Wednesday, April 18, 2001
By TOM PAULSON
SEATTLE POST-INTELLIGENCER REPORTER
SAN FRANCISCO -- The evidence was hidden beneath a typical Northwest tangle of second-growth forest and brush -- evidence of the massive quake that struck Seattle about 1,100 years ago.
Until a few years ago, scientists were unable to look directly at the cracks in the Earth's crust known as the Seattle fault.
Then came the breakthrough -- by accident. A new aerial mapping technology that was supposed to show ground-water patterns revealed much more -- making the fault visible on the ground's surface for the first time. It's an accident geologist Ralph Haugerud wants to see repeated often.
Speaking here today at the annual meeting of the Seismological Society of America, Haugerud is advocating broader use of this new technology in the study of earthquake risks.
It's called LIDAR, a strained acronym for "light detection and ranging," meant to sound like radar. The decade-old technique, originally developed by NASA, involves flying an airplane over land and peppering it with millions of laser pulses that send back precise topographical readings.
The 6.8-magnitude Nisqually quake, which served as a wake-up call for the Puget Sound region Feb. 28, will be a primary topic at this annual meeting of scientists. But sorting out the evidence left from prehistoric quakes is also critical to understanding current risks.
In 1996, LIDAR identified a previously unknown section of the Seattle fault zone.
"That's how we discovered Toe Jam," said Haugerud, of the Seattle office of the U.S. Geological Survey.
The Toe Jam fault scarp, named for Toe Jam Hill Road on south Bainbridge Island, was the first surface rupture to be discovered on the Seattle fault system. Many scientists believe the system, a series of shallow faults running east-west from Bremerton to beyond Redmond, has the potential for a major earthquake.
Had they been in California, scientists likely could have spotted the surface fault line many years ago simply by flying over and taking a look, or perhaps taking photographs. Even a child can identify the San Andreas fault from an airplane.
But in the Pacific Northwest, nearly every piece of undeveloped terrain is covered with lush vegetation or forest, disguising the topography. LIDAR provides a picture of the ground hidden beneath, using a sophisticated computer program that produces an image of the land stripped clean of all its trees, bushes and greenery.
The laser doesn't penetrate the vegetation, Haugerud explained. This laser is a highly focused light beam that can't even get through a blade of grass, he said. Instead, the computer program sorts through the millions of laser signals, rejecting the readings that hit leaves or branches and using only those points of light that actually reach the ground.
"We have to throw away two-thirds of the points," Haugerud said.
It's a cumbersome and complex system, he said, but it provides an amazingly precise view of land surface that cannot be obtained otherwise.
It wasn't seismology that initially brought LIDAR to the Northwest. It was water management in Kitsap County.
"They weren't looking for earthquake faults," said Jerry Harless, head of geographic mapping for the Puget Sound Regional Council. They were looking to create better topographic maps for land, water and resource management, Harless said.
In 1996, when Harless was working for Kitsap County, his colleague Greg Berghoff at Kitsap Public Utilities District contracted with the firm Airborne Laser Mapping to do a LIDAR scan of Bainbridge Island. The main idea was to identify ground-water recharging and runoff patterns.
But Berghoff had training as a geologist and, when he got the LIDAR map of the southern end of Bainbridge, he noticed an odd, bold line running east-west along Toe Jam Hill and contacted geologists at the University of Washington.
"They got all excited," Harless said. "They were trying to map the Seattle fault at the time ... but most of its eastern half is already developed and covered up."
Here was a chance to study the Seattle fault at the surface. Scientists from the UW and USGS descended upon Toe Jam Hill with fervor -- and with the property owner's permission -- to dig trenches and examine the sheared and folded rock layers of a fault normally buried deep beneath the land and water surfaces.
"We learned a lot about the fault's history," Haugerud said.
Because this laser-mapping technique can provide such a precise topographic map with so many potential uses, a group of odd bedfellows has formed to push for wider use.
The USGS, Kitsap PUD, the City of Seattle and NASA (which helped develop LIDAR) have formed the Puget Sound LIDAR Consortium to eventually map all of Puget Sound.
Much of Kitsap County and King County has been flown already. There's a proposal to do a LIDAR map of the areas hardest hit during the Nisqually quake.
Haugerud noted that another potential fault scarp, like Toe Jam, has been identified due east of that fault scarp thanks to a recent LIDAR scan of mainland Kitsap County. It's near an area known as Waterman Point and looks to be related to the Seattle fault as well, but scientists haven't yet got in there to trench it and confirm.
Because of the surprising revelation of Toe Jam, scientists in Puget Sound are already sold on the value of LIDAR for geologic and seismologic studies. Haugerud is presenting the findings today at this conference to help others better appreciate the power of this new tool.
"We want more LIDAR data," he said. "We're spreading the gospel of LIDAR."
How LIDAR works
LIDAR can see what's beneath the thick canopy of trees that characterize much of Bainbrige Island by filtering out certain types of data that is collected when a LIDAR plane flies over the area.
Reflection & resolution
As the plane crosses a survey area, it projects a laser beam onto the surface, which reflects back to a receiver.
The speed and length of the returning lightwave -- combined with Global Positioning System navigation -- reveals the contours of the surface. Shorter waves (which might represent trees or roofs of houses) are deleted and the underlying topography is revealed. Because the laser can collect 30,000 data points per second, the resolution of the final image is more defined, which led to the discovery of the faults hidden under the trees.
Other applications
In addition to revealing possible fault lines, LIDAR and high-resolution topography are useful for:
Hazards studies to show areas
susceptible to landslide or flood
Ground composition and earth history
Land-use management
Public works and endangered species projects (estimating storm-water
runoff, locating stream channels, etc.)
-- Anonymous, April 18, 2001