A new comprehensive classification of the Piton de la Fournaise activity spanning the 1985-2010 period. Search and analysis of short-term precursors from a broad-band seismological station
Résumé
Piton de la Fournaise volcano (La Réunion Island) is one of the most active basaltic volcanoes, with an average of one eruption every 10 months. This study provides the first exhaustive compilation of all volcanic events (intrusions, eruptions, seismic crises) and related parameters at Piton de la Fournaise in the 1985-2010 period. This compilation has been correlated with the analysis of the records from the very broad-band seismological RER station (Geoscope network), located 8.5 km north of the summit. Our approach allowed us to identify short-term long period seismic precursors for most eruptions and intrusions. After a signal filtering process that consists in removing the instrumental response and the theoretical Earth tides effect, these precursors can be distributed into 4 classes that depend on their waveform and are globally considered as tilt related with magma transfer inside the sub-aerial part of the volcano edifice. The shapes and characteristics of these transient phenomena (time delay, duration or class) exhibit particular features that can be partly related to other simple eruption or intrusion parameters (location, altitude, volume). Statistical analyses of all events (intrusions and eruptions) are then derived. Estimates of acceleration rates of tilt signal at the RER station have been retrieved for eruptions and intrusions, with the challenge of providing a way to differentiate one from the other in real-time. Acceleration rates seem to correlate with eruptive lava flow volume and a threshold value can be determined allowing us to discriminate between intrusions and eruptions, illustrating the interest of analyzing them for real-time monitoring. The correlation with the initial seismic crisis marking the opening of magma ascent path was investigated, showing that the delay between the RER transient phenomenon and the start of the seismic crisis has been increasing since the major caldera formation event of 2007. This longer delay may be due to a combination of drastic changes in the internal structure of the edifice: a concomitant decrease in volume of magma batches and a deeper origin of magma setting off volcanic unrest. Our study highlights the additional role of external factors like loading and unloading related to the rainy season and stress field evolution due to Earth tides in influencing magma propagation and volcanic activity.