10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska THE INFLUENCE OF GLACIAL PROCESSES ON SEDIMENTARY DEPOSITIONAL HISTORY AND EARTHQUAKE GROUND MOTIONS IN ANCHORAGE, ALASKA E. C. Cannon 1 and U. Dutta 2 ABSTRACT Tectonic and glacial processes have played a major role in the development of the Cook Inlet sedimentary basin in which Anchorage, Alaska is located. The northeast-trending basin structure is controlled by deformation associated with the Alaska-Aleutian megathrust and by collisional tectonics. Multiple Quaternary glaciations have significantly modified the landscape though erosion, transport, and deposition. Due to the temporal variations in glaciations over the Anchorage area, depositional environments produced deposits ranging from glaciomarine to terrestrial glacial moraine and outwash deposits. Analyzing the formation, transport and deposition, and classification of units within the sedimentary basin contributes to the understanding of earthquake ground motions in the Anchorage area. This paper provides an overview of the geologic structure, glacial history, composition and distribution of sedimentary units, and geologic development of the Anchorage area, based on published literature, to develop an updated geologic model. The geologic model is used to examine average shear-wave velocity to 30 m depth (Vs30) data (local Vs30 measurements and the PEER NGA database) with respect to the sedimentary units and landforms in the basin. In addition, site-condition results from the USGS Global Vs30 Map Server are compared to the geologic model. Site responses to earthquake ground motions in the Anchorage area are influenced by the depositional history of the sedimentary units in the basin, controlled in large part by the horizontal and vertical distribution of sediments resulting from Quaternary glacial processes. 1 Senior Project Geologist, Golder Associates Inc., 2121 Abbott Road, Suite 100, Anchorage, AK 99507 2 Associate Professor, School of Engineering, University of Alaska Anchorage, 3211 Providence Drive, Anchorage, AK 99508 Cannon EC, Dutta, U. The Influence of Glacial Processes on Sedimentary Depositional History and Earthquake Ground Motions in Anchorage, Alaska. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.
10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska The Influence of Glacial Processes on Sedimentary Depositional History and Earthquake Ground Motions in Anchorage, Alaska E. C. Cannon 1 and U. Dutta 2 ABSTRACT Tectonic and glacial processes have played a major role in the development of the Cook Inlet sedimentary basin in which Anchorage, Alaska is located. The northeast-trending basin structure is controlled by deformation associated with the Alaska-Aleutian megathrust and by collisional tectonics. Multiple Quaternary glaciations have significantly modified the landscape though erosion, transport, and deposition. Due to the temporal variations in glaciations over the Anchorage area, depositional environments produced deposits ranging from glaciomarine to terrestrial glacial moraine and outwash deposits. Analyzing the formation, transport and deposition, and classification of units within the sedimentary basin contributes to the understanding of earthquake ground motions in the Anchorage area. This paper provides an overview of the geologic structure, glacial history, composition and distribution of sedimentary units, and geologic development of the Anchorage area, based on published literature, to develop an updated geologic model. The geologic model is used to examine average shear-wave velocity to 30 m depth (Vs30) data (local Vs30 measurements and the PEER NGA database) with respect to the sedimentary units and landforms in the basin. In addition, site-condition results from the USGS Global Vs30 Map Server are compared to the geologic model. Site responses to earthquake ground motions in the Anchorage area are influenced by the depositional history of the sedimentary units in the basin, controlled in large part by the horizontal and vertical distribution of sediments resulting from Quaternary glacial processes. Introduction The City of Anchorage, Alaska is situated in a geologically-complex region resulting from the interplay of active tectonics and seismicity with geologic and glacial processes. The sedimentary composition and structure of deposits found here reflect the varied depositional processes that have occurred including ice, water, gravity, and wind transport. Site responses to ground motion from earthquakes are influenced by several factors, including the composition and structure of sedimentary deposits. To evaluate and improve our understanding of ground motion characteristics in Anchorage, we develop a detailed geologic model that can be used to explore and explain ground motion data. 1 Senior Project Geologist, Golder Associates Inc., 2121 Abbott Road, Suite 100, Anchorage, AK 99507 2 Associate Professor, School of Engineering, University of Alaska Anchorage, 3211 Providence Drive, Anchorage, AK 99508 Cannon EC, Dutta, U. The Influence of Glacial Processes on Sedimentary Depositional History and Earthquake Ground Motions in Anchorage, Alaska. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.
