We got a letter from one of our readers this week pointing out a seismic series near Agua volcano and Ciudad de Antigua in Guatemala.
“I’ve been living in Guatemala for a while, staying in Antigua. On April 27, the area around, and on, Agua volcano started to shake. We felt at least 8 quakes that day. They were very short (1-2 sec) and different from what the usual earthquake feels like. The locals never felt anything like that before. These quakes lasted for a week. Please see the official report I’ve attached to this e-mail.
Later I’ve checked the official reports and they showed some more quakes in the same area (April 7-8, also attached). There’s no new data after that.
I know nothing about volcanoes, and I’ve been trying to find out more about what’s going on but there’s little information. Different web sites didn’t even mark those quakes on their earthquake lists and maps, just few. But the official report looks scary.
Are these quakes related to Agua volcano and how? Could they be an alert? How dangerous it is to stay in Antigua?
Maybe these are silly questions for someone with the knowledge. But I’m dying out of curiosity (and a bit of too much adrenaline). I hope someone can explain/comment it. I’d really very much appreciate some info.”
Before I try to answer these questions, I will give a brief background of the surrounding geologic setting.
Background without Steve Zissou
Guatemala is heavily influenced by the subduction volcanism in the country, creating a range of volcanoes running the length of the country, and creating the volcanic highlands.
The subduction process has created a seismic hotbed with numerous large earthquakes occurring at a high rate, destroying 3 out of 4 capital cities in the country. The one not destroyed by an earthquake was destroyed by a lahar from Agua.
This means that large parts of Guatemala are crisscrossed with tectonic faultlines. In short, things tend to move about quite often in Guatemala, something that is amply evidenced by all the church ruins in Antigua.
If we turn to the volcanoes around Antigua that most people know about you have 4 of them. These are the Acatenango-Fuego double volcano, Agua and Pacaya. Currently both Fuego and Pacaya are erupting at the same time, something that is pretty common.
But, and this is a big but, these four volcanoes are dependent on a fifth far larger and less well known volcano. The name is Amatitlán, it is a large serial VEI-7 caldera mostly known for the lake that it contains.
In most ways it is different from its neighbours, firstly it does not look like a volcano at all, it is more of a hole in the ground. It erupts rarely, but always spectacularly. Either it will extrude a lava dome the size of a small mountain, or it will just have a large explosive event creating a new vent, or a combination of both things.
As far as we know Amatitlán has erupted 27 times in the last 250 000 years, giving an average of almost 10 000 years between eruptions. All of the known eruptions have been large. Another way to look at it, the entire Mayan civilization up to this date has existed in between cataclysmic eruptions from Amatitlán. Thankfully no eruption seems to be around the corner from this volcano.
The reason that there has been no large eruption is found out on an en echelon fissure swarm running outwards from the caldera, containing what can be perceived as distal flank vents. Pacaya is the first one of these, and it is a true flank vent situated on the caldera rim of Amatitlán.
The next over is the dormant Agua, that is believed to have erupted 10 000 years ago. And at the end of the fissure swarm we find Fuego and Acatenango.
As long as Fuego and Pacaya is erupting, the pressure at Amatitlán will be kept below the eruption threshold, so in a way Acatenango, Fuego and Pacaya works as large pressure relief valves.
Amatitlán is also highly seismically active, but I will not go on about this here. For those interested I am leaving the link to a previous article I wrote about it.
The current seismicity and answers
The current earthquake swarm is not that large for the area, containing only 5 M4 events, and the largest is only an M4.2. And this is not a lot for the area.
The origin is unlikely to be caused by regular tectonic activity. First of all, the earthquakes are not following a known faultline, they are also widely dispersed over quite a bit of area without any discernible pattern indicating a faultline.
Even though the earthquakes are small to moderate they would be quite noticeable for the population of Antigua since the field of earthquakes run out from the volcano of Agua in under the city, and towards Amatitlán.
As I am writing this, I had hoped for one of Andrej Fliis wonderful plots, but it has run a bit late, and I needed to write this article this weekend, so I freely admit that I am lacking a crucial piece of evidence right now. I will rectify this later as the plot is done with an update.
This means that I can’t ascertain fully which of the below two things are happening.
