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Earthquakes in Hawaii are byproducts of volcanism; almost all of them occur beneath or near the island of Hawaii and its active volcanoes. Traveltimes of longitudinal waves indicate that the Mohorovicic discontinuity is depressed only a few kilometers beneath the Hawaiian Ridge, where it lies about 15 km below sea level. The pattern of ground deformation around the summit of Kilauea Volcano suggests that magma rising from a deep source accumulates slowly in a reservoir a few kilometers beneath the caldera and then is discharged rapidly during an eruption. This reservoir is the principal immediate source of lava discharged through a rift zone during a flank eruption. Periodic swarms of deep earthquakes from a zone in the mantle about 60 km beneath the summit of Kilauea outline a possible source of magma for the volcano. Swarms of small shallow earthquakes herald the opening of fissures within the volcano and provide short-term warnings of impending eruptions. More persistent swarms of somewhat larger, shallow earthquakes stem from widespread failure in rocks surrounding the summit reservoir during severe downwarping of the summit when the reservoir is deflated by an important flank eruption.