Submarine hydrocarbon seepage in the wake of the 2010 Maule earthquake, Central Chile

Abstract:

Subduction zone earthquakes are often discussed as potential triggers for the sudden release of hydrocarbons from marine sediments. However, a causal link that goes beyond theoretical considerations is difficult to establish because of the inaccessibility of offshore epicentral areas, and the infrequent occurrence of large earthquakes. The Central Chile margin is known to host marine gas hydrates and gas-bearing sediments, and for some locations methane seeps have been reported. In February 2010 a 500 km long stretch of this margin ruptured during the Mw 8.8 Maule earthquake. In order to investigate the impact of the Maule earthquake on hydrocarbon seepage we combine geophysical (seismic reflection and Parasound profiles, swath bathymetry), modeling (Coulomb stress estimates) geochemical (methane, sulfate, sulfide, alkalinity) and visual (ROV surveys) data and observations from the rupture area. Essential parts of the data base were collected on R/V SONNE Cruise 210 six months after the earthquake. Results from Coulomb stress calculations suggest that co-seismic slip on the subduction interface was sufficient to cause extensional fractures and normal faults at the seafloor, as spectacularly documented in some ROV videos. First results from geochemical analyses indicate highest methane concentrations in the water column as well has high sulfide concentrations in surface sediments in the vicinity of these fractures. This, together with the consideration that in a forearc setting under horizontal extension only small overpressures in the forearc cover sediments are needed to cause natural hydrofracturing, indicates that seismic shaking during the 2010 Maule earthquake may have increased or even triggered hydrocarbon seepage offshore Central Chile.