Hidden Dykes detected on Ultra Long Period seismic signals at Piton de la Fournaise volcano?

Broadband seismic data enable us to test whether Ultra Long Period (ULP) signals can be used to investigate magma chamber pressure state and to monitor volcanic eruptions. By systematically investigating seismic signals at GEOSCOPE station RER, we find ULP signals associated with volcanic events, during the last 2 decades. We tentatively propose to interpret these signals as events highlighting the activity of the upper magma feeding system of the Piton de la Fournaise volcano and as being related to the deep magma chamber. Some of these unusual signals were previously detected and usually interpreted in term of tilt [Battaglia, J., Aki, K., Montagner, J.-.J.-P. Tilt signals derived from a GEOSCOPE VBB station on the Piton de la Fournaise volcano, Geophys. Res. Lett. 27 (5) (2000) 605–608.]. They are detected using STS1 seismometers sensitive in the bandwidth 10− 3 to 10− 2 Hz.This alternative model enables to quantify each event in terms of a pressure drop in the upper magma reservoir of the volcano. Two kinds of seismic waveforms can be associated with two types of fracturing events. The first one is related to the fast rise of a magma batch coming from the deep magma chamber and the second is the result of a slow rise in pressure inside a shallow reservoir. In both cases, we are able to detect these events on very long period seismic data. The interpretation of these signals supports the idea of a single magma source at the sea level. Thus, we propose to classify all the eruptions occurring during the last two decades into two groups, corresponding to the two primary pressure states of the magma chamber.