Spatial distribution of b-values for microseismicity in the SWIR Longqi hydrothermal field and magmatic-tectonic interpretation

Natural microseismicity serves as a potent tool for exploring smaller-scale hydrothermal and tectonic phenomena. Investigating seismic activities within the hydrothermal fields of mid-ocean ridges(MORs) offers profound insights into earth's internal dynamics. However, studies on natural earthquakes at ultra-slow spreading ridges, especially the Southwest Indian Ridge (SWIR), remain relatively scarce. To investigate the microseismic distribution, heat flow pathways, and tectonic characteristics of the Longqi hydrothermal field, a typical representative of SWIR, this paper processed one month of passive source OBS data from the DY43 cruise through microearthquake detection and relocation, obtaining a catalog of over 3000 earthquakes, significantly expanding the earthquake database for the Longqi field and improving the magnitude completeness. And the b-value calculation and imaging of the earthquake catalog were carried out using the maximum likelihood method and grid search method, respectively. The research results indicate that: ① The overall b-value of the SWIR Longqi field is 0.989; ② The b-value at the center of the Longqi hydrothermal vent is approximately 0.8, while the b-value around the vent is around 1.2; ③ High and low b-value areas alternate at a depth of 10km along the ridge axis; ④ There is an anomalously low b-value area of around 0.5 at depths of 12-16 km to the north across the ridge axis. Combining previous research results on b-values at MORs, this paper suggests that the background b-value of less than 1 in the Longqi field is consistent with its tectonic-type hydrothermal origin. The detachment fault beneath the Longqi hydrothermal vent leads to high stress and a low b-value, while the microseismic activity around the vent originates from rock fracturing caused by the combined effects of cold seawater and hydrothermal fluids. The uneven distribution of high and low b-values in the deep part of the hydrothermal field may reflect the uneven distribution of subsurface magma. The low b-value area in the north is speculated to be due to high stress resulting from torsional compression at the bottom of the detachment fault. In summary, it can be anticipated that the spatial distribution of b-values will serve as an indicator and reference factor for stress, fault structure, and magmatic-hydrothermal activity in MOR hydrothermal field in the future.