13 - Seismic Signals Associated with Pyroclastic Flows, Rockfalls, and Lahars

Abstract

Volcanic hazard is significantly related to the propagation of density currents, such as pyroclastic flows and lahars. These flows represent the motion of natural sediments generated along volcanic slopes that propagate with high velocity and can devastate everything in their path. Both types of volcanic flows have produced destructive effects during human history. In the list of the 27 largest volcanic catastrophes between 1700 and 1986 that were responsible for at least 1000 deaths, 11 were associated with pyroclastic flows and 11 with lahars. Therefore, the ability to monitor and track these phenomena in real time is important for the mitigation of volcanic risk. Pyroclastic flows and rockfalls are common for both explosive and effusive activity at andesitic and dacitic volcanoes. They may be generated by a number of different mechanisms. Among them is the gravitational collapse of a growing lava dome, the collapse from an eruption column, and the explosive destruction of growing lava dome. The pyroclastic flows are distinguished from rockfalls by their larger size, longer runout (≥ 0.5 km), greater production of fine ash and appreciable buoyant hot ash clouds. The fall-and-flow of fragments of lava dome or flow fronts and the numerous rockfalls generate seismic signals. Pyroclastic flows and rockfalls are common for both explosive and effusive activity at andesitic and dacitic volcanoes. The earthquakes associated with pyroclastic flows and rockfalls occur in dense groups, sometimes completely filling the seismogram. Relationship between the seismic signals, produced by pyroclastic flows and rockfalls, and eruption dynamics has been discussed.