Electromagnetic signals on active volcanoes: Analysis of electrical resistivity and transfer functions at Taal volcano (Philippines) related to the 2010 seismovolcanic crisis

International audienceElectromagnetic (EM)methods are powerful for imaging lithology, geological discontinuities, and hydrothermaland plumbing systems of volcanoes. Thesemethods can also be efficiently used for monitoring temporal changesrelated to the time evolution of volcanic activity. However, themagnetotelluric (MT) technique has not been yetinvestigated on many volcanoes. In this study, we take advantage of simultaneous magnetic and electric recordsmade on Taal volcano in The Philippines to prospect if resistivity changes were related to the 2010–2011seismovolcanic crisis. The seismicity began on April 19, 2010 and abruptly raised up on April 29. The crisisvanished nine months later onMarch 2011, and was characterized by the appearance of a source of deformationat 5 kmdepth which gave birth to a secondary deformation source located in the hydrothermal systemat about2.5 kmdepth someweeks later. Two stations, DAK andMCL, located respectively on the northern flank and insidethe summit crater, were simultaneously recording the horizontal North-South and East-West components ofboth magnetic and electric fields from March 11 to June 13, 2010. 1D inversion modeling shows that resistivityat the depth of 5 kmwhere dikes and deformation sources are positioned, is estimated smaller a ten ofΩm. Temporalchanges of the apparent resistivity and power spectra of the components of the EMfield are analyzed usingboth a single station approach at the two DAK and MCL stations, and a remote reference technique with a far(~3000 km) and a near station (~1.1 km). Local noises due to radio transmitters at stations prevent studying signalsbelow 10 s of period. The day-to-day time-lapse analysiswith the single station method shows several cleardecreases of the apparent resistivity at the station inside the crater, and high fluctuations at the station outside ofit, both in relation to the crisis for periods less than few hundred seconds . Application of the remote referencetechniquewith far and close remote stations leads to assume that therewas no clear change at the station locatedin the crater, while some fluctuating resistivity changes could have occurred at the second station DAK. Furthermore,the MT responses undoubtedly show that other local EM signals have taken place on the volcano beforeand during the crisis. In particular, electric signals linked to the reactivation of fissures and disturbances of groundfluids are present