Gas in Beppu Bay inferred from acoustic reflection anomalies.

Abstract

Acoustic reflection profiles across Beppu Bay indicate gas zones at shallow depth within the sediments in some areas of the Bay. The clearest gas anomaly is situated at less than 10m depth, coinciding with the deepest part of the Bay. A mud volcano appears to exist over this anomaly, which may have resulted from excessive gas accumulation near the sediment surface. Continuous gas bubbling has been observed in this area of the Bay for at least the last 50 years. It is also possible that gas contributed to sediment fluidization during the slumping that resulted in the disappearance of Okinohama Port in the 1596 earthquake. In the central part of the Bay, gas appears to be less abundant or absent, and the reflection profiles reveal active listric faulting. The main decollement surface is inferred to be somewhere between 500 and 1, 000m depth. The presence of listric faulting means that the seismic risk due to movement on these faults may have been overestimated in the past. The deformation in the sediments of the central part of the Bay indicates predominantly horizontal extension, whereas near the margins significant vertical deformation is occurring. This is particulary evident along the southwest margin, where a deeply penetrating fault zone appears to prove vertical permeability for gas flow towards the surface. Both carbon dioxide and methane gases accompany the discharge of groundwater wells in the adjacent Oita City, whereas in Beppu City wells carbon dioxide is predominant and methane is rare. Both gases may be present in the sediment beneath Beppu Bay. The CO2 may be caused by prograde metamorphism due to increasing temperature at mid to lower crustal depths, whereas the CH4 is attributed to the maturation of organic material in sediments at 3-4km depth.