Data from NASA's airborne Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) have shown that some faults in the area west of Calexico, Calif., have continued to move at Earth's surface, most likely in the many aftershocks that have rumbled after the initial April 4 quake.
This fault motion is likely to be what is known as "triggered slip," caused by changes in stress in Earth's crust from the main quake rupture.
The new maps created from the UAVSAR data, called interferograms, were created by combining data from flights on April 13, 2010, and July 1, 2010.
The image above shows an interferogram swath measuring 54 by 12.5 miles (87 by 20 kilometers) overlaid on a Google Earth image. Each colored contour, or fringe, of the interferogram represents 4.7 inches (11.9 centimeters) of surface displacement. The different shades in the image represent ground surface motions of up to a few inches upward or downward.
Yellow shaded regions moved to the south or downward; regions in blue moved to the north or upward; and regions shaded in magenta showed no motion. Major fault lines are marked in red, and recent aftershocks are denoted by yellow, orange and red dots, with older earthquakes shown as gray dots.
Many aftershocks have been located west of Calexico. In this area, there are many small "cuts," or discontinuities, in the interferogram color. These are caused by ground motions on small faults that have occurred since April 13, ranging from half an inch to a few inches (less than a centimeter to a few centimeters).
Andrea Donnellan, based at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and principal investigator of the UAVSAR project to map and assess seismic hazards in Southern California, said the latest flight provides valuable new data that researchers can use to monitor the continued readjustment of Earth's crust since April's major quake.
"The region was reflown with UAVSAR to monitor continued activity, including quiet motions — which are movements of faults that do not result in earthquakes — as Earth's crust readjusts, as well as large aftershocks, such as the magnitude 5.7 quake observed on June 14," Donnellan said.
The April 4, 2010, El Mayor-Cucapah quake was centered 32 miles (52 km) south-southeast of Calexico, Calif., in northern Baja California. The quake, the region's largest in nearly 120 years, was also felt in southern California and parts of Nevada and Arizona. There have been thousands of aftershocks, extending from near the northern tip of the Gulf of California to a few miles northwest of the U.S.-Mexico border. The area northwest of the main rupture, along the area of California's Elsinore fault, has been especially active.
Geologists used the first UAVSAR interferogram, which included the April 4 quake, to map many new, small fault ruptures in the field. JPL geophysicist Eric Fielding said, "This new interferogram shows that some of the faults have continued to slip since our overflight on April 13th. Such mapping is important for understanding the fault structure in this area between the main fault ruptures on April 4th farther south in Baja California and the faults farther to the north in Southern California, including the Elsinore and San Jacinto faults."
Fielding's studies of interferograms from Japanese and European Space Agency satellites indicate the largest fault movement visible in the new UAVSAR interferogram occurred between May 21 and June 6 along a northeast-southwest trending fault known as the Yuha fault, visible to the right of the center of the interferogram. This fault previously had slipped about 1 to 2 inches (2 to 4 cm) in the first days after the April 4 earthquake, as was shown in the earlier UAVSAR and satellite interferograms. Since April 13, the fault has slipped about another 1 inch (2 cm).
The fact that the Yuha fault has a trend toward the northeast is also significant, Fielding added, because it is very different from the northwest-trending major Elsinore and San Jacinto fault systems and the faults in Mexico that ruptured in the main earthquake.
"This adds to evidence that the faults in Mexico are not directly connected to the Elsinore and San Jacinto faults and may explain why the magnitude 7.2 April 4 earthquake stopped before it reached California," Fielding said.
The UAVSAR radar flies at an altitude of 41,000 feet (12.5 km) on a Gulfstream-III aircraft from NASA's Dryden Flight Research Center, Edwards, Calif.