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Open-File Report O-13-12, 3D geology and shear-wave velocity models of the Portland, Oregon, metropolitan area, by Warren P. Roe and Ian P. Madin.

Order publication DVD, $30, from Nature of the Northwest. DVD includes:

from the report introduction:
Understanding local geologic and geotechnical conditions is essential for accurately assessing earthquake and landslide hazards in the Portland metropolitan area. This recognition led the Oregon Department of Geology and Mineral Industries (DOGAMI) to develop earthquake hazard geology maps in the 1990s that provided 1:24,000 scale surficial geologic mapping along with isopachs of the thickness of critical surficial deposits and depth-to-bedrock (Madin, 1990). These maps formed the basis for a series of subsequent relative earthquake hazard maps that used a variety of analytical techniques to produce maps of relative susceptibility to earthquake induced liquefaction, landsliding and amplification of ground shaking (Mabey and others, 1993, 1995a,b,c,d,e, 1997). The geologic maps and thickness data were based on a modest number of geotechnical boring logs, water well logs, and measured shear wave velocity profiles. Recognizing that far more geotechnical data and better mapping tools (like high-resolution topography from lidar) had become available in the intervening years, in 2004 DOGAMI began collecting new surface and subsurface geologic data for the Portland metropolitan area with funding support from the U.S. Geological Survey (USGS) National Earthquake Hazard Reduction Program (NEHRP). The purpose of the NEHRP project has been to provide current high-resolution and high-quality geology data to support urban earthquake hazard assessments and modeling. NEHRP-funded mapping projects preceding this report include an initial regional compilation (Madin, 2004); the Oregon City (Madin, 2009), Dixie Mountain (Madin and Niewendorp, 2008), and Linnton (Madin and others, 2008) 7.5-minute quadrangles; and a regional lidar-based surficial geologic map (Ma and others, 2012). The NEHRP project has been carried out in close cooperation with the USGS Pacific Northwest Geologic Hazards and Urban Mapping Program, under which a new regional geologic map is being prepared.

As described in this report, the final step in the long-term NEHRP project was to create three-dimensional (3D) models of shallow geology and a shear-wave velocity (Vs) model of the upper 98.4 ft (30 m) (Vs30) of the greater Portland metropolitan area. The purpose of building these models was to better constrain and delineate geologic hazards associated with surficial geology such as ground-shaking amplification, earthquake-induced liquefaction and lateral spread, and earthquake-induced landslides. A second goal was to design and develop a drill hole database that met our requirements for creating 3D models. We designed the database to be flexible enough to accommodate future geographic, geologic, or geotechnical additions and to serve as a repository for various types of subsurface investigations. This 3D modeling project was begun in 2012 and completed in 2013.

Using ArcGIS version 10.1 software, we created a randomly placed dense set of points covering the project area and overlaid them with generalized and modified geologic map polygons from Ma and others (2012) and a high-resolution (better than 1 ft [0.3 m]) digital elevation model (DEM). We combined these points with subsurface interpretations from over 10,000 drill holes we located and interpreted for this project. Using the geographic coordinates, elevation information, and geologic interpretations at each point, we fitted surfaces through these points to create digital 3D models of each geologic unit’s top, bottom, and thickness. From these 3D models, and with further analysis of Vs data and trends, we created a Vs30 model for the project area. This report describes modeling parameters and assumptions, geologic observations, the 3D and Vs30 models, the drill hole database built to support them, and the overall modeling process. Digital results of the project are in accompanying ArcGIS files, an ArcGIS file geodatabase, a Microsoft Access database, and a Microsoft Excel spreadsheet.

[...] The report is divided into two main sections: 3D Physical Models and Shear-Wave Velocity Models. The 3D physical models are 3D depictions of generalized geologic units derived from mapped geologic units (defined by Ma and others, 2012; see Table 2) combined with subsurface geologic interpretations made using the drill hole database compiled for this study. The 3D depictions are in the form of ArcGIS Triangulated Irregular Networks (TINs) and rasters and are referred to in the report as “3D models.” In the second section, shear-wave velocity models describe raster-based Vs30 models derived directly from the 3D physical models and are referred to in the report as the “Vs30 model(s).” In both sections, the term “model” is used to emphasize that results are modeled from geologic interpretations and are subject to spatial, vertical, and qualitative errors in the data inputs (e.g., “interpreter’s bias” in geologic interpretations). Related references to “modeling” indicate actual processes used to create the 3D physical and Vs30 models. A distinction is also drawn between mapped geologic units and “model geologic units” (or “model units”) defined for this project, which are derived from mapped geology. Model units are similar to mapped geologic units but vary conceptually. For example: in reality, Boring lavas are likely have difficult-to-model volcanic conduits, intrusive bodies, and interfingering relationships with sedimentary units. However, as model units, Boring lavas are treated as physically and temporally distinct stratigraphic units, which is a generalization adequate for the purposes of this project. We use “model unit” to emphasize these differences and to distinguish model-based geology from field-mapped geology.

