Rule-book rewrite for megaquakes

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Rule-book rewrite for megaquakes From firework fault-lines to super slippage, Japan’s megaquake is blowing away our old ideas Ferris Jabr

THE Tohoku earthquake that rocked Japan last month has sent geologists reeling. As the first analyses of what may well become the best studied earthquake in history start to filter through, there is already talk of rewriting the rule book on how “megathrust” quakes happen. And all countries around the world that sit on subduction zones may now need to reconsider whether they are at risk of a similar devastating event. At the annual meeting of the 6 | NewScientist | 23 April 2011

Seismological Society of America in Memphis, Tennessee, on 14 April, geologists from around the world presented early analyses of the magnitude-9 megathrust earthquake. “Few earthquakes of this size have been subjected to this kind of intense postmortem,” says Emile Okal of Northwestern University in Evanston, Illinois. “The Japanese have an incredibly dense network of GPS and seismic coverage.” The findings, which describe the event in unprecedented detail, have convinced some that they

must throw away standard as the energy released by the theoretical frameworks, which quake rippled around the world. simply cannot explain how They found the quake’s rupture the Earth flinched and heaved behaviour to be far more complex beneath the ocean floor near than any other. Japan. The event, they say, Typically a subduction demands that we change not only earthquake – in which one tectonic our scientific understanding of plate pushes beneath another – large subduction quakes, known rips in one or two directions along as “megaquakes”, but also our a fault: on a north-south fault line, assessment of the regions around say, a rupture heads north, south, the world at risk of such events. or north and south at the same “There are many things we time. But Kiser and his colleagues thought we knew and it’s now found that the Tohoku quake painfully clear we just don’t,” “Few earthquakes of says Okal. Barbara Romanowicz, this size have ever been director of the Berkeley subjected to this kind of Seismological Laboratory at the intense post-mortem” University of California, agrees: “A lot of ideas will be shattered because of this quake.” ripped left, right and centre along Eric Kiser of Harvard University the fault like the starbursts of a and his colleagues studied fireworks display (see top diagram). measurements by the US “When we imaged the main Transportable Array – a fleet shock, the propagation of energy of 400 high-end seismographs was all over the place,” says Kiser. dotted around North America – “We believe this is the most

In this section n Genes responsible for premature birth, page 8 n “Gravity lines” trace warped space-time, page 14 n A supercode for spies, page 22

Bursting at the seam During a quake, ruptures typically travel linearly along the fault, but the rupture pattern of the Japan megaquake looks like the multiple starbursts of a firework

THREE MAIN RUPTURES RELEASED THE LARGEST AMOUNTS OF ENERGY IN THE FIRST 3.5 MINUTES

NORTH AMERICAN PLATE

EPICENTRE

Sendai

PACIFIC PLATE

Shallow fault area

TOKYO

200 km

Deep fault area

SOURCE: KISER ET AL

SMALLER RUPTURES

In cross-section, other data show these ruptures fall into two general areas The shallower area experienced the greatest slip and triggered the tsunami

The deeper area generated the highest-frequency seismic waves, causing the shaking felt on land

The North American plate is forced upwards, causing a tsunami

NORTH AMERICAN PLATE

PACIFIC OCEAN SHALLOW FAULT PACIFIC PLATE

DEEP FAULT Not to scale

a megathrust quake. Traditionally, young, hot, swiftly subducting plates are considered far more likely to produce megathrust earthquakes than their older, cooler, denser and more sluggish counterparts. The ocean crust off the north-east coast of Japan is about 140 million years old. It’s hardly the Usain Bolt of tectonic plates, yet it generated Japan’s largest recorded quake. More remarkable still – the event may not be an exception, but could define a new rule. The subducting India plate that caused the 2004 Indian Ocean earthquake and tsunami is 80 to 90 million years old and is not particularly swift, says Okal. Yet it generated the third-largest earthquake ever recorded – a magnitude-9.2 event. “The

SOURCE: KANAMORI

any other factor, he thinks, this aggregate rupture accounts for the quake’s phenomenal size. He points out that the entire rupture zone within which the starburst rips took place was about 40,000 square kilometres, far smaller than what might typically produce a magnitude-9 quake. That area becomes much larger – roughly 100,000 square kilometres – if you include the region covered by the hundreds of aftershocks that hit in the weeks following 11 March. Had this entire area ruptured on 11 March, the quake would have easily exceeded magnitude 9, Kiser says. Back in Memphis, Hiroo Kanamori of the California Institute of Technology in Pasadena says that the relatively small zone that ruptured on 11 March can be split into two areas. His analysis has highlighted one rupture zone along the Pacific Ocean’s Japan trench, which he believes was largely responsible for the tsunami, and another rupture deeper along the fault –Effect clear, cause clarifying– line and closer to the coast that caused most of the shaking. “We have never seen anything like complex rupture behaviour ever this,” he says (see lower diagram). observed.” The team reckons the Takeshi Sagiya of Nagoya pattern may partially explain University in Japan and Guangfu why the quake was so ferocious. Shao of the University of California Some fault zones are highly at Santa Barbara presented heterogeneous, with patches where the rock catches like Velcro analyses that point to another factor which contributed to the and others where it slips as vast amounts of energy released though oiled, says Matt Pritchard by the event. Their latest estimates of Cornell University in Ithaca, of how far the tectonic plates slid New York. It is possible that past one another suggest that at its the Japanese fault consists of a maximum the slip was 60 metres – dangerous mixture of Velcro and a figure so big that every researcher “oily” patches. The Velcro keeps New Scientist contacted asked in the plates glued together and astonishment for the figure to be absorbs the strain of subduction, repeated. Such a massive shift is but when they gave way in unprecedented in the recorded March, oily patches allowed the history of earthquakes. plates to slip all over the place – Kanamori says it’s possible that accounting for the complex a structure on the sea floor, like rupture and boosting the quake’s a seamount, locked the plates energy release. Kiser’s results have revealed that together allowing stress to build up for thousands of years before on 11 March, the bursts of energy it was released in one huge burst. ripped four separate patches that The huge slip happened on a have all individually generated fault that was not a candidate for quakes in the past. More than

standard models say give me the age of the subducting plates and the rate of subduction and I will tell you the maximum magnitude of an earthquake in that area. It says an old plate moving slowly can’t produce much beyond a magnitude 7,” says Okal. “This is now a model we essentially have to abandon.” “What we have to realise now,” says Pritchard, “is that pretty much any subduction zone is a candidate for a magnitude-9 quake.” Romanowicz agrees: “As prepared as Japan was for earthquakes, it did not expect such a large quake in that particular place. This is the lesson to learn. It’s not just about Japan: many other places in the world could generate giant earthquakes in ways that people just aren’t paying attention to.” n 23 April 2011 | NewScientist | 7