The Kobe earthquake forced both the Japanese government and researchers to take stock. In the past few years, Japanese scientists have been trying to find out why they failed in predicting Kobe and other earthquakes that have hit Japan over the years. Now that effort is paying off. In a recent issue of the journal Science, Jin-Oh Park and his colleagues at the Japan Marine Science and Technology Center in Yokosuka, near Tokyo, uncovered a major cause of the most unpredictable earthquakes. It was a “splay” fault, a secondary fault that branches off a main one and can cause damage hundreds of kilometers from obvious trouble spots. This was the first time scientists confirmed the existence of a splay fault. By shedding light on the vexing phenomenon, experts think they may be able eventually to predict many types of earthquakes that have tricked geologists in the past. “This brings us a step closer to knowing about earthquake mechanism, and could lead to a better chance of prediction,” says Shozo Harada, a Diet member, who has helped draft anti-quake laws.
Even before Kobe, Japanese scientists had come to suspect that predicting earthquakes was not as simple as once thought. They were expecting the next big temblor to arrive at the most obvious place–in the populous Tokai region, west of Tokyo, rather than Kobe, which is even farther to the west. The mistake was understandable, given that the Tokai region lies near the fault produced by two colliding tectonic plates–the Philippines Sea Plate and the Eurasian Plate, which was (and still is) long overdue for an earthquake. In the 1970s, a University of Tokyo researcher warned that a massive earthquake would hit Japan’s Tokai region “any day.” At the time the warning so spooked Japan’s politicians that Tokai almost monopolized Japan’s earthquake prediction and disaster prevention efforts for the next 25 years. “While [the government] focused on Tokai, all the recent major quakes, including the Nihonkai-Chubu earthquake (which killed more than 100 in 1983) and Kobe, which was a surprise to everybody, occurred elsewhere,” observes Masataka Ando, a prominent seismologist at Nagoya University.
The Kobe disaster gave scientists a good reason to check out those neglected areas. Two winters ago, Korean-born Park was poring over seismic data that had been gathered during a trip in the Pacific Ocean south of the Kii Penninsula, between Nagoya and Osaka in western Japan, when he saw “something suspicious”–a splay fault, perhaps? To be sure, Park needed data from a wider area. His team went back out to sea and strung out an array of floating air guns, which each sent a different low-frequency sound wave (like a mini-earthquake) toward the ocean floor. Microphones picked up the reflections of the sound waves from different layers of sediment beneath the ocean floor. When the team crunched the data and displayed it on a computer screen, they saw a splay fault that runs for 100 kilometers off the Kii Peninsula.
Splay faults are cracks in the earth’s crust that occur when one plate moves over another–in the case of Japan, the Philippines Sea Plate collides with the Eurasian Plate, pushing it up. “Tear” faults occur at right angles to the edge of the upper plate, carving it into sections. The splay faults form when the sections crack again (this time like a piece of pie that’s picked up from one end). Not only are splay faults unpredictable, but they also give rise to a particular kind of earthquake that is prone to causing tsunamis.
The location of the newly discovered fault suggests that it is the probable cause of the magnitude 8.1 Tonankai earthquake of 1944, which sent a tsunami washing ashore and claimed over 1,200 lives. Park warns that the fault is still active and poses a serious quake and tsunami threat to the coastal cities of western Japan. His team has since gone on to discover a whole network of splay faults that branch out from the main Eurasian-Philippines Sea fault, which runs along the Nankai Trough, an underwater valley. These splay faults have probably been causing the big earthquakes that hit the Kii Peninsula every hundred years or so.
Scientists now think that the Nankai Trough poses a 40 to 50 percent probability of generating a devastating temblor within the next 30 years, and they’ve identified two of the most likely Big Ones: the Tonankai earthquake, which would hit the coastline near Nagoya, Japan’s fourth largest city; and the Nankai earthquake, which would strike near Osaka, the second largest city. In July, Tokyo budgeted $11 million to study the faults and the resulting earthquakes and tsunamis they could cause.
Park and his colleagues are part of an international program that is studying earthquakes by drilling the ocean floor. The agenda includes plans to recover and analyze samples of fault rocks from the Nankai Trough. These samples may uncover traces of earthquakes that occurred centuries ago–data that might prove invaluable in figuring out earthquake cycles and making better predictions. “In earthquake studies, centuries-old data helps the latest research,” says Nagoya University’s Ando. In addition, they’ll plant instruments in the seabed that will monitor the ocean floor, which may tip them off to future quakes. The large-scale project will take a decade to complete, but perhaps this time scientists will finally take the true measure of this dangerous phenomenon.
