STRESS, FAILURE AND FLUID FLOW DEDUCED FROM EARTHQUAKES ACCOMPANYING ERUPTIONS AT PITON DE LA FOURNAISE VOLCANO

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

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

J. Volcanol. Geotherm. Res. 70(1996) 145-167

Abstract.

In the present episode of eruptive activity, evidence from seismicity for sustained magma inflow from depth into the edifice of Piton de la Fournaise is lacking. Pre-eruptive main deformation and shallow seismicity help to identify very small volumes of magma that are in motion beneath the rim of Dolomieu summit crater, and oriented along the azimuth of the future vents. Small magma pockets may reside in the cone above sea-level, or rather may be expelled repeatedly due to crystallisation in a small, low velocity, aseismic, region below sea-level under the high velocity central plug of the cone in which pre-eruptive earthquake swarms are located. In cross section the hypocenters define two steep sheets diverging from the aseismic zone at sea-level towards 1.5 km elevation above sea-level (or 1 km beneath the 2632mhigh cone). However, failure induced by increased pressure in the suggested chamber does not account for observed focal mechanisms.

The occurence and timing of magma transport are attested by eruption, and seismic activity may be related to magma transport. Focal mechanisms document strike-slip, not normal faulting or tensile failure. Vertical propagation of the edge of a feeder dike may enhance strike-slip motion above the edge, in a region where effective normal stress is decreased by thermally-induced groundwater flow. The strike-slip mechanisms could also be caused by a tensile-shear widening of the horizontal section of vertical conduits.

Fournaise strikeslip earthquakes occur in two orientations with P axes orthogonal between them, within a single pre-eruptive crisis. Earthquakes are distributed in the same volume but mechanisms switch from one to the other type systematically with time, indicating a reversal of stress conditions. The orientations of P axes with respect to the epicentral trend suggest that in the later parts of crises leading to eruptions, a compression of the medium occurs after a dilation in the first part. The activated zone might respond successively to the arrival and the departure of the magma on its way from the reservoir at depth to the vent, radial to the cone.

For preprint, please mailto lepine@ipgp.jussieu.fr