New work reinforces megaquake’s harsh lessons in geoscience

Science : Analyses of Japan’s March earthquake have overturned long-held views of fault behavior, writes Richard Kerr for Science, and they indicate that another disaster may occur. Papers published this week by Mark Simons of Caltech and colleagues, Satoshi Ide of the University of Tokyo and colleagues, and Mariko Sato of the Japan Coast Guard in Tokyo and colleagues help show why the earthquake confounded expectations.The offshore fault north of Tokyo was expected to fail segment by segment in large, but not huge, earthquakes; historically this had been the case, with a quake of magnitude 7 or 8 happening as frequently as every few decades, or as seldom as every few hundred years. The 11 March quake saw the descending ocean plate and the overlying plate carrying Japan slip past each other by as much as 60 meters—at least twice as much as the maximum slip reported for the magnitude-8.8 earthquake in Maule, Chile, last year. It’s not just the amount but also the pattern of slip that’s surprising. Five contiguous segments of fault broke at once, with the extreme slip concentrated in just two of them, located far offshore. Because areas around the high-slip patch failed repeatedly in smaller quakes, while that patch remained quake-free, seismologists assumed that this meant that portion of the fault was slipping slowly and without building up strain; now it seems that patch of fault had been pinned in place. The offshore, shallow portion of the fault to the south, which is half as far from Tokyo as the epicenter of the last quake, remains unbroken. The most recent rupture transferred stress southward. Now researchers must determine whether that stress has merely accelerated slippage at the southern portion of the fault, or if that portion of fault is locked in place with no way to dissipate accumulating strain. If that is the case, another large earthquake much closer to Tokyo than the last will occur.