We thank Sue Hough and Lisa Wald for reviewing this manuscript on short notice. The first figure was done using GMT (Wessel and Smith;
1991);
and some of the analysis was done using ZMAP by Wiemer (2000). E. H. and K. H. are supported by U.S. Geological Survey Grant 01HQGR0038 and USGS Cooperative Agreement 01HQAG0007. Contribution 8845;
Division of Geological and Planetary Sciences;
California Institute of Technology;
Pasadena;