LGS: seismic hazard assessment experience

Service Areas

Representative Experience - Seismic Hazard Assessment

Seismic Hazard Analysis of Project Simandou, West Africa

LGS is currently conducting a seismotectonic evaluation of West Africa as part of a team led by the Scott Wilson Group for Rio Tinto’s Project Simandou. Project Simandou is an ambitious attempt by one of the world's leading mining companies to extract iron ore in southeastern Guinea and transport it hundreds of kilometers by rail to an undeveloped port on the Atlantic Ocean. LGS is assessing the earthquake hazard to proposed railway alignments, rail tunnels, several proposed port locations and the mining concession itself in the Simandou Range. The investigation requires an evaluation of the liquefaction potential of coastal sediments and the susceptibility of bridge foundations to lateral spreading during moderate magnitude earthquakes (see photo of West Africa). A database of historic and modern earthquakes has been complied and used to develop a seismotectonic model of the country. This model will be used to provide a context for assessing the probability of occurrence of future events that might impact the project. Faults suspected of recent activity have been identified in a review of aerial photography and field mapping of the structures is to be conducted in Phase 2 along with an evaluation of the tsunami hazard from offshore earthquakes.

Fault and Seismic Hazards Investigations of the North-of-the-Delta Offstream Storage Project, Northern California, USA

LGS was the Seismicity Task Leader on a multidisciplinary team headed by William Lettis & Associates (WLA) investigating faults and seismic hazard in the northeastern Coast Ranges of California. The North-of-the-Delta Offstream Storage (NODOS) project, a joint venture of the California Department of Water Resources and the U.S. Bureau of Reclamation, is investigating potential sites for surface water impoundment north of the Sacramento-San Joaquin Delta to increase water supplies and improve Delta water quality. The WLA - LGS team evaluated surface-faulting hazards and maximum levels of earthquake strong ground shaking at two dam sites. A seismotectonic model of the northeastern Coast Ranges and northwestern Sacramento Valley was developed using available geological and seismological data, supplemented by reconnaissance and detailed investigations to obtain site-specific data in areas of potential fault activity. These investigations included photogeologic interpretation, quantitative geomorphic analysis, field mapping, and analysis of historical and instrumental seismicity. Paleoseismic trenching investigations were conducted to assess the late Quaternary activity of several faults that pass through and near the dam sites. Maximum earthquake magnitudes were calculated for active or potentially-active faults within 50 km of the sites. This information, along with the earthquake magnitude-frequency distribution and hypocentral distance, was used to calculate the probability of exceeding maximum levels of vibratory ground motion during the operating life of the proposed dams. See a regional cross section of seismic activity in northern California.

Seismotectonic Evaluation of the Lake Almanor and Butt Valley Dams, Northern California, USA.

LGS was the Seismicity Task Leader on a multidisciplinary team headed by William Lettis & Associates to provide a seismotectonic framework to evaluate earthquake hazards to the Lake Almanor and Butt Valley dams in the northern Sierra Nevada for Pacific Gas & Electric (PG&E). These old dams were constructed of hydraulic fill under less stringent engineering standards, making them vulnerable to liquefaction in the event of moderate seismic loading. These dams were a critical part of PG&E's power generation system and their failure could have had catastrophic consequences for downstream hydropower infrastructure. LGS integrated the historical and instrumental earthquake data from California and Nevada to compile a seismicity catalog for the northern Sierra Nevada and northeastern Sacramento Valley, and evaluated the occurrence, style and rate of seismic activity in the study region. This analysis included evaluation of earthquake catalog completeness, preparation of epicentral maps, seismicity cross-sections, focal mechanism analysis, and development of regional earthquake recurrence intervals. The seismotectonic evaluation developed in this project led to the development of a transtensional model to reconcile the kinematics of the entire Sierra Nevada frontal fault system. See graphic of the model. (Unruh, Humphrey and Barron, 2003, Transtensional model for the Sierra Nevada frontal fault system, eastern California, Geology, v. 31, no. 4, p. 327-330.).