Tuesday, October 15, 2013

7.1 Magnitude Quake Strikes The Philippines

Dozens were dead, and many roads and buildings damaged when a 7.1 magnitude quake struck Bohol Island in the Philippines Tuesday morning. As many as 49 people so far have been confirmed dead, and up to 33 people were reported missing. Approximately 164 people were reported injured. The quake occurred about 20km deep underneath the island of Bohol, in the center of the Philippine archipelago, violently shaking Bohol island and neighboring (to the NW) Cebu island, while the rest of the islands felt moderate shaking. Many aftershocks of varying magnitudes are occurring and will likely continue for several weeks.

There are many nearby holocene volcanoes, some of which have shown historical activity. On the island of Negros I West of the epicenter, several volcanoes likely shook and could be destabilized by the quaking, including the Pleistocene to early Holocene Cuernos de Negros, Kanalon volcano, Mandalagan volcano, and Silay. Out of all of these volcanoes, only Kanalon volcano has had confirmed eruptions in historical time.

East of the epicenter, on the island of Leyte I, there are several volcanoes as well, including Cabalían, Mahagnao, and Biliran, all of which currently display fumerolic or geothermal activity, and all of which have had eruptive activity within the past two centuries.


Image from Google Earth with USGS quake shake map overlay, detailing the strength of the shake force around the Philippine archipelago and associated volcanoes.

Earthquakes this large can cause disruptions in magmatic systems. Although rarely will a volcano erupt directly after a large quake event, changes in pressures in the magma chambers, fractures in the mountains (which can cause landslides and other destabilization), and tectonic stress, can contribute over time to an increase in eruptive potential. While it is not an extreme danger at this point, monitoring on volcanoes near the epicenter would be a good proactive step to take in order to cover all of the bases.

The Philippines is home to many famous and dangerous volcanoes. In 1991, Mount Pinatubo, previously a rather unknown and unremarkable lava dome complex which was heavily forested, erupted violently, sending columns of ash skyward, and pyroclastic flows down its flanks, accompanied by deadly lahars. The eruption vented so much material into the atmosphere so quickly, that the ash circumnavigated the globe, and actually lowered global temperatures for about a year, effectively pausing the effects of global warming. This was the second largest eruption in the 20th century after Alaska's Novarupta event.

Mount Mayon is also quite an active volcano in the Philippines, having had many frequent eruptive episodes throughout history. Currently, no volcanoes except Mayon are displaying eruptive activity, but Taal volcano in Luzon has been showing unrest in recent years. This is most certainly a dangerous volcano on the island. Having last erupted in the 1960s-1970's, and causing many fatalities as pyroclastic flows moved quickly over the caldera lake's surface to the shores, the volcano has since been showing fluctuations and signs of D/I (Deflation/Inflation) events. The temperature of the central caldera cone's lake has risen and fallen, acidity levels have fluctuated, and GPS shows many deformation events. It is likely that within the next decade or two, Taal will erupt again.

As it stands currently, no alert levels have been raised for any currently active volcanoes in the Philippines after the quake, and there is no imminent risk of eruption.

The Philippines archipelago lies along the so-called Pacific Ring of Fire, which encircles the entire Pacific ocean plate. The majority of the worlds large earthquakes and active volcanoes occur along this ring, which spans form the West Coast of the Americas, to Russia's Kamchatka and Kurile Islands, to Japan, to the Philippines, to Indonesia, and New Zealand.

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