Precise relocation of earthquakes preceding the March 9, 1998 eruption of the Piton de la Fournaise volcano

The March 9, 1998 eruption of the Piton de la Fournaise volcano (Réunion island) was preceded by a seismic crisis unusually long for this volcano. It lasted for more than 35 hours and included about 3100 micro-earthquakes with a maximum magnitude of 2.2. During the crisis, the events migrated slowly and monotonically from a depth of about 5 km below sea level to the surface, leading to the onset of the eruption. The arrival times of 1001 events have been determined using waveform similarity and a set of reference events manually picked. This technique allows to greatly increase the number of located events by including the small magnitude ones and also reduces the scattering related to the inconsistency in the manual phase picking. The program hypo71 has been used with a 1D velocity model to obtain preliminary locations. From about 5 km b.s.l. to sea level, the obtained locations define an almost continuous narrow vertical pass with a horizontal extend of the order of a few hundred meters (about 200-400 meters). Above sea level the earthquakes do not indicate any clear migration. In order to further improve the precision of the locations, and define eventual active structures, we searched for multiplets among the located events. A total of about 770 earthquakes can be grouped into 16 multiplets of more than 5 events centered at different depths along the migration path, some of them including events with reversed focal mechanisms. The application of precise relative relocation techniques using waveform cross-correlation differential times enlightens several streaks or planar features with various orientations. For some of the multiplets, clear upward migration of the earthquakes is observed along the defined features.