MENLO PARK, Calif., Sept. 4 (UPI) -- Taking advantage of the largest swarm of earthquakes ever recorded, geoscientists have uncovered mechanisms that may help them better predict big shakers and project the damage they could inflict.
The international team studying thousands of underwater temblors that ripped through an island-strewn strip of the Pacific Ocean 75 miles south of Tokyo two summers ago honed in on forces that can make the earth move, sometimes with calamitous consequences.
Understanding the grinding stresses that can crack Earth's crust like an eggshell and leave the ground trembling for miles is key to minimizing the element of surprise that usually accompanies such disasters, hazards experts told United Press International.
"The Earth's crust can deform catastrophically in earthquakes, but it's difficult to predict exactly what causes such failure," said geophysicist Chris Marone of Penn State University in University Park, Pa., who reviewed the findings. "Analyzing thousands of small shocks might help us better understand how earthquakes occur."
Taking a novel approach to the task, Ross Stein of the U.S. Geological Survey in Menlo Park, Calif., Shinji Toda of the Active Fault Research Center in Tsukuba, Japan, and Takeshi Sagiya of the Geographical Survey Institute, also in Tsukuba, studied the connection between seismicity and stresses in a "super-swarm" of more than 7,000 shocks that rocked a region in Japan's Izu islands volcanic chain between June and August of 2000.
They found that, as predicted by a theory they were testing, the greater the stress rate, the more frequent the shakers.
"We are always trolling for opportunities to test theories of what causes earthquakes, so the massive Izu 2000 swarm seemed like manna from heaven," Stein recalled.
"We believe that it helps to explain why swarms produce earthquakes so far away -- 25 to 30 miles -- from the source of the volcanic intrusion," he told UPI. "We believe it can help us forecast the rate and distribution of damaging swarm earthquakes. And more generally, we think it helps to explain why we sometimes have swarms and other times main shock-aftershock sequences."
Most earthquakes consist of a main jolt followed by smaller tremors, or aftershocks, that wane with time. But some occur in swarms, a sustained succession of rapid-fire shocks that eventually screech to a stop.
Swarms are common in volcanic areas -- such as Japan, Hawaii, the Pacific Northwest, Alaska, Yellowstone and parts of northern and southern California -- where sudden, searing rivers of molten rock can carve paths of destruction through Earth's crust. Less often, they also can occur on tectonic faults, such as the notorious, 30-million-year-old San Andreas rupture that suddenly slipped on one side and swelled on the other by up to 21 feet (6.4 meters) on April 18, 1906, setting off the great earthquake and fire that crumbled the city of San Francisco.
"We offer a new explanation for the occurrence of swarms, attributing them to a sustained increase in the rate at which the crust is stressed," Stein, Toda and Sagiya said. "We attribute main shock-aftershock sequences to a sudden but permanent increase in stress, rather than a change in stressing rate."
Such a stress-seismicity correlation may provide a formula for forecasting swarm or aftershock damage, the scientists said in the British journal Nature. Because large earthquakes appear to be seismically identical to small shocks, previews and damage predictions will remain limited in scope, at least for the moment, they cautioned.
Nonetheless, Marone said, the method presents "an opportunity to test earthquake theories that could lead the way to fundamental breakthroughs."
The team tackled one of the biggest challenges in earthquake analysis: working out what conditions trigger the crust warping that sends the ground shivering. Rarely do scientists in the field know the initial stresses around a fault, or fracture in Earth's crust that can move many feet in a matter of seconds to produce an earthquake.
"In our view, swarms result from a sustained increase in the rate the crust is being stressed, and main shock-aftershock sequences result from a sudden jump in stress rather than a change in stressing rate," Stein told UPI.
"The most common way the stressing rate changes is when magma (molten rock) is either forced into a magma chamber or into cracks deep in the crust, causing the crust to deform or warp. But a tectonic fault such as the San Andreas can experience a sustained stressing rate increase if the fault creeps (slips steadily rather than only in jumps), which sometimes happens."
By comparing seismic activity before and after the magma infusion, the investigators found rates jumped by a factor of nearly 1,000 in the area of highest stressing, with some spots suffering a daily equivalent of 1,000 earthquakes of magnitude 3 or larger.
"What was most surprising to us was that the largest shocks in the swarm themselves produce aftershocks, but during swarms the duration of the aftershock sequences was drastically shortened -- from a year normally, to a day," Stein said.
The findings support a scenario painted by James Dieterich, former USGS chief scientist for earthquake hazards. His theory predicted the rate of earthquakes rises in direct proportion to the rate at which the crust is stressed and that the duration of aftershocks of magnitude-6 main shocks during a swarm would shorten as the rate of stress on the crust increased.
"Probably (the findings) can be interpreted in terms of increasing stressing rate," Sagiya told UPI. "Non-swarm earthquakes occur as a result of stress accumulation continuing for more than 100 years. The stressing rates for those earthquakes thus are very small. On the other hand, in volcanic areas, activity of underground magma easily can cause a very large stressing rate."
Earthquakes swarm in volcanic areas because of the movement of molten rock through the crust.
"The magma may move into cracks that have previously opened, or it may inject forcibly akin to the process of hydrofracturing in oil fields," Don Swanson, USGS scientist-in-charge at the Hawaiian Volcano Observatory in Hawaii National Park, told UPI. "It appears that the Nature paper deals with a forcible intrusion."
The infusion of melted rock set off a staccato-like burst of 7,000 shocks with a reading of 3 or larger on the Richter scale, a measure of the magnitude of seismic wave from an earthquake, the scientists found. Of the total, 45 registered magnitude 5 and six had a 6 Richter reading. A temblor with magnitude greater than 4.5 can cause damage to buildings and other structures. Severe earthquakes measure greater than 7 -- the 1906 shaker that leveled much of San Francisco registered 7.8.
From the results, the team inferred the swarm off the Japanese mainland was caused by a blade-like injection of molten rock into Earth's crust over an area some 10 miles long and 10 miles deep.
"Fortunately, the magma did not reach the Earth's surface, but the blade (or 'dike') was forced open by the magma pressure a total of about 65 feet," Stein said.
The swarm was the largest ever observed and one of the best ever recorded, scientists noted.
"We are unaware of any swarm anywhere in the world that can match Izu 2000's energy release, let alone its rate of energy release," Stein said. "It appears to be unprecedented."
The extraordinary event was meticulously tracked by Global Positioning System sensors the Japanese had placed on the islands and by a network of seismometers strategically scattered over the land and on the ocean floor. In addition, the investigators used a record of earthquake-induced seismic waves dating back to 1980 to measure changes in the rate of earthquake occurrence.
"This was a great experiment performed by nature. Fortunately, our development in measurement of crustal deformation by GPS permitted us to observe the event, and gave us a good opportunity of testing Dietrich's theory," Motoo Ukawa, director of volcanic eruption prediction research at the National Research Institute for Earth Science and Disaster Prevention in Tsukuba, told UPI.
"If validated (the theory) can be used to forecast seismicity and earthquake hazards in all settings and countries," Toda said.
Only a handful of U.S. sites boast monitoring capabilities equal to those of the Japanese islands, but a proposal now before Congress seeks to place 1,000 GPS receivers across the western United States.
"The practical prediction of large/great earthquakes will remain a difficult problem in this century," Ukawa concluded. "But we will be able to make a great advance in earthquake prediction on the basis of theories like Dietrich's and the development of measurement (systems like) GPS ... and other instruments."
