| Historical seismicity, ground deformation, thermal activity, and eruptions at large Quaternary calderas of the world are common, spatially and temporally complex, and usually caused by combinations of tectonic, magmatic, and hydrologic processes. Study of historical unrest helps to define what is ordinary and what is extraordinary, and to document processes of unrest that act over periods of hours to millennia. At least 1,299 episodes of unrest have occurred at 138 calderas >5 kilometers in diameter during historical time. The most commonly reported unrest includes local earthquakes or earthquake swarms; volcanic tremor; uplift; subsidence; tilt; ground fissuring; changes in the temperature of soil, water, or gas; changes in fumarolic activity; and eruptions. Of 94 well-reported episodes of unrest within the past 30 years, 45 culminated in eruptions. However, at silicic calderas that had not erupted for at least 100 years, only 10 of 60 episodes of unrest led to eruptions. -- Newhall and Dzurisin, 1988 |
