Copies of this map are available from the Department of Geology and Mineral Industries
This is one of four companion publications presenting earthquake hazard maps for small to intermediate-sized communities in western Oregon. Each publication includes a geographic grouping of urban areas.
The results and conclusions of this report are necessarily based on limited geologic and geophysical data. The hazards and data are described in this report. At any given site in any map area, site-specific data could give results that differ from those shown on this map. This report cannot replace site-specific investigations. Some appropriate uses are discussed in the report. The hazards of an individual site should be assessed through geotechnical or engineering geology investigation by qualified practitioners.
Since the late 1980s, the understanding of earthquake hazards in the Pacific Northwest has significantly increased. It is now known that Oregon may experience damaging earthquakes much larger than any that have been recorded in the past (Atwater, 1987; Heaton and Hartzell, 1987; Weaver and Shedlock, 1989; Yelin and others, 1994). Planning the response to earthquake disasters and strengthening homes, buildings, and lifelines for power, water, communication, and transportation can greatly reduce the impact of an earthquake. These measures should be based on the best possible forecast of the amount and distribution of future earthquake damage. Earthquake hazard maps such as those in this publication provide a basis for such a forecast.
The amount of damage sustained by a building during a strong earthquake is difficult to predict and depends on the size, type, and location of the earthquake, the characteristics of the soils at the building site, and the characteristics of the building itself. At present, it is not possible to accurately forecast the location or size of future earthquakes. It is possible, however, to predict the behavior of the soil at any particular site. In fact, in many major earthquakes around the world, a large amount of the damage has been due to the behavior of the soil. In this report, "soil" means the relatively loose and soft geologic material that typically overlies solid bedrock in western Oregon.
The maps in this report identify those areas in selected Oregon communities that will be at higher risk, relative to other areas, during a damaging earthquake. The analysis is based on the behavior of the soils and does not depict the absolute earthquake hazard at any particular site. It is quite possible that, for any given earthquake, damage in even the highest hazard areas will be light. On the other hand, during an earthquake that is stronger or much closer than our design parameters, even the lowest hazard categories could experience severe damage.
This report includes a nontechnical description of how the maps were made and how they might be used. More technical information on the mapmaking methods is contained in the Appendix.
The printed report includes paper-copy Relative Earthquake Hazard Maps for each urban area, overlaid on U.S. Geological Survey topographic base maps at the scale of 1:24,000. In addition, for each area, three individual hazard component maps are included as digital data files on CD-ROM. The digital data are in two formats: (1) high-resolution -.JPG files (bitmap images) that can be viewed with many image viewers or word processors and (2) MapInfo(r) and ArcView(r) GIS vector files.
These maps were produced by the Oregon Department of Geology and Mineral Industries and were funded by the State of Oregon and the U.S. Geological Survey (USGS), Department of the Interior, under USGS award #1434-97-GR-03118. 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.
Earthquakes from three different sources threaten communities in western Oregon (Figure 1). These sources are crustal, intraplate, and subduction-zone earthquakes. The most common are crustal earthquakes, which typically occur in the North American plate above the subduction zone at relatively shallow depths of 6-12 mi (10-20 km) below the surface. The March 1993 earthquake at Scotts Mills (magnitude [M] 5.6) (Madin and others, 1993) and the September 1993 Klamath Falls main shocks (M 5.9 and M 6.0) (Wiley and others, 1993) were such crustal earthquakes.
Deeper intraplate earthquakes occur within the remains of the ocean floor (the Juan de Fuca plate) that has been subducted beneath North America. Intraplate earthquakes caused damage in the Puget Sound region in 1949 and again in 1965. This type of earthquake could occur beneath much of western Oregon at depths of 25-37 mi (40-60 km).
