FRANCIS A. (TONY) DAHLEN, professor of geosciences and a leading theoretical seismologist, died June 3 of cancer. He was 64. Dahlen, a member of the faculty since 1970, conducted pioneering research on the seismic waves generated by earthquakes to help explain the structure of the Earth’s interior. Dahlen also was praised as a teacher and administrator. He chaired the Department of Geosciences from 2001 to 2006