Why the deadly tsunami in Indonesia took everyone by surprise

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A tsunami that killed hundreds of people and displaced thousands more in Indonesia struck without warning — and there's no way to tell when it will be safe for people to return home, says volcanologist Janine Krippner.

The eruption of the Anak Krakatau volcano on Saturday evening caused part of the island in the Sunda Strait to collapse into the sea, reportedly generating tsunami waves of more than two metres, killing at least 430 people in Sumatra and Java.

Indonesia has since raised the volcano's threat level and more than doubled its no-go zone.

Because most tsunamis are caused by earthquakes, not volcanoes, nobody saw this one coming, says Krippner, a volcano expert at West Virginia's Concord University.

Here is part of her conversation with As It Happens guest host Helen Mann. 

What is it that Anak Krakatau is doing that has led to this increased threat level?

Anak Krakatau has been active for a long time. In fact, this year it's been erupting nearly constantly since around June. But there was a larger eruption a few days ago and part of the volcano collapsed into the sea. 

What triggered the collapse, we just don't know.

Is that unusual that that would happen?

It's relatively rare.

When it happens on land, we obviously don't have the impact of the ocean, so we don't have tsunamis. 

What went through your mind when you heard about this and when you saw some of the data?

Honestly, my gut sank thinking about all of those people who have been impacted. It's an absolutely horrific, horrific event.

It's not like the normal kind of tsunamis. This particular kind was generated a lot closer to the coast, so people just had no warning.

If this is already a rare event, having a volcano-caused tsunami, how is it that another one might potentially happen so soon after the first?

The problem is that we can't tell how stable or unstable the volcano is after this event. We have the volcano that we can see above the ocean floor, but it also goes much further down below.

How long has Krakatau been behaving in this way?

The collapse is not usual, but the eruption style of the last few months is very normal for Krakatau.

In fact, it's been erupting above the surface on and off since 1929. Before that, it was below the surface of the ocean. 

So should officials have been on higher alert that a collapse or some other potential event was possibly pending?

I don't know that there was any way that they could have seen this coming.

With the high level activity being normal at this volcano, this collapse has not happened previously. So why it happened on this particular day, there's just no knowing at this point.

Volcanoes are essentially piles of rock and lava. They're inherently unstable systems. But, as we see with the hundreds of thousands of volcanoes around the world that are not collapsing every day, this is a rare event. 

So can you take us through the sequence of events and what would have actually transpired under the ocean?

A large amount of the volcanic edifice collapsed into the ocean. So what happens then is it's displacing a large amount of water.

If you think of throwing a pebble into a pond, you get a series of waves that propagate outwards from that pebble because of the water being displaced by the pebble. So that series of waves then travelled at very high speeds towards land.

From what I've heard, especially in one location, there were not the usual signs of a tsunami, like the water receding. The first wave of that sequence was the warning. And it's not enough. If people don't see that wave, they have absolutely no warning whatsoever.

In these areas, often the earthquake is the warning. But because this is not an earthquake, people did not heed that signal.

Is there, do you think, any way that volcanologists can reassure people when it would be safe for them to return to these areas?

I'd really like if we could get a survey of the volcano to see if we can understand any unstable areas.

But, unfortunately, the volcano really hasn't stopped erupting. So it's a very dangerous area to be at the moment.

Written by Sheena Goodyear with files from Associated Press. Interview produced by Chris Harbord. Q&A has been edited for length and clarity.