Modeling the three-dimensional structure of macroscopic magma transport systems: Application to Kilauea Volcano, Hawaii

We report the results of modeling the three-dimensional internal structure of Kilauea's magmatic passageways. The approach uses a clear plexiglass model containing equally-spaced levels upon which well-located seismic hypocenters are plotted. Application of constraining geologic and geophysical criteria to this distributed volume of earthquakes permits the interpretation of seismic structures produced by fracturing in response to locally high fluid pressures. Four magma transport and storage structures have been identified within and beneath Kilauea: (1) Primary conduit. The conduit transporting magma into Kilauea's summit storage reservoir rises from the model base (14.6 km) to the 6.5 km depth level. It is a zone of intense fracturing and inferred intrusion, whose horizontal sections are elliptical in planform. Over its height, the average major axis of the component horizontal sections is 3.3 km, with an average minor axis of 1.7 km. This yields an aspect ratio of ξ = 0.52. At the 14.6 km level, the strike of the major axis is N67°E. During passage from the upper mantle through the oceanic crust, this axis rotates in a right-handed sense, until the strike is N41°W at the 6.5 km level. (2) Magma chamber complex floor. The interval from 6.5 to 5.7 km, immediately over the primary conduit, is aseismic. This suggests differentially high fluid-to-rock ratios, and relatively weak pathways for further vertical transport into higher levels of the storage complex, as well as lateral leakage eastward into the Mauna Ulu staging area—for later vertical ascent beneath the upper east rift zone. Seismicity within the immediately subjacent rocks that form the top of the primary conduit (at 6.5 km) suggests that this inferred magma-rich horizon forms the effective floor of the summit storage complex. (3) Magma chamber crown. Intense seismicity over the 1.1–1.9 km depth interval defines an elliptical region in plan view. The top of this region has a broad apex with an average major axis of 1.5 km, and an average minor axis of 1.1 km producing an aspect ratio of ξ = 0.73. This region coincides with the epicentral position of summit vertical displacement maxima during inflationary deformation and is interpreted as a region of intense diking, crowning the summit storage complex. (4) Upper east rift zone pipe. From a depth of 5.7 km, a cylindrical region of seismicity rises beneath the upper east rift zone, to the 1.9 km level, where it merges with the subhorizontal upper east rift zone duct. This pipe-like zone has a mean diameter of 1.4 km and a vertical axis that pierces the surface of the volcano at the intersection of the Koae fracture system and the upper east rift zone. We suggest this pipe forms a principal connection between the base of the summit storage reservoir, and near-surface storage compartments and ducts in the upper east rift zone. In this role, the pipe is suggested to have contributed heavily to Mauna Ulu's magma supply during 1969–1974. The model of magma transport and storage thus constructed provides a context in which other types of data (e.g., ground deformation and electrical conductivity) may be interpreted as well as a means of refining petrological inferences based on lava chemistry.