This research will address the determination of down-dip slip distribution on fault surfaces during large earthquakes. This has been a difficult problem in the past because of limitations in the bandwidth of seismic instrumentation. This technique will be to use recorded tsunamis for oceanic earthquakes, along with any land-based geodetic data, to constrain the down-dip earthquake slips. Earthquakes that will be investigated are the 1944 Tonankai and the 1946 Nankaido earthquakes along the Nankai Trough, Japan, the 1923 Kanto, Japan, earthquake, the 1964 Alaska earthquake, and the 1960 Chilean earthquake. This research is a component of the National Earthquake Hazard Reduction Program.