DEVASTATING JOLT IN JAPAN : Danger Zone Lies Beneath Japan : Geology: Quake experts say the convergence of four enormous tectonic plates makes area the world’s ‘messiest.’

Japan’s worst earthquake in almost 50 years came near the crush zone of four continent-sized pieces of the Earth’s surface, where the underlying faults are even more complicated and potentially hazardous than in Southern California.

Scientists knowledgeable about plate tectonics call the area the “messiest” place in the world. Powerful temblors are common as the plates are ripped apart and heated, with shocks normally striking much deeper beneath the surface than they do along the West Coast.

Japan and California may be separated by thousands of miles of ocean, but scientists note that citizens of both stand on essentially the same ground.

The islands of Japan are located on what geologists call the Eurasian Plate, which slides over both the western edges of the Philippine Sea Plate and the Pacific Plate. From the north, the edge of the North American Plate slams into the Eurasian Plate.

Los Angeles is located at the eastern edge of the Pacific Plate, and San Francisco is on the western edge of the North American Plate.

While California and Japan may share the ultimate source of their seismic uncertainty, their respective earthquake experts have chosen markedly different approaches in efforts to mitigate the potential for quake damage.

“One of the things we see going on in Japan is a very significant investment in new technology, one that we have not been able to make in this country,” said William L. Ellsworth, a seismologist at the U.S. Geological Survey who is evaluating earthquake hazards in Japan.

But despite Japan’s enormous investment in high-tech satellite sensors, a national early warning network of seismic monitors and state-of-the-art digital communications links, most earthquake experts were apparently caught off guard when Tuesday’s quake occurred. The networks are not yet extensive enough, some experts said.

In an effort to ease the titanic stresses building along the plates, the Earth fractured brutally along a shallow fault 13 miles below the surface and about 20 miles offshore near a small island in Japan’s Inland Sea.

The earthquake was far more powerful than last year’s 6.7-magnitude Northridge quake, but unlike what occurred in that one, the Earth’s energy was expended in mostly horizontal motion, which minimized the potential for tidal waves. Some quake experts suspect the fault plane may extend beneath Kobe, where the heaviest damage and casualties were concentrated.

The spectacle of entire buildings lying on their sides and of highways tumbled from their foundations in Kobe has again proven that engineers worldwide have not solved the problem of how to prevent devastation from a quake, experts say.

When the Northridge earthquake downed portions of five major transportation arteries, for example, Japanese engineers predicted that their roadways could withstand such abuse. But Tuesday’s powerful Kobe quake toppled buildings as well as elevated highways, knocking over the Hinshin Expressway--the major link between Osaka and Kobe--as though it were a Tinker Toy.

The Japanese opted for stiffer structures that can be twice as strong--and more expensive--than those in the United States. Their expressways require 50% more steel and sit on squat, wide columns. American experts design for flexibility, intending bridges to sway, not to resist the powerful motion of an earthquake.

“The good construction in Japan at the present is more conservative than our construction--they design for about two to three times the strength we design for,” said Vitelmo Bertero, a UC Berkeley professor of structural engineering who worked in Japan for more than a year.

Based on preliminary reports and television footage, experts say that several factors contributed to the devastation in Kobe, a port city of 1.5 million. Much of the soil, built on many layers of sediment and deposits, is soft. In previous earthquakes, engineers have found that areas with wet, loosely compacted soil or landfill--such as the Marina district of San Francisco during the 1989 Loma Prieta quake--suffered tremendous damage.

The vibrations and motion caused by an earthquake shake the wet ground, causing soil or sand that normally is stable to become more like quicksand, slipping and settling in a process called liquefaction. In Japan, because land is scarce, highways are usually elevated and bridges are often built on soft or wet soil.

On Tuesday, television cameras relayed terrifying images of entire buildings resting on their sides--apparent victims of liquefaction, said Ronald Polivka, a senior consultant with EQE International, a San Francisco-based firm specializing in earthquake engineering.

In California, millions have been spent retrofitting older buildings and bridges. But in Kobe, where older buildings and structures appear to have suffered the worst damage, no such program is in operation. Many of the buildings are wooden structures or unreinforced masonry, without shear walls or the structural support offered by the stucco or plaster common in Southern California homes.

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Seismic Stress Points

Japan, located on the western edge of the Pacific “Rim of Fire,” is at the center of one of the world’s most treacherous geological formations. Four mammoth tectonic plates collide under the islands of Japan, generating more than 1,000 small quakes every month and frequent powerful temblors. Tuesday’s magnitude 7.2 earthquake occurred along a shallow strike-slip fault 13 miles below the surface and about 20 miles offshore near Awaji Island in Japan’s Inland Sea. The quake was preceded by a series of smaller temblors in the 10 hours before the main shock, but there is no indication so far that these were actually foreshocks, scientists said.

While Japan has experienced numerous previous earthquakes, until now the region around Kobe has been relatively quiet. Above, previous earthquakes since 1960 ranging from 4.0 to 7.4 are spotted in red.

Strike-slip fault

EURASIAN PLATE

NORTH AMERICAN PLATE

PACIFIC PLATE

PHILIPPINE SEA PLATE

Note: Los Angeles rests on the Pacific Plate nearly 5,300 miles away.

Source: Alan Jones, State University of New York at Binghamton