Determination of Site Response in Anchorage, Alaska, on the Basis of Spectral Ratio Methods

Martirosyan, A.; Dutta, Utpal; Biswas, N. N.; Παπαγεωργιου, Αποστολοσ; Combellick, R. A.

doi:10.1193/1.1464535

Cited by 32 publications

(37 citation statements)

References 17 publications

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“…The results are shown in Figures 3-7 for five representative sites located in site classes C, C/D, and D. These site classes were estimated by Martirosyan et al (2002) based on the results of an average of 30 m shear-wave velocity data of the Anchorage basin (Dutta et al, 2000). Of the five sites, one site (K203) is located in the area of site class C on the eastern side of the basin.…”

Section: Resultsmentioning

confidence: 99%

“…Because the observed SRf for the Anchorage basin were computed as previously mentioned with respect to the rock sites (An01 and K216) located on the metamorphic terrane of the Chugach Mountains in the eastern part of the basin, we used a 1D-layered Earth model of this site to compute T ref f. Martirosyan et al (2002) has shown that the rock sites of the Anchorage basin do not have unit site response above 7.0 Hz, as these sites overlay weathered rocks due to constant freezing and thawing in the area. Recent borehole drilling data at the rock site indicate that a 4 m thick weathered rock overlays the hard rock.…”

Section: Forward Modeling and Model Parameterizationmentioning

confidence: 99%

“…Both the WM and SM stations have been distributed within the basin covering different geological formations (Fig. 1 The Fourier amplitude spectra (0.5-11.0 Hz) of the direct S-wave data from earthquakes recorded by both the WM and SM networks were used to compute SRf using standard spectral ratio (Martirosyan et al, 2002) and generalized inversion (GI) methods (Dutta et al, 2001). SRfs were computed assuming that the reference stations (An01 for WM sites and K216 for SM sites) have unit site response in the aforementioned frequency range.…”

Section: Datamentioning

confidence: 99%

“…Under this project, a number of seismological problems of earthquake engineering interest were investigated. These in-clude the estimation of S-wave site response (Dutta et al, 2001;Martirosyan et al, 2002), site classification from the shear-wave velocity structure of the uppermost 30 m of soil column using surface measurements of high-frequency Rayleigh-wave phase velocities (Nath et al, 1997;Dutta et al, 2000;Martirosyan et al, 2002;Badal et al, 2004), estimation of the shear-wave velocity structure of deeper sediments up to a depth of 1000 m from long period array measurements of microtremors (Dutta, Satoh, et al, 2007), frequency dependence of S-wave attenuation , and source parameters of local earthquakes .…”

Section: Introductionmentioning

confidence: 99%

“…In the present study, the simulated annealing (SA) inversion technique Stoffa, 1991, 1995) has been applied to the available SRf data (Dutta et al, 2001;Martirosyan et al, 2002) for the Anchorage basin to find optimal values of shear-wave velocity, thickness, density and shear-wave quality factor of the subsurface sediments (0-600 m) in such a way that an L-2 norm error between the observed and computed SRf attains a minimum. The prior bounds of the aforesaid parameters were set during the inversion process based on the available geological information.…”

Section: Introductionmentioning

confidence: 99%

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Investigation of Shallow Sedimentary Structure of the Anchorage Basin, Alaska, Using Simulated Annealing Inversion of Site Response

Dutta

Sen

Biswas

et al. 2009

Bulletin of the Seismological Society of America

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This study deals with shallow sedimentary structure of the Anchorage basin in Alaska. For this purpose, inversion of site response [SRf] data in the frequency range 0.5-11.0 Hz from various sites of the basin has been performed using the simulated annealing method to compute subsurface layer thickness, shear-wave velocity (β), density, and shear-wave quality factor. The one-dimensional (1D) models for the aforementioned parameters were obtained with preset bounds on the basis of available geological information such that the L-2 norm error between the observed and computed site response attained a global minimum. Next, the spatial distribution of the important parameter β was obtained by interpolating values yielded by the 1D models. The results indicate the presence of three distinct velocity zones as the source of spatial variation of SRf in the Anchorage basin. In the uppermost part of the basin, the β values of fine-grain Quaternary sediments mainly lie in the range of 180-500 m=sec with thickness varying from 15 to 50 m. This formation overlies relatively thick (80-200 m) coarse-grain Quaternary sediments with β values in the range of 600-900 m=sec. These two Quaternary units are, in turn, overlain on Tertiary sediments with β > 1000 m=sec located at depths of 100 and 250 m, respectively, in the central and western side along the Knik Arm parts of the basin. The important implication of the result is that the sources of spatial variation of SRf in the Anchorage basin for the frequency band 0.5-11 Hz, besides in the uppermost 30 m, are found to be deeper than this depth. Thus, use of commonly considered geological formations in the depth intervals from 0 to 30 m for the ground-motion interpretation will likely yield erroneous results in the Anchorage basin.

