Featured Research

from universities, journals, and other organizations

Seismic Profiling Reveals Ancient River Valley And Prehistoric Faulting Beneath Portland, Ore.

Date:
May 3, 1999
Source:
United States Geological Survey
Summary:
An ancient river valley a mile wide and 250 feet deep, as well as breaks in geologic strata beneath two suspected fault zones have been revealed beneath Portland, Ore., by a team of scientists from the U.S. Geological Survey, the University of Washington and Portland State University.

An ancient river valley a mile wide and 250 feet deep, as well as breaks in geologic strata beneath two suspected fault zones have been revealed beneath Portland, Ore., by a team of scientists from the U.S. Geological Survey, the University of Washington and Portland State University.

Related Articles


These new data will be presented by USGS researcher, Tom Pratt, at the annual meeting of the Seismological Society of America, meeting in Seattle May 3-5.

In a presentation slated for 11:30 a.m., Tuesday, May 4, Pratt will explain how he and fellow researchers used an echo-sounding method called seismic reflection profiling to produce images that revealed the underground features. Seismic profiling is similar to the ultrasound commonly used in medical imaging, but it is applied to the earth on a much larger scale. The technique uses computer-processed echoes returned from the subsurface after the ground was struck or a special energy source produced a loud ‘click' in the water.

The scientists looked as much as 500 feet into the earth, using equipment towed behind a small boat on the Columbia and Willamette Rivers. In a separate effort they used equipment on land to look beneath three of Portland's residential streets; Rex Street, Berkeley Street and Monteith Street.

The ancient river valley is believed to have formed about 15,000 years ago, possibly when an ice dam east of the Cascades broke and released huge volumes of water into the Columbia River. The valley was carved into the rocks beneath and just south of the modern Columbia River. The flood waters also left as much as 250 feet of silt and mud covering the 15,000 year-old ground surface beneath much of the Portland area. The thickness of these soft sediments, which may amplify ground shaking during large earthquakes, were also mapped by the scientists.

Pratt said the 15,000 year old ground surface appears to be broken by prehistoric earthquakes on two suspected fault zones near the Willamette River. "The East Bank fault is suspected, from tiny changes in the earth's magnetic field and differences in the depth of rocks in drill holes, to lie along the north side of the Willamette River. The Portland Hills fault is believed to lie at the base of the Portland Hills and to cause the hillside near downtown Portland to be a long, straight feature," Pratt said.

Profiles near the mouth of the Willamette River, near the university, and near Ross Island, southeast of downtown Portland, show abrupt, 3-to 30-foot changes in the depth to the 15,000 year old ground surface at the faults. Pratt said changes such as these are expected, if the faults have moved.

Although Pratt and his fellow scientists say they cannot rule out other processes, such as erosion, as a cause of these changes, their location beneath suspected fault zones suggests that earthquakes have ruptured the surface in the past 15,000 years and that the faults are active. He said it is not known how often earthquakes could occur on the faults. "Earthquakes are thought to be infrequent on these faults because few modern earthquakes have occurred near them," Pratt said, "but our work raises the possibility that past earthquakes are more frequent than previously assumed. Our study is a first test to see whether the faults are active, but it is not definitive. Follow-up studies will be needed to verify the results."

Seismic profiles across another suspected fault zone, the Frontal fault, that crosses the Columbia River near the east end of Reed Island, did not show clear evidence for past earthquakes. Pratt said this could imply that the fault is not active, but he cautioned that the location of this fault is not well known and small amounts of motion may go undetected by the seismic profiling technique.

As the nation's largest water, earth and biological science, and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize the loss of life and property from natural disasters, contribute to the sound conservation and the economic and physical development of the nation's natural resources, and enhance the quality of life by monitoring water, biological, energy, and mineral resources.


Story Source:

The above story is based on materials provided by United States Geological Survey. Note: Materials may be edited for content and length.


Cite This Page:

United States Geological Survey. "Seismic Profiling Reveals Ancient River Valley And Prehistoric Faulting Beneath Portland, Ore.." ScienceDaily. ScienceDaily, 3 May 1999. <www.sciencedaily.com/releases/1999/05/990503041403.htm>.
United States Geological Survey. (1999, May 3). Seismic Profiling Reveals Ancient River Valley And Prehistoric Faulting Beneath Portland, Ore.. ScienceDaily. Retrieved October 25, 2014 from www.sciencedaily.com/releases/1999/05/990503041403.htm
United States Geological Survey. "Seismic Profiling Reveals Ancient River Valley And Prehistoric Faulting Beneath Portland, Ore.." ScienceDaily. www.sciencedaily.com/releases/1999/05/990503041403.htm (accessed October 25, 2014).

Share This



More Earth & Climate News

Saturday, October 25, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

EU Gets Climate Deal, UK PM Gets Knock

EU Gets Climate Deal, UK PM Gets Knock

Reuters - Business Video Online (Oct. 24, 2014) EU leaders achieve a show of unity by striking a compromise deal on carbon emissions. But David Cameron's bid to push back EU budget contributions gets a slap in the face as the European Commission demands an extra 2bn euros. David Pollard reports. Video provided by Reuters
Powered by NewsLook.com
Deep Sea 'mushroom' Could Be Early Branch on Tree of Life

Deep Sea 'mushroom' Could Be Early Branch on Tree of Life

Reuters - Innovations Video Online (Oct. 24, 2014) Miniature deep sea animals discovered off the Australian coast almost three decades ago are puzzling scientists, who say the organisms have proved impossible to categorise. Academics at the Natural History of Denmark have appealed to the world scientific community for help, saying that further information on Dendrogramma enigmatica and Dendrogramma discoides could answer key evolutionary questions. Jim Drury has more. Video provided by Reuters
Powered by NewsLook.com
Raw: Tornado Rips Roofs in Washington State

Raw: Tornado Rips Roofs in Washington State

AP (Oct. 24, 2014) A rare tornado ripped roofs off buildings, uprooted trees and shattered windows Thursday afternoon in the southwest Washington city of Longview, but there were no reports of injuries. (Oct. 24) Video provided by AP
Powered by NewsLook.com
Fast-Moving Lava Headed For Town On Hawaii's Big Island

Fast-Moving Lava Headed For Town On Hawaii's Big Island

Newsy (Oct. 24, 2014) Lava from the Kilauea volcano on Hawaii's Big Island has accelerated as it travels toward a town called Pahoa. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:

Strange & Offbeat Stories


Plants & Animals

Earth & Climate

Fossils & Ruins

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins