This research was conducted at the University of Texas at Dallas rocks physics laboratory, and at the Atlantic Richfield Company laboratories in Piano, Texas. We thank Kent Nielsen for the use of his facilities and Billy Smith for keeping the instruments running at both the UTD and ARCO labs. DLH acknowledges the support and help of Dale Sawyer in making the research logistics workable. We thank two anonymous reviewers for their helpful comments. This work was supported by USSSP grant 149-20761b to DLH.