1. U.S. Geological Survey, Cascades Observatory, 1300 Cardinal Court, Building 10, Suite 100, Vancouver, Washington 98683, nbeeler@usgs.gov
2. Department of Earth, Environmental and Planetary Sciences, Brown University, 324 Brook Street, Providence, Rhode Island 02912
3. Now at Department of Earth and Planetary Sciences, University of California Davis, One Shields Avenue, Davis, California 95616.