Abstract
Typical eruptions of Taal Volcano in the Philippines are frequent and violently explosive. The last devastating phreatomagmatic eruption from 1965 to 1977 resulted in the death of about 200 persons. For this and other earlier events, no warnings of impending eruptions were issued. Since this time, volcanic crises involving episodes of seismicity, deformation, gas release, temperature change, etc., have occurred frequently. Each of these did not, but possibly could have, evolved into an explosive eruption. One such event occurred between April 2010 and March 2011. This paper reports various types of geophysical data (electric/magnetic field, ground temperature, seismicity and ground deformation) taken during this period of activity together with the current state of geophysical knowledge about the volcano in order to throw light on activity evolution. While the 2010 seismovolcanic crisis was preceded by unusual changes in electric field, ground tilting and uplift, the major signals from all data were concomitant with the evolution of the crisis. On April 20, 2010, felt earthquakes first started to occur. The numbers increased sharply after April 29 with repeated pulses of seismicity that ended finally in March 2011. Overall, we can identify three periods of seismic activity, from April 20 to August 4–8, from August 4–8 to November 18, and from November 18 to March 28, 2011. Although only a single tiltmeter was in operation, it recorded three phases of deformation that correspond generally with the pulses in seismic activity. Phase 1 appears to correspond to inflation at a depth of about 5 km just to the NW of Main Crater Lake from April 20 to June 11, 2010. This resulted in an eastward tilting of the northern flank of the volcano. This apparently triggered rapid but shallower inflation (Phase 2) in the hydrothermal region of the volcano under the northern edge of Main Crater Lake from June 11 to July 13 just to the SE of the initial deeper source as indicated by northward tilting of the volcano’s northern flank. Progressive deflation began after July 13. By March 28, 2011, the deformation state recovered to the pre-crisis level and felt seismic activity ceased. Apparent related changes in seismicity, tilt, magnetic and electric fields occur during smaller NS inflations in the Phase 3 deflation—Phase 3a (July 13 to August 4–8), Phase 3b (August 4–8 to November 18) and Phase 3c (November 18 to March 28, 2011). These likely result from interrelated stress change, fluid/gas migration and thermal activity at depth and in the hydrothermal system. The rapidity of changes in the state of the volcano over a matter of hours during this entire process indicates rapid communication occurs between processes at depth and in the hydrothermal system and this could easily result in the dramatic evolution to violent phreatic explosions with little warning.
Highlights
Taal Volcano is a huge active stratovolcano, located at the southwestern end of the Macolod corridor in the large convergence zone between the Eurasian and Philippine tectonic plates (Fig. 1a) (Galgana et al 2007)
Apparent related changes in seismicity, tilt, magnetic and electric fields occur during smaller NS inflations in the Phase 3 deflation—Phase 3a (July 13 to August 4–8), Phase 3b (August 4–8 to November 18) and Phase 3c (November 18 to March 28, 2011)
We initially focused on the effects of the five largest rainfalls of more than 50 mm on electric field and tilt
Summary
Taal Volcano is a huge active stratovolcano, located at the southwestern end of the Macolod corridor in the large convergence zone between the Eurasian and Philippine tectonic plates (Fig. 1a) (Galgana et al 2007). These three sets of large and inter-correlated transient signals recorded at both DAK and MCL stations identify the start of each of three major phases in the 2010 seismovolcanic crisis at Taal Volcano: (1) the initial increase in seismicity (April 29), (2) the change within a day (June 10–11) of the tilt from an eastward to a northward direction at DAK for the month, and (3) the beginning of the deflation of the volcano (July 13) These observations indicate that (1) the 2010 volcanic activity impacted a large part of the volcano centered on the northern part of the crater, (2) major fissures located in the northern part of MCL, the inner part of the crater and the main 1992–94 active fissures located in the northern geothermal field were simultaneously reactivated during major onsets of activity, and apparently are linked to processes within the volcano at depth during this time (see Zlotnicki et al 2017). We note that CO2 degassing reached its maximum in March 2011 around the end of the Phase 3c activity (Arpa et al 2013)
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