Effect of Surficial Geology on Earthquake Ground Motions From the 2018 Mw 7.0 Anchorage Earthquake in Anchorage, Alaska

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

The November 30, 2018 M_w=7.0 Anchorage earthquake was widely felt across southcentral Alaska. The City of Anchorage experienced perceived shaking ranging between strong and very strong levels, based on the United States Geological Survey ShakeMap for the earthquake. The maximum peak ground acceleration (PGA) recorded by strong ground motion stations in Anchorage was over 0.5 g in southeast Anchorage. Short period (0.2 second) spectral accelerations exceeded 1.2 g at stations in southwest and west Anchorage. One-second spectral accelerations exceeded 0.4 g in the Anchorage downtown area. This distribution and variation of intensity of shaking across the Anchorage area is a function of glacial and post-glacial deposits, and associated geologic processes related to deposition and erosion. The primary surficial geologic units in the Anchorage area include the Elmendorf glacial outwash deposits, cohesive and non-cohesive facies of the Bootlegger Cove Formation, Late Pleistocene glacial deposits, Holocene alluvial deposits, and bedrock of the Chugach Mountains. In addition, after the Late Pleistocene glaciers retreated, peat and organic deposits formed in many areas. The pattern of PGA values from the Anchorage earthquake generally indicates that the mid- to lower values of PGA occurred at stations located on glacial, glaciodeltaic, and alluvial deposits. The mid- to high values of PGA occurred at stations located in proximity to the Bootlegger Cove Formation deposits. One of the largest measured PGA values occurred in southeast Anchorage at a station located in the foothills of the Chugach Mountains. This area is mantled by glacial deposits but also contains fine-grained sand and silt deposits, and peat/organic deposits. The range and distribution of strong ground motion shaking across Anchorage highlights the importance of geologic factors contributing to ground motion responses.

Presenting Author: Eric C. Cannon

Authors