Don, good post, as is the earlier long one. The marine geophysicist who I happen to be married to is working on the North Anatolia fault (1999 quakes) offshore, using high resolution seafloor bathymetry and subbottom data...lots of evidence of gas....including gas seeping out of the active faults strands themselves...I was displaying a profile for her and tried a certain kind of amplitude gain, and the details in the water column jumped out....you could see scattered echoes, which are fish, but then vertical alignments of echoes right above fault strands...either the fish decided they would just stack themselves up above faults, being geologic fish and all, or it is gas seeping out of the faults. The marine geophysicist says that there are lots of publications mentioning gas and life that is living off the seepage along this fault. It would be good to know if the gas is coming from a deep or shallow source. It would be more expensive than onshore monitoring, but monitoring the flux of gas and change of composition on a daily basis might be interesting for earthquake prediction....We are in the middle of a proposal to look at seepage along the thrust fault front south of the northern Channel Islands, California...same general thrust system that extends beneath Los Angeles...
Chris