1. 1U.S. Geological Survey, Earthquake Science Center, Pasadena, California, U.S.A.
2. 2Department of Geology, Puerto Rico Seismic Network, University of Puerto Rico Mayagüez, Mayagüez, Puerto Rico
3. 3Seismological Laboratory, California Institute of Technology, Pasadena, California, U.S.A.
4. 4Now at, British Antarctic Survey, Cambridge, United Kingdom