A new technique utilizing theoretical wave forms has been developed to determine precise shear wave travel times. This technique was applied to long-period World-Wide Standard Seismograph Network and Canadian network seismograms of five large nuclear explosions to obtain a surface focus shear wave data set containing about 100 travel times for distances greater than 30¿. Very little scatter is present in the data from Novaya Zemlya and the Nevada test site, and so a reliable inversion to a lower mantle velocity structure is permitted. This velocity model, based on the 59 travel times from Novaya Zemlya, has significanlty more structure than earlier models. The model, S1, has proved to be appropriate for free oscillations as well as for travel times. This model should be useful in studying both lateral inhomogeneites and the mineralogical composition of the earth's mantle.