It's been just over two months since Anak Krakatau, the young volcano growing from the ruins of Indonesia's infamous Krakatau, sent a devastating tsunami sweeping the shores of Sunda Strait.

After a brief lull, eruptive activity resumed in mid-February. The Indonesian government has warned fishermen to remain at least 5 kilometers away from the seething island, though they have lifted restrictions and allowed residents to rebuild their homes on tsunami-ravaged shores. It seems the poor haven't been given the resources they need to avoid disaster.

It's too early to say what's in store for the dangerous young volcano. It may be decades before it builds itself up enough to collapse again. But chances are good that the Sunda Strait hasn't seen its last tsunami. And I can guarantee that somewhere on Earth, within at least some of our lifetimes, another volcano will suffer a catastrophic collapse that sends enormous waves of water surging for vulnerable coastlines, killing hundreds, thousands, or even tens of thousands of unsuspecting people.

And those of you living on quiet coasts far from any active volcano aren't immune to the threat.

In this brief series, we're going to investigate what volcanic tsunamis are, how and where they're most likely to happen, what the risk factors and frequency are, and how we can prepare for them. We'll be exploring some possibilities for early warning systems, and talk a bit about what you can do if you find yourself in the path of one. We'll begin today with the basics.

First and most obviously: a volcanic tsunami is a mass movement of water caused by a volcano. Note that I didn't say erupting volcano. This is because a volcano doesn't have to erupt to cause a tsunami. Let's have a look at the ways we know volcanoes can generate tsunamis, explosively or not:

During eruptions, volcanic tsunamis can be caused by underwater explosions and shock waves caused by large explosions - even ones that occur above the waterline. Shock waves coupling with sea waves can produce tsunamis up to three meters in height. So, sometimes the energy of the eruption all by itself can make the sea behave in extremely dangerous ways.

Pyroclastic flows and lahars hitting the ocean can also displace water on an alarming scale. Lahars can dump quite a lot of material into the waters near a volcano very quickly, but pyroclastic flows are the stars of this show. These lethal flows of searing gasses and entrained volcanic debris can send a cubic kilometer or more of material careening into formerly serene seas, displacing massive amounts of water that have nowhere to go but away.

Earthquakes, whether tectonic or volcanic in origin, can cause landslides and debris avalanches. It doesn't matter if these mass movements of rock, ash, and other material originate above or below sea level: water gets shifted, and tsunamis happen.

Lava benches can collapse without warning, generating localized but deadly waves.

Caldera collapse, like the catastrophic conclusion of Krakatau's 1888 eruption, can displace massive amounts of water, causing enormous tsunamis that ravage parts of the coast that wouldn't ordinarily be considered low-lying.

Flank failures and sector collapses can be as bad or worse, and don't need an eruption of any size to happen. Most volcanoes are steep, made of conglomerations of rubble and loose material, often have hydrothermal systems that alter and weaken even the most solid lava, and suffer destabilizing intrusions from new magma. New material is often added far faster than erosion can cope with. And submarine lava flows build the steepest slopes of all, due to rapid cooling and/or accumulation on coral reefs. With several of these factors combining on one volcano, you don't need an eruption or earthquake to bring an entire sector down. Gravity can manage the job alone. And then you end up with the familiar problem of too much debris shifting the water at once.

Any one of the above factors can cause a volcanic tsunami, but they often happen in combination. This makes it difficult to predict the timing and scope of such tsunamis. But how much of a threat are they?

Volcanic tsunamis are relatively rare; we've recorded just 90-100 during the 19th and 20th Centuries combined. There are almost that many volcanic eruptions per year, with most years chalking up between 60 and 80 eruptions. So as a proportion of overall volcanic activity, they're just a small fraction. However, they have an outsized effect on mortality: they've caused 20-25% of all the known fatalities directly connected to volcanoes since the mid-1700s. Tens of thousands of people can die in a single event.

Volcanic tsunami hazards aren't limited to just a few small areas of the globe. Entire ocean coastlines are at risk, including the Pacific and central Atlantic. "Severe tsunami hazards exist on all coastlines that face both the open sea and large and relatively young volcanic islands," Franziska Whelan and Dieter Kelletat state in their paper on mega-tsunamis. On a planet that is more than 70% covered in water and experiencing vigorous plate tectonics, that leaves a great many coastal populations at risk.

Despite their outsized impact, volcanic tsunamis are not often considered in assessments of volcanic hazards. Hazard maps and evacuation routes don't usually take them into consideration, even in regions where the risk is known to be high. Relatively few scientists have investigated them, and volcanologists typically don't include tsunamis in their work. Things have improved somewhat over the past two decades, but we are still woefully lacking in preparation and awareness.

Next, we'll explore regions at risk, determine the geologic factors that signal potential volcanic tsunami hazards, and then we'll investigate what an effective Tsunami Early Warning System that includes volcanically-generated tsunamis might look like. Such a system can't come soon enough for people living in the hazard zone.

References:

Bellotti, G., Di Risio, M., and De Girolamo, P. (2009): Feasibility of Tsunami Early Warning Systems for small volcanic islands. Natural Hazards and Earth System Science, vol. 9, issue 6, 1911-1919

Giachetti, T. et al (2012): Tsunami hazard related to a flank collapse of Anak Krakatau volcano, Sunda Strait, Indonesia. Geological Society, London, Special Publications, 361, 79-90

Paris, R.: (2015): Source mechanisms of volcanic tsunamis. Philosophical transactions. Series A, Mathematical, physical, and engineering sciences. 373.

Paris, R. et al (2014): Volcanic tsunami: a review of source mechanisms, past events and hazards in Southeast Asia (Indonesia, Philippines, Papua New Guinea). Natural Hazards, 70, 447-470.

Whelan, F. and Kelletat, D. (2003): Submarine slides on volcanic islands–a source for mega-tsunamis in the Quaternary. Progress in Physical Geography: Earth and Environment, 27(2), 198–216