Great subduction-zone earthquakes occur around the world where the plates that make up the surface of the Earth collide. When the plates collide, one plate slides (subducts) beneath the other, where it is reabsorbed into the mantle of the planet. The dipping interface between the two plates is the site of some of the most powerful earthquakes ever recorded, often having magnitudes of M 8 to M 9 on the moment magnitude scale. The 1960 Chilean (M 9.5) and the 1964 Great Alaska (M 9.2) earthquakes were subduction-zone earthquakes (Kanamori, 1977). The Cascadia subduction zone, which lies off the Oregon and Washington coasts, has been recognized for many years. No earthquakes have occurred on the Cascadia subduction zone during our short 200-year historical record. However, in the past several years, a variety of studies have found widespread evidence that very large earthquakes have occurred repeatedly in the past, most recently about 300 years ago, in January 1700 (Atwater, 1987; Yamaguchi and others, 1997). The best available evidence indicates that these earthquakes occur, on average, every 500 to 540 years, with an interval between individual events that ranges from 100-300 years to about 1,000 years (Atwater and Hemphill-Haley, 1997). We have every reason to believe that they will continue to occur in the future.
Together, these three types of earthquakes could cause strong shaking through most of western Oregon. Maps are available that forecast the likely strength of shaking for all of Oregon (Geomatrix Consultants, 1995; Frankel and others, 1996; Madin and Mabey, 1996). However, these maps show the expected strength of shaking at a firm site on bedrock and do not include the significant influence of soil on the strength of shaking. They forecast a uniform level of shaking and damage in most communities, and as such they do not provide a useful tool for planning earthquake hazard mitigation measures.
Damaging earthquakes will occur in the cities and towns of western Oregon. This fact was demonstrated by the Scotts Mills earthquake (M 5.6) in 1993 (Madin and others, 1993). Although we cannot predict when the next damaging earthquake will strike, where it will occur, or how large it will be, we can evaluate the influence of site geology on potential earthquake damage. This evaluation can occur reliably even though the exact sources of earthquake shaking are uncertain.
The most severe damage done by an earthquake is commonly localized. One or more of the following phenomena generally will cause the damage in these areas:
These effects can be evaluated before the earthquake occurs, if data are available on the thickness and nature of the geologic materials and soils at the site (Bolt, 1993). Knowing the exact nature and magnitude of these effects is useful to technical professionals, and such data (in digital format) are included in this publication. For others, what is more significant is that these effects increase the damage caused by an earthquake and localize the most severe damage.
Urban areas were mapped if they had a population greater than 4,000, were in Uniform Building Code (UBC) Seismic Zone 3 or 4, and were not likely to be the subject of a more detailed future hazard mapping program. The goal of this project was to provide an inexpensive general hazard assessment for small communities that could not afford their own mapping program but were not large enough to justify a major state-funded mapping effort. Such major, full-scale projects have been undertaken for the Portland, Salem, Eugene-Springfield, and Klamath Falls urban areas; they typically take several years and cost several hundred thousand dollars. In contrast, this project involved about two weeks of work and a few thousand dollars for each urban area mapped.
For each urban area selected, the hazard map area (inside the thick black line) was defined by the urban growth boundary plus a 3,300-ft (1-km)-wide buffer.
The most important element of any earthquake hazard evaluation is the development of a three-dimensional geologic model. For analysis of the amplification and liquefaction hazards, the most important feature is the thickness of the loose sand, silt, and gravel deposits that usually overlie firm bedrock. For an analysis of the landslide hazard, the steepness of the slopes and presence of existing landslides is important. For each urban area, the geologic model was developed as follows:
The best available geologic mapping was used to determine what geologic materials were present and where they occurred. Air photos were used to help make these decisions where the
mapping was poor or of low resolution. All data were plotted digitally on USGS Digital Raster Graphics (DRG) maps (the digital equivalent of USGS 1:24,000-scale topographic maps).
Drillers' logs of water wells were examined to determine the geology beneath the surface and map the thickness of the loose surficial deposits and the depth to firm bedrock. Water wells were located according to the location information provided on the logs, which often is accurate only to within about 1,000 ft. Field location of the individual logs would have been prohibitively expensive.
The water well data were combined with the surface data to produce a three-dimensional geologic model, describing the thickness of the various geologic materials in the top 100 ft (30 m) throughout each urban area. For this procedure, MapInfo(r) and Vertical Mapper(r) Geographic Information System (GIS) software programs were used. The models take the form of a grid of thickness values spaced every 165 ft (50 m).
The resultant models were reviewed by geologists knowledgeable about each area, who judged whether the models were reasonable and consistent with the data.
