What is the meaning of the (suddenly quiescent) earthquake swarm at Yellowstone National Park? Here are takes from two more top volcanologists. First up is Stephen Self, who presented a report on the hazards from a supervolcano eruption to the British government in 2005 and does work for the U.S. Nuclear Regulatory Commission:
I think that it is part of normal background seismicity at a large caldera volcano such as Yellowstone. Such swarms probably take place every few 10s of years, but we have been monitoring these volcanoes at the present level for only about 10-20 years. Thus a swarm may seem new or unusual whereas it's really part of business-as-usual. It may signify magma moving around underneath, or hot water, but these movements take place intermittently and quasi-steadily for many 100s of thousands of years. If this swarm were followed quite quickly (months-years) by another, and then another, and the intensity and frequency was seen to rise, then a re-think of this opinion may be in order.
Next up is Bil McGuire of University College London, also considered one of the U.K.'s top volcanologists. He has appeared frequently in television documentaries both there and in the United States:
I have indeed been paying attention to the Yellowstone situation and there have now been more than 500 small quakes since December 26th. This is certainly somewhat unusual activity, compared to recent decades, but not particularly unusual for so-called 'restless' volcanic calderas such as Yellowstone. The Campi Flegrei caldera in the Bay of Naples, for example, experienced many thousands of earthquakes in the 70s and 80s, along with surface swelling of 1 - 2 metres, all without eruption.
At Yellowstone, the quakes may have a number of causes, including movements along an active fault or the fracturing of rock in response to the migration of hot water or magma. Even if the latter, however, the chances are that the magma will stay beneath the surface, cool and solidify. The last eruptive activity here was a good 70,000 years ago, so the annual probability of an eruption is very small, although a steam blast left a 5 km wide crater just 13,000 years ago. So-called super-eruptions have return periods of 600 - 800,000 years or so (the last was 640,000 years ago), so the probability of another super-eruption in any single year is extremely small.
Having said this, it is likely that another eruption will happen at Yellowstone sometime. Not only do we not know when this will happen, but we are also not well versed in differentiating the signs of unrest associated with the normal activity of the caldera from those that presage a forthcoming eruption.
And for those of you just tuning, here is the last bulletin from the Yellowstone Volcanic Observatory on Jan. 3:
Over 500 earthquakes, as large as M 3.9, have been recorded by an automated earthquake system since the inception of this unusual earthquake sequence that began Dec. 27, 2008. More than 300 of these events have been reviewed and evaluated by seismic analysts. Depths of the earthquakes range from ~ 1km to around 10 km. We note that the earthquakes extend northward from central Yellowstone Lake for ~10 km toward the Fishing Bridge area, with a migration of recent earthquakes toward the north. Some of the dozen M3+ earthquakes were felt in the Lake, Grant Village and Old Faithful areas. Personnel of the Yellowstone Volcano Observatory continue to evaluate this earthquake sequence and will provide information to the NPS, USGS and the public as it evolves.This earthquake sequence is the most intense in this area for some years. No damage has been reported within Yellowstone National Park, nor would any be expected from earthquakes of this size. The swarm is in a region of historical earthquake activity and is close to areas of Yellowstone famous hydrothermal activity. Similar earthquake swarms have occurred in the past in Yellowstone without triggering steam explosions or volcanic activity. Nevertheless, there is some potential for hydrothermal explosions and earthquakes may continue or increase in magnitude. There is a much lower potential for related volcanic activity.