火山爆发前会变安静

When dormant ¹ volcanoes are about to erupt, they show some predictive characteristics-- seismic ² activity beneath the volcano starts to increase, gas escapes through the vent ³, or the surrounding ground starts to deform ⁴. However, until now, there has not been a way to forecast eruptions ⁵ of more restless volcanoes because of the constant seismic activity and gas and steam emissions ⁷. Carnegie volcanologist Diana Roman, working with a team of scientists from Penn State, Oxford ⁸ University, the University of Iceland, and INETER* has shown that periods of seismic quiet occur immediately before eruptions and can thus be used to forecast an impending ⁹ eruption ⁶ for restless volcanoes. The duration of the silence can indicate the level of energy that will be released when eruption occurs. Longer quiet periods mean a bigger bang. The research is published in Earth and Planetary Science Letters. The team monitored a sequence of eruptions at the Telica Volcano in Nicaragua in 2011. It is a so-called stratovolcano, with a classic-looking cone ¹⁰ built up by many layers of lava ¹¹ and ash. They started monitoring Telica in 2009 with various instruments and by 2011 they had a comprehensive network within 2.5 miles (4 kilometers) of the volcano's summit. , ,The 2011 eruptive event was a month-long series of small to moderate ash explosions. Prior to the eruption, there was a lack of deep seismicity or deformation ¹², and small changes in sulfur ¹³ dioxide gas emissions, indicating that the eruption was not driven by fresh magma. Instead, the eruption likely resulted from the vents ¹⁴ being sealed off so that gas could not escape. This resulted in an increase in the pressure that eventually caused the explosions., ,Of the 50 explosions that occurred, 35 had preceding quiet periods lasting ¹⁵ 30 minutes or longer. Thirteen explosions were preceded by quiet intervals ¹⁶ of at least five minutes. Only two of the 50 did not have any quiet period preceding the explosion. , ,"It is the proverbial calm before the storm," remarked Roman. "The icing on the cake is that we could also use these quiet periods to forecast the amount of energy released."