Even though it was erupting, more material was coming into the volcano than was going out, and that was causing the pressure to increase. That's often followed by some kind of breakout somewhere, either enhanced activity or new eruptive vent. That was going on for, I think, several months. And then, immediately preceding the breakouts in the Leilani Estates area [one of the residential areas affected by the latest Kilauea activity], there were earthquakes migrating into that region. There was evidence of ground tilting, so we knew magma was on the move. We didn't yet know whether it would lead to an eruption. But, of course, once you see that, you're not surprised that it starts erupting.
Are volcanoes like earthquakes in the sense that geologists can look at tension building up on a fault and more or less predict when the next big one will occur?
It's different in a number of respects. With earthquakes, for example, in the San Andreas Fault — I worked on that as well — and we can measure strain building up. But we haven't yet seen anything reliably that indicates a change in behavior before an earthquake. Strain will build up and in some cases for rather large earthquakes, it may be 200 years between events or longer, potentially.
In the Pacific Northwest, we think it may be 500 years between the big magnitude 9 [earthquakes], but we see energy building up at a pretty constant rate, and we've been trying very hard to see if we can find something that indicates an acceleration or a change in behavior that leads to the earthquake, other than foreshocks. We haven't seen anything that's consistent and reliable. There are hints here and there, but nothing that we can reliably point to.
Short-term earthquake forecasting is much, much more challenging, whereas volcanic eruptions can't occur unless magma moves into the subsurface. Unlike earthquake prediction — which in terms of short-term prediction is currently not feasible and may never be feasible on a short timescale — volcano prediction is feasible and is common if volcanoes are instrumented. But not all volcanoes are instrumented. You can't do it if you don't have anything to measure.
Is it possible to predict the severity of a volcanic eruption?
There are a couple ways we can get it. One way is just to know what volcanoes have done in the past. If you have a particular volcano, like Kilauea, we have a fairly long historical record. But you can also go back and look at the deposits from past eruptions and see whether they were passive lava flows or they were explosive ash-forming eruptions. We know, for example, that Kilauea has generated explosive ash-forming eruptions in the past, and part of that is from Hawaiian oral tradition, and part of that is from just mapping the geology, mapping the deposits.