Treasure Island is a 162 Ha artificial island formed by hydraulic filling on a shoal adjacent to a large rock outcrop known as Yerba Buena Island in San Francisco Bay. The composition and consistency of the hydraulic fill varies across Treasure Island, but it is basically loose, fine to medium, silty sand, with occasional clayey zones. There is an improved area on the island, where vibroflotation and compaction piles were used to densify the fill. Treasure Island experienced lateral spreading in an unimproved area on the North side of the island during the 1989 Loma Prieta Earthquake. Seismologically, it is located roughly midway between the Peninsula segment of the San Andreas Fault to the West and the Northern segment of the Hayward fault to the East. An earthquake of magnitude greater than 7.0 is predicted for one of these fault segments within the next thirty years, with an aggregate probabillity in excess of 50%. Intensity at Treasure Island is predicted to be MM VIII for these events.
The Fire Station #1 site on Treasure Island is quasi-free field, being a large vacant lot behind a one-story wood-frame structure on a surface foundation. The National Science Foundation funded, through the Earthquake Hazard Mitigation Program, the installation of a deep accelerometer array on Treasure Island. This effort is being carried out in collaboration with the Strong Motion Instrumentation Program of the California Division of Mines and Geology (CDMG). CDMG already had a surface instrument at the Fire Station #1 site, which produced a record during the Loma Prieta event. A total of five other instruments have been installed: at bedrock and at four locations within the soil profile. Eight piezometers have also been provided at various depths in the hydraulic fill. An inclinometer casing has been installed to record horizontal displacements. An extensive site investigation has been carried out at the fire station. Boring data also exists for other locations.