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Analysis of anomalously large high-frequency amplification in Chugiak, AK, from the 2018 Anchorage earthquake and aftershocks

Kim B. Olsen, Te-Yang Yeh, & Jamison H. Steidl

Submitted September 10, 2023, SCEC Contribution #13214, 2023 SCEC Annual Meeting Poster #169

A dense, pseudo-linear nodal ~3km long array was deployed in Chugiak, Alaska (30 km NE of Anchorage, AK) by the USGS (2021-08-09 to 2021-09-08) around the permanent station ARTY, where anomalously large high-frequency ground motions were recorded during the 2018 Mw 7.1 Anchorage, AK earthquake and subsequent aftershocks. The topography along the transect is characterized by a series of terraces and steep slopes, formed by glacial activity. The nodal array data confirms the anomalously large ground motions recorded at the permanent station, and show similar PGA amplification patterns for aftershocks with different magnitude and azimuth. In this study, we have performed 3D finite-difference simulations up to 10 Hz, including 5m-resolution surface topography, to decipher the cause of these unexpectedly large amplifications. We used the 3D ambient noise tomography model by Berg et al. (2020), where we embedded the Cook Inlet basin structure into the model using the generic velocity and density relations from Brocher (2008) along with the Mesozoic unconformity depth map from Skarlatoudis et al. (2022). Our results show that PGAs can be modulated by up to 50% by topography, with the largest effects along the local topographic gradients. However, the spectral energy between 4-10 Hz in the data is severely underpredicted by the simulations when considering the surface topography alone, suggesting the presence of site effects from low-velocity material at shallow depths. We then used a series of trial-and-error simulations, varying the shear wave speed and depth extent in the top 12 m of a near-surface low-velocity layer along the area where the larger PGAs were recorded. The addition of the shallow low-velocity layer in the simulations improved the PGA pattern in the synthetics along the nodal array considerably. Thus, our results suggest 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.

Key Words
topographic effects, site effects, high-frequency amplification

Olsen, K. B., Yeh, T., & Steidl, J. H. (2023, 09). Analysis of anomalously large high-frequency amplification in Chugiak, AK, from the 2018 Anchorage earthquake and aftershocks. Poster Presentation at 2023 SCEC Annual Meeting.

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