In this paper, we present an overview of the geologic and tectonic setting of the Anchorage region. We provide a summary of the previous geological and glacial research that has been completed, which forms the foundation of our efforts. Based on these publicallyavailable geologic data, we present our updated geologic model for Anchorage and describe the key features of the model. We then use the geologic model to investigate the distribution of surface and subsurface average shear-wave velocity data to 30 m depth (Vs30) [1,2] as a function of sedimentary deposits, and discuss our results with respect to previous investigations. We also compare Vs30 measurements in Anchorage with calculated values from the USGS Global Vs30 Map Server [3]. Geologic and Tectonic Setting Anchorage is located in Cook Inlet, a northeast-trending forearc basin associated with the Aleutian subduction zone [4]. The Pacific oceanic plate is moving northwest at approximately 54 to 57 mm/yr with respect to the North American plate in the northern Gulf of Alaska [5]. Plate convergence results in right-lateral, strike-slip faults in addition to reverse faults and compressional fold structures. Quaternary faults and folds include the Castle Mountain fault and the Cook Inlet fault-related folds [6]. Seismicity is generated within the downgoing Pacific plate, at the subduction zone interface, and from deformation within the upper North American plate. Bedrock in the Anchorage area consists of the Permian to mid-cretaceous McHugh Complex, a mélange consisting of graywacke, argillite, limestone, chert, and volcanic rocks [7]. The mélange is overlain by sedimentary rocks of the Tertiary Kenai Group, composed of carbonaceous siltstone, sandstone, and conglomerate [8]. The rocks of the McHugh Complex are part of the Chugach terrane, separated from the Wrangellia composite terrane to the northwest by the Border Ranges fault [9]. The Border Ranges fault was originally a north-dipping subduction thrust fault but subsequently reactivated with strike-slip and normal fault motion. The Border Ranges fault is classified as a pre-quaternary fault [6]. The late Quaternary glacial history for approximately the last 200,000 years in Upper Cook Inlet consists of the Eklutna, Knik and Naptowne Glaciations [10]. The most recent late Quaternary glaciation, the Naptowne Glaciation, began about 27 to 32 cal kya (calibrated thousands of years ago), reached a maximum extent about 23 cal kya, and ended about 11.0 cal kya [11]. Between approximately 17.5 and 16.0 cal kya, the glacial front had receded north of Anchorage, allowing for subsequent deposition of glaciodeltaic deposits and the glacioestuarine Bootlegger Cove Formation (BCF) [11,12]. The Elmendorf moraine reached a maximum southern extent in the Anchorage area about 15 cal kya [11], with the limits of glacial outwash deposits found further to the south. Previous Geologic Work Numerous researchers have developed the geologic and glacial history knowledge base of the Anchorage area. Miller and Dobrovolny [13] published the first detailed geologic map of the Anchorage area based on their work from 1949 to the 1950 s; identified key features such as the Elmendorf moraine and outwash deposits, and Bootlegger Cove Clay; and presented geologic
cross-sections based on limited borehole data. Following the M9.2 Great Alaska earthquake in 1964 and additional development in the city, Schmoll and Dobrovolny [14] published an updated geologic map. With the compilation of geotechnical borehole databases, Ulery and others [15] presented geologic maps of southwestern Anchorage displaying the top and base depth, and isopach data for the cohesive units of the Bootlegger Cove Formation (renamed from the Bootlegger Cove Clay to represent the compositional range within the formation). Later, Updike and Ulery [16] released a surficial geologic map of southwest Anchorage with map and crosssections based on the borehole databases. Combellick [8] also used an extensive borehole database to develop a surficial geologic map and cross-sections for central and eastern Anchorage. In addition, Schmoll and others [17] developed a geologic map of Fire Island west of Anchorage. A similar pattern of research advancement is seen in the development of understanding the glacial history of the Anchorage area. Some of the earliest mid-20th century publications include Péwé and others [18] and Karlstrom [19] working to establish the Pleistocene glacial framework, along with Miller and Dobrovolny [13]. Karlstrom [20] provided a summary of the four major Pleistocene glacial stades: Caribou Hills, Eklutna, Knik, Naptowne. Reger and