The offshore M4 was even shallower (especially given the 0.8 km of water), so only 700 m below sea floor +/- 600 m. And, much of that is relatively young sediment. I think it was likely deeper.

I was doing field work on Santa Cruz Island (California) back when the Navy had an acoustic station at Valley Anchorage (it, and associated buildings, are complete gone). I was working on the ~13,000 year old landslide there and was hearing a "whoomphing" deep sound. The sound was emitted in the water somewhere (cables etc headed out into water).

Chris