Volcanic tremor during eruptions: Temporal characteristics, scaling and constraints on conduit size and processes

We investigated characteristics of eruption tremor observed for 24 eruptions at 18 volcanoes based on published reports. In particular, we computed reduced displacements (D_R) to normalize the data and examined tremor time histories. We observed: (a) maximum D_R is approximately proportional to the square root of the cross sectional area of the vent, however, with lower than expected slope; (b) about one half of the cases show approximately exponential increases in D_R at the beginnings of eruptions, on a scale of minutes to hours; (c) one half of the cases show a sustained maximum level of tremor; (d) more than 90% of the cases show approximately exponential decay at the ends of eruptions, also on a scale of minutes to hours; and (e) exponential increases, if they occur, are commonly associated with the first large stage of eruptions. We estimate the radii of the vents using several methods and reconcile the topographic estimates, which are systematically too large, with those obtained from D_R itself and theoretical considerations. We compare scaling of tremor D_R with that for explosions and find that explosions have large absolute pressures and scale with vent radius squared, whereas tremor consists of pressure fluctuations that have lower amplitudes than the absolute pressure of explosions, and the scaling is different. We explore several methods to determine the appropriate scaling. This characteristic helps us to distinguish the type of eruptions: explosive (Vulcanian or Strombolian) eruptions versus sustained or continuous ash (e.g. Plinian) eruptions. Average eruption discharge, estimated from the total volume of tephra and the total duration of eruption tremor, is well correlated with peak discharge calculated from cross sectional area of the vent and velocity of volcanic ejecta. These results suggest similar scaling between different eruption types and the overall usefulness of monitoring tremor for evaluating volcanic activity.