High Resolution Active Source Seismic Experiment Across the Costa Rican Segment of the Central American arc

Volcanic arcs that form above subduction zones represent the accumulated magmatic output of the subduction process. Understanding of this output, along with a well-constrained input, is critical if we are to reach an understanding of the melting processes that occur within the `subduction factory'. During early 2005 we carried out a large active source seismic experiment in Costa Rica to examine melting within the Costa Rican segment of the Central American arc. Two transects were surveyed: 1) Across-arc from the Pacific to Caribbean oceans and 2) Along the strike of the volcanoes from Panama to Nicaragua. A total of 742 vertical component geophones were deployed at an average spacing of 200~m along the across-arc transect and 372~m on the along-arc transect. Seismic sources were spaced approximately evenly along the lines with a total of 20 shots recorded across the arc and 15 along the arc. We present here inital results from the 153~km long across-arc transect. Shot sizes ranged from 300 to 1025~kg of buried explosive, depending on local site conditions, and consistently allow us to observe strong arrivals to >70~km offset. In many cases refracted energy is observed at >100~km offset. The largest shots did not always produce the best observations; shot elevation relative to the surrounding environment and presence of water within the hole were found to be critical factors for energy transmission. Deep reflection events can be observed on several shots and may be Moho reflections. For the first time we can show the velocity strucure across the Central American arc to the back-arc basin: we have very good upper crustal velocity constraints along the entire transect.