2020 Simeonof Earthquake: A Case Study in the Seismic Design of Marine Structures

Abstract: Between 1938 and 1996, eight earthquakes with magnitudes between 7.1 and 9.2 ruptured almost the entirety of the Aleutian subduction zone between the North American and Pacific plates. These earthquakes stretched some 1,800 miles from Anchorage, AK to Attu Island, AK, save for a 75 mile segment around the Shumagin Islands, known as the “Shumagin Gap”. Beginning with the Simeonof Earthquake in 2020, four earthquakes occurred between July 2020 and July 2023 in the vicinity of and within the Shumagin Gap that have shed light on the geo-tectonic conditions in this region after nearly 100 years of inactivity. These events ranged in magnitude from 7.2 to 8.2.

Bridge inspectors investigated the geotechnical and structural conditions at 17 bridges and ferry terminals in the region following the Simeonof Earthquake, which occurred on July 21, 2020, 75 miles east of Sand Point, AK. Two docks in the community of Sand Point provide the opportunity for a performance based comparison of modern design standards to previous eras. The new Sand Point City Dock II was constructed in 2019 using the displacement-based seismic design methodologies outlined in ASCE 61-14. The old Sand Point City Dock, built in 1983, is located immediately adjacent to the new dock on the same breakwater and the seismic design standard is unknown, but likely forced-based. While the new dock suffered only minor damage and has remained operational with few repairs, the old dock has been closed since the earthquake due to failed connections at critical locations. This paper examines the philosophies governing force-based and displacement-based design and compares the design features of the two structures in Sand Point that led to disparate outcomes.

How does this project / topic contribute to the advancement of the industry and profession?: This topic provides insight into the performance of modern seismic designs subject to a seismic event. I have found few examples in the literature modern structures in operation that have experienced significant ground motions. This will also provide the opportunity to examine outcomes, both desirable and undesirable, based on various allowances in ASCE 61-14.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: No, but it does examine the efficacy current methodologies.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: This paper is the result of inspections conducted immediately after an earthquake on the Aleutian Islands. The most challenging aspect was travel logistics due to limited options, the remote location, and adverse weather conditions. Our team was an experienced group, but we had to be flexible and rely on the expertise of locals, such as our pilot.

Who is the target audience for this paper?: Engineers who design marine structures for seismic loads.