STRUCTURE OF THE LOWER EAST RIFT ZONE OF KILAUEA VOLCANO, HAWAII, FROM SEISMIC AND GRAVITY-DATA

Two seismic refraction surveys were carried out in 1976 and 1977 on the east rift zone of Kilauea volcano as part of an exploratory program for geothermal resources. The short traverse seismic refraction survey of January 1976 delineated the upper surface structure of the east rift, revealing velocities of 2.5 km/s under the Kalapana line and 3.1 km/s under the Leilani line beneath a surface layer of low, but variable velocity. This survey was not successful in determining the depth of the high-velocity material. The longer traverse seismic refraction survey of 1977 outlined a high-velocity body at a depth of 2.1-2.3 km. Crustal layer velocities of 3.0, 5.25 and 7.0 km/s were observed with kinks" in the travel-time curves suggesting that the 7.0 km/s material is intruding into the 5.25 km/s zone. The high-velocity body was interpreted to be a complex of dikes containing solidified magma from past volcanic eruptions that provide energy for the hydrothermal processes associated with the rift zone. Analysis of gravity data from a survey of 1975 provided constraints on the horizontal extent of the dike complex indicating a width varying from 12 to 17 km. Density contrasts of 0.4-0.6 were obtained by using velocity-density systematics demonstrating that the extruded surface lava is quite different from that which remains at depth. One possible conclusion is that magma differentiation has taken place. © 1979."