Wikimedia commons, Arco de Santa Catalina in Antigua, Guatemala.
Very interesting article!
http://volcanocafe.wordpress.com/2013/05/05/a-tale-of-three-cities/
I’m Guatemalan and I’m doing a PhD studying Fuego volcano, so I was glad to see someone writing about Guatemalan volcanoes and about the risk related to their activity. I do have a several comments about the article, and I hope they won’t be taken as “malicious hair splitting”, but rather to further the discussion about these fascinating ideas.
Regarding the 1541 lahars, I guess nobody knows exactly what happened, but the early chronicles (see Remesal, 1619, and Feldman, 1993 for a compilation) agree that very intense rain happening for several days prior to the lahars, whether there was also an earthquake is not as clear, since references to the “earthquake that came from above” may just relate to the lahar and associated rumbling and shaking phenomena. The simplest hypothesis is that this resulted from an extreme rainfall event, which recur every so many years during the tropical storm season (the event happened on September 11, at the thickest of the storm season), an probably had nothing to do with volcanic activity, and maybe not even with any seismic activity. Recent examples of similar events at Agua include lahars and mudflows during tropical storm Stan in October of 2005, the rainy season of 2006, affecting the town of Palin on the E flank, tropical storm Agatha in 2010, affecting the town of San Juan Obispo on the N flank, etc. All such events only cause a few deaths compared with the 1541, but examples on other nearby volcanoes, like the lahar that destroyed Panabaj, on the flank of Toliman and Atitlan volcanoes during Stan in 2005 shows that such events have the full potential to cause a similar disaster, entirely related to extreme rainfall. But maybe an earthquake also contributed… but this we will probably never know.
The controversy about a crater lake breach is also difficult to settle, but I don’t think there is any good evidence for the crater lake hypothesis. None of the original sources (eyewitnesses) that I know mention a crater lake, although I neither aware of any description of Agua’s summit before Remesal’s account of a climb in 1615. Two other lines of evidence suggest to me that the crater lake breach hypothesis is unlikely. First there is no evidence of lake sediments on the current crater (although the crater is partially filled with colluvial material from the inner walls), suggesting that a long term lake probably didn’t exist within the crater. Secondly, the breach in the crater drains to the wrong barranca, and therefore a crater lake breach probably wouldn’t end up in Ciudad Vieja (or what today would be San Miguel Escobar), it would instead end up either in San Pedro Las Huertas or San Juan El Obispo. As I mentioned before, a crater lake is unnecessary to explain a lahar associated to heavy rain, and for the lack of evidence of such a lake, it seems more likely to me that the lahars were just caused by the collapse and transformation of saturated flank material.
The story of the Antigua moving, first contemplated in 1717 and finally done in 1776, after the Santa Marta earthquakes, is full of political intrigue and power struggles between the religious and civil powers of the time, and the natural events (eruptions and earthquakes) were used by both parties (those who wanted Antigua to remain as the capital, and those who wanted to move it) for or against the move, and in the end the decision was probably motivated by many other reasons than just the obvious “natural hazards threat”, as always, things are more complex than they seem at first. There have been a few very interesting analysis on this, and for an in depth analysis you can read Christophe Belaubre work (http://www.revistas.una.ac.cr/index.php/historia/article/view/1754). In any case the new Capital General, Martin Mayorga, finally prevailed over Cortes y Larraz and the capital was moved!
The volcanological relationship between Acatenango and Fuego is a difficult one to assess, given that we know relatively little about both volcanoes. Geochemically they seem different in some aspects, with Acatenango having some more silicic rocks, but there is broad overlap in compositions.
A plot of the whole rock TAS that I made from the CENTAM database
I suspect that this difference would decrease if we do more sampling of Fuego especially of the older rocks, as the sample dataset may be skewed towards younger products (e. g. 1974), and in general I think that so far we have only scratched a tiny bit of Fuegos eruptive history, this may be difficult because having Fuego been so active in the last few thousand years it may be difficult to find outcrops of older rocks.
I did not understand the comment on the blog stating that “some researchers have extemporized that the magmatic system of Fuego runs through the magmatic system of Acatenango. The reason for this theory is to explain that some of the eruptions of Fuego carry magmatic signatures from Acatenango, but Acatenango never have the magmatic signature of the bulk of Fuegos eruptions”. What does that mean and where does the idea come from? I am aware of the hypothesis by some authors (e. g. Chesner and Rose, 1984) that magmas from both Fuego and Acatenango may have in common that they pond at the base of the crust, and then ascend through the crust to shallower independent reservoirs, but I think that’s different from what the blog post says. Maybe I’m not getting it right?
Overall I don’t see any evidence that Acatenango produces bigger eruptions than Fuego, and in that sense I don’t know what to make of the blog statement that “Historically Acatenango has not suffered from frequent eruptions. Instead the eruptions have tended towards being larger than the eruptions of its twin Fuego.” To my knowledge, the only Acatenango eruption for which there is any quantitative estimate of volume (eruption P-4 in Basset, 1996) has a min volume between 6.3 x 10^7 and 1.3 x 10^8 m^3, which would put it in the VEI 3 – 4 category range, similar to the 1971 and 1974 Fuego eruptions, and probably similar to those in 1932, 1880, 1717, 1581-82. The 370 BC pyroclastic flow mentioned in the blog entry, and which I assume would correspond to the sample AC.196 from Basset (1996) with a radiocarbon age of 2330 yBP, reached a distance of ~ 10 km from the Pico Mayor (Acatenango) crater, and although it would have been a large pyroclastic flow, the distance it reached is comparable to many of Fuegos pyroclastic flows. I think overall, both volcanoes can produce similar large eruptions, which is in itself very worrisome.