Existing landslides were mapped where depicted on existing geologic maps or where air photos showed clear signs of landslide topography.
Slope data were derived from USGS Digital Elevations Models (DEMs) with elevation data every 100 ft (30 m). They were then used in MapInfo(r) and Vertical Mapper(r) to map the steepness of slopes.
The details of the local geology and data sources for each urban area are described in the "Urban Area Summaries" section of this report.
The soils and soft sedimentary rocks near the surface can modify bedrock ground shaking caused by an earthquake. This modification can increase (or decrease) the strength of shaking or change the frequency of the shaking. The nature of the modifications is determined by the thickness of the geologic materials and their physical properties, such as stiffness.
This amplification study used a method first developed for the National Earthquake Hazard Reduction Program (NEHRP) and published by the Federal Emergency Management Agency (FEMA, 1995). This method was adopted in the 1997 version of the Uniform Building Code (ICBO [International Conference of Building Officials], 1997) and will henceforth be referred to as the UBC-97 methodology. The UBC-97 methodology defines six soil categories that are based on average shear-wave velocity in the upper 100 ft (30 m) of the soil column. The shear-wave velocity is the speed with which a particular type of ground vibration travels through a material, and can be measured directly by several techniques. The six soil categories are Hard Rock (A), Rock (B), Very Dense Soil and Soft Rock (C), Stiff Soil (D), Soft Soil (E), and Special Soils (F). Category F soils are very soft soils requiring site-specific evaluation and are not mapped in this study, because limited funding precluded any site visits.
For the amplification hazard component maps, we collected shear-wave velocity data (see Appendix for data and methods) at one or more sites in each urban area and used our geologic model to calculate the average shear-wave velocity of each 165-ft (50-m) grid cell in the model. We then assigned a soil category, using the relationships in Table 1.
According to the UBC-97 methodology, none of the urban areas in this study had Type A soils. UBC-97 soil category maps for each urban area are presented in the accompanying digital map set.
Liquefaction is a phenomenon in which shaking of a saturated soil causes its material properties to change so that it behaves as a liquid. In qualitative terms, the cause of liquefaction was described very well by Seed and Idriss (1982): "If a saturated sand is subjected to ground vibrations, it tends to compact and decrease in volume; if drainage is unable to occur, the tendency to decrease in volume results in an increase in pore water pressure, and if the pore water pressure builds up to the point at which it is equal to the overburden pressure, the effective stress becomes zero, the sand loses its strength completely, and it develops a liquefied state."
Soils that liquefy tend to be young, loose, granular soils that are saturated with water (National Research Council, 1985). Unsaturated soils will not liquefy, but they may settle. If an earthquake induces liquefaction, several things can happen: The liquefied layer and everything lying on top of it may move downslope. Alternatively, it may oscillate with displacements large enough to rupture pipelines, move bridge abutments, or rupture building foundations. Light objects, such as underground storage tanks, can float toward the surface, and heavy objects, such as buildings, can sink. Typical displacements can range from centimeters to meters. Thus, if the soil at a site liquefies, the damage resulting from an earthquake can be dramatically increased over what shaking alone might have caused.
The liquefaction hazard analysis is based on the age and grain size of the geologic unit, the thickness of the unit, and the shear-wave velocity. Use of the shear-wave velocity to characterize the liquefaction potential follows Andrus and Stokoe (1997). Liquefaction hazard categories were assigned according to Table 2. In all communities we assumed that the susceptible units were saturated. This is reasonable and conservative, since most of the susceptible units are either alluvial deposits in floodplains, coastal deposits, or silt deposits in areas of low relief and high rainfall in the Willamette Valley.
The hazard due to earthquake-induced landsliding was assessed with slope data derived from USGS DEMs with 100-ft (30-m) data spacing and from mapping of existing slides, either from air photo interpretation or published geologic maps. The analysis was based on methods used by Wang, Y., and others (1998) and Wang, Z., and others (1999) but was greatly simplified because no field data were available. Earthquake-induced landslide hazard categories were assigned according to Table 3.
