Seismic Tomography Study of the Kawerau Region and Whakatane Graben (Bay of Plenty, New Zealand)

Abstract

The crustal seismic velocity structure of the Kawerau geothermal field was investigated by a seismic tomography study using travel times of aftershocks associated with the 2 March, 1987 Edgecumbe earthquake. A total of 887 travel times from 89 events were used. Using the square of the slowness as a parameter to obtain continuous velocity-depth functions, inversion solutions were obtained using a damped least square algorithm to solve the large system of equations. The study shows that the Kawerau Field lies at the SW end of a crustal low velocity structure which extends from Kawerau to the north (over 30 km); the structure is well defined between 3 and 6 km depth. The seismic velocity anomaly occurs in brittle greywacke basement rocks beneath the tectonically very active Whakatane Graben. The low velocities probably reflect intense fracturing associated with active NNE trending faults. A low velocity anomaly also occurs beneath the E Kawerau field; however, it can not be separated from the much larger anomaly beneath the Graben.