Updike [10] summarized the glacial history of Upper Cook Inlet. Further constraints on the timing of late Wisconsin glacial events, including deposition of the BCF in Upper Cook Inlet, are provided by Reger and others [12]. Reger and others [11] summarized the late Quaternary glacial history of the Kenai Peninsula, including Upper Cook Inlet. Updated Anchorage Geologic Model Based on publically-available geologic data, we have assembled an updated geologic model for the Anchorage area (Fig. 1). The three main features of the geologic model include: (1) combining and reconciling several geologic datasets, including subsurface geologic information, to develop a three-dimensional geologic model, (2) building the model using a Geographic Information System (GIS) format, and (3) incorporating the National Elevation Dataset 1/3 arc second digital elevation model [21] into the geologic model. Data from the following geologic reports are included in the geologic model: east and central Anchorage [8], southwest Anchorage [16], Fire Island [17]. To reconcile differences in geologic interpretations between these reports, and also to include geologic data coverage in the northern and southern parts of Anchorage, we include [14]. Top and base depth, and isopach data for the cohesive facies of the BCF are incorporated from [15]. Faults are from [6] and references sited therein. We follow the geologic unit convention of [8] to define the geologic units in our geologic model. General compositional and density/consistency descriptions include: Alluvium sand, gravel, minor silt, Holocene; loose Artificial Fill - gravel, sand, silt, local organics; loose to dense Bedrock - Mesozoic metamorphic rock and Tertiary sedimentary deposits, undifferentiated Colluvium - gravel, sand, silt, some clay; loose
Glacial Drift - sand, gravel, silt, clay; dense to very dense Glacioestuarine or Eolian Deposits - silt, fine sand, local medium to coarse sand and gravel; includes the non-cohesive facies of the BCF; dense to very dense Glacioestuarine or Lacustrine Deposits - silt, clay, local sand and gravel; includes the cohesive facies of the BCF; very soft to stiff Glaciofluvial, Glaciodeltaic, and Alluvial Fan Deposits - sand, gravel, local minor silt and clay; includes the Elmendorf outwash deposits Landslide Deposits displaced sand and gravel deposits underlain by silt and clay Tidal Deposits - fine sand, silt Glacial features of our geologic model include the Skilak and Elmendorf moraine limits [10,11], and Elmendorf outwash deposit limits [8,14,16]. Two features of the BCF are included in the model: (1) top and base depth, and isopach data for the fine-grained cohesive facies of the BCF in southwest Anchorage [15], and (2) a boundary representing approximately 10 m (30 feet) or greater thickness of the fine-grained cohesive facies of the BCF in central and southwest Anchorage [8]. Discussion of Updated Geologic Model Our updated geologic model is largely based on the initial efforts of [13,14], followed by additional refinement through extensive research of borehole databases [8,15,16]. The main features of our geologic model, many of which have been identified in the previous reports, are as follows: Cohesive Facies of BCF A zone of cohesive BCF facies extends from north of downtown Anchorage south to southwestern Anchorage along Turnagain Arm. Maximum thickness of the cohesive BCF facies in southwest Anchorage is approximately 60 to 70 m (roughly 195 to 230 feet). The eastern and western boundaries of the zone representing greater than approximately 10 m (30 feet) thickness of cohesive BCF facies are shown in Fig. 1 in central and southwest Anchorage. Elmendorf Moraine and Outwash Deposits The southern limit of the Elmendorf moraine is located north of downtown Anchorage. Elmendorf outwash deposits extend southwest towards the Lake Hood-Turnagain Heights area. The outwash deposits are on the order of 15 m (50 feet) thick in the downtown area, and cover both glacial drift found to the east, and BCF deposits generally found to the west. The limits of the Elmendorf moraines and outwash deposits, in addition to the Skilak moraines, are shown in Fig. 1. Glacial Deposits The majority of the Anchorage area is underlain at depth by glacial drift, and glaciofluvial and ice contact deposits. Depth to Bedrock Bedrock crops out along the base of the Chugach Mountains to the east. Depth to bedrock is approximately 460 m (1,600 feet) at Pt. Campbell [8] and about 365 m (1,200 feet) at the mouth of Turnagain Arm [22]. The top of bedrock may dip to the northwest at an overall slope on the order of 40H:1V to 50H:1V across the Anchorage area.
