Alexandre Nercessian, Alfred Hirn, Jean-Claude Lépine and Martine Sapin.

Sismologie Expérimentale, Dpt.Sismologie, UA 195 CNRS, and Observatoires Volcanologiques, Institut de Physique du Globe, 4 place Jussieu, 75252 Paris Cedex 05, France.

J. Volcano. Geotherm. Res. 70 (1996) 123-143


Arrival times of seismic waves from local earthquakes are inverted for jointly locating the sources and defining the 3D velocity heterogeneity of Piton de la Fournaise.

The lateral heterogeneity of the 2632 m high edifice is resolved as a high velocity plug of 1.5 km diameter surrounded by a low velocity ring which may be interpreted as due to the effusive part of the construction of Fournaise on the flank of the older volcano Piton des Neiges. Wave mode conversion detected on 3component seismograms provides evidence for boundaries of contrasted velocities.

Preeruptive swarm earthquakes cluster in the high velocity zone, under Dolomieu summital crater. Low strength and cohesion of the surrounding material account for the lack of seismicity for the final one to three kilometers radial flow of magma to the vents in Enclos Fouqué.

Beneath the high velocity plug the existence of a body with low velocity for P, and even for S is well constrained but with a poor definition of walls and bottom because of the lack of distributed deep earthquakes for sampling. The few earthquakes well-located in this depth region occur preferentially around 1.5 km depth beneath sea-level in the region of the cone and may be considered as constraining beneath it the deeper limit of an aseismic part of that low velocity body. The coincidence in time of their occurrence with the swarms above sea-level and the eruptions suggests a magmatic activation of the low velocity aseismic volume of 1.5 km size beneath sea-level under the high velocity plug of the cone. Further down the concentration of seismicity in two swarms between 2 and 4 km under the eastern flank does not allow the structure to be well sampled.

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