This study and the preceding NEHRP project work were funded by the USGS NEHRP program through assistance awards 03HQAG0013, 08AG0140, and G11AC20312. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

References

Ma, L., Madin, I. P., Duplantis, S., and Williams, K. J., 2012, Lidar-based surficial geologic map and database of the greater Portland area, Clackamas, Columbia, Marion, Multnomah, Washington, and Yamhill counties, Oregon, and Clark County, Washington: Portland, Oreg., Oregon Department of Geology and Mineral Industries Open-File Report O-12-02, 30 p, scale 1:63,360.

Mabey, M. A., and Madin, I. P., 1995, Downhole and seismic cone penetrometer shear-wave velocity measurements for the Portland metropolitan area, 1993 and 1994: Portland, Oreg., Oregon Department of Geology and Mineral Industries Open-File Report O-95-07, 67 p.

Mabey, M.A., Madin, I. P., Youd, T. L., Jones, C. F., 1993, Earthquake hazard maps of the Portland quadrangle, Multnomah and Washington counties, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Geologic Map GMS-79, 98 p., scale 1:24,,000.

Mabey, M. A., Madin, I. P., Meier, D. B., and Palmer, S. P., 1995a, Relative earthquake hazard map of the Mount Tabor quadrangle, Multnomah County, Oregon, and Clark County, Washington: Portland, Oreg., Oregon Department of Geology and Mineral Industries Geologic Map GMS-89, 5 p., scale 1:24,000.

Mabey, M. A., Madin, I. P., and Meier, D. B., 1995b, Relative earthquake hazard map of the Beaverton quadrangle, Washington County, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Geologic Map GMS-90, 6 p., scale 1:24,000.

Mabey, M. A., Madin, I. P., and Meier, D. B., 1995c, Relative earthquake hazard map of the Lake Oswego quadrangle, Clackamas, Multnomah, and Washington counties, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Geologic Map GMS-91, 6 p., scale 1:24,000.

Mabey, M. A., Madin, I. P., and Meier, D. B., 1995d, Relative earthquake hazard map of the Gladstone quadrangle, Clackamas and Multnomah counties, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Geologic Map GMS-92, 5 p., scale 1:24,000.

Mabey, M. A., Madin, I. P., Black, G. L., and Meier, D. B., 1995e, Relative earthquake hazard map of the Linnton quadrangle, Multnomah and Washington counties, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Geologic Map GMS-104, scales 1:24,000, scale 1:55,000.

Mabey, M.A., Black, G. L., Madin, I. P., Meier., D. B., Youd, T. L., Jones, C. F., and Rice, J. B., 1997, Relative earthquake hazard map of the Portland metro region, Clackamas, Multnomah, and Washington counties, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Interpretive Map IMS-1, various scales.

Madin, I. P., 1990, Earthquake-hazard geology maps of the Portland metropolitan area, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries, 21 p., scale 1:24,000.

Madin, I. P., 2004, Geologic mapping and database for Portland area fault studies, final technical report: Portland, Oreg., Oregon Department of Geology Open-File Report O-04-02, 18 p.

Madin, I. P., 2009, Geologic map of the Oregon City 7.5’ quadrangle, Clackamas County, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Geologic Map GMS-119, 46 p., plus app., scale 1:24,000.

Madin, I. P., and Burns, W. J., 2013, Ground motion, ground deformation, tsunami inundation, coseismic subsidence, and damage potential maps for the 2012 Oregon resilience plan for Cascadia subduction zone earthquakes: Portland, Oreg., Oregon Department of Geology and Mineral Industries Open-File Report O-13-06, 36 p.

Madin, I. P., and Niewendorp, C. A., 2008, Preliminary geologic map of the Dixie Mountain 7.5’ quadrangle, Multnomah, Washington and Columbia counties, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Open-File Report O-08-07, 43 p., scale 1:24,000.

Madin, I. P., Ma, L., and Niewendorp, C. A., 2008, Preliminary geologic map of the Linnton 7.5’ quadrangle, Multnomah and Washington counties, Oregon: Portland, Oreg., Oregon Department of Geology and Mineral Industries Open-File Report O-08-06, 35 p., scale 1:24,000.