Vs30 Distribution We evaluate the distribution of average shear-wave velocity to 30 m depth (Vs30) data in the Anchorage area based on our updated geologic model. We reviewed a total of fifty Vs30 instrumentally-derived measurements from local Vs30 measurements [1] and the Pacific Earthquake Engineering Research Center (PEER) Next Generation Attenuation (NGA) database [2]. Twenty-two records are from surface and downhole instruments used to determine subsurface shear wave structure using the CXW surface measurement technique [1]. An additional twenty-eight records are from the PEER NGA database [2] recorded from the Nenana Mountain earthquake (Central Alaska Range) of October 23, 2002. The instrumentally-derived Vs30 values are spatially distributed across the Anchorage area and across the geologic units. From the fifty measured Vs30 values analyzed, the smallest measured Vs30 value is 191.3 m/s, located in west Anchorage in a spruce bog area where the cohesive facies of the BCF is estimated to be approximately 10 to 15 m (30 to 50 feet) thick. The largest measured Vs30 value is 582.0 m/s, located in eastern Anchorage where bedrock may be overlain by an estimated 60 to 90 m (200 to 300 feet) of glacial drift. Fig. 2 shows the locations of Vs30 stations across Anchorage including color-coded measured Vs30 values. The Vs30 data follow a general pattern of velocities greater than 360 m/s (Site Class C) located to the east along the Chugach Mountains and adjacent to the mountain front, and velocities less than 360 m/s (Site Class D) located in the downtown area and southwest Anchorage. This pattern of site classes and velocities is similar to the spatial distribution of Vs30 values noted by [1]. The boundary representing the change between Site Class C and Site Class D (360 m/s) generally follows the eastern limit of the cohesive BCF facies zone greater than 10 m thick. Our observations of measured Vs30 values generally agree with the ranges of shear wave velocities determined for soils in Anchorage [23] 100 to 200 m/s for soft and highly organic soils, 200 to 375 m/s for stiff clays and fine grained soils, and greater than 375 m/s for sandy and gravelly soils. Frequency-specific site response (SR) values are also found to correlate with the spatial distribution pattern of Vs30 data. Three studies analyzed site response (SR) data from the Anchorage basin using recorded local earthquake data from Anchorage strong motion stations [24-26]. According to these studies, at 1 Hz (low frequency), the SR values increase from 1.5 from the eastern side of Anchorage to around 3.0 in the west-central and northwestern parts of the city. SR values of approximately 1.5 are located on the deposits of older glacial drift, classified as Site Class C [1]. However, the areas with SR values greater than 2.0 are underlain by the BCF classified as Site Class D soils in the western portion of the city. At 5 Hz (high frequency), the trend of spatial distribution of SR changes from the pattern noted at low frequency. High SR values of approximately 3.0 are observed in the south and southeastern parts of the city adjoining Turnagain Arm, and SR values greater than 2.0 are located near the center of the city. A positive correlation between the Vs30 data and SR values is observed at low frequency, while no correlation occurs between the SR and Vs30 data at high frequency [1].
