Nonlinear Soil Response in the Anchorage Area During the 2018 M7.1 Anchorage, Alaska Earthquake

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Ground motions from the 2018 M7.1 Anchorage earthquake and its aftershocks were recorded on an extensive strong motion array in the Anchorage, AK area and reveal nonlinear soil behavior at larger magnitudes. In order to better understand the nonlinear soil response during these earthquakes, we have investigated the site characteristics of the rock and near-rock accelerometer sites using water well logs, horizontal to vertical spectral ratios (HVSRs) and uphole travel times (available for one site only) to help constrain soil site response models. The one rock site (K216) has a downhole accelerometer at 9 m. Mainshock uphole travel time for an S to P converted phase is 0.01 s and for the S-wave arrival is 0.02s (record sampling is 200 sps). This implies a time-average Vp of ~900 m/s and Vs of ~450 m/s over the top 9 m of weathered rock at the site. A HVSR peak for K216 near 0.09 s is compatible with this interpretation. At three near-rock sites (ABBK, AMJG and ARTY) water well logs and HVSRs, along with the Delaney Park Downhole Array soil type Vs measurements of Thornley et al. (2019), help constrain the Vs soil profile at these sites. Bedrock depths from well logs are at least 40 m at ABBK (5 km NW of K216), about 29 m at AMJG and 34 m at ARTY. HVSRs at ABBK and AMJG have a peak at about 0.3 s indicating a time-averaged Vs above bedrock of at least 525 m/s for ABBK and about 450 m/s for AMJG. Due to a steep hillside location, the HVSR at ARTY is not usable due to vertical component contamination, but the well log at the site constrains the depth to bedrock and the presence of till (~900 m/s) above bedrock. 1D soil response modeling using these soil profile constraints and the downhole record at K216 are compatible with observed response spectra, indicating that the 0.6 - 2 g mainshock PGAs at the near-rock sites are mostly due to site geology. The 0.3 g rock site PGA indicates that short period nonlinear soil response for the mainshock at Anchorage basin sites (0.1 to 0.3 g) is about equal (and opposite) to the weak-motion site amplifications at these sites.

Presenting Author: Chris H. Cramer

Authors