Geologic map of the Cook Inlet region, Alaska, including parts of the Talkeetna, Talkeetna Mountains, Tyonek, Anchorage, Lake Clark, Kenai, Seward, Iliamna, Seldovia, Mount Katmai, and Afognak 1:250,000-scale quadrangles

Wilson, Frederic H.; Hults, Chad P.; Schmoll, Henry R.; Haeussler, Peter J.; Schmidt, J. W.; Yehle, Lynn A.; Labay, Keith A.

doi:10.3133/sim3153

Cited by 8 publications

(5 citation statements)

References 30 publications

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“…Iliamna Volcano is a 3,053 m high, andesitic stratovolcano in the southwestern Cook Inlet of Alaska (Figure 1). The modern volcanic edifice consists of a stratified assemblage of andesitic lava flows, lahars, and pyroclastic flows, and debris‐avalanche deposits that unconformably overlie Jurassic plutonic and sedimentary rocks (Detterman & Reed, 1980; Waythomas & Miller, 1999; Wilson et al., 2012). The volcano has a substantial snow and ice cover, and is mantled by four 5‐ to 10‐km long glaciers extending from its upper flanks with a combined ice volume of 15 km 3 and an estimated ∼1 km 3 of ice above 1,000 m elevation (Figures 1 and 2; Trabant, 1999).…”

Section: Geologic Backgroundmentioning

confidence: 99%

Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

2021

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Water‐saturated, hydrothermally altered rocks reduce the strength of volcanic edifices and increase the potential for sector collapses and far‐traveled mass flows of unconsolidated debris. Iliamna Volcano is an andesitic stratovolcano located on the western side of the Cook Inlet, ∼225 km southwest of Anchorage and is a source of repeated avalanches. The widespread snow and ice cover on Iliamna Volcano make surface alteration difficult to identify. However, intense hydrothermal alteration significantly reduces both the electrical resistivity and magnetization of volcanic rock and can therefore be identified with airborne geophysical measurements. We use airborne electromagnetic and magnetic data to map snow and ice thickness and identify underlying alteration zones at Iliamna Volcano, Alaska. Resistivities were calculated to an average depth of >300 m, and a 3‐D susceptibility model extends from the surface to the base of the volcano, about 3,000 m below the summit. Geophysical models image low resistivity (<30 ohm‐m) and low susceptibilities near the summit of Iliamna and below its older vent complex, with the low susceptibilities indicating alteration up to ∼800 m in thickness. Thin conductors (∼50–100 m thick) on the edifice slopes coincide with recorded locations of repeated debris avalanches over the past ∼60 years and are attributed to saturated zones at high elevation. Three‐dimensional slope stability models based upon the geophysically constrained alteration distribution suggest the edifice of Iliamna is unstable and could lead to collapse scars ∼400 m deep near the current and former vent complexes.

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Section: Geologic Backgroundmentioning

confidence: 99%

Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

2021

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“…The simplified surficial geology is presented in Figure 2. The Chugach Mountains, composed of lightly metamorphosed greywacke, rise at the city’s eastern border (Wilson et al, 2012). Dense glacial till overlies the greywacke and is found at the surface in the lower reaches of the Chugach Mountains to the east, but it is encountered at depth to the west (Combellick, 1999; Updike and Ulery, 1986).…”

Section: Geologymentioning

confidence: 99%

Engineering site response analysis of Anchorage, Alaska, using site amplifications and random vibration theory

et al. 2022

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Earthquake records collected at dense arrays of strong-motion stations are often utilized in microzonation studies to evaluate the changes in site response due to variability in site conditions across a region. These studies typically begin with calculating Fourier spectral amplification(s) and then transition to performing engineering site response analyses. It has proven difficult to utilize Fourier spectral amplification(s) to define the appropriate elastic response spectr(um)/(a) for a site or sites. This is because, first, the ground motions recorded at these strong-motion stations have lower intensity and hence do not show the nonlinear site effects observed during higher-intensity earthquakes and, second, Fourier and response spectral amplitudes measure different aspects of ground motions. The strong-motion stations in Anchorage, Alaska, have been recording earthquakes in the region for the last three decades. This study utilizes a database of 95 events from 2004 to 2019 to calculate Fourier spectral amplifications at 35 stations using the generalized inversion technique (GIT). Estimated response spectra have been evaluated at each site by applying those Fourier spectral amplifications to a response spectrum of a reference station through random vibration theory (RVT). Correction factors are also applied within the approach to account for nonlinear site effects. This RVT-based approach is tested using ground motions recorded during the MW 7.1 2018 Anchorage Earthquake, and close matches between measured and predicted response spectra are found. The method is then compared with site response analyses using a calibrated 1D equivalent linear (EQL) model of the Delaney Park Downhole Array site. Estimated spectra using the RVT-based approach are, finally, compared with those using Next Generation Attenuation Subduction (NGA-Sub) and NGA-West2 ground-motion models. The proposed method provides a coherent and straightforward way to use GIT-derived Fourier spectral amplifications to directly estimate site-specific response spectra, accounting for nonlinear site effects and without requiring engineering characterization of subsurface soil conditions.

