Recent Ocean Drilling Program (ODP) results in the Oregon accretionary prism and on the Blake Ridge indicate that the zone containing free gas beneath the hydrate-bearing near-surface sediments is considerably thicker than previously thought. In this paper, we present results from travel time inversion of refracted seismic waves that show very low (100 in the normal accretionary complex sediments west of Hydrate Ridge. These results suggest that refraction seismic techniques are a powerful way to constrain the depth to which free gas is present in sediments beneath the hydrate stability zone. Defining the extent of the free gas zone is an important factor for estimating the total volume of gas present and for evaluating its impact on slope stability and potential contribution to global climate change. ¿ 2001 American Geophysical Union