The Relative Earthquake Hazard Map is a composite hazard map depicting the relative hazard at any site due to the combination of the effects mentioned above. It delineates those areas that are most likely to experience the most severe effects during a damaging earthquake. Areas of highest risk are those with high ground amplification, high likelihood of liquefaction, existing landslides, or slopes steeper than 25°. Planners, lenders, insurers, and emergency responders can use these simple composite hazard maps for general hazard mitigation or response planning.
It is very important to note that the relative hazard map predicts the tendency of a site to have greater or lesser damage than other sites in the area. These zones, however, should not be used as the sole basis for any type of restrictive or exclusionary development policy.
The Relative Earthquake Hazard Maps were created to show which areas will have the greatest tendency to experience damage due to any combination of the three hazards described above. For the purpose of creating the final relative hazard map for each urban area, the zones in each of the three component maps were assigned numerical values according to
Table 4.
For every point (in a 165-ft [30-m] grid spacing) on the map, the zone rating for each individual hazard type was squared, and the resulting numbers were added together. Then the square root of this sum was taken and rounded to the nearest whole number. A result of 4 or more was assigned to Zone A, 3 to Zone B, 2 to Zone C, and 1 to Zone D.
While the production of the individual hazard maps is different from previous DOGAMI relative earthquake studies (Wang and Priest, 1995; Wang and Leonard, 1996; Mabey and others, 1997), the method of production of the final relative hazard map is very similar. Thus, these relative hazard maps are directly comparable to DOGAMI studies in Eugene-Springfield, Portland, Salem, and Siletz Bay.
The GIS techniques used to develop these maps involved several changes between vector data and raster data, with a data grid cell size of 165 ft (50 m) for the raster data. As a result, the relative hazard maps often had numerous zones that were very small, and probably not significant. The final maps were hand-edited to remove all hazard zones that covered less than 1 acre.
The Relative Earthquake Hazard Maps delineate those areas most likely to experience damage in a given earthquake. This information can be used to develop a variety of hazard mitigation strategies. The information should, however, be carefully considered and understood, so that inappropriate use can be avoided.
One of the key uses of these maps is to develop emergency response plans. The areas indicated as having a higher hazard would be the areas where the greatest and most abundant damage will tend to occur. Planning for disaster response will be enhanced by the use of these maps to identify which resources and transportation routes are likely to be damaged.
Efforts and funds for both urban renewal and strengthening or replacing older and weaker buildings can be focused on the areas where the effects of earthquakes will be the greatest. The location of future urban expansion or intensified development should also consider earthquake hazards.
Requirements placed on development could be based on the hazard zone in which the development is located. For example, the type of site-specific earthquake hazard investigation that is required could be based on the hazard.
Lifelines include road and access systems including railroads, airports, and runways, bridges, and over- and underpasses, as well as utilities and distribution systems. The Relative Earthquake Hazard Map and its component single-hazard maps are especially useful for expected-damage estimation and mitigation for lifelines. Lifelines are usually distributed widely and often require regional as opposed to site-specific hazard assessments. The hazard maps presented here allow quantitative estimates of the hazard throughout a lifeline system. This information can be used for assessing vulnerability as well as deciding on priorities and approaches for mitigation.
The hazard zones shown on the Relative Earthquake Hazard Maps cannot serve as a substitute for site-specific evaluations based on subsurface information gathered at a site. The calculated values of the individual component maps used to make the Relative Hazard Maps may, however, be used to good purpose in the absence of such site-specific information, for instance, at the feasibility-study or preliminary-design stage. In most cases, the quantitative values calculated for these maps would be superior to a qualitative estimate based solely on lithology or non-site-specific information. Any significant deviation of observed site geology from the geologic model used in the analyses indicates the need for additional analyses at the site.
It is important to recognize the limitations of a Relative Earthquake Hazard Map, which in no way includes information with regard to the probability of damage to occur. Rather, it shows that when shaking occurs, the damage is more likely to occur, or be more severe, in the higher hazard areas. The exact probability of such shaking to occur is yet to be determined.
Neither should the higher hazard areas be viewed as unsafe. Except for landslides, the earthquake effects that are factored into the Relative Earthquake Hazard Map are not life threatening in and of themselves. What is life threatening is the way that structures such as buildings and bridges respond to these effects.
