Microearthquake evidence for reaction-driven cracking within the TAG hydrothermal deposit

We detected 32,078 very small, local microearthquakes (average ML = -1) during a 9-month deployment of five ocean bottom seismometers (OBS) on the periphery of the Trans-Atlantic Geotraverse (TAG) active mound. Seismicity rates were constant without any mainshock-aftershock behavior at ~243 events per day at the beginning of the experiment, 128 events per day after an instrument failed, and 97 events per day at the end of the experiment when whale calls increased background noise levels. The microearthquake seismograms are characterized by durations of <1 second and most have single-phase P-wave arrivals (i.e., no S-arrivals). We accurately located 6,207 of the earthquakes, with hypocenters clustered within a narrow depth interval from ~50-125 mbsf on the south and west flanks of the deposit. We model the microearthquakes as reaction-driven fracturing events caused by anhydrite deposition in the secondary circulation system of the hydrothermal mound, and show that under reasonable modeling assumptions an average event represents a volume increase of 31-58 cm^3, yielding an annual (seismogenic) anhydrite deposition rate of 27-51 m^3.