Over the past three years we've witnessed two large earthquake swarms at Yellowstone (2010 and 2009). Recent history tells us that these earthquake swarms are common, but we would like to know how frequently they have occurred in the more distant past. USGS researcher Bill Evans may have found an answer.
A Gassy Link to Past Earthquake Swarm
With colleagues from the USGS, Lone Pine Research, and Yellowstone National Park, Evans tracked the effects of the 1978 swarm at Mud Volcano. That intense swarm lasted about 7 months and was followed by increased heat output and dieback of vegetation in an area called "Cooking Hillside" (see map at left). Geothermal areas (regions of increased heat and gas discharge) at Yellowstone emit abundant carbon dioxide (CO2), some of which is incorporated into growing vegetation during photosynthesis. Evans wondered whether changing amounts of geothermal CO2 emissions might leave a record in the growth rings of nearby trees. He and his colleagues focused on a still surviving tree in the area (see tree photo below). They found that the proportion of carbon-14, an isotope abundant in Earth's atmosphere, decreased dramatically in the tree rings after the 1978 swarm. This suggests increased uptake of carbon- 14-poor CO2 coming from increased output of geothermal gas through the local soils. The increased CO2 was sufficient to change the composition of the air that the tree was respiring, but not enough to harm it.
By comparing the tree rings to present day rings and the current amount of CO2 discharge at Mud Volcano, they concluded that gas discharge more than doubled at the time of the swarm (see plot below). Moreover, because the timing of increased gas discharge immediately accompanied the swarm, the authors concluded that the swarm itself may have been caused by increased gas pressures within the geothermal system that underlies Mud Volcano and much of the caldera. Similar studies have identified changes in gas discharge through studies of tree rings at other volcanoes (e.g., Mammoth Mountain, CA).
Evans and colleagues hope to apply the technique at other areas within Yellowstone, including ones where very old trees may hold clues to earthquake swarms and other geological activity that preceded human recordings. Ideally, this and similar techniques can give us a better understanding of the record of earthquakes in Yellowstone's more distant past. Please read the full article in Geology for further information.
Evans WC, Bergfeld D, McGeehin JP, King JC, and Heasler H (2010). Tree-ring 14C links seismic swarm to CO2 spike at Yellowstone, USA, Geology v.38, p.1075-1078.