VULCANIAN ERUPTIONS AND HAZARD ASSESSMENT OF BALLISTIC IMPACTS

Vulcanian eruptions are frequent, short-lived explosions that are produced by rapid decompression of pressurized magma. These explosions often occur by the disruption of a dense caprock plugging the vent, which originates ballistic projectiles that are ejected at high velocities and represent a common hazard associated to this kind of eruptions. In order to improve hazard assessment, we need a better understanding of the relationship between porosity, gas overpressure, ejection velocities and maximum range of the ballistic projectiles. We present a 1-D model of Vulcanian eruptions that considers the energy balance in decompression of a pressurized magma below a caprock, followed by fragmentation and acceleration of pyroclasts. We tested the model via fragmentation experiments at 850 °C and initial pressure (<25 MPa) with a shock-tube apparatus. We measured the ejection velocity of a caprock propelled by the expansion of an underlying gas-particle mixture produced by in situ fragmentation by rapid decompression of natural samples. Then we combined this model with a ballistic model which considers drag coefficient data measured experimentally for volcanic particles. Finally, we applied the combined model to different Vulcanian eruptions at Popocatépetl volcano, Mexico, and calibrate it with the maximum range reached by the ballistic projectiles and their corresponding travel times measured from videos of the explosions. Our study relates the zones that could be affected by ballistic projectiles with the initial pressure, which can be estimated from seismic and geophysical monitoring, providing valuable information for more refined hazard assessment of active explosive volcanoes.