Tilt signals derived from a GEOSCOPE VBB station on the Piton de la Fournaise volcano

We report some very long period transients observed on a very broadband seismic station of the GEOSCOPE network, which is situated 8 km away from the summit of the Piton de la Fournaise volcano. We transform the ground velocity measured by the seismic station into tilt and test the result by comparing with data from Blum-type classical pendulum tiltmeters located in the same vault. The comparison validates the use of the horizontal seismometers for measuring tilt for periods shorter than 25 hours. The method is applied to the transients observed prior to several eruptions and magmatic intrusions since 1991. Because of the moderate distance between GEOSCOPE station and the summit of the volcano, the obtained tilt measurements provide information about eruptions and intrusions which complement the information on the geometry and location of the magmatic source provided by the summit tilt network.