USGS - U.S. Geological Survey

10/08/2020 | Press release | Distributed by Public on 10/08/2020 13:01

From Volcanoes to Vineyards - New Geologic Map Reveals Portland's Deep History

The U.S. Geological Survey's bi-state map, which covers more than 2500 square miles and would measure 13-feet wide if printed in full, provides geologic information essential for evaluating natural hazards, resources and land use. The map was produced in cooperation with the Oregon Department of Geology and Mineral Industries and the Washington Geological Survey and is based on decades of field and laboratory work by 14 geoscientists from the USGS, Portland State University, Oregon State University and DOGAMI.

'Many believe that we've mapped all the geology in the country,' said USGS Director Jim Reilly. 'As in all things, maps benefit from a better understanding of the details whether they be in the differences in the geologic units themselves or better precision in how we visualize the terrains. The new mapping will give a more detailed view of the geological conditions in the area for local managers and agencies charged with characterizing and managing resources and hazards. It also provides important information for members of the public seeking recreational sites and in understanding some of what defines why the area is one of our nation's premier wine-making regions.'

Underpinning the map is a modern geologic database for the greater Portland metro area delineating its tectonic setting, active faults, volcanoes, landslide hazards and distribution of geologic materials.

'Planners and managers can use the database to improve seismic hazard- and resource assessments in this economically important region,' said Ray Wells, a research geologist with the USGS who led the mapping team.

'From the Coast Range to the Cascades, this map helps us understand the geologic setting of this tectonically active region, which overlies the Cascadia subduction zone,' said Wells. He noted that this area is home to a variety of geologic hazards, a wide distribution of geologic resources and groundwater systems, all of which are better understood today as a result of this information.

'For example,' Wells continued, 'Scoggins Dam, southwest of Hillsboro, Oregon, is slated for repair or replacement, and the USGS, in cooperation with the Bureau of Reclamation, is already providing information from the map database to help in Reclamation's remediation effort.'

'Reclamation is analyzing seismic challenges at Scoggins Dam,' said Chris Regilski, dam safety coordinator for Reclamation's Columbia-Pacific Northwest Region. 'Having additional detailed geological mapping of this area provides us with good science to inform our design and to better understand the risks at this dam.'

This new map will be the basis for a wide range of future geohazard studies because it provides a consistent geologic framework across the area, as well as unprecedented topographic detail and accuracy in areas mapped with lidar. 'The Oregon Department of Geology and Mineral Industries has already used these data to produce a first-ever earthquake loss estimate for the entire Portland Metro region,' said Ian Madin, an earthquake hazard geologist with DOGAMI.

But it's not just land- and resource managers who benefit from the map's information. The western part of the mapped area is also home to several well-known American wine-growing regions. 'Those viticulturists,' said Wells, 'can now use the map to improve understanding of their soil types and how variations in geology can contribute to terroir, or the characteristics of place that contribute to wine quality.'

'Geologic maps provide essential information for planning and decision making. With this new and updated geologic information, communities will have a better understanding of the geologic hazards and resources where they live, work, and recreate,' said Casey Hanell, the State Geologist of Washington. 'This effort is a great example of leveraging partnerships among state geologic surveys and the USGS to provide high-quality geologic information for safe, resilient and prosperous communities.'

The mapped area also includes the southern part of the Mist Natural Gas Storage Field in the northwest corner, the Columbia South Shore Well Field aquifer in the Portland Basin, the Columbia River Basalt aquifer of the Tualatin Basin, and Tualatin Basin Aquifer Storage and Recovery projects, along with numerous transportation, electrical transmission and pipeline corridors.

The area's geologic hazards are produced by subduction of the Pacific Ocean plate underneath the North American continental plate. The relentless build-up of stress along the sloping plate boundary, or 'megathrust,' deforms the leading edge of the North American plate and is released in megathrust earthquakes, as last occurred in the year 1700. In addition, rising magma from the deeper parts of the subduction zone continuously feeds the active volcanoes of the Cascades Arc, including Mount Hood and Mount St. Helens, both visible from Portland.

Strong ground shaking from earthquakes has impacted the metro area, most recently from the 1993 magnitude 5.7 Scotts Mills earthquake. Great offshore earthquakes as large as magnitude 9 on the Cascadia megathrust have shaken the metro area every 500 years or so, most recently in 1700.

This project was supported by FEDMAP and STATEMAP, part of USGS's National Cooperative Geologic Mapping Program, and by the Earthquake Hazards Program. FEDMAP projects are based on national requirements for geologic map information in areas of multiple-issue need or areas of compelling single-issue need, national requirements for geologic map information in areas where mapping is required to solve critical earth science problems, and the needs of land management agencies of the Department of Interior. STATEMAP cooperative agreements are funded through an annual competitive grant process.