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“…Figure 2a shows that PGA has a nonuniform spatial distribution throughout the Anchorage basin. Unconsolidated deposits in Anchorage are highly variable and create an intricate configuration of subsurface conditions, including areas composed of glacioestuarine deposits, alluvial fan deposits, deltaic deposits, the BCF, and moraine and kame deposits (Wilson et al, 2012), while the eastern flank of the Chugach Mountains provide the highest PGAs recorded (i.e. between 0.4 and 0.6 g ).…”

Section: Spatial Variability Of Ground Motions In the Anchorage Metropolitan Areamentioning

confidence: 99%

“…The acceleration spectra for the recorded motions exhibited similar responses with PGA ranging from 0.20 to 0.30 g , with the exception of the Port Access Bridge station, which recorded a PGA of about 0.40 g (Table 1) and exhibited a large spectral spike of 2.1 g between periods of 0.2 and 0.3 s. Figure 2a does not show particularly strong high-frequency motions near to stations NSMP 8038 and K223. However, surface geology maps of the area where the PoA is located (Figure 1) show the presence of alluvial fan and colluvial deposits (Holocene and Upper Pleistocene; Wilson et al, 2012). Shallow and potentially strong impedance contrast between this material and harder rock may explain large response spectral peaks in the short period range.…”

Section: Performance Of Improved Ground Under Cyclic Loadingmentioning

confidence: 99%

“… a Descriptions of the geological units shown in the map after Wilson et al (2012) are provided as follows: Qaf – Alluvial fan deposits; Qat – Alluvium along major rivers and in terraces; Qbc – Bootlegger Cove Formation; Qd – Eolian deposits; Qdl – Deltaic deposits; Qes – Estuarine deposits; Qgc – Glacioalluvium; Qge – Glacioestuarine deposits; Qgl – Glaciolacustrine deposits; Qgm – Modified glacial deposits; Qig – Intermediate age glacial deposits; Qlc – Landslide and colluvial deposits; Qsl – Lacustrine, swamp, and fine silt deposits; Qtf – Modern tidal flat and estuarine deposits; Jsch – Greenschist and blueschist; KMn – McHugh and Uyak Complexes (undivided).…”

Section: Introductionmentioning

confidence: 99%

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Geotechnical lessons from the M_w 7.1 2018 Anchorage Alaska earthquake

et al. 2021

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The 2018 Mw 7.1 Anchorage, Alaska, earthquake is one of the largest earthquakes to strike near a major US city since the 1994 Northridge earthquake. The significance of this event motivated reconnaissance efforts to thoroughly document damage to the built environment. This article presents the spatial variability of ground motion intensity and its correlation with subsurface conditions in Anchorage, the identification of liquefaction triggering in the absence of surficial manifestations (such as sand boils or sediment ejecta), cyclic softening failure in organic soils, and the poor performance of anthropogenic fills subjected to cyclic loading. In addition to lessons from observed ground deformation and geotechnical effects on structures, this article provides case studies documenting the satisfactory behavior of improved ground subjected to cyclic loading and the appropriateness of current design procedures for the estimation of seismically induced sliding displacements of mechanically stabilized earth walls.

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Geologic map of the Cook Inlet region, Alaska, including parts of the Talkeetna, Talkeetna Mountains, Tyonek, Anchorage, Lake Clark, Kenai, Seward, Iliamna, Seldovia, Mount Katmai, and Afognak 1:250,000-scale quadrangles

Cited by 8 publications

References 30 publications

Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

Engineering site response analysis of Anchorage, Alaska, using site amplifications and random vibration theory

Geotechnical lessons from the M_w 7.1 2018 Anchorage Alaska earthquake

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Geologic map of the Cook Inlet region, Alaska, including parts of the Talkeetna, Talkeetna Mountains, Tyonek, Anchorage, Lake Clark, Kenai, Seward, Iliamna, Seldovia, Mount Katmai, and Afognak 1:250,000-scale quadrangles

Cited by 8 publications

References 30 publications

Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

Airborne Geophysical Imaging of Weak Zones on Iliamna Volcano, Alaska: Implications for Slope Stability

Engineering site response analysis of Anchorage, Alaska, using site amplifications and random vibration theory

Geotechnical lessons from the Mw 7.1 2018 Anchorage Alaska earthquake

Contact Info

Product

Resources

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Geotechnical lessons from the M_w 7.1 2018 Anchorage Alaska earthquake