Source: Indonesia tsunami caused by collapse of volcano, experts confirm | World news | The Guardian Saturday’s tsunami in Indonesia was caused by a chunk of the volcanic Anak Krakatau island slipping into the ocean, it was confirmed on Monday, as officials at the country’s natural disaster agency said it must develop a new tsunami early warning system. At least 281 people were killed, hundreds injured and many buildings were heavily damaged when the tsunami struck, almost without warning, along the rim of the Sunda Strait, between Java and Sumatra islands. Anak Krakatau had been spewing ash and lava for months before a 64-hectare section of its south-west side collapsed, an Indonesian official said. “This caused an underwater landslide and eventually caused the tsunami,” Dwikorita Karnawati, the head of the meteorological agency, said. Images captured by the European Space Agency’s Sentinel-1 satellite showed that a large portion of the southern flank of the volcano had slid off into the ocean, scientists said. This of course brings up the La Palma risk which may not be so large for the western Atlantic basin as closer African and European shores. Source2: https://websites.pmc.ucsc.edu/~ward/papers/La_Palma_grl.pdf Bob Wilson
For additional massive landslide / tsunami risks, see also the Cape Verde Islands, and the Hilina Slump in Hawaii. Multiple webpage links here: Hawaii volcanics | PriusChat On a much smaller scale, the benches created by Kilauea's lava flows have repeatedly fallen off into the ocean. But these are usually just a few acres at a time, so the greatest risks are to sightseers standing atop them, and boats getting too close.
A very bad one. In this case showing adoption of technology to make and distribute horrifying videos far outstrips technology of detection and warning. But it is worse because source area here is so close to coastal areas where people got bashed. Even with all the tech (which Indonesia now wants to buy), advance notifications will be tight on time. Anak Krakatoa is getting pushed up so fast that more landslides seem certain. Obviously not the only area with such risks. But this one has large nearby coastal populations, so short warnings at best. Also, for reasons I don't understand, big sloshes in those regional seas do not appear to travel much farther. Islands 'in the way' or sea-floor topography must be involved. == Anyway the usual topics arise. As soon as possible, international aid delivery. Nearest 'conflict sea' is not very far away, so contestants there could take a break and help out. Tsunami, tidal waves, or whatever one wishes to call large sea intrusions are rare. And large. It has not been easy to get tech in place to detect and quickly notify. Where sloshes present themselves with hours of notice, that seems reasonably well sorted. But in case it's minutes, almost not at all. Maybe a bit like the "last mile" problem in signals connectivity. It's the really expensive part.
Just looking at a map, it seems that the wave source is mostly (about 80%) surrounded and contained by Java and Sumatra. The initial populated targets are quite close, just 25 to 30 miles away, while a number of others at 50-70 miles are at the ends of wave-magnifying 'funnels'. The initial wave can escape that containment in only two directions. The much larger window is to the southwest into the Indian Ocean. The next target is Madagascar nearly 4000 miles away, where the wave height should be attenuated so much that only tide gauges should notice. A far narrower window, the Sunda Strait, points northeast to Borneo about 400 miles away. The distance alone should greatly attenuate the wave if it all goes straight. But diffraction exiting the Strait into the Java Sea may spread it out to a wider angle, attenuating wave height even more. These ideas don't even consider sea floor topography at all. But what little I can find suggests that area to be comparatively shallow, which should make for slower and more attenuated wave propagation. And if the Sunda Strait is significantly more shallow than the Java Sea, the wave exiting into the later is weaker still. Then add in any more sea floor detail, and the multiple islands in the way, ... I suppose someone will model this in the coming year.
Things@4 are the important ones without doubt. If these seas are not yet well modeled there is room in lofty journals to tell stories. Attributing so much action on Pacific ring of fire is a bit vague. Tectonic plate boundaries have kinks and hinges that are more frisky than elsewhere. A much larger blowout was not many Christmases ago. Island-enclosed sea sloshes have a flip side. Tsunami from elsewhere can't really get inside, or at least this is my view.
Which seas are shallow or deep has other, different effects. When Earth was more icy, seas were lower and many current islands were continental extensions. Known since Darwinian times but geologists and biologists are still cranking out publications. About where critters can walk and where they'd need to swim. Weird folk like me spout off about how everything is connected to everything else. We really mean it.
