Pre-eruptive migration of earthquakes at the Piton de la Fournaise volcano (Reunion Island)

We present a study of the seismicity which preceded the 1998 March 9 eruption of the Piton de la Fournaise volcano. The eruption was preceded by a seismic crisis unusually long for this volcano which lasted for more than 35 hr and included about 3100 events with a maximum magnitude of 2.2. We located 583 volcano-tectonic (VT) events by manually picking their arrival times. The locations outline a monotonic upward migration of the hypocentres from about 5 km below sea level to the surface, leading to the onset of the eruption. This good correlation of the seismicity with eruptive activity suggests that the seismic migration was triggered by the ascent of magma toward the eruption site. The location of the earthquakes outlines an almost vertical conduit below the volcano providing new insights into the volcanic plumbing system.