New evidence for deep magma migration in Hawaii: Seismic tomography of Kilauea and Loihi volcanoes

Kilauea volcano exhibits a long, curvilinear east rift zone (ERZ) that channels magma from summit magma chambers to eruptive vents along its length. The rift zone is characterized by shallow dikes and deeper magma cumulates. Previous onshore and offshore studies have improved our understanding of how magma migrates from the summit to the distal vents, but the details have not been clearly resolved. To clarify magmatic processes, we present a 3-D P-wave velocity model of the combined subaerial and submarine portions of the southern Island of Hawaii using a simultaneous inversion of first-arrival times from airgun shots and earthquakes, recorded by the USGS-Hawaiian Volcano Observatory seismic network. High-velocity materials (6.5-7.0 km/s) lie around Kilauea's summit and the upper and middle rift zones. The extent of these velocities is highly correlated with seismicity and ground movements, suggesting olivine cumulates precipitated within the volcanic edifice. These high velocities are separate from a distinct high- velocity body of 6.5-6.9 km/s beneath the lower ERZ and the upper Puna Ridge. This separation implies that distal magma cumulates might be connected to a summit conduit at depth, probably at the base of the edifice or within the oceanic crust and lithosphere. Similar high velocities are observed within the crust beneath Loihi, probably related to young cumulates parallel to its rift zones. Low-velocity anomalies beneath the summits of Kilauea and Loihi reflect accumulated melts within the upper mantle. Our observations have important implications for magmatic growth of Hawaiian volcanoes: During the earliest stage of volcanic growth, magma from the mantle accumulates beneath the oceanic crust, and intrudes laterally into the oceanic lithosphere and crust, defining future rift zones. Surface eruptions build volcanic edifices upon the crust, with continued intrusions along rift zones. Such deep magma intrusion may also impart lateral stresses causing lithospheric earthquakes, as recently documented near Kilauea and Loihi volcanoes. Thus, Hawaiian rift zones might have a more complex plumbing system than previously thought, comprising a shallow zone of dikes and a deep zone of magma intrusion within the oceanic lithosphere.