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Lidar uncovers hidden earthquake damage

Photonics Spectra
Apr 2012
Ashley N. Paddock, ashley.paddock@photonics.com

MEXICALI, Mexico, and DAVIS, Calif. – A new tool reveals how earthquake faults behave and how, down to a few inches, earthquakes change the landscape.

A team of geologists from the US, Mexico and China has reported the most comprehensive before-and-after picture yet taken of an earthquake zone, using data from the 7.2-magnitude quake that struck near Mexicali in April 2010.

“We can learn so much about how earthquakes work by studying fresh fault ruptures,” said Michael E. Oskin, a geology professor at the University of California, Davis.

The team, working with the National Center for Airborne Laser Mapping, flew over the area, using new lidar equipment to measure surface features to within a few inches. In less than three days, they made a detailed scan over approximately 140 sq mi, Oskin said.

They knew that the Mexican government had mapped the area with lidar in 2006, he added. When the earthquake occurred, he and Ramón Arrowsmith from Arizona State University applied for and got funding from the National Science Foundation to carry out an immediate aerial survey to compare the results. Traditional ground surveys of the fault rupture also were conducted by John Fletcher and graduate student Orlando Teran from the Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE). These helped guide planning of the aerial lidar survey and interpretation of the results.


Agricultural fields and canals of the Colorado River delta after the earthquake.


From the ground, features such as the 5-ft escarpment created when part of a hillside abruptly moved up and sideways are readily visible. But the lidar survey further reveals warping of the ground surface adjacent to faults that previously could not easily be detected, Oskin said. For example, it revealed the folding above the Indiviso fault running beneath agricultural fields in the floodplain of the Colorado River.

“This would be very hard to see in the field,” Oskin said.

The geologists used the virtual reality facility at UC Davis’ W.M. Keck Center for Active Visualization in Earth Sciences to handle and view the data from the survey. By comparing pre- and postearth-quake surveys, they could see exactly where the ground moved and by how much.

The survey revealed deformation around the system of small faults that caused the earthquake and supplied measurements that provide clues to understanding how these multifault earthquakes occur.

The 2010 Mexicali earthquake did not occur on a major fault – the San Andreas, for example – but ran through a series of smaller faults in the Earth’s crust. These minor faults are common around major faults but are “underappreciated,” Oskin said.


Mapped fault surface ruptures (black lines) mark breaks in the Earth’s crust. Courtesy of Michael Oskin et al, www.keckcaves.org.


“This sort of earthquake happens out of the blue,” he added.

The new lidar survey shows how seven of these small faults came together to cause a major earthquake, Oskin said.

This is the first earthquake to be imaged by “before” and “after” lidar, said Ken Hudnut, a geophysicist with the US Geographical Survey and co-author of the paper. He made the first use of airborne lidar about 10 years ago to document surface faulting from the Hector Mine earthquake.

The findings appear in Science (doi: 10.1126/science.1213778). The postevent data set collected by the team is publicly available at: http://opentopography.org/.


GLOSSARY
lidar
An acronym of light detection and ranging, describing systems that use a light beam in place of conventional microwave beams for atmospheric monitoring, tracking and detection functions. Ladar, an acronym of laser detection and ranging, uses laser light for detection of speed, altitude, direction and range; it is often called laser radar.
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