As you mention in the blog, the 370 BC pyroclastic flow would have cause a lot of damage if it would have happened in recent times, mainly because it’s on the other side of San Pedro Yepocapa (you can download the google earth .kmz file with the location of the sample as given by Basset, 1996, from here: http://www.geo.mtu.edu/~rpescoba/downloads/Pyroclastic_Flow_AC196_Basset_1996.kmz
Now the topic of evacuations is a very hairy one. It is very clear that a big eruption from Acatenango can easily destroy the towns around the volcano, but when such an eruption may happen, and therefore when to evacuate, is a very difficult question to answer. There have been at least two forma hazard assessments done for Fuego and Acatenango, the last one by Jim Vallance et al. (2001), which can be accessed here: http://vulcan.wr.usgs.gov/Volcanoes/Guatemala/Publications/OFR01-431/ Some larger (thousands of people) towns that could be within reach of eruptions like those happening in the past include San Pedro Yepocapa, Acatenango, San Antonio Nejapa, San Miguel Dueñas, and Alotenango, depending not only on the size and character of the eruption, but on the location of the vent (e. g. Yepocapa vs Pico Mayor). Calling for the evacuation of several tens of thousands of people, especially if the crisis extends for days or even weeks, and may not end in a catastrophic eruption, can be a very difficult decision. The people at CONRED (Guatemalan disaster reduction agency), the local authorities and most than anyone else the people at risk, would certainly face a hard choice. And one could ask hypothetical questions, like: should an evacuation be called for a crisis like the 1924 – 1927 eruption? What about the 1972 eruption? And if so, who should evacuate? Compared to what has happened at Fuego in recent years (e. g. September 13, 2012), these were really minor eruptions, but the potential for a catastrophic one is always there.
The record of eruptions at Fuego is also a controversial matter, which I hope I will be able to clarify at least a bit with part of my dissertation. The GVP lists 7 eruptions possibly having a VEI of 4, which are basically the same as those classified as “fuerte” by Meyer-Abich (1956), plus the 1974 eruption and excluding the 1953 eruption, which is given a VEI 3 in the GVP; but some of those older eruptions, like the 1737 eruptions may not have been that big or may not even have happened at all. It seems that at least the 1581-82, 1717, 1880, 1932, and 1974 eruptions were most likely in the VEI 3 to 4 category, and probably many more were at least in the VEI 3 category (e. g. the many eruptions happening between 1702 and 1717).
The blog entry states that “Two VEI-4 eruptions are documented, the last in 1974 when it had numerous pyroclastic flows killing residents in nearby villages.” Is this saying that the pyroclastic flows killed people? Or is it saying that the eruption overall (possibly due to other hazardous phenomena) caused those deaths? To my knowledge, there are no confirmed fatalities due to the 1974 eruption pyroclastic flows, although there were a few casualties from collapsing roofs due to airfall tephra accumulation. There have been also casualties due to the lahars.
I also have to take issue with your comment stating “What is lacking is good mitigation with pre-prepared evacuation maps from the valleys most affected by pyroclastic flows and lahars”. I don’t exactly know what you mean with “evacuation maps”, but I don’t think how maps would really solve the problem of preparedness and crisis management in this particular case. With all their limitations, CONRED, INSIVUMEH, the local authorities, and of course the people in the villages themselves have made a big effort over the last 10 – 15 years to improve their capacity to respond to volcanic crises. Whether or not such efforts are fruitful or not is difficult to evaluate, and may only be assessed with some degree of clarity in the aftermath of a (hopefully “successfully” managed, whatever that means) real volcanic crisis. My perception is that this will depend a lot on how the potentially destructive crisis unfolds from the volcanological point of view. As the situation is currently my gut feeling tells me that, if it develops very quickly without clear and scary precursors, producing large pyroclastic flows that reach the communities, it may end in tragedy. But if it develops more gradually and there are enough (clear and scary) premonitory signals, there are better chances that a large amount of people will evacuate. Heavy tephra fall in their communities may be in the end what could save people from the pyroclastic flows! Again, the September 13, 2012 eruption is something we should look at more closely to get a better sense of people’s potential response to a future crisis. In any case I see the current efforts, including the simulation exercise done at Fuego last week, as a good thing and a step in the right direction.
I think your comment “Also the will to evacuate and being evacuated is slightly low locally, something that can be understood if one think about that the people in the villages are really poor without the means to support themselves if they evacuate.” is a really important one, and I totally agree with this. The issue here can’t be isolated as a solely volcanic hazard driven problem, especially in the context of uncertainty.
Finally, your closing remark “If you think this was bad choice for cities” is a very interesting one, because it shows the perspective of someone who obviously is focused on the volcanic (and seismic?) hazard issue, but is not seeing the complexity and myriad of other things that may go into choosing the “best” place to settle a city. I agree with you (I have to, I’m a volcanologist/geo-hazardologist) that it would make sense to avoid geological hazards as much as possible, but I also recognize that avoiding climate related disease, having nearby fertile soil and water (especially in the context of the XVIII century), having some protection against potential military threats, etc, etc, and many more etc’s, certainly also played an important role. Putting the capital city in the northern lowlands of Peten, far away from most volcanic and seismic hazards may in the end not play out that well, if you don’t believe me just ask the Mayans…
RUDIGER ESCOBAR WOLF
P.S. All science and knowledge is driven by debate. It was an honour to have the insights and comments from someone who actively studies such an interesting volcano as Fuego. /CARL D.S.
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