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Phreatic explosion at Mayon volcano, Philippines, 6 May 2013

Summary: At Mayon, a well monitored volcano, after months of quiescence a spontaneous explosion occurred on 6 May 2013, killing five people including 3 German Tourists and injuring many more. This brief statement summarizes the geologic situation, dangerous access to volcanoes known to be active, and recommends a debate and revision of existing volcano activity alert levels in order to develop international standards and prevent false safety interpretations by non-experts. 



Geologic background: Mayon volcano is located in eastern Luzon, Philippines, and is part of an alignment of ten large volcanic cones in a west-dipping subduction zone. These volcanoes built up on Tertiary-Quarternary sediments underlain by metamorphic rocks of the Luzon peninsular. The rock types of these volcanoes cover a broad range, from basalt and andesite to dacite and rhyolite. Of all this volcanic chain, only two are active, Mayon and Bulusan. A major fault (San Vicente-Linao fault) strikes along the southwestern side of Mayon, together with a number of smaller NW-SE trending faults. 



Mayon volcano and Tourism: Mayon is a symmetric stratovolcano with steep flanks elevating up to 2460 m above sea level and popular with tourists for camping and climbing. Since written records, Mayon is the most active volcano in the Philippines, and one of the most active ones on the world. A small crater has formed at the summit, being the site of most effusive and explosive eruptions ranging in VEI from 1 to 3. Flank eruptions are rare, but accompanied the larger eruptions. Access to Mayon starts from a base of about 300 m such that Tourists are for at least 48 hours on the volcano flanks or in direct vicinity to the summit. 


Historical eruptions: The largest historical eruption, VEI=4, occurred 200 yrs ago in 1814, and produced some 5x 108 m³ tephra, generating pyroclastic flows killing 1,200 approx. Mayon had some 50 eruptions since 1616, with a mean eruption interval of 4 years. Volcano explosivity index (VEI), a measure of the eruption magnitude, ranges from VEI=1 to VEI=4 in the same period, with every 14 years one eruption VEI=3 or even VEI=4. Such statistics may be misleading and does not illustrate non-stationary behavior of volcanoes, however. The last VEI=3 eruption occurred in 2001, the last VEI=4 eruption in 1897. Heavy rainfalls in 2006 caused lahars and landslides, killing over 1,000 people. Phreatic explosions such as in 6 May 2013 are common, accompanying or predating magmatic eruptions, such as in 2008, 2006, 2004, 2003, 2000, some of which exceeding 2.5 km ash plume heights (as in 2008). Because phreatic explosions are not necessarily related to shallow magma accumulating under the summit crater, their occurrence may be spontaneous, and be triggered also by external forces (rainfall). In November 2012 Mayon showed new signs of unrest, which ceased again however soon thereafter.



Alert level and danger zone(s): Activity at Mayon is defined in 6 classes, from being “quite” (level 0), low-level unrest (level 1), “unrest of magmatic origin” (level 2), “high unrest and magma close to crater” (level 3), “eruption possible in days” (level 4), to “hazardeous eruption” (level 5). The level 0, although indicating “quiescence” is interpreted by the observatories at PHIVOLCS to have no indications for future eruptions, i.e. no ash emission, inflation, harmonic tremor hydrologic changes at springs or crater glow. As none of these have been recorded before and after 6 May 2013, level 0 remains. However, Mayon is one of the few volcanoes worldwide where a so-called permanent danger zone (PDZ) is defined. This means that up to a radius of 6 km any access and approach to the volcano is dangerous, and could lead to serious injury and death. Discussions about possible relocation of all people living in this danger zone continue since years. This danger zone is in place also because of spontaneous gas driven explosions, which are precursors for magmatic eruptions. Gas driven explosions, in turn, are known to volcanologists to have no or little precursors themselves.



The 6 May 2013 explosion: The Philippine Institute of Volcanology and Seismology regularly issues volcanic activity advisories. The 6 May 2013 explosion was identified as a "small phreatic event" that lasted about 73 seconds and sent ash some 500 m into the air. The cloud was partly dark grey to brownish, providing a hint about the significant ash and particle concentration. Although Mayon is well monitored, no intensification of volcanic activity was observed in the days or hours before or after the explosion. The alert level therefore remains on level 0.



Recommendations and further actions: Our evaluation leads to two main concluding points and recommendations:

  1. At Mayon, tourists approached a volcano known to be dangerous, and stayed within the permanent danger zone for days, this with knowledge but not with authorization from officials. This is a problem not specific at Mayon but also elsewhere, where touristic tours and guides are organized to volcanoes, often especially during phases of increased activity. We emphasize that active volcanoes remain in large parts unpredictable to scientists, and consequently also to non-experts risking their own health and the lives of rescue teams if accessing.
  2. Alert levels at volcanoes are not standardized and confusing to non-experts. At Mayon a 6-class scale is defined including a class 0, whilst other volcano observatories and countries define 5 (f.i. New Zealand), 4 (f.i. USA, Indonesia) or 3 (f.i. Mexico) classes only. The class 1 defines the volcano as “quite”, which at some other volcanoes really means quite. At Mayon, however, even at class 0, the permanent danger zone is recommended to avoid. Non-experts who may inform themselves about the activity status before visiting volcanoes may not know about the local but lifesaving differences in these alert level classification schemes. CEDIM consequently recommends to internationalize the alert levels and introduce standard definitions applicable globally.

 

Dr. habil. Thomas Walter, Dr. Birger-G. Lühr (GFZ, Section 2.1 -  Earthquake Risk and Early Warning)