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Hundreds of earthquakes were recorded during a nine‐month ocean bottom seismometer deployment surrounding Lō'ihi submarine volcano, Hawai'i. The 12‐station ocean bottom seismometer network widened the aperture of earthquake detection around the Big Island, allowing better constraints on the location of seismicity offshore Hawai'i. Although this deployment occurred during a time of volcanic quiescence for Lō'ihi, it establishes an important basis for background seismicity of the volcano. Offshore seismicity during this study was dominated by events located in the mantle fault zone at depths of 25–40 km. These events reflect rupture on preexisting faults in the lower lithosphere caused by stresses induced by volcano loading and flexure of the Pacific Plate (Pritchard et al., 2007,https://doi.org/10.1111/j.1365‐246X.2006.03169.x; Wolfe et al., 2004,https://doi.org/10.1029/2003GC000618). Tomography was performed using double‐difference seismic tomography and showed shallow velocities to be slower than the regional velocity model (HG50; Klein, 1981,https://pubs.geoscienceworld.org/ssa/bssa/article/71/5/1503/118231/A‐linear‐gradient‐crustal‐model‐for‐south‐Hawaii). A broad, low‐velocity anomaly was observed from 20–40‐km depth, and is suggestive of the central plume conduit that supplies magma to Lō'ihi and the active volcanoes of the Big Island. A localized high‐velocity body is observed 4–6‐km depth beneath Lō'ihi's summit, extending 10 km to the north and south. Following Lō'ihi's active rift zones and crossing the summit, this high‐velocity body is characteristic of intrusive material. Two low‐velocity anomalies are observed below the oceanic crust, interpreted as melt accumulation beneath Lō'ihi and magmatic underplating beneath Hawai'i Island.