The map depicts trends and tendencies. In all cases, the actual threat at a given location can be assessed only by some degree of site-specific assessment. This is similar to being able to say demographically that a zip code zone contains an economic middle class, but within that zone there easily could be individuals or neighborhoods significantly richer or poorer.
Because the maps exist as "layers" of digital GIS data, they can easily be combined with earthquake source information to produce earthquake damage scenarios. They can also be combined with probabilistic or scenario bedrock ground shaking maps to provide an assessment of the absolute level of hazard and an estimate of how often that level will occur. Finally, the maps can also be easily used in conjunction with GIS data for land use or emergency management planning.
This study does not address the hazard of tsunamis that exists in areas close to the Oregon coast and is also earthquake induced. The Oregon Department of Geology and Mineral Industries has published separate tsunami hazard maps on this subject (Priest, 1995; Priest and Baptista, 1995).
The Dallas geologic model was developed using surface geologic data from Baldwin (1964), Gannet and Caldwell (1998) and O'Connor and others (in press) and subsurface data from logs of 51 approximately located water wells.
The geology consists of Quaternary silt, sand, and gravel overlying Miocene sedimentary and volcanic bedrock (Tbs). The Quaternary deposits can be grossly divided into an older sand and gravel alluvial unit (QTac) and a younger unit of flood silt (Qmf) deposited by catastrophic Missoula floods (Bretz and others, 1956; Waitt, 1985). The model consists of a body of QTac and a body of Qmf. (Units are described in Appendix 1.)
Shear-wave velocities were assigned as follows:
The Hood River geologic model was derived from surface mapping at 1:62,500 by Beaulieu (1977) and air photo interpretation and subsurface data from 26 approximately located water wells.
The geology consists of several deposits of Quaternary sediments on top of bedrock (Tbv) that includes Miocene Columbia River basalt and Pliocene-Pleistocene lava flows and debris flows derived from the Mount Hood volcano. There are two units of Quaternary deposits: Sand, silt, and clay deposits along the shores of the Columbia River, and a body of clay on the broad flat in the middle of the area are mapped as unit Qaf. Gravel deposits at the south end of the area are mapped as QTac. (Units are described in Appendix 1.)
Shear-wave velocities are assigned as follows:
This model was developed using surface geologic from Gannett and Caldwell (1998), Yeats and others (1991), and Brownfield and Schlicker (1981). Subsurface data were obtained from 165 approximately located water well logs.
The geology consists of three units of Quaternary material deposited on top of bedrock. The bedrock (Tbs) consists of basalt flows of the Columbia River Basalt Group, tuffaceous marine sedimentary rocks of the Oligocene and Eocene Keasey and Pittsburgh Bluff Formations, tuffaceous sedimentary rocks and pillow basalts of the Eocene Nestucca Formation, and Eocene basalt and gabbro sills and dikes. The oldest Quaternary unit (QTaf) consists of Pleistocene fluvial sand, silt, clay, and gravel and is overlain by two layers of Quaternary silt deposited by catastrophic outburst floods from glacial Lake Missoula (Bretz and others, 1956; Waitt, 1985). The two units of Missoula flood silts can be distinguished by a consistent color change in driller's logs (upper brown-tan=Qmf1, lower blue-gray=Qmf2) and have different shear-wave velocity values, based on field measurements. The geologic model consists of a body of Qmf1, a body of Qmf2, and a body of QTaf. (Units are described in Appendix 1.)
Shear-wave velocity values are assigned as follows:
The Monmouth-Independence geologic model was developed using geologic data from Bela (1981), Gannet and Caldwell (1998) and O'Connor and others (in press). Subsurface geology was inferred using data from 49 approximately located water wells.
The surface geology was divided into three units: Holocene sand and gravel (Qac) deposited on the floodplain of the Willamette River, Late Pleistocene silt (Qmf) deposited by catastrophic floods from lake Missoula (Bretz and others, 1956; Waitt, 1985), and Eocene sandstone and claystone bedrock (Tbv). A unit of Pliocene-Pleistocene sand and gravel (QTaf) was recognized only in the subsurface. The model includes a body of Qac, a body of Qmf, and a body of QTaf. Qac and Qmf completely cover QTac, which pinches out to the west. (Units are described in Appendix 1.)
