Microseismicity and 3-D Mapping of an Active Geothermal Field, Kilauea Lower East Rift Zone, Puna, Hawaii

Abstract

The local fault and dike structures in Puna, southeastern Hawaii, are of interest both in terms of electricity production and volcanic hazard monitoring. The geothermal power plant at Puna has a 30 MW capacity and is built on a section of the Kilauea Lower East Rift Zone that was resurfaced by lava flows as recently as 1955 and 1960.
The Puna Borehole Network was established in 2006 in order to provide detailed seismic data about the Puna geothermal field. The array consists of eight 3-component borehole seismometers. The instrument depths range from 24 to 210 m (80 to 690 ft); five stations with 2 Hz geophones and three with 4.5 Hz geophones.
The large majority of events were < M0.5 and occurred at depths between 2 and 3 km. The size, location, and depth of the microearthquakes suggest that power plant activity affects local seismicity. Earthquake depths increase from NE to SW, trending up the rift zone towards the Kilauea summit. Depths also increase from the study area to the SE, consistent with active normal faulting along Kilauea’s south flank. Shear wave polarization indicates that the active, fluid-filled fracture system trends SW-NE, consistent with the orientation of the LERZ. Double difference relocation suggests an intersecting network of fractures with both NE and approximately NW trends.
3-D tomographic analyses of P-wave velocity, S-wave velocity, and the Vp/Vs ratio are also presented. An area of anomalously fast P-wave velocity at the relatively shallow depth of 2.5 km may be evidence for a dense, gabbroic intrusion. This intrusion may underlie and be the parent of a body of dacitic melt that was discovered under the geothermal field during drilling.