — the third-deepest trench in the Atlantic, crossing the ocean from Brazil through the equator to West Africa. The trench (7,761 m (25,463 ft) deep, 300 km (190 mi) long) was formed by the Romanche Fracture Zone, which was responsible for the strange 2016 quake. These underwater fault lines are far from seismic sensors, making them difficult to study. However, sensors had just been installed for the Romanche fault and caught the 2016 monster, measured at a magnitude 7.1 and traveling in one direction before mysteriously making a boomerang turn and heading back at supershear speed (11,000 mph!) that the scientists referred to as “breaking the seismic sound barrier” (gif of the seismic pattern here).
“Either both fault patches were sufficiently preseismically stressed to promote seismogenic failure or the deeper SE1 rupture instantaneously increased the static stress, immediately causing the shallow SE2 portion of the fault to fail.”
What turned this quake around and increased its speed? The researchers speculate that the first half was a very deep and powerful quake with a lot of “fracture energy” that not only opened a fracture but opened it with enough force that it deflected it into a shallower part of the trench that was heading in the opposite direction. With less resistance at this shallow depth, the rebounding quake gained tremendous speed. The end result was never-seen-before boomerang quake. Can something like this happen on the surface? Like around Yellowstone?
“This might be actually more common than we think.”
That’s a qualified and speculative ‘maybe’ from geophysicist Yoshihiro Kaneko of GNS Science in New Zealand, who was not part of the study (published in the journal Nature Geoscience) but was interviewed by National Geographic. He says the Romanche boomerang was similar to the magnitude 9.0 Tohoku earthquake in 2011 and the powerful series that shook Kumamoto in 2016.
Are you worried yet?