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

1989; Taylor & Francis; Volume: 32; Issue: 1 Linguagem: Inglês

10.1080/00288306.1989.10421389

1175-8791

Russell Robinson,

Geological and Geochemical Analysis

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, Mt Edgecumbe. Activity southwest of the gap may be part of secondary aftershock sequences because three large aftershocks soon after the main event occurred in that region. The gap in activity may then just be a coincidence. Computed depths of the aftershocks range from 0. 2 to 9. 6 km, most being from 4. 0 to 5. 9 km. The depth cutoff of 9. 6 km probably reflects a high temperature gradient as would be expected in a volcanic region. As is typical for normal faulting events, the main shock initiated near the bottom edge of the aftershock zone. The aftershocks near the main surface faulting appear to lie in the footwall and do not define the fault plane itself. Focal mechanisms have been obtained for 27 events; they are primarily of normal faulting type with varying degrees of strike-slip motion (right-lateral if a northeast-southwest fault plane is picked). The tension (T) axes are mostly horizontal and strongly clustered in the 150 ± 20° azimuth. The compression (P) axes are clustered in a vertical direction or, if not vertical, have a northwest-southeast azimuth. This stress regime reflects the tectonic situation of back-arc spreading and rift formation combined with a degree of shearing due to the oblique plate convergence zone to the east.