Vs30 Topographic Slope Proxy Topographic slope can be used as a proxy to provide Vs30 estimates [27,28]. The assumptions are that for soils, (1) shear-wave velocity generally increases as the grain size of the soil increases, and (2) deposition of larger grains occurs on steeper slopes. For lower angle slopes where deposition of fine-grained material is anticipated, lower shear-wave velocities would be expected. However, these assumptions do not account for other factors, such as glaciation and glacial erosion, and other geologic influences on depositional processes. We compared the local Vs30 measurements [1] and PEER NGA Vs30 values [2] to the USGS Vs30 data [3] for Anchorage from the USGS Vs30 Global Server. The topographic data in [3] is based on Shuttle Radar Topography Mission (SRTM) 30-second digital elevation model data. We selected the active tectonics coefficients from the USGS Vs30 Global Server. For the location of each measured Vs30 value [1,2], we extracted the predicted Vs30 value from the USGS dataset [3], to compare the measured and predicted Vs30 values using a percent error calculation. We calculated the estimated Vs30 percent error for each station by subtracting the measured value from predicted value, dividing the result by the measured value, and multiplying by 100. A zero percent error indicates that the predicted and measured Vs30 values are equal. A negative percent error indicates that the predicted Vs30 value underestimates the measured value, and conversely a positive percent error indicates that the predicted Vs30 value overestimates the measured value. Based on fifty Vs30 stations, the average Vs30 percent error is -7%, with a standard deviation (1-sigma) of 29%. Thirty-two stations have Vs30 percent errors within ±1-sigma, 9 stations have Vs30 percent errors greater than 22% (+1-sigma), and 9 stations have Vs30 percent errors less than -36% (-1-sigma). The negative average percent error for Vs30 values may indicate that the Vs30 Global Server active tectonic coefficients [3] should be adjusted using Anchorage topographic and measured Vs30 data for Vs30 analysis in the Anchorage area. The standard active tectonic coefficients [28,29] are derived from Vs30 and topographic slope correlations from California, Utah, Italy, and Taiwan. For the Vs30 stations where the Vs30 percent errors are less than -36%, these sites are typically located on low-angle terrain underlain by glacial drift, and we suggest that faster shearwave velocities in the glacial drift contribute to faster measured Vs30 values. For the Vs30 stations where the Vs30 percent errors are greater than 22%, some sites are located on moderate slopes, and additional sites are located in the non-cohesive facies of the BCF with local hummocky topography the topographic slope estimates from the digital terrain model for these sites may be influencing the predicted Vs30 values resulting in higher predicted values. Finally, some sites are located in the cohesive facies of the BCF and soil site conditions may be such that low measured Vs30 values occur locally.