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Section: Resultsmentioning

confidence: 99%

Section: Forward Modeling and Model Parameterizationmentioning

confidence: 99%

Section: Datamentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Investigation of Shallow Sedimentary Structure of the Anchorage Basin, Alaska, Using Simulated Annealing Inversion of Site Response

Dutta

Sen

Biswas

et al. 2009

Bulletin of the Seismological Society of America

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Site Response in Las Vegas Valley, Nevada from NTS Explosions and Earthquake Data

Rodgers

Tkalčić

McCallen

et al. 2006

Pure appl. geophys.

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We report site response in Las Vegas Valley (LVV) from historical recordings of Nevada Test Site (NTS) nuclear explosions and earthquake recordings from permanent and temporary seismic stations. Our data set significantly improves the spatial coverage of LVV over previous studies, especially in the northern, deeper parts of the basin. Site response at stations in LVV was measured for frequencies in the range 0.2-5.0 Hz using Standard Spectral Ratios (SSR) and Horizontal-Vertical Spectral Ratios (HVR). For the SSR measurements we used a reference site (approximately NEHRP B ''rock'' classification) located on Frenchman Mountain outside the basin. Site response at sedimentary sites is variable in LVV with average amplifications approaching a factor of 10 at some frequencies. We observed peaks in the site response curves at frequencies clustered near 0.6, 1.2 and 2.0 Hz, with some sites showing additional lower amplitude peaks at higher frequencies. The spatial pattern of site response is strongly correlated with the reported depth to basement for frequencies between 0.2 and 3.0 Hz, although the frequency of peak amplification does not show a similar correlation. For a few sites where we have geotechnical shear velocities, the amplification shows a correlation with the average upper 30-meter shear velocities, V 30 . We performed two-dimensional finite difference simulations and reproduced the observed peak site amplifications at 0.6 and 1.2 Hz with a low velocity near-surface layer with shear velocities 600-750 m/s and a thickness of 100-200 m. These modeling results indicate that the amplitude and frequencies of site response peaks in LVV are strongly controlled by shallow velocity structure.

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Site Response Analysis of Anchorage, Alaska Using Generalized Inversions of Strong-Motion Data (2004–2019)

Thornley

Douglas

Dutta

et al. 2022

Pure Appl. Geophys.

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Anchorage, Alaska, is located in one of the most active tectonic settings in the world. The city and region were significantly impacted by the MW 9.2 Great Alaska Earthquake in 1964, and they were recently shaken by a MW 7.1 event in 2018. The city was developed in an area underlain by complex soil deposits of varied geological origins and stiffnesses, with the deposits’ thicknesses increasing east to west. Situated at the edge of the North American Plate, with the actively subducting Pacific Plate below, Anchorage is susceptible to both intraslab and interface earthquakes, along with crustal earthquakes. Strong-motion stations were installed across the city in an attempt to capture the variability in site response. Several previous studies have been performed to evaluate that variability but have not included larger magnitude events and have not benefited from the current density of instrumentation. The work presented here provides background information on the geology and tectonic setting of Anchorage and presents details related to the dataset and methods used to perform the site-response analysis. This study has collected strong-motion recordings from 35 surface stations across Anchorage for 95 events spanning from 2004 to 2019, including the MW 7.1 Anchorage Earthquake in 2018. The more than 1700 three-component recordings from those 95 events with moment magnitudes ranging from 4.5 to 7.1 were used to evaluate site response variability across the city. Using the Generalized Inversion Technique and a reference rock site, spectral amplifications were calculated and analyzed for frequencies between 0.25 and 10 Hz for each strong-motion station. The study results were used to develop contour maps at 1 Hz and 5 Hz, using logarithmic-band averages, to describe the variability of spectral amplifications at these two frequencies of interest. The results were also compared to geologic conditions across Anchorage, and the overlaying of different soil deposits can be seen to have an impact on the spectral amplification at the sites. The results of this study provide improvements on past microzonation studies and, using sensitivity analyses, offer support for the use of small and moderate earthquakes to evaluate spectral amplifications.

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Determination of Site Response in Anchorage, Alaska, on the Basis of Spectral Ratio Methods

Cited by 32 publications

References 17 publications

Investigation of Shallow Sedimentary Structure of the Anchorage Basin, Alaska, Using Simulated Annealing Inversion of Site Response

Investigation of Shallow Sedimentary Structure of the Anchorage Basin, Alaska, Using Simulated Annealing Inversion of Site Response

Site Response in Las Vegas Valley, Nevada from NTS Explosions and Earthquake Data

Site Response Analysis of Anchorage, Alaska Using Generalized Inversions of Strong-Motion Data (2004–2019)

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