1. Davies, D., and Sheppard, R. M., Nature, 239, 318 (1972).
2. Julian, B. R., Technical Note 1970–45 (MIT Lincoln Laboratory, 1970).
3. Davies, D., and Julian, B. R., Geophys. J., 29, 185 (1972).
4. Sorrels, G. G., Crowley, J. B., and Veith, K. F., Bull. Seism. Soc. Am., 61, 27 (1971).
5. Julian, B. R., and Sengupta, M. K., Nature, 242, 443 (1973).