麻豆淫院

(麻豆淫院Org.com) -- Using satellite radar data, NASA-funded scientists, including UC Riverside鈥檚 Gareth Funning, have observed for the first time the healing of subtle, natural surface scars from an earthquake in Iran that occurred on a 鈥渂uried鈥� fault extending several miles below the surface鈥攁 fault whose fractures are not easily observed at Earth鈥檚 surface.

Reporting March 5 in Nature, the scientists, led by geophysicist Eric Fielding of NASA鈥檚 Jet Propulsion Laboratory (JPL) in Pasadena, Calif., describe how so-called 鈥渂uried鈥� faults are not so hidden after all.

Using as a case study the magnitude 6.6 earthquake that killed more than 30,000 people and devastated Bam, Iran, in 2003, the scientists analyzed radar images from the European Space Agency鈥檚 Envisat satellite to study the land surface above a fault buried about half a mile under Earth鈥檚 surface. They discovered a shallow, narrow surface depression that formed and evolved after the quake.

鈥淯sing data that cover the first three and a half years after the earthquake, we observed subsidence and lateral contraction of the Earth鈥檚 crust surrounding the fault on which the earthquake occurred,鈥� said Funning, an assistant professor in the Department of Earth Sciences. 鈥淎fter testing many models, we concluded that this can only be a result of volume reduction in the crust - the closing of cracks that were opened up during the earthquake. In other words, we observed the crust healing after the earthquake. This has been inferred before using seismological methods, but never before seen from surface movements.鈥�

The researchers鈥� results have implications for assessing the risk of future earthquakes associated with known buried faults. Such faults can be found around the world but are often missed by geologists or assumed not to be active. A buried fault is thought to be responsible for the major 1994 Northridge earthquake in Southern California.

鈥淭he study of so-called 鈥榙amage zones鈥� surrounding faults is an important topic in Earth Sciences,鈥� said Funning, who joined UCR in 2007. 鈥淪uch damage, which is observed on many faults in California, particularly the San Andreas, can influence the behavior of earthquakes on those faults - where they initiate, the direction in which they break, and the strength of the shaking near the fault.鈥�

In the case of the Bam earthquake, previous seismic and satellite studies showed that the fault under Bam had slipped by about 2 to 3 meters at the time of the earthquake. But when scientists from Iran went out in the field after the earthquake, the cracks they found at the surface only showed 25 centimeters of slip or less. If indeed there had been 2 to 3 meters of slip at depth, the surface must have somehow absorbed that slip.

The researchers suspected the fault zone below could reveal itself in a slight deformation of Earth鈥檚 surface because the pressure and stress during an earthquake causes rocks in the fault zone to expand and become more porous. After the quake, the ground will 鈥渉eal鈥� over a period of years, settling and forming a depression.

To investigate the extent and rate of surface deformation after the 2003 earthquake, the researchers turned to the Advanced Synthetic Aperture Radar (ASAR) instrument on Envisat. (ASAR images are used to precisely measure elevation by bouncing a beam of microwave radiation off Earth鈥檚 surface and observing the reflection back to the satellite.) The researchers compared images from the 3.5 years following the Bam quake to see how the surface elevation changed, using a technique known as interferometric synthetic aperture radar (InSAR).

InSAR revealed a shallow, ditch-like depression on the surface - measuring between 200 to 400 meters wide and about 3 centimeters deep - directly above the ruptured fault, indicating that the deformation was associated with the earthquake.

The team also modeled the sinking throughout the fault zone, using a model that is normally used to study crustal compaction and expansion around volcanoes. By analyzing an array of points along the fault to estimate how compaction produced the features at the surface, the researchers concluded that the 2 to 3 meters of slip at depth was absorbed by a 鈥渄amage zone,鈥� close to Earth鈥檚 surface. This means that the earthquake slip was spread over a wide volume of rock in the surface layers instead of a single fault.

The study is helping the researchers anticipate the future behavior of the fault. Initially, they were concerned that if stress at depth was not relieved at the surface, then a subsequent earthquake could result. Because the rupture鈥檚 stress was absorbed in the damage zone, the researchers believe the fault that shook Bam in 2003 is no longer a risk.

鈥淭here鈥檚 always the chance that a nearby, related fault could rupture, as eastern Iran is full of faults that are active at some scale,鈥� JPL鈥檚 Fielding said. 鈥淏ut this one beneath Bam is the type that ruptures every 2,000 years or longer, and the stress on it seems to have been relieved.鈥�

Provided by University of California, Riverside