Using satellite radar data, NASA-funded scientists have observed, for the first time, the healing of subtle, natural surface scars from an earthquake that occurred on a "buried" fault several miles below the surface-a fault whose fractures are not easily observed at Earth's surface.
Reporting in the March 5 issue of Nature, geophysicist Eric Fielding of NASA's Jet Propulsion Laboratory in Pasadena, Calif., describes how so-called "buried" faults are not so hidden after all. Using the magnitude 6.6 earthquake that devastated Bam, Iran, in 2003 as a case study, Fielding and his university colleagues analyzed radar images from the European Space Agency's Envisat satellite to study the land surface above a fault that is buried about 1 kilometer (half a mile) under Earth's surface. They discovered a shallow, narrow surface depression that formed and evolved after the quake, which killed more than 30,000 people.
The results have implications for assessing the risk of future earthquakes associated with known buried faults, which can be found around the world but are often missed by geologists or assumed not to be active. Buried faults are thought to be responsible for the major 1992 Landers and 1999 Hector Mine earthquakes in Southern California.
Previous seismic and satellite studies showed that the fault under Bam had slipped by about 2 to 3 meters (6.6 to 9.8 feet) 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 (9.8 inches) 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.
Fielding and colleagues suspected the fault zone below could reveal itself in a slight deformation of Earth's 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 "heal" 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 instrument on Envisat. Researchers use images from that instrument to precisely measure elevation by bouncing a beam of microwave radiation off Earth's surface and observing the reflection back to the satellite. Fielding and colleagues then 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, or InSAR.
"The advantage of InSAR is that you get a map of the pattern," said Fielding, "whereas a single surveying station on the ground would just reveal that something funny was going on at one place."
Indeed, InSAR revealed a shallow, ditch-like depression on the surface -- measuring between 200 to 400 meters (219 to 437 yards) wide and about 3 centimeters (1.2 inches) deep -- directly above the ruptured fault. "Using InSAR, we know that the deformation and the earthquake are associated," he said. “The depression deepened for at least 3.5 years after 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 "damage zone," close to Earth's surface. This means that the earthquake slip was spread over a wide volume of rock in the surface layers instead of a single fault.
"There's a big, crushed-up mass of the rock that absorbs this slip that occurred at depth, and it is only visible at the surface as a subtle deformation after the earthquake," Fielding said.
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's stress was absorbed in the damage zone, the researchers believe the fault that shook Bam in 2003 is no longer a risk.
"There's always the chance that a nearby, related fault could rupture, as eastern Iran is full of faults that are active at some scale," Fielding said. "But 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."
Other researchers on the study include Paul Lundgren of JPL; Roland Bόrgmann of the University of California, Berkeley; and Gareth Funning of the University of California, Riverside.
NASA is studying designs for a future Earth observation mission called Deformation, Ecosystem Structure and Dynamics of Ice. A key objective of the mission would be to enable InSAR measurements of deformation on fault zones around the world to better understand the processes that cause earthquakes.
JPL is managed for NASA by the California Institute of Technology in Pasadena.
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