Analysis of Anomalously Large High-Frequency Amplification in Chugiak, Ak, From the 2018 Anchorage Earthquake and Aftershocks
Description:
A dense, pseudo-linear nodal ~3km long array was deployed in Chugiak, Alaska (30 km NE of Anchorage, AK) by the USGS from 2021-08-09 to 2021-09-08 around the permanent NetQuakes station ARTY where an anomoulously large peak-ground acceleration (PGA) of 1.98 g was recorded during the 2018 Mw 7.1 Anchorage, AK earthquake, as compared to ~0.3 g at a nearby site. The nodal array data for aftershocks with different magnitudes (ML 1.8 - 4.3) and azimuths confirm the anomalously large ground motions recorded at the permanent station and show similar PGA amplification at stations up to ~1 km further to the east of ARTY. The topography and Quaternary geology along the transect are characterized by a series of flat surfaces and steep slopes, interpreted as kame terraces and outwash fans, which formed during deglaciation prior to 14 kya. In this study, we perform 3D finite-difference simulations up to 10 Hz, including the 1m-resolution surface topography, to explore the cause of these unexpectedly large amplifications. The simulations, computed with a near-surface geotechnical layer (GTL) tapered to 350 m depth, match the nodal data reasonably well at stations west of ARTY as well as those near the eastern end of the array towards the mountains, while this model underpredicts in the area covered by glacial sediments. The addition of a layer of glacial sediments near the surface along the array and surroundings with depth-to-bedrock constrained by water wells and assuming a Vs30 of 400 m/s improves the fit to data from the array considerably, except for underprediction on the N-S component. Our analysis also reveals that PGAs along the array can be modulated by up to 50% by topography, with the largest effects along the local topographic gradients. Our study shows that the anomalously large high-frequency amplification recorded at and near ARTY is caused by a combination of topographic effects and near-surface low-velocity material.
Session: Physics-Based Ground Motion Modeling - II
Type: Oral
Date: 5/3/2024
Presentation Time: 11:00 AM (local time)
Presenting Author: Kim
Student Presenter: No
Invited Presentation:
Authors
Te-Yang Yeh Corresponding Author tyeh2@sdsu.edu San Diego State University |
Kim Olsen Presenting Author kbolsen@sdsu.edu San Diego State University |
Jamison Steidl jsteidl@usgs.gov University of California, Santa Barbara |
Peter Haeussler pheuslr@usgs.gov U.S. Geological Survey |
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Analysis of Anomalously Large High-Frequency Amplification in Chugiak, Ak, From the 2018 Anchorage Earthquake and Aftershocks
Category
Physics-Based Ground Motion Modeling