Shear-wave velocities are assigned as follows:
The Newberg-Dundee geologic model was developed using surface geologic data from Gannet and Caldwell (1998) and airphoto interpretation. Subsurface geology was inferred from 151 approximately located water wells and 2 shear-wave velocity profiles.
The geology is complex, particularly in the subsurface. Surficial deposits consist of two layers of Quaternary silt deposited by catastrophic Missoula floods (Bretz and others, 1956; Waitt, 1985), and sand and silt alluvium deposited on the floodplain of the Willamette River. The flood silt can be divided into an upper oxidized low-velocity layer (Qmf1) and a lower unoxidized higher-velocity layer (Qmf2). The Quaternary alluvium was combined with the Qmf1 unit because of the difficulty of distinguishing the two units in well logs and because their shear-wave velocities are likely to be similarly low.
The Quaternary units overlie a complex geology consisting of Pliocene-Pleistocene alluvial sand, silt, and clay (QTaf), laterite developed on Columbia River basalt, Columbia River basalt, and Miocene marine sediments (Tbs). There are almost certainly significant faults in the area, because the thicknesses of the above units change abruptly at several locations, but mapping these faults was not possible in this study. Numerous well logs reported red and brown clay, silt, siltstone, sandstone, and mudstone. These descriptions could conceivably be assigned to the laterite, Pliocene-Pleistocene sediments, or Miocene sediments. The complexity of these units could not be resolved in this study. Instead, we assumed that the velocities of the alluvial and the laterite units were similar and combined them under unit name QTaf. We also assumed that the velocities of unit Tbs and the Columbia River basalt were similar and combined them under unit name Tbs. The final model consists of a body of Qmf1, a body of Qmf2, a body of QTaf, and one of Tbs. (Units are described in Appendix 1.)
Shear-wave velocity is assigned as follows:
The Sandy geologic model was developed using geologic mapping (Trimble, 1963) and air photo analysis. No late Quaternary unconsolidated deposits are in the target area, and the geology consists almost entirely of Pleistocene to Miocene volcaniclastic sandstone and conglomerate and fluvial mudstone (Tbs). (Units are described in Appendix 1.) Several moderate-sized landslides occur along the steep walls of the Sandy River canyon.
Shear-wave velocity values are assigned as follows:
The geologic model was developed using surface geologic data from Brownfield (1982), Gannet and Caldwell (1998) and air photo interpretation. Subsurface geology was interpreted from 39 approximately located water wells and two shear-wave velocity profiles.
The geology consists of up to 7 m of Quaternary silt and sand (Qmf), including modern stream deposits and material deposited by catastrophic Missoula floods (Bretz and others, 1956; Waitt, 1985). The Qmf fills a shallow valley cut into shale and siltstone of the Eocene Nestucca and Yamhill Formations (Tbs). (Units are described in Appendix 1.) Although one velocity profile (site Willa01, QTaf) showed evidence of an intermediate-age and -velocity layer between the Qmf and Tbs layers, it was not possible to map this unit in other boreholes, and so it was ignored. A few ancient landslides are found in the hills along the northern edge of the area.
Shear-wave velocity is assigned as follows:
The St. Helens-Columbia City-Scappoose geologic model was developed using airphoto interpretation and existing geologic mapping from Wilkinson and others (1946). Subsurface geology was inferred from 88 approximately located water wells.
The geology of the area is somewhat complex and difficult to model. Quaternary surficial deposits consist of a fine-grained unit and a coarse grained unit. The fine-grained unit (Qaf) consists of silts and sands deposited by the modern Columbia River and by the latest Pleistocene Missoula floods (Bretz and others, 1956; Waitt, 1985). Thick deposits of clay northwest of Scappoose were included in this unit, but could be older. Also, deposits of red clay and silt over Columbia River basalt bedrock that are probably laterite were included in the fine unit. The coarse unit (Qac) consists of pebble to boulder gravel with variable amounts of silt, sand, and clay; it is probably largely gravel deposited by the late Quaternary Missoula floods but may include some older Pliocene-Pleistocene gravels. Bedrock includes Columbia River basalt, Miocene marine sedimentary rocks, and Pliocene-Pleistocene conglomerate and sandstone, all of which are combined in a single bedrock unit (Tbv). (Units are described in Appendix 1.) Several landslides were inferred from airphotos along the bluffs north and west of St. Helens.
