Hydrate Ridge is a 25-km-long and 15-km-wide ridge in the Cascadia accretionary complex, formed by the oblique subduction of Juan de Fuca plate beneath North America at a rate of ~4.5 cm/yr. Sediment on the subducting plate contains large volumes of sandy and silty turbidities (MacKay, 1995). It is characterized by a northern summit at a water depth of 600 m and a southern summit at a water depth of 800 m (Shipboard Scientific Party, 2003). As in many accretionary complexes, gas hydrate is present in this environment (Shipboard Scientific Party, 1994). Hydrate Ridge appears to be capped by hydrate, as indicated by a strong BSR (Tréhu et al., 1999) and by recovered samples of massive hydrate (Bohrmann et al., 1998).
Site 1244 is located in ~890 m of water in the eastern flank of Hydrate Ridge ~3 km northwest of the southern summit (Fig. F1), and a BSR is present at a depth of 124 mbsf. Site 1245 is located in 870 m of water on the western flank of Hydrate Ridge ~3 km northwest of the southern summit, and a BSR is present at a depth of ~134 mbsf. Site 1247 is located in 845 m of water on the western flank of Hydrate Ridge, and a BSR is present at ~121–124 mbsf. All three sites are well within the GHSZ (Shipboard Scientific Party, 2003). Unlike the summit region, there is no evidence of venting of bubbles at the seafloor at these sites.