Scientists Expose 'Buried' Fault That Caused Deadly 2003 Quake

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Jet Propulsion Laboratory, Pasadena, Calif.
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Iqbal Pittalwala 951-827-6050
University of California, Riverside
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Robert Sanders 510-643-6998
University of California, Berkeley
rsanders@berkeley.edu

NEWS RELEASE: 2009-039 March 4, 2009

Scientists Expose 'Buried' Fault That Caused Deadly 2003 Quake

PASADENA, Calif. – 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 Bürgmann 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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