Tech TrendsFinding Fault28 Feb, 2006 By: Kenneth Wong Cadalyst
USGS Develops 3D Model of the Earth's Upper Crust
Tom Brocher, A Geophysicist by profession, talks to the Earth. Using a special vocabulary, he interrogates the rocks to understand what's happening below the ground, as far as 27 miles down. And the earth responds in sporadic murmurs, returning signals that reveal its subterranean character.
"We send seismic waves down," Brocher says. "They bounce off various boundaries and discontinuities below the surface. We look for structures that trap seismic waves and make the earthquake shaking stronger and last longer. A classical example of that is a place where, because of fault motion, some part of the earth has sunk in, and erosion from the surrounding hills has filled the depression with mud and debris. Because all those deposits are much softer, once seismic waves go into those 'basins,' as they are called, the waves rattle a lot, much like water splashing around inside a bathtub."
Based on many such exchanges over the years, Brocher and his colleagues at the USGS (U.S. Geological Survey, www.usgs.gov) assemble a 3D view of the earth's upper crust (figure 1). Their latest project, now available for download, is a fault-block model of the San Francisco Bay area (www.sf06simulation.org).
Figure 1. The USGS fault-block model of the San Francisco Bay area is now available for download. First responders will find insights from conducting quake simulations using the 3D model.
Historical and Hypothetical Quakes
"We first developed an accurate geological model," Brocher explains. "We say, 'Okay, this kind of rock is here at two miles below the surface, then replaced by something else, then there's a fault underneath it, and so on.'" In essence, Brocher and his team were constructing a hierarchy of fault blocks, layer by layer. Each rock type is assigned appropriate physical properties and seismic velocities. The resulting 3D crust is a model as well as a database, an accumulation of geologic and seismographic data dating back as far as 100 years. The model can recreate, for instance, the Loma Prieta earthquake of 1989. By the same token, it can also simulate a quake of a certain magnitude along a certain fault line. The insight gained from these simulations, Brocher points out, is tremendously valuable to emergency responders. And the model is by no means final; as studies at the USGS produce more data, it is continually updated.
The USGS's model of the Bay Area was built in EarthVision, a commercial 3D visualization product developed by Dynamic Graphics (www.dgi.com). Petroleum companies such as BP (www.bp.com) use the software for geospatial modeling. With features for easy import and display of seismic data, the software appeals to researchers from a variety of fields, from archeology to oceanography.
Andrew Lawson and Human Seismometers
Professor Andrew Lawson did some of the grueling work nearly a century ago (http://seismo.berkeley.edu/history/history2.html). The Lawson Report, published in 1908, was an exhaustive study of the great San Francisco quake of 1906. Jack Boatwright, one of Brocher's colleagues at the USGS, took the graphics, tables and charts in Lawson's tome and converted them into seismographic values so they could become a part of the 3D model.
"Back in 1906, we had very few instrumental records, but we had quite a lot of human observations of damages," Brocher says. These individual eyewitnesses, or human seismometers, might not be able to express the motion they felt in numerical terms, but their recollection of certain details—such as swaying chandeliers, falling furniture and duration of the shaking—offered clues about the quake's intensity in different areas (figure 2).
Figure 2. An intensity map shows how the impact of the 1906 quake was felt in different geographic regions.
If you aspire to be a human seismometer, Brocher invites you to fill out the USGS questionnaire called "Did You Feel It?" (http://pasadena.wr.usgs.gov/shake/).
"After an earthquake, we usually get about 10,000 to 20,000 responses," says Brocher. "As it turns out, when averaged, these human seismometers are typically very accurate."
What Can You Do With It?
Primarily intended for researchers, the USGS's Bay Area fault-block model is a tool to help mitigate loss of life and property.
"We've assigned properties to rocks for calculating seismic activity," Brocher says, "but someone else can take the same model, assign different properties to the layers and use it to figure out groundwater flow, for instance. Then they can create a 3D model that shows how the water flows underground when it rains hard in the Santa Cruz Mountains."
If you are an amateur earthquake chaser who owns a copy of ArcGIS (from ESRI, www.esri.com), you can study the topology along the Hayward Fault by downloading the USGS map at http://pubs.usgs.gov/ds/2006/177/HF_GIS_data.html.
What Makes a Geophysicist Tick?
"If you want interesting geology," Brocher says, "you have to go to a place where the earth is in motion. Here, in California, you find rocks that are very young—young, for a geologist, means a million years or so—but they're already smashed up into blocks and pieces." That's because about half a dozen major faults run along the 180 X 80 mile region, wreaking havoc in the subsurface. "If you go Kansas, for instance, where nothing happens for several hundred millions of years, things tend be one dimensional; you find the same geological unit for hundreds of miles, flat as a stack of pancakes."
No doubt the good people of Kansas would rather enjoy their pancakes in peace than face the frequent jolts that Californians regularly endure.
"If you'd visited a geologist's office ten years ago," says Brocher, "you'd see boxes of maps, fossils and rocks, dusty and filled to the brim of the shelves. Now, geology has gone completely digital. And that's what has sparked the effort for 3D geology."
What's Down There?
When Brocher was purchasing his home, he made the most of his seismographic studies. He made sure his home was situated in a plot safely tucked away from the fault lines and from areas prone to mudslides. Because I live in San Francisco, right in the heart of earthquake-ville, it seemed silly not to take advantage of Brocher's wisdom while I had him on the phone. So I told him where I lived and asked him if I had any cause for concern. He immediately identified my neighborhood as a sand dune. "It's not very solid," he warns me, "so, if the shaking is hard enough and it's filled with water, you'll feel a lot of motion." That means I'll make a great human seismometer.
Kenneth Wong is a former editor of Cadence magazine. As a freelance writer, he explores innovative use of technology and its implications. E-mail him at firstname.lastname@example.org.