1. Department of Geosciences, 1910 University Dr., Boise State University, Boise, Idaho 83725-1535, USA e-mail: wsnyder@boisestate.edu
2. Lamont-Doherty Earth Observatory of Columbia University, 212 Seismology 61 Route 9W, Palisades, New York 10964, USA
3. Department of Geosciences, 1910 University Dr., Boise State University, Boise, Idaho 83725-1535, USA