Very-low-frequency resistivity, self-potential and ground temperature surveys on Taal volcano (Philippines): Implications for future activity

International audienceTaal volcano is one of the most dangerous volcanoes in the Philippines. Thirty-three eruptions have occurredthrough historical timewith several exhibiting cataclysmic phases.Most recent eruptions are confined to VolcanoIsland locatedwithin the prehistoric Taal collapse caldera that is nowfilled by Taal Lake. The last eruptive activityfrom1965 to 1977 took place fromMt. Tabaro, about 2 kmto the southwest of the Main Crater center. Since thistime, episodes of seismic activity, ground deformation, gas release, surface fissuring, fumarole activity and temperaturechanges are recorded periodically around themain crater, but nomajor eruption has occurred. This periodof quiescence is the third longest period without eruptive activity since 1572. In March 2010, a campaignbased on Very-Low-Frequency (VLF) resistivity surveys together with repeated surveys of self-potential, groundtemperature and fissure activity was intensified and the results compared to a large-scale Electrical ResistivityTomography experiment. This work fortunately occurred before, within and after a new seismovolcanic crisisfrom late April 2010 to March 2011. The joint analysis of these new data, together with results from previousmagnetotelluric soundings, allows a better description of the electrical resistivity and crustal structure beneaththe Main Crater down to a depth of several kilometers. No indication of growth of the two geothermal areas locatedon both sides of the northern crater rim was apparent from 2005 to March 2010. These areas appear controlledby active fissures, opened during the 1992 and 1994 crises, that dip downward towards the core of thehydrothermal system located at about 2.5 km depth beneath the crater. Older mineralized fissures at lower elevationsto the North of the geothermal areas also dip downward under the crater. Repeated self-potential andground temperature surveys completed between 2005 and 2015 shownewgeothermal and hydrothermal activityin the areas of these older mineralized fissures that occurred during the April 2010 to March 2011seismovolcanic crisis. This dramatically extends the geothermal activity further to the North on the volcano.The occurrence of these newly activated fissures after a long period of quiescence and indications of inflationin mid-2010 under the North rim of the Main Crater suggests that new eruptive activity near the North rim ofthe crater could occur in the future