S-Wave Velocity Structure of Sediments in Anchorage, Alaska, Estimated with Array Measurements of Microtremors

Abstract

The array measurements of microtremors were carried out at nine sites to estimate the subsurface S -wave velocity ( β ) structures of the sedimentary deposits beneath the metropolitan area of Anchorage, Alaska. The data were recorded by ten three-component accelerometers arranged in a triangular manner for three different array sizes. The phase velocities ( C ( f )) were estimated at each site from the vertical component of the recorded microtremor data by using the frequency-wavenumber technique. The C ( f ) data from different arrays were combined after checking their consistency for a series of overlapped frequency bands from different arrays and were inverted using a stochastic least-squares inversion technique to estimate the 1D β -structure underneath each site. The inversion results show that the engineering basement ( β >750 m/sec) lies at a relatively shallower depth (∼40 m) in the eastern part of the basin along the foothills of the Chugach Mountains (cm) and at deeper depths toward the southcentral (∼100 m) and western (∼150 m) parts of the basin in accord with the general dip of the basin. Below the engineering basement, a well- developed low-velocity zone (lvz) with β -values in the range of 900–1040 m/sec is found to be present in the eastern as well as along the Knik Arm side in the western part of the basin at a depth of 200 m and 900 m, respectively. Moreover, the central part of the basin is associated with a weakly developed lvz below the engineering basement depth. In the rest of the basin, the β -value increases gradually with depth. The spatial variations of β in the basin at different depths from 20 to 500 m are represented by using 2D interpolation of the β -structures obtained from the inversion of C ( f ) data. The depth to the crystalline basement of the basin, however, could not be ascertained and it seems to be much deeper than the maximum depth (2000 m) resolved by the data gathered in this study. To validate the results of inversion, the spectral ratio between the horizontal and vertical components (h/v) of the recorded microtremor data at each site has been compared with the computed h/v of Rayleigh waves based on the respective β -structure. The results showed good agreement in the frequency range of about 0.4–6.0 Hz. In this frequency range, the h/v peaks are due to the overall effect of the velocity contrasts between layers representing the subsurface β -structure.