seismic processes across North America
By Geoff Koch
December 12, 2003
But to geophysicist Paul Segall it is something more.
''This kind of thing happens once in a career,'' says Segall, a professor of geophysics at Stanford University who chairs the PBO advisory board that determined just how this sprinkling should occur.
PBO is part of the $219 million EarthScope project to understand no less than the structure and evolution of the entire North American continent. The National Science Foundation (NSF), which is funding the project, notes that EarthScope was launched exactly 200 years after the Lewis and Clark Expedition.
''This time, however, instead of toting sextants and compasses to map the surface, scientists will bring seismometers, state-of-the art drilling equipment, satellites and Global Positioning System (GPS) receivers to map Earth's interior,'' according to the NSF website.
Tremendous violence occurs at the boundary between the Pacific and North American plates of the Earth's crust. As the plates come together near the West Coast, they continue to smash into each other in slow motion - a process that has been going on for millions of years and has given rise to many prominent features of the North American landscape, including the San Andreas Fault and the Cascade Mountains.
This boundary will be the focus of PBO. ''It is where the action is,'' Segall said.
The grinding action generates subtle deformation across the continent. Eventually, hundreds of sensors will be in place to measure this deformation and help geologists understand the shifting strains in the ground. But first, Segall and his colleagues need to answer some questions, including two that will be discussed at the American Geophysical Union (AGU) conference in San Francisco on Dec. 10.
Setting up sensors, reading results
Lest they be accused of not being creative, consider how geologists usually decide where to place expensive equipment such as GPS receivers.
''Typically it is by gut feel,'' Segall said.
In a project compared to Lewis and Clark's Corps of Discovery, gut feel just won't do. So Segall, along with University of Oregon geologist David Schmidt and Stanford graduate student Jessica Murray, built mathematical models to figure out exactly how these sensors should be arrayed across the landscape.
It's a good thing they did. Their models suggested several changes in how to configure the network, particularly in the seismically active Pacific Northwest.
Segall compared the work to optimize the sensor network to the work of astronomers to design telescopes. Stargazers pick one type of instrument to look at objects nearby in the solar system and another type to peer into deep space, he said.
Similarly, the sensors might be set up differently depending on the specific seismic effects, depths and time scales his colleagues are interested in. According to Segall, the models that Schmidt will present at the AGU conference will make it easier for competing research interests to compromise.
Another fundamental issue facing researchers is how to interpret the data returned by the sensors - an analog to the age-old problem in science of distinguishing signal from noise. Devices called strainmeters that measure deformation deep in the Earth are planned to be used in the project.
But the devices are very sensitive. It is difficult to determine if the data represent widespread aftereffects of a volcanic eruption or a small, insignificant crack appearing in the rock near the device itself, Segall explained: ''They feel everything.''
At the AGU conference, Segall will talk about the challenges of understanding the data returned by these devices. Given that PBO sensors are scheduled to be installed early next year, there is an urgency behind this math-heavy topic.
''There are only a few dozen strainmeters around the world today, but we are set to install around 200 of them,'' he noted.
From sextants to seismometers
Segall is excited about the technology set to be deployed for the project. ''PBO scientists will be able to monitor activity at North America's plate boundaries with unprecedented accuracy at periods ranging from the seconds it takes for small earthquakes to rupture to the decades it takes for magma to accumulate beneath volcanoes,'' he said.
And he won't be the only one participating in this once-in-a-career event. According to NSF, all EarthScope data will be made available in close to real time on the web for the benefit of academics and amateurs alike.
During the AGU conference, Segall also will discuss his recent research on earthquakes in Iceland. Working with scientists from Harvard University and the National Energy Authority in Iceland, Segall compared how subtle deformations at ground level and water levels in wells changed over time following an earthquake.
Segall used a unique combination of new and old technology. Color-coded satellite images revealed changes in the crust after the quake, while galoshes-clad researchers on the ground took measurements from wells that dot the Icelandic landscape. He found a clear correlation between the two sets of data. In effect, the shifting water levels revealed the shifting stresses in the Earth's crust following the quakes.
Peter Gilles, geologist at the University of California-Los Angeles and NASA's Jet Propulsion Laboratory, said the results might have a practical use.
''One of the main concerns
after a major earthquake is the possible triggering of other
earthquakes on nearby faults,'' Gilles said. ''Understanding
how the stress is modified on adjacent faults has direct implications
on risk assessment after an event.''
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