earthquake_satellites_000316 TOULOUSE, France - European scientists are gaining a better understanding via Earth-observing satellites of last year's devastating earthquake in Turkey, which killed more than 20,000 people.
"Satellite images are essential for the study of earth tremors of such magnitude because they enable the region to be observed at a larger scale," said Bertrand Meyer, a researcher on the in Paris-based Institut de Physique du Globe de Paris (IPGP) team.
Such big pictures can't save lives or bring back Turkey's quake victims, but they are essential to understanding the physics of this dangerous region and evaluating its seismic risk, he said. For instance, what caused the ocean to submerge an entire section of the coastal town of Golcuk within seconds of the quake?
Understanding answers to that and other questions could help engineers design and build homes and offices that are less deadly during temblors.
An earthquake evaluation tool: Satellites
Some seismologists consider satellite imagery the best tool for understanding Earth's surface physics since the invention of the seismograph in 1855. The breakthrough satellites provide, though, still falls short of the goal of precisely anticipating earthquakes.
"The satellite is an essential and indispensable tool to evaluate the risk of earthquakes, but there are no methods of forecasting them," Meyer said.
To evaluate the seismic risk in a given region and understand how strain builds up requires lengthy teamwork, in which different space technologies are combined with other approaches -- like geology and seismography -- to investigate very complex surface and underground mechanisms.
The day the ground trembled
On August 17, 1999 a massive, magnitude-7.3 earthquake hit northwest Turkey. This was the most powerful tremor experienced in Europe in the last 50 years, nearly as powerful as the magnitude-7.9 San Francisco quake that killed 700 people in 1906 (estimates vary on this death toll).
A second temblor, on November 12, 1999 further east in the same region, victimized thousands more people.
Rapid response
Within hours of the quake, rescuers frantically dug through debris. Firefighters battled a raging fire at Turkey's largest oil refinery at Izmit.
Three days later, Meyer, Jean-Bernard de Chabalier and Rolando Armijo, who heads the IPGP department, went to Golcük, the town most seriously hit. Golcük is not far from the epicenter of the quake. The researchers were confronted by an unexpected phenomenon: A large neighborhood of this coastal town was suddenly and irreversibly
submerged by the Sea of Marmara.Taking advantage of the French space agency's Earth-imaging satellites (Spot), they inspected overhead views that could see objects as small as 30 feet (10 meters) across, allowing them to compare images of Izmit, Golcük and the Istanbul region before and after the quake.
Among the data collected by Spot was a miniature pull-apart of plates that explained how within 10s of seconds of the quake in Golcük a whole section of the town suddenly dropped a couple of meters. The sea flowed into the streets over 820 feet (250 meters) in length during this catastrophe.
Their work mapping the seismic cracks and ruptures also was helped initially by U.S. satellite LandSat's 93-foot (28.5-meter) resolution photos. For the researchers, a two-step strategy of comparing satellite and ground data consisted of:
- Using satellite photos as space-maps to locate ground observations like earthquake-produced disruptions, cracks, disrupted roads and walls
- Correlating satellite views before and after the quake to underline disruptions, and then integrating these new seismic events in a larger picture, in terms both of time and geographical scale
"From space, we can see how [earth] ground fractures are building up over geological periods and how they evolve over hundreds of kilometers," explains Meyer.
Another approach involves the use of the Global Positioning Satellite (GPS) system.Satellite images and field surveys made it possible to document the history of the fault over millions of years -- a period of time much longer than its seismic cycle time (the period of time that elapses between successive earthquakes). In Turkey, seismic cycle time is naturally about 250 years on the same fault, Meyer said. As a result, it is believed that over the last 5 million years,
two plates -- the Eurasian and Anatolian-Aegean -- slid together and overlapped each other a distance of 52 miles (85 kilometers). Only satellites can provide this result. In analyzing smaller quakes, satellite images help in accurately mapping the cracks caused by temblors. The Spot satellites, however, are limited to a 33-foot (10-meter) resolution.
Located at an altitude of 511 miles (822 kilometers), Spot 4, Spot 2 and Spot 1 provide black-and-white images over a 23-square-mile (60-square-kilometer) footprint with a resolution of 33 feet (10 meters). Combined they can be programmed to photograph the same area every day.
Dream of prediction with radar satellites
Currently one of the most interesting technologies for earthquake studies could lie with radar satellites, such as ERS 1 and ERS 2 (European Remote Sensing satellites). Unlike Spot satellites, they can penetrate cloud cover. And their precision may one day give seismologists the means to tell that a major quake is imminent.
"There is a small hope in the future to decipher, through radar interferometry, signs of the triggering mechanism of big earthquakes," Meyer said.
Working in darkness or with bad weather conditions, radar satellites are able to measure accurately the overall displacement of the ground after an earthquake over large areas with a 60-mile by 60-mile (100-square-kilometer) footprint. These radar pictures, which use electromagnetic beams, rather than visual light, are processed at the ESA-ESRIN facility at Frascati, Italy -- near Rome.
Their highly accurate interferometry technique, which measures the reflection and the phase of the beams on the ground, provides precision to within 1 inch (2.5 centimeters) of displacement over the shaken area.
So far, the technology has successfully picked up minute movements after an earthquake, as in the case of the Landers fault in California, where displacements were detected for two years after the 1992 quake. Now the focus is on refining the same approach to detect -- if they exist -- minute pre-quake signs.
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