Summary The City of Anchorage is situated in a geologically-complex area with influences from tectonic, seismic, geologic, and glacial processes. To evaluate and improve our understanding of earthquake ground motions in Anchorage, we developed a detailed geologic model that can be used to explore and explain ground motion data. Our geologic model is a compilation of previous geologic reports [8,13-17] that we combined into a GIS format. In addition, we include information on glacial history from [10-12]. The main features of the updated geologic model include the depth distribution of the cohesive BCF; limits of the Elmendorf and Skilak moraines, and Elmendorf outwash deposits; spatial distribution of surficial geologic units; and depth to bedrock. We analyzed local Vs30 measurements [1] and PEER NGA instrumentally-derived Vs30 data [2]. The Vs30 data follow a general pattern of velocities greater than 360 m/s (Site Class C) located to the east along the Chugach Mountains and adjacent to the mountain front, and velocities less than 360 m/s (Site Class D) located in the downtown area and southwest Anchorage. We compared the measured Vs30 values to the predicted Vs30 values from the USGS Vs30 Global Server [3] and found that for about one third of the Vs30 sites, factors such as local topography, or the presence of glacial drift or the cohesive facies of the BCF, may be responsible for predicted Vs30 values underestimating or overestimating measured Vs30 values outside the 1-sigma range of the statistical analysis. Our updated geologic model is a preliminary step towards developing a basin-scale geologic model which can be used to study earthquake ground motions in the Anchorage area. Further refinement of the geologic model constrained by three-dimensional data will strengthen the usefulness of the model. Acknowledgments The primary author thanks John Thornley of Golder Associates Inc. for discussions related to development of this paper. The Generic Mapping Tools (GMT) were used for analysis and figure development [29]. References 1. Dutta U, Biswas N, Martirosyan A, Nath S, Dravinski M, Papageorgiou A, Combellick R. Delineation of spatial variation of shear wave velocity with high-frequency Rayleigh waves in Anchorage, Alaska. Geophysical Journal International 2000; 143: 365-375. 2. PEER NGA. Pacific Earthquake Engineering Research Center, PEER NGA Database Flatfile. Regents of the University of California 2005; Website: http://peer.berkeley.edu/nga/flatfile.html 3. USGS. Global Vs30 Map Server. U.S. Geological Survey 2013; Website: http://earthquake.usgs.gov/hazards/apps/vs30/ 4. Haeussler PJ, Bruhn RL, Pratt TL. Potential seismic hazards and tectonics of the upper Cook Inlet basin, Alaska, based on analysis of Pliocene and younger deformation. Geological Society of America Bulletin 2000; 112 (9): 1414-1429. 5. Carver G, Plafker G. Paleoseismicity and Neotectonics of the Aleutian Subduction Zone An Overview, in: Freymueller JT, Haeussler PJ, Wesson RL, Ekström G (eds.), Active Tectonics and Seismic Potential of Alaska 2008; Monograph 179: 43-64, American Geophysical Union, Washington D.C.
6. Koehler RD. Quaternary Faults and Folds (QFF). Alaska Division of Geological & Geophysical Surveys Digital Data Series 2013; 3, Website: http://maps.dggs.alaska.gov/qff/ 7. Karl SM, Bradley DC, Combellick RA, Miller ML. Field guide to the Accretionary Complex and Neotectonics of South-Central Alaska, Anchorage to Seward 2011; Alaska Geological Society, Anchorage, AK. 8. Combellick RA. Simplified geologic map and cross sections of central and east Anchorage, Alaska. Alaska Division of Geological & Geophysical Surveys Preliminary Interpretive Report 1999; 1999-1. 