Seismic monitoring of sub-seafloor fluid processes in the Haima cold seep area using an Ocean Bottom Seismometer (OBS)

The use of ocean bottom seismometers provides an effective means of studying the process and the dynamic of cold seeps by continuously recording micro-events produced by sub-seafloor fluid migration. We deployed a four-component Ocean Bottom Seismometer (OBS) at an active site of the Haima cold seep from 6 November to 19 November in 2021. Here, we present the results of this short-term OBS monitoring. We first examine the OBS record manually to distinguish (by their distinctive seismographic signatures) four types of events: shipping noises, vibrations from our remotely operated vehicle (ROV) operations, local earthquakes, and short duration events (SDEs). Only the SDEs are further discussed in this work. Such SDEs are similar to those observed in other sea areas and are interpreted to be correlated with sub-seafloor fluid migration. In the OBS data collected during the 14-day monitoring period. We identify five SDEs. Compared to the SDE occurrence rate observed in other cold seep regions, five events is rather low, from which it could be inferred that fluid migration, and subsequent gas seepage, is not very active at the Haima site. This conclusion agrees with multi-beam and chemical observations at that site. Our observations thus provide further constraint on the seepage activity in this location. This is the first time that cold seep-related SDEs have been identified in the South China Sea, expanding the list of sea areas where SDEs are now linked to cold seep fluid migration.