On Monday, airline passengers were the first to observe the eruption of the just-barely-above-the-water volcano that forms the islands of Hunga Tonga and Hunga Ha'apai, in Tonga. Three days later, a magnitude 7.9 earthquake occurred on the Tonga trench.
Was the timing of these two events just coincidence?
Why do I think the events were unrelated? Well:
- Earthquakes happen all the time. Thankfully for Tongan residents, I am using a geological definition of "time" here, but still: A major earthquake on a major plate boundary does not require any special explanation.
- The earthquake was a perfectly ordinary plate boundary event. Everything we know about this earthquake so far - its hypocentral location, the direction of slip along the fault - is completely consistent with the other earthquakes we've recorded in that part of the world, most of which have not been associated with volcanic eruptions. Part of the top of the Pacific plate slipped a little further underneath the bottom of the Indo-Australian plate, nothing volcanic about it.
- The earthquake began 270 km away from the volcano. That's many times the size of the structure we call Hunga Tonga-Hunga Ha'apai.
- I can't think of a good physical mechanism for a volcano to produce the postulated spooky-earthquakes-at-a-distance. We don't understand all of what makes an earthquake happen now rather than later, but we can make a list of ways that an explosion of magma 270 km away might be felt at the plate interface. It's a pretty short list: Were there any significant earthquakes associated with the eruption, whose passing seismic waves might have triggered the 7.9? No. A couple of earthquakes occurred before the eruption, but they were small - only M5 - and I haven't looked at them in detail, or seen any firm evidence that they were directly associated with the volcano, either.
So while it's possible that research in the distant future might uncover an explanation for the close timing of these two events (and there is quite a lot of exciting work happening now about the relationships between volcanism, seismicity, and fluids in subduction zones, but it's mostly discussing the kinds of relationships that must be carefully teased from decades of observation), I quite like Occam's razor on this one. There is no reason to reach beyond our current understanding of volcanoes and earthquakes to explain what makes perfect sense as a coincidence.
Meanwhile, if you're hoping for more volcano stuff, there is some high-quality Tonga volcano porn here (still pictures) and here (video). Volcanologist and new Scibling Erik Klemetti has more to say about the eruption, too.