Green Gabbro

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?

Yes.

i-366589314560aaacbc54d956ed5a64a9-GE-map-Tonga.png

Why do I think the events were unrelated? Well:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Comments

  1. #1 Casz
    March 20, 2009

    Thanks for the clear explanation. I had thought they were just, well because of the timing and lack of any other knowledge:)

  2. #2 Kim
    March 20, 2009

    Yay! I’m glad you took this on.

  3. #3 hoski
    March 20, 2009

    I believe you are correct, but still the same driving force is behind both events (plate tectonics) – but still, the timing must be a coincidense.

  4. #4 Anonymous
    March 20, 2009

    I’m glad you tackled this so eloquently. In case you didn’t see this, the BBC is trying to still sell the direct EQ-eruption angle. It reallly bugs me.

  5. #5 Erik
    March 20, 2009

    And I have no idea why the system decided I was “anonymous”. The last comment (and this one) comes from Erik over at Eruptions.

  6. #6 Jdhuey
    March 20, 2009

    Sometimes a cigar is just a cigar and sometimes a coincidence is just a coincidence. Nice post.

  7. #7 John.St
    March 21, 2009

    From what I have experienced in Chile and Western Argentina, where we have close to 60 earthquakes per month and quite a few volcanic eruptions, earthquakes always occur within 12-13 years before or after the eruptions, which means that earthquakes are closer related to Swedish kings’ birthdays than to volcanic activity.

  8. #8 Rrr
    March 21, 2009

    earthquakes are closer related to Swedish kings’ birthdays than to volcanic activity

    In that case, look out around May Day, since HMK Carl XVI Gustaf was born April 30.

  9. #9 Heidi Houston
    March 24, 2009

    It’s possible, and maybe even plausible
    that the earthquake and the eruptions are related.

    From a casual perusal, the eruptions are about 250 km
    north of the M7.9 hypocenter, and the subduction zone is pretty
    active so the timing could be coincidental.

    OTOH, large earthquakes have been linked to eruptions for decades.
    Both the dynamic shaking and static stress changes
    due to a large megathrust earthquake
    could encourage volcanic eruptions in the arc – for one thing,
    trench-perpendicular horizontal compressive stress in the arc
    region is reduced by megathrust slip.

    However, in this case the volcanic eruption *preceded*
    the M7.9 earthquake by several days.
    A plausible, but speculative and probably untestable relation
    between the eruption and the earthquake, is that
    some aseismic slip could have occurred on
    the subduction zone to the north nearer to the eruption,
    triggering the eruption and also the earthquake to the south.

    And another possible connection is that the M7.9′s shaking and stress reduction
    in the vicinity of the eruption could intensify the eruptions
    (I don’t know if they did or did not intensify).

    Here is the USGS answer to a FAQ:

    USGS Frequently Asked Questions

    Question: Can earthquakes trigger volcanic eruptions?
    Answer:

    Volcano eruptions have occurred shortly after earthquakes and they may be linked, but scientists are still debating the topic. Notably, an Andean volcano (Cordon Caulle) began erupting 2 days after the magnitude 9.5 1960 Chile earthquake.

    Eruptions of mud volcanoes have occurred in the Andaman Islands following the recent magnitude 9.0 megathrust earthquake. Mud volcanoes consist of surface mud extrusions that vary in size from meters to several kilometers. They sometimes resemble magmatic volcanoes in appearance but they generally consist of low lying mud flows. Mud volcanoes do not involve magma. They emit mud at significantly cooler temperatures than lava, well below the ~800 degrees Celsius temperatures that characterize volcanic eruptions. Eruptions from mud volcanoes can reach heights of several hundred meters and consist of mud and sometimes burning hydrocarbon gasses. They are often associated with gas and oil fields. Mud volcanoes were known to exist in the Andaman Islands before the earthquake and in many other regions of the world.

    Deadly mud volcano eruptions are extremely rare because their eruptions generally do not affect large areas. One deadly eruption in Bozdagh, Azerbaijan reportedly killed six shepherds who were camping in the caldera of a mud volcano and about 2,000 of their sheep.

    Finally,
    Some great photos of the Tongan eruptions can be found here:
    http://www.boston.com/bigpicture/2009/03/undersea_eruptions_near_tonga.html

    and some great photos of last years eruptions of Chaiten:
    http://www.boston.com/bigpicture/2008/06/chaiten_volcano_still_active.html

  10. #10 Boris Johnson
    June 23, 2009

    I was lucky enough to be living and working in Tonga at the time of this eruption and earthquake. You could see steam and smoke rising for days after. Not only was there an earthquake and volcano but there was also a tsunami warning that reached all the way to Fiji.

    See my blog @ http://btjtravel.blogspot.com/ for other stories about Tonga.

  11. #11 armuss
    December 24, 2009

    ‘ was there too but havent l’ve any shak’ng

  12. #12 Shakeel
    May 10, 2012

    Fantastic Post

    For Latest Petrophysics visit

    http://petphy.blogspot.com/