Observations of regional seismicity and local harmonic tremor at Brothers volcano, south Kermadec arc, using an ocean bottom hydrophone array

Four ocean bottom hydrophones (OBHs) were deployed for 7 months on the caldera floor of Brothers volcano, located within the southern Kermadec intraoceanic arc roughly 350 km northeast of New Zealand. The volcanic edifice is 13 × 8 km at the seafloor, with a 3 km wide caldera that has a floor depth of 1850 m and which is surrounded by 290‐ to 530‐m‐high walls encompassing a ∼350‐m‐high dacite cone. Three of the OBHs recorded low‐frequency (0.5–110 Hz) acoustic T waves from regional and local earthquakes, as well as harmonic tremor from within the Brother volcano. The fourth OBH was not recovered intact. The T wave‐derived locations for 964 regional earthquakes show that the majority of events cluster beneath the dacite cone in the southern quadrant of the caldera and the east flank of Brothers volcano. In addition, regional seismicity was observed along a NE‐SW trending fault structure to the southeast and northwest of the volcano in a small basin in the Kermadec back arc and along the Kermadec arc and fore arc. A total of 2470 discrete harmonic tremor events were recorded on all three OBHs with a fundamental frequency of 3 ± 0.5 Hz. The majority of tremor signals were detected on OBH‐2, implying that a greater number of hydrothermal fluid conduits/chambers exist within the southern caldera quadrant in comparison with the eastern or western quadrants of the caldera.