Major infrastructure projects are designed to withstand seismic events. The Alaska Energy Authority (AEA) is designing the Susitna-Watana dam to withstand the maximum credible earthquake at the proposed project site. (The maximum credible earthquake is defined as the largest earthquake that appears capable of occurring at a specific location.)
Defining the maximum credible earthquake requires an understanding of potential seismic sources in the region and the potential effects of an earthquake on the proposed dam. Potential seismic sources are characterized by defining locations, frequency and magnitudes of future earthquakes likely to affect the project. Additionally, ground motion analyses will help define the strength and duration of shaking likely to be imposed on the dam. Seismic design analyses are performed to evaluate the response of the dam to the seismic loads and the findings are incorporated into the final design to satisfactorily resist[EF1] the loads imposed by future seismic events.
AEA is conducting geologic and seismic investigations, building on an extensive seismic hazard assessment that was conducted as part of the 1980s project. In addition, AEA is mapping potential earthquake sources and collecting data on recent seismic events. In 2012, through a partnership with the Matanuska-Susitna Borough, the Susitna River has been digitally mapped with Light Detection and Ranging (LiDAR) which will enhance the current understanding of local faults.
AEA is conducting geologic and seismic investigations to update the seismic studies undertaken in the 1980s and will use that information in the design. Current studies include updating the seismic source characterization, seismic hazard evaluation (using both probabilistic and deterministic analysis), and ground motion evaluations. Furthermore, detailed evaluations of faulting and geologic conditions are being conducted using remote imagery products such as Light Detection and Ranging (LIDAR) to map potential traces of faults. New and extensive geologic and geophysical research is being incorporated and new field studies and subsurface investigations are being performed. Improvements in seismic monitoring have lead to a greater understanding of earthquake occurrence in the region. Seismic monitoring will be further enhanced. The Alaska Earthquake Information Center has installed seismograph stations near the proposed dam site to improve and augment earthquake detection and location capabilities. New data from recent earthquakes such as the 2011 magnitude 9.0 Tohoku earthquake will be used to improve ground motion estimates for both regional and local earthquake sources.
Features will be incorporated into the design and engineering of the Susitna-Watana Hydro dam and related structures to protect them against damage during extreme earthquakes. AEA is proposing a roller-compacted concrete (RCC) dam with a wide base at the lower parts of the cross section to limit stresses on the structure during an earthquake.
AEA will also hire an independent board of consultants who are international experts in dam construction and design to perform a peer review. The Federal Energy Regulatory Commission also has an extensive review process to ensure that the proposed dam is engineered to safely pass all credible loading conditions.