Stratovolcanoes in Guatemala tends to be rather steep and unstable since they are mostly built by magmatic rubble. This means that quite often parts of the flanks of the Guatemalan volcanoes tend to fail, and large amounts of volcanic debris start to flow for large distances.
Both Pacaya, Acatenango and Fuego has done that, but so far Agua has staid put. Flank failures are among the most dangerous events that can happen at a volcano. Most people know about the explosive failure of Mount St Helens in America, but not everyone is familiar with the far larger explosive flank failure in 1902 at Gagxanul (Volcán Santa Maria) in Guatemala.
There are though quite a few things against this being related to any future flank collapse of Agua.
The main reason is that the current activity is pointing toward the volcanic highland side of Agua instead of towards the coastal lowlands. On the highland side the edifice of Agua is only 2000 metres, but towards the coastal plain it is 3500 metres tall. This means that the gravity is constantly pulling the mountain towards the sea, and that it is buttressed against the highlands in the other direction.
So, currently I will rule out that Agua would catastrophically fail.
Normally when you track a volcano prior to an eruption you look for deep small earthquakes indicating that magma is arriving from depth. Sadly, the national seismic grid in Guatemala is not good enough to track these minute earthquakes.
The equipment at hand has for natural reasons been aimed towards known active culprits to detect what they are doing, especially since they tend to kill people. This leaves the less active, or dormant volcanoes without coverage.
This is not in any way any criticism against the geological authorities in Guatemala, it is just a recognition of a fact. That being said, Agua being situated between the grids aimed at Pacaya and Fuego is reasonably well monitored for being a long dormant volcano. In fact, it is better monitored than 90 percent of all US volcanoes.
Anyway, time to stop rambling. There is another way to detect magma, and that is to look for shallow emplacements in the magma reservoir, as magma arrive a magma chamber will start to creak and groan as the pressure builds up. And those creaks and groans are the earthquakes often associated with volcanoes.
If it is a more active volcano the earthquakes are often reasonably situated under the volcano, and you will get a ball shaped series of earthquakes around a void that is the magma in the chamber.
In long dormant volcanoes you instead tend to get more confusing shapes of earthquakes since there is no longer any easy way for the magma to enter the magma reservoir, or the magma must build a new magmatic system.
At El Hierro in the Canaries we saw an array of dykes and sills form as magma intruded as sheets in the bedrock following the point of least resistance.
Since we can rule out this being faultline tectonics, and it is unlikely that it is related to a future flank collapse, we can therefore ponder the idea that this is a dyke/sill system forming as magma is intruding towards Agua.
For a non-volcanologist this may sound alarming, but it is not especially alarming. First of all, volcanoes like Agua tend to go through cycles of dormancy and renewed activity. During a phase of renewed activity, they can have dozens of eruptions spanning a few hundred years, and then quietly go back to dormancy. After all, erupting is what volcanoes are supposed to do.
There is also another thing to remember. Volcanoes do not erupt out of the blue. They tend to be quite noisy prior to an eruption, and the amount of noise tend to increase significantly the longer a volcano has been dormant.
Gagxanul is a spiffing example of this, it took at least 3 months of incredibly strong seismic activity before it finally erupted. In all probability Gagxanul had several seismic crises prior to the eruption, but they where not recorded since nothing came out of them.
What we are seeing now is probably just a small sheet intrusion that will not amount to anything on its own, at least in the near future. For Agua to be able to roar back into life we will have to see swarms like this lasting months, and at a far higher seismic level.
So, to answer the most pressing question for our reader. Is this dangerous? No. At least not in the immediate future. If Agua would really come back to life, we will know about it weeks in advance and the authorities (that are quite good) will know about it weeks or months in advance, and they will do any necessary evacuations prior to the beginning of any serious fireworks.
Currently I am not worried at all, and I have most of my family in the vicinity. I hope I have dispelled any worries for our readers about visiting, or living, in Antigua.
And for any of our readers pondering visiting Antigua, please go to the Blue Sky Bar and climb up to the roof bar along the narrow creaking cast iron staircase (mind your head if you are tall). As you sit there you can have drinks mixed by some of the worlds best bartenders while looking at Fuego erupting in the night with stars shining above you. Don’t mind the two smiling idiots looking at each other all the time, it is probably just me and my wife.