Descr: We will conduct an active-source seismic investigation of the volcanic arc, backarc, and downgoing plate in the Costa Rican portion of the Central American Focus Site. Detailed knowledge of arc structure constitutes a major gap in our knowledge of this Focus Site. Our program focuses on the central Costa Rican segment of the arc, the site of important transitions in lava chemistry, because the narrow isthmus here is well-suited for detailed seismic imaging using onshore-offshore techniques. This location also enables the incorporation of data from the regional Costa Rican seismic network with our proposed areal active-source array. This combination will result in the most detailed 3D image of arc crustal structure to date. This experiment will involve TWO SEPARATE FIELD SEASONS: an initial season in late January - end February 2005, which will only entail onshore explosion refraction work, and a second season in 2006 that will entail onshore-offshore shooting from the R/V Ewing's replacement vessel. The exact timing of the second field season is to be determined, pending the ship schedule. This request is ONLY for the FIRST field season. That season will involve two refraction lines -- one across the isthmus from the Pacific Ocean to the Caribbean, and one along the axis of the volcanic arc, along the crests of the volcanoes (from Irazu to Rincon). The 820 Texans will be deployed twice, once on each line. Approximately 40-45 explosive shots will be detonated.
