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Geophenomena

Predicting landslides
Hot spring pops up in Paso Robles
Dry craters in Bhuj


Predicting landslides

Landslides kill thousands of people around the world each year and wreak costly property damage, especially in heavily populated mountainous regions such as the Alps. A new service funded by the European Space Agency might be able to reduce some of those losses by using satellites to predict where and when landslides might occur.

The Service for Landslide Monitoring project, or SLAM, began operating in February over Italy and Switzerland with the goal of identifying and monitoring slope movement. Using images from the European Remote Sensing Satellite (ERS), researchers have been able to create detailed landslide hazard maps that pinpoint unstable slopes and make it easier to predict potential slope failures.

This is the first time synthetic aperture radar (SAR) and interferometry have been used together to forecast landslides, says Gerald Wieczorek, a civil engineer with the U.S. Geological Survey’s Landslide Hazards Program. SAR obtains images of an area from different positions along the satellite’s path, emulating a much larger antenna. Interferometry is the process of combining radar images taken at different times.

“The interferometric techniques are able to detect precursory deformations which don’t produce any superficial evidences in the terrain, such as terraces, tensional cracks or damage to buildings,” says Nicola Casagli of Italy’s National Group for the Prevention of Hydrogeological Hazards, a research network working with Italy’s Civil Protection Department and a SLAM user.

Several factors can trigger landslides, including heavy rain, earthquakes, volcanic activity, changes in groundwater flow and human-induced slope disturbances. However, the early detection of landslides that can move at speeds ranging from millimeters per year to kilometers per hour has proved challenging.

In the past, researchers have used monitoring devices to detect the minuscule ground shifts that occur on a slope prior to a landslide. Unlike SAR interferometry, however, the devices cannot cover large regions simultaneously and are usually limited to slopes already known to be unstable.

“There are some very sensitive ground-based monitoring techniques that, if the appropriate instruments had been installed, would detect slight preceding motion,” Wieczorek says. “But the instruments would have to be there and in the right place.”

SLAM provides users with two unique methods of observation. The first, called differential interferometry, provides maps of displacement over time. “By comparing, pixel by pixel, the phase values of two SAR images acquired in the same area at different times, it is possible to obtain information about the ground deformations that occurred within the observed area, with centimetric accuracy,” Casagli says.

The second method, called the permanent scatterers technique, provides accuracy to the millimeter, Casagli says. Instead of using two single SAR images, researchers use a series of at least 25 images and look for fixed points, such as walls or large boulders, which occur in every image. They can then compare these permanent scatterers with various points in the landscape, allowing them to identify even minute changes.

One of the first test cases for SLAM will be coverage of the Arno River Basin, a 9,000-square-kilometer area with a population of 2.5 million, where more than 3,000 landslides have occurred, mostly due to erosion, rain and earthquakes.

Ideally, for hazard management purposes, researchers would be able to predict the location, time, type, size, velocity and run-out distance of a landslide. In order to determine as many of these variables as possible, SLAM researchers will compare new SAR imagery with historical imagery recorded since ERS launched a decade ago, and combine that information with geological models of slope failures. “Although several assumptions are made in order to simplify the complex behavior of a mass movement, such models can provide a useful tool, especially for emergencies management,” Casagli says.

Sara Pratt
Geotimes contributing writer

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Hot spring pops up in Paso Robles

The town of Paso Robles lies in the southern end of the Salinas Valley, on California’s central coast. The town owes its existence to natural hot springs in the region, which people visited for their “curative powers.” Even the town’s main street, Spring Street, is named for these features. Now, a new hot spring has come to town, opened by the magnitude-6.5 earthquake that killed two people and leveled part of the town’s historic center last December.

The San Simeon earthquake last December opened a new hot spring in Paso Robles, Calif., but digging has not helped clarify the spring’s origin. Photo by Robert Anderson, California Seismic Safety Commission.

Bubbling since the Dec. 22 earthquake, the sulfur spring has spewed thousands of gallons a day of hot water laced with boron into the Paso Robles City Hall parking lot. Despite excavating a pit 150 feet wide by 150 feet deep, the city has yet to locate the source of the spring. Such features may be the most difficult earthquake effect to characterize, says Lew Rosenberg, county geologist for San Luis Obispo County. “It’s easy to see a surface crack or fault, but where the water comes from is more mysterious and elusive,” he says.

One of the questions geologists must answer, Rosenberg says, is how this new spring is connected to the town’s other springs and the fault system. Over the past few months, the water pressure has dropped in springs built into a resort hotel across the street. U.S. Geological Survey mapping shows that both Spring Street and a string of springs are aligned along the Rinconada fault, on which the town sits. It is unclear, Rosenberg says, if the new spring is part of the same seepage system.

The city has hired a consultant to use thermal imaging in order to find hot springs and the thermal connections between them. Although the geothermal activity is nowhere near as great as California’s Geysers field farther north (which is connected to volcanic activity), the water is coming from depths that circulate within the fault zone, several hundred feet deep.

The county also is looking for a solution to disposing of the chemical-laced waters discharged from the unexpected spring: For now, the stream of warm, high-boron sulfuric water goes directly to the Salinas River, but both the high temperature and boron content could affect the river’s fish.

Rosenberg and his co-workers have documented a variety of other phenomena connected to the recent earthquake, from liquefaction effects near Pismo Beach to large earthquake-induced landslides. “Geologically, it’s been really neat,” he says, and continued work on this central portion of California’s fault systems is important, in order to “help us connect what we know about northern and southern California geology.”

Naomi Lubick

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Dry craters in Bhuj

The magnitude-7.7 earthquake that struck Bhuj, India, in 2001, released a pressurized slurry of water and soil, creating small sand volcanoes and cracks. Water continued to flow, sometimes for days afterward, all as a result of liquefaction. But the earthquake also triggered delayed explosions of dry liquefaction craters — a surprising phenomenon, says Martitia Tuttle, a Maine-based consulting geologist who also has studied liquefaction after large earthquakes in San Francisco and Quebec.

A dry liquefaction crater in Bhuj, India, that appeared after a large 2001 earthquake. Photo by Martitia Tuttle.

“It’s not unusual to see a delayed surface manifestation of underground liquefaction,” Tuttle says. But the dry craters are harder to explain.

Paul Rydelek of the Center for Earthquake Research and Information (CERI) at the University of Memphis, and Tuttle published their description of the dry blowouts and other ejecta in the Jan.8 Nature. Although the team remains uncertain about the exact dynamics of how dry craters might occur, they hypothesized that gas trapped in the clay and sand layers — pressurized and then jostled by an earthquake — might lead to explosive ejection of the gas and material.

“It’s bizarre, and I have to say that when I first saw it I didn’t believe it,” says Eugene Schweig, a U.S. Geological Survey geologist located at CERI and a co-worker of Tuttle’s. But Schweig says the observations clearly show that dry craters can occur, and the hypothetical cause — collapse of a gas-filled cavity — is appealing. The question remains as to how the gas gets there.

Naomi Lubick

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