Very-low-frequency resistivity, self-potential and ground temperature surveys on Taal volcano (Philippines): Implications for future activity
Résumé
Taal volcano is one of the most dangerous volcanoes in the Philippines. Thirty-three eruptions have occurred
through historical timewith several exhibiting cataclysmic phases.Most recent eruptions are confined to Volcano
Island locatedwithin the prehistoric Taal collapse caldera that is nowfilled by Taal Lake. The last eruptive activity
from1965 to 1977 took place fromMt. Tabaro, about 2 kmto the southwest of the Main Crater center. Since this
time, episodes of seismic activity, ground deformation, gas release, surface fissuring, fumarole activity and temperature
changes are recorded periodically around themain crater, but nomajor eruption has occurred. This period
of quiescence is the third longest period without eruptive activity since 1572. In March 2010, a campaign
based on Very-Low-Frequency (VLF) resistivity surveys together with repeated surveys of self-potential, ground
temperature and fissure activity was intensified and the results compared to a large-scale Electrical Resistivity
Tomography experiment. This work fortunately occurred before, within and after a new seismovolcanic crisis
from late April 2010 to March 2011. The joint analysis of these new data, together with results from previous
magnetotelluric soundings, allows a better description of the electrical resistivity and crustal structure beneath
the Main Crater down to a depth of several kilometers. No indication of growth of the two geothermal areas located
on both sides of the northern crater rim was apparent from 2005 to March 2010. These areas appear controlled
by active fissures, opened during the 1992 and 1994 crises, that dip downward towards the core of the
hydrothermal system located at about 2.5 km depth beneath the crater. Older mineralized fissures at lower elevations
to the North of the geothermal areas also dip downward under the crater. Repeated self-potential and
ground temperature surveys completed between 2005 and 2015 shownewgeothermal and hydrothermal activity
in the areas of these older mineralized fissures that occurred during the April 2010 to March 2011
seismovolcanic 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 inflation
in mid-2010 under the North rim of the Main Crater suggests that new eruptive activity near the North rim of
the crater could occur in the future.