On the tsunami that killed over 400 people, from the Guardian:
Why wasn’t there a warning system?
In the case of tsunamis caused by earthquakes, the shaking earth can act as a warning but it’s much trickier to anticipate tsunamis from volcanic eruptions.
“There’s usually a big draw down in water below low tide levels, so you if you’re on the coast you’ve got a matter of minutes to get to high ground,” Kennedy said.
Kennedy said if a buoy network had been in place around Anak Krakatau, a one-to-two minute warning of a pending wave was the most anyone could expect.
“The expense of doing that everywhere is just impossible,” he said.
That set me thinking. Anak Krakatoa in the Sunda Strait is 44 km from the coast of Java, 35 km from the coast of Sumatra. The speed of a tsunami can be calculated as the square root of water depth times gravitation acceleration g. The Sunda Strait is about 100 metres deep, so the speed of a tsunami would be about 31.5 metres per second (113 kph), and would take 23 minutes to travel 44 km. So it really ought to be possible to get more than “a one-to-two minute warning” of a wave approaching from the volcano.
How about putting a few of these around volcanic islands like Anak Krakatau?
The SEAWATCH Deep Sea Module (SDSM) is designed specifically for the detection of tsunamis and can be fitted to existing SEAWATCH systems or be an integrated part of new systems. The SDSM consisting of a pressure sensor, a processor, batteries and an acoustic modem, is located on the seafloor.
I’d guess that this measures water pressure (and therefore depth), and water pressure would increase when a tsunami passes over. There are three islands around Anak Krakatau, and if three of these detectors were placed on the sea floor in a triangle (below) 10 km from it, they’d be able to detect a tsunami about 5 minutes after it started spreading.
That would have allowed a 20 minute warning for Tanjung Lesung to the south, where there were many casualties.
If you had the detectors being electronically monitored around the clock, you’d probably be able to work out rapidly the location of the source of the tsunami, and its principal direction and size, and be able to issue immediate warnings to the most endangered coastal areas via TV and radio, email and text message.
Furthermore, using the data from the detectors you might be able to estimate the likely height of the tsunami as it arrived at the coast, and using coastal topological data make estimates of how far inland it might reach, and worst case estimates of the likely number of casualties that could be expected, passing on these estimates to hospitals and emergency services.
If the data from the tsunami detectors was publicly available, some enterprising people might offer a personalised warning service to paying customers. Beachfront hotel owners would probably be more than happy to pay for 15 or 20 minutes advance warning of an approaching tsunami, in order to protect their guests. In fact most businesses and properties along the coast would want such a service. And individual people might want a service that would individually alert them by email or phone, and perhaps also offer advice as to where best to flee, and what to take with them.
And I can’t see that the basic system would be prohibitively expensive. There would be the capital costs of 3 or 4 tsunami detectors around Anak Krakatoa, the connection of them to shore computers which would do all the necessary geographical and topographical calculations, and some other computing systems to send out warnings. Running costs would be minimal, because there wouldn’t need to be a permanent staff of managers or engineers.
There are probably any number of volcanic islands like Anak Krakatoa which merit similar alarm systems. Stromboli in the Mediterranean might well be one. And tsunami detectors around such islands might well serve as elements in a wider global tsunami detection system. Eventually, there would be tsunami warning systems along all coastlines everywhere in the world, including places that never get any tsunamis.
The Krakatoa tsunami was predicted 6 years ago:
Numerical modelling of a rapid, partial destabilization of Anak Krakatau Volcano (Indonesia) was performed in order to investigate the tsunami triggered by this event. Anak Krakatau, which is largely built on the steep NE wall of the 1883 Krakatau eruption caldera, is active on its SW side (towards the 1883 caldera), which makes the edifice quite unstable. A hypothetical 0.280 km3 flank collapse directed southwestwards would trigger an initial wave 43 m in height that would reach the islands of Sertung, Panjang and Rakata in less than 1 min, with amplitudes from 15 to 30 m. These waves would be potentially dangerous for the many small tourist boats circulating in, and around, the Krakatau Archipelago. The waves would then propagate in a radial manner from the impact region and across the Sunda Strait, at an average speed of 80–110 km h−1. The tsunami would reach the cities located on the western coast of Java (e.g. Merak, Anyer and Carita.) 35–45 min after the onset of collapse, with a maximum amplitude from 1.5 (Merak and Panimbang) to 3.4 m (Labuhan).
This seems to be more or less exactly what happened on 22 Dec 2018.