Aftershocks of the 1987 Edgecumbe earthquake, New Zealand: Seismological and structural studies using portable seismographs in the epicentral region

Abstract The Edgecumbe earthquake (1987 Mar 2d01h42m34s UT; M L 6. 3, M S 6. 6) was a normal faulting event accompanied by surface fauft breaks. The earthquake occurred in an onshore area of active back-arc extension characterised by recent volcanic activity. Over 100 aftershocks have been located using 11 portable, and 2 permanent, seismographs in the epicentral region. The events occurred during the period from 3 to 10 days after the main shock, and most have magnitudes of 3. 0 or greater. They have been located using a velocity model derived from the arrival-time data itself. Station terms, which form a part of that model, reflect the lateral variation in near-surface geology. The epicentres define a region 65 km long, in the northeast-southwest direction, and 10 km broad. This is much longer than the observed surface faulting or what would be expected from the main-shock magnitude. There is a gap in the aftershock distribution near the Kawerau Geothermal Field and the recently active andesite volcano,...