Deep-plunge wave triggered first fault slip ever linked to such an event
Shortly after a powerful earthquake shook Japan, the entire country moved a few millimeters to the east. The cause, the researchers report on June 18 in Sciencewas a seismic wave that plunged toward the Earth’s core and returned, causing faults to slide — the first recorded case of a wave reflected from the core setting a fault in motion.
Earthquakes regularly move landmasses. But they don’t usually move entire countries, and seismic waves bouncing off the planet’s core aren’t the cause. Such S waves reflected from the core, as they are called, pass through the 2,900 kilometers of Earth’s rocky mantle to reach the edge of the core, then return.
Seismologist Sunyoung Park and his colleagues detected one such wave by extracting archival seismic and GPS data. Tohoku earthquake in Japan, March 11, 2011. This wave appeared about 15 minutes after the magnitude 9.0 mainshock and was accompanied by a coincident ground shift, recorded by hundreds of GPS sensors across Japan. “We see this permanent compensation,” says Park, of the University of Chicago.
Such ground shifting means the wave did more than just pass through, says Caltech seismologist Zachary Ross, who was not involved in the research. “This implies that there are a certain number of faults.”
And because the change occurred across the whole of Japan – from the island of Hokkaido in the north to the island of Kyushu in the south – much of the plate boundary must have opened up, Park and colleagues conclude. In fact, the team determined that two separate plate boundaries, totaling at least 3,000 kilometers in extent, likely broke apart.
That makes sense, says Andrea Donnellan, a geophysicist at Purdue University in West Lafayette, Indiana, who was not involved in the research. A seismic wave can trigger the release of a tectonic constraint which has been built over decades, centuries or even millenniashe said. “I think it’s very plausible.”
This is the first time that an S wave reflected from the core has been shown to trigger fault slip, Park says. “It’s a type of seismic risk that we hadn’t thought about before.” Such a long rupture length is also unprecedented: it is more than twice that of the massive 2004 Sumatra earthquake.
In the case of the Tohoku earthquake, the slip caused by the S wave reflected from the core was probably not noticeable. This is because its energy was distributed over a very large area and it occurred relatively slowly, over a period of about three minutes. But future events, researchers say, may not be so benign.