9. Trop JM, Ridgway KD. Mesozoic and Cenozoic tectonic growth of southern Alaska: A sedimentary basin perspective. Geological Society of America Special Paper 2007; 431: 55-94 10. Reger RD, Updike RG. Upper Cook Inlet Region and the Matanuska Valley, in: Péwé TL, Reger RD (eds.) Guidebook to Permafrost and Quaternary Geology along the Richardson and Glenn Highways between Fairbanks and Anchorage, Alaska 1983; Guidebook 1: 185-263, Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK. 11. Reger RD, Sturmann AG, Berg EE, Burns PAC. A guide to the late Quaternary history of northern and western Kenai Peninsula, Alaska 2007; Guidebook 8, Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK. 12. Reger RD, Combellick RA, Brigham-Grette J. Late-Wisconsin events in the Upper Cook Inlet region, southcentral Alaska, in: Combellick RA, Tannian F (eds.), Short notes on Alaska Geology 1995. Alaska Division of Geological & Geophysical Surveys Professional Report 1995; 117D: 33-45. 13. Miller RD, Dobrovolny E. Surficial geology of Anchorage and vicinity, Alaska. U.S. Geological Survey Bulletin 1959; 1093. 14. Schmoll HR, Dobrovolny E. Generalized geologic map of Anchorage and vicinity, Alaska. U.S. Geological Survey Miscellaneous Geologic Investigations Map 1972; 787-A. 15. Ulery CA, Updike RG, USGS Office of Earthquakes. Subsurface structure of the cohesive facies of the Bootlegger Cove formation, southwest Anchorage. Alaska Division of Geological & Geophysical Surveys Professional Report 1983; 84. 16. Updike RG, Ulery CA. Engineering-geologic map of southwest Anchorage, Alaska. Alaska Division of Geological & Geophysical Surveys Professional Report 1986; 89. 17. Schmoll HR, Dobrovolny E, Gardner CA, 1981, Preliminary geologic map of Fire Island, municipality of Anchorage, Alaska. U.S. Geological Survey Open-File Report 1981; 81-552. 18. Péwé TL. Multiple glaciation in Alaska; a progress report. U.S. Geological Survey Circular 1953; 289. 19. Karlstrom TNV. Upper Cook Inlet Region, Alaska, in: Péwé TL (ed.), Multiple glaciation in Alaska; a progress report. U.S. Geological Survey Circular 1953; 289: 3-5. 20. Karlstrom TNV. Quaternary geology of the Kenai Lowland and glacial history of the Cook Inlet region, Alaska. U.S. Geological Survey Professional Paper 1964; 443. 21. NED. National Elevation Dataset, The National Map Viewer. U.S. Geological Survey 2013; Website: http://viewer.nationalmap.gov/viewer/ 22. Hartman DC, Pessel GH, McGee DL. Stratigraphy of the Kenai group, Cook Inlet. Alaska Division of Geological & Geophysical Surveys Open-File Report 1974; 49. 23. Nath SK, Chatterjee D, Biswas NN, Dravinski M, Cole DA, Papageorgiou A, Rodriquez JA, Poran CJ. Correlation Study of Shear Wave Velocity in Near Surface Geological Formations in Anchorage, Alaska. Earthquake Spectra 1997; 13 (1): 55-75. 24. Dutta U, Martirosyan A, Biswas N, Papageorgiou A, Combellick R. Estimation of S-Wave Site Response in Anchorage, Alaska, from Weak-Motion Data Using Generalized Inversion Method. Bulletin of the Seismological Society of America 2001; 91 (2): 335 346. 25. Martirosyan A, Dutta U, Biswas N, Papageorgiou A, Combellick R. Determination of the site response in Anchorage, Alaska, on the basis of spectral ratio methods. Earthquake Spectra 2002; 18: 85 104. 26. Dutta U, Biswas N, Martirosyan A, Papageorgiou A, Kinoshita S. Estimation of earthquake source parameters and site response in Anchorage, Alaska from strong motion network data using generalized inversion method. Physics of the Earth and Planetary Interiors 2003; 137: 13 29. 27. Allen TI, Wald DJ. Topographic Slope as a Proxy for Seismic Site-Conditions (VS30) and Amplification around the Globe. U.S. Geological Survey Open-File Report 2007; 2007-1357. 28. Wald DJ, Allen TI. Topographic Slope as a Proxy for Seismic Site Conditions. Bulletin of the Geological Society of America 2007; 97 (5): 1379-1395. 29. Wessel P, Smith WHF. New, improved version of Generic Mapping Tools released. EOS Transactions of the American Geophysical Union 1998; 79 (47): 579.
Figure 1. Generalized geologic map of the Anchorage, Alaska area, based on: geologic units [8,14-17], glacial geology [10], Border Ranges fault [6], shaded relief digital elevation model [21].
Figure 2. Map of instrumentally-derived measurements of Vs30 (circle symbols) from local Vs30 measurements [1] and the PEER NGA database [2]; and predicted Vs30 values (map background) from the USGS Vs30 Global Server pre-defined mapping (slope type: tectonic) for Anchorage, Alaska [3]. Vs30 velocity and site class legend based on [27].