Comparison of the Site Response in Anchorage from Mainshock and Aftershocks of 30 November 2018, Anchorage Earthquake

Session: Forthcoming Updates of the USGS NSHMs: Hawaii, Conterminous U.S. and Alaska
Type: Oral
Date: 4/28/2020
Time: 03:30 PM
Room: 230 + 235
Description: 

Anchorage earthquake (M7.1) on November 30, 2018, is the largest earthquake to shake Anchorage since the 1964 Great Alaska Earthquake. The epicenter of the mainshock was about 14 km northwest of Anchorage and at a depth of 41.8 km. Approximately thirty strong motions stations deployed at various parts of the Anchorage bowl covering major geological units have recorded the ground motions. In addition to the mainshock, these strong-motion stations also recorded numerous aftershocks of magnitude between 4.5 to 5.8 with an excellent signal to noise ratio. The suites of ground motion data collected from both the main and aftershocks form the basis of the present analysis. The standard spectral ratio method in the frequency band of 0.1-10.0 Hz using Fourier amplitude spectra of 10-sec recorded S-wave data starting from the arrival of the S-wave was used to compute the site response (SR) at each site with respect to a reference rock site (K216) located on the Chugach Mountains at the eastern part of the Anchorage basin. The complexities of the ground motions at various parts of the basin are studied by comparing the SR of the mainshock and aftershocks at each site along with their subsurface soil profiles. A significant low SR in the frequency range between 2.5-5.0 Hz is observed at a few locations in Anchorage basin during the mainshock when compared with the logarithmic averaged SR computed from all the aftershocks, indicating a strong non-linear soil behavior at these sites. However, for other sites, the SR values from the mainshock are observed to be well within ±1 standard deviations of the averaged SR from the aftershocks. Besides SR, the mean (RotD₅₀) and maximum (RotD₁₀₀ ) direction of the response are computed at each period independently from the pseudo-spectral acceleration data from two recorded horizontal components. The presentation will also provide a detailed analysis of the spatial distribution of the RotD₅₀ and RotD₁₀₀ and their ratio (RotD₅₀/ RotD₁₀₀ ) as a function of the period and their relation with the site class or the subsurface soil type.

Presenting Author: Utpal Dutta

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