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A seismic swarm which occurred in December 1994 near the Aoba volcano, Vanuatu, is investigated in detail by analyzing local and regional broad band seismic records. The seismic area is situated in a poorly instrumented region, although major seismic and volcanic events are possible. The data collected by four permanent short period stations (the nearest one being installed in the Espiritu Santo Island located 80 km from the volcano) allowed us to locate epicenters with a 2–3 km precision, and to document the present local seismicity. The quality of available regional broadband data allows us to determine reliable focal parameters, though the magnitude of the strongest event is 4.8. Data have been selected from the permanent Geoscope and Iris stations located in the south-western Pacific region, and from two temporary networks (one installed in New Caledonia and Vanuatu by I.R.D. and the E.O.S.T, and another one installed in Fiji and Tonga Islands by the Washington University, St Louis, USA). Results obtained either with local (epicentral distance d km ) body waves or regional (500 km ) surface waves lead us to conclude that the swarm is associated with the volcanic edifice, but is not of volcanic type according to the definitions given by Minakami [Phys. Volcanol. (1974) 1] and Schick [Bull. Volcanol. 44 (1981) 491]. The observed mechanisms evidence an important local influence of the volcano itself on the stress field and the observed near horizontal P axis argues against a strong increase of phreatic activity. In addition, seismic and volcanic observations are analyzed in the framework of the interpretation of earthquake swarms occurring in the vicinity of active volcanoes. |
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