ESA satellite imagery credited with breakthrough in earthquake research

California scientists credit synthetic aperture radar imagery
from the European Space Agency with making possible new ways
to depict earthquake fault zones and uncovering unusual
earthquake-related deformations.

Their study of imagery from a 1999 earthquake in the western
US could provide a new way to identify active faults and help
track when the last earthquake occurred in a fault zone.

Writing in last week’s issue of Science magazine, researchers
from the Scripps Institution of Oceanography at the University
of California in San Diego, and the California Institute of
Technology detailed their studies of the so-called “Hector
Mine” earthquake, a magnitude 7.1 earthquake that tore through
28 miles of faults in the Mojave Desert. Named after a nearby
abandoned mine in the remote area, the earthquake caused
virtually no damage. It was, however, the “perfect” event to
use satellite and radar technologies to document unique
characteristics of faults, said Scripps’ Yuri Fialko, the
study’s lead author.

The earthquake was comprehensively imaged with interferometric
synthetic aperture radar (InSAR), which uses a series of
satellite recordings to detect changes in the Earth’s surface.
The most surprising finding that came out of the analysis of
the imagery was the first evidence that faults can move
backwards.

“Even small stress perturbations from distant earthquakes can
cause faults to move a little bit, but it’s only been known
to cause this motion in a forward sense,” Fialko said in a
Scripps announcement of the study’s publication in Science.
“Here we observed the faults coming backwards, due to
relatively small stress changes, which is really quite
unusual.”

The article argues that the backward motion on the faults is
caused by the dissimilar nature of material within the faults,
rather than frictional failure. The results, Failko said,
will guide new seismic studies to areas with contrasting
fault materials and can then be used to identify potentially
active faults.

Co-author Peter Shearer of Scripps credited the study’s
detailed results to the “breakthrough” offered by InSAR
technology.

“Prior to InSAR, all we had were spot measurements of the
deformation field,” the Scripps scientist said. “With InSAR
we have millions of points and thus a continuous picture of
the deformation across southern California.”

Using the satellite data, the study was able to document both
vertical and horizontal terrain displacements of several
millimetres to several centimetres across kilometre-wide
zones centred on faults.

“The findings became possible due to highly successful
satellite missions of the European Space Agency,” the
scientists were cited as saying by the Scripps announcement.

In the Science article, the authors pointed out that the
earthquake area had been imaged repeatedly by ESA’s ERS-1
and ERS-2 satellites over the past 10 years. The research
team generated and analysed all possible interferometric
pairs that included the earthquake date, ending up with
15 interferograms from a descending orbit, and five
interferograms from an ascending orbit.

The first synthetic aperture radar was launched in 1991 as
one of three main instruments on ESA’s ERS-1 satellite. It
was followed by a second on ERS-2 in 1995. These highly
successful ESA satellites have collected a wealth of
valuable data on the Earth’s land surfaces, oceans, and
polar caps. Today, several hundred research groups
worldwide use ERS data to further their studies. With ERS
data, the InSAR technique represents a major breakthrough
in Earth sciences, allowing scientists to understand better
earthquakes and other natural events.

Europe’s latest environmental satellite, Envisat, was
launched earlier this year carrying an advanced SAR (ASAR).
Envisat’s ASAR instrument is the first permanent spaceborne
radar to incorporate dual-polarisation capabilities — the
instrument can transmit and receive signals in either
horizontal or vertical polarisation. This dramatically
improves the capability of SAR to discriminate between
different types of terrain compared with the sensors on the
earlier ERS generation of satellites, while offering a
continuity of service to users working with the InSAR
technique.

ESA satellite imagery credited with breakthrough in earthquake research

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