Shear-wave velocity is assigned to the various units as follows:
Geological models were reviewed by Marshall Gannett and Jim O'Connor of the U.S. Geological Survey (USGS) Water Resources Division, Ken Lite of the Oregon Water Resources Department, Dr. Ray Wells of the USGS, Dr. Curt Peterson of Portland State University, Dr. Jad D'Allura of Southern Oregon University, and Dr. John Beaulieu, Gerald Black, and Dr. George Priest of the Oregon Department of Geology and Mineral Industries. The reports were reviewed by Gerald Black and Mei Mei Wang. Marshall Gannett and Jim O'Connor provided unpublished digital geologic data that were helpful in building the geologic models. Dr. Marvin Beeson provided unpublished geologic mapping. We are very grateful to all of these individuals for their generous assistance.
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[See MS-Excel file "Table A-1.xls" - Table A-1, Measured shear-wave velocities]
This section describes our technique for collecting and applying the shear-wave velocity data shown in the preceding table (Table A-1). The table is also available on the accompanying CD-ROM disk as a Microsoft ExcelTM spreadsheet.
SH-wave data were collected by means of a 12-channel Bison 5000 seismograph with 8-bit instantaneous floating point and 2048 samples per channel. The data were recorded at a sampling rate between 0.025 and 0.5 ms, depending upon site conditions. The energy source for SH-wave generation is a 1.5 m section of steel I-beam struck horizontally by a 4.5-kg sledgehammer. The geophones used for recording SH-wave data were 30-Hz horizontal component Mark Product geophones. Spacing between the geophones is 3.05 m (10 ft). We used the walkaway method (Hunter and others, 1984), in which a group of 12 in-line geophones remained fixed and the energy source was "stepped out" through a set of predefined offsets. Depending upon site-geological conditions, the offsets of 3.05 m (10 ft), 30.5 m (100 ft), 61.0 m (200 ft), 91.5 m (300 ft), 122 m (400 ft), and 152.4 m (500 ft) were used. In order to enhance the SH-wave and reduce other phases, 5-20 hammer strikes on each site of the steel I-beam were stacked and recorded for each offset.
The SH-wave data were processed on a PC computer using the commercial software SIP by Rimrock Geophysics, Inc. (version 4.1, 1995). The key step for data processing is to identify the refractions from different horizons. Figure A-1 shows the composited SH-wave refraction profile generated from the individual offset records, at site McMin03 (Table A-1) near Dayton, Oregon.
[See TIFF file "FigureA1.tif" - Figure A-1. Composited SH-wave refraction profile at site McMin03.]
Four refractions, R1, R2, R3, and R4 are identified in the profile.
Arrival times of the refractions were picked interactively on the PC using the BSIPIK module in SIP. The arrival time data picked from each offset record were edited and combined in the SIPIN module to generate a data file for velocity-model deduction.
Figure A-2 shows the arrival times for the refractions identified in the profile (Figure A-1). The shear-wave velocity model is generated automatically using the SIPT2 module.
[See TIFF file "FigureA2.tif" - Figure A-2. Arrival time curves of the refractions at site McMin03.]
Figure A-3 shows the shear-wave velocity model derived from the refraction data at site McMin03 (Figure A-1). The model is used to calculate an average shear-wave velocity.
[See TIFF file "FigureA3.tif" - Figure A-3. Shear-wave velocity model interpreted from refraction data at site McMin03.]
The average shear-wave velocity (Vs) over the upper 30 m of the soil profile is calculated with the formula of the Uniform Building Code (International Conference of Building Officials, 1997):
Vs = 30m/Sum{di/Vsi}
Where: di = thickness of layer i in meters and
Vsi = shear-wave velocity of layer i in m/s.
Based on the average shear-wave velocity and the UBC-97 soil profile categories as shown in Table 1 above (page 4), the UBC-97 soil classification map is generated with MapInfo(r) and Vertical Mapper(r). Soil types SE and SF can not be differentiated from the average shear-wave velocity. SE and SF are differentiated based on geologic and geotechnical data, and engineering
judgement.
