23 March 2011
Last week, I commented on the suggested connection between the eruption at the Indonesian volcano Karangetang and the earthquake in Japan. That eruption was already building on its own, and it’s likely that there was little or no connection between the two. (There has since been an eruption at Japan’s Shinmoedake volcano, and I think it’s much more likely that that eruption has some connection to the earthquake; possible mechanisms are discussed in my previous post.)
Statistical analysis and volcano monitoring has established that there are both eruptions which were likely triggered by large earthquakes, and given us some plausible mechanisms for how this might happen, although this is still a rather rare event. Ron Schott brought up an interesting point in a comment, however: The mechanisms that I discussed are generally regarded as operating in the short-term – i.e., a few days to weeks after an earthquake (perhaps even a few months). But what about long-term earthquake triggering – are there connections between volcanic eruptions and earthquakes which happened years before? Are there any plausible mechanisms for long-term triggers, and how would they operate? I did a little research to see if I could find answers to either of these questions.
It turns out that there is even less known about the possibility of delayed eruption triggering than there is about short-term triggering. I could only find two papers that even discussed it, and one referenced the other. The initial paper, Remote seismic influence on large explosive eruptions (Marzocchi, 2001) found statistically significant links between eruptions and earthquakes on two timescales: 0-5 years and 30-35 years. Marzocchi suggests that the short-term link (and delay) could be “attributed to the inertia of the volcanic system in reacting to static stress changes” – in other words, it takes a little while for the mechanisms caused by stress changes to destabilize the volcanic system enough to cause an eruption. For the 30-35 year lag, however, Marzocchi doesn’t suggest any potential mechanisms, merely notes that the timescale is similar to the estimated relaxation time of a viscous asthenosphere. I’m not entirely sure if this is a plausible connection, or what mechanism would be associated with “asthenospheric relaxation”. (I guess it probably refers to long-term regional stress changes rather than immediate stress changes caused by the passage of seismic waves, but I could be wrong.)
The only other paper I could find which mentioned long-term triggering referenced the Marzocchi paper, with this comment:
“Prior studies did not find evidence for triggered response times longer than about five days, but this does not necessarily mean they do not occur. We might expect eruptions triggered after a longer delay to be less frequent than those triggered within days of an earthquake, with potential variability between arc settings. Thus, such events may not be detectible above the natural background variability of eruption rate in global data sets. Thus, while individual instances of delayed triggered eruptions have been proposed, demonstrating whether a general relationship between earthquakes and eruptions exists on timescales longer than a few days has proven difficult.”
In other words: There could be instances of long-term triggering of volcanic eruptions, but not only is it difficult to find many statistically significant instances, it will be very difficult to explain how the triggering happens. The “asthenospheric relaxation” idea seemed very offhand to me, but if anyone knows of a paper that explains how it could be related to eruptions, I’d be happy to update the post with additional info.
Marzocchi, W., 2001, Remote seismic influence on large explosive eruptions. Journal of Geophysical Research, v. 107, n. B1, p. 6-1 to 6-7. DOI: 10.1029/2001JB000307
Watt, S. F. L., Pyle, D. M., and Mather, T. A., 2009, The influence of great earthquakes on volcanic eruption rate along the Chilean subduction zone. Earth and Planetary Science Letters, v. 277, p. 399-407. DOI: 10.1016/j.epsl.2008.11.005
Additionally, the USGS has a nice FAQ page about the earthquakes and eruption triggering – it basically summarizes the topics I covered in my last post. Have a look!