San Francisco, California Interferogram
This image is an interferogram that was created using pairs of images taken by Synthetic Aperture Radar (SAR) that have been combined to measure surface deformation or changes that may have occurred in the time between data for the two images were taken. The images were collected by the European Space Agency's (ESA) Remote Sensing Satellites (ERS-1 and ERS-2) in June 1992 and September 1997 and were combined to produce these image maps of the apparent surface deformation, or changes, over the southern San Francisco Bay, California. The radar image data are shown as a gray-scale image, with the interferometric measurements that show the changes rendered in color. Only the urbanized area could be mapped with these data. The sharp color changes across the Hayward Fault (marked by a red line), show about 3 centimeters (1.2 inches) of gradual displacement or movement of the southwest side of the fault. The fault moved horizontally towards the northwest during the 63 months between the acquisition of the two SAR images. This fault movement is called "aseismic creep" because the fault moved slowly without generating an earthquake. Scientists are using the SAR interferometry along with other data collected on the ground to monitor this fault motion in an attempt to estimate the probability of another earthquake on the Hayward Fault, which last had a major magnitude 7 earthquake in 1868. Researchers are using SAR interferometry to study earthquakes and to monitor subsidence and rebound in the Santa Clara Valley and other areas.
The Santa Clara Valley (near the bottom of the image) area around San Jose also shows apparent motion of the ground surface in the interferogram. The blue and purple area to the northwest of San Jose seemed to move upward relative to the yellow and red areas to the southeast by as much as 8 centimeters (3.2 inches). The cause of this apparent motion is not yet confirmed, but the rise of groundwater levels during the time between the images may have caused the reversal of a small portion of the subsidence that this area suffered in the past. Much of the Santa Clara Valley subsided dramatically between 1916 and 1966 with a maximum of 3.8 meter (12 feet) drop in downtown San Jose.
Data courtesy of ESA, acquired through Radarsat International.
Eric J. Fielding, Section 324, Jet Propulsion Laboratory, California Institute of Technology,
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