1. Department of Geological Sciences; Brown University; 324 Brook Street, Box 1846, Providence Rhode Island 02912 USA
2. National Science Foundation; 4201 Wilson Boulevard, Room 785N, Arlington Virginia 22230 USA
3. Lamont-Doherty Earth Observatory; PO Box 1000, 61 Route 9W, Palisades New York 10964 USA
4. Department of Geophysics; Instituto Nicaragüense de Estudios Territoriales; Apartado 2110, Managua Nicaragua
5. Observatorio Vulcanológico y Sismológico de Costa Rica; Universidad Nacional; Apartado 2346-3000, Heredia Costa Rica