Volcanic Tremor: Seismic Signals of (Almost) Unknown Origin

Abstract

There exists strong evidence that the origin of volcanic tremor is caused by pressure fluctuations which are directly associated with degassing processes of the magma. The tremor signals contain features that are also found in self-excited and self-sustained feed-back systems. Two-phase flow (liquid magma and gas/steam) phenomena offer good possibilities at modeling pressure sources which are in agreement with tremor observations. However, the observation of volcanic tremor alone does not allow us to draw specific conclusions on the source mechanism. Different physical seismic source processes associated with two-phase flow are in concordance with signal characteristics of volcanic tremor. This implies severe restrictions in the application of volcanic tremor in understanding the dynamics of volcanic activity. The problem is enhanced by the fact that, in general, only a limited frequency band of the flow-induced seismic signals is observed (ca. 1-10Hz). The extension of, mainly, the lower cut-off frequency to values of 0.01 Hz or less might strongly improve the possibility of establishing less speculative source models for the origin of volcanic tremor than is possible at present.

“It is an old observation that like causes produce like effects, but it is just as well known that in nature very different causes can produce identical effects. It is indisputable that in these causes all observations are included which we normally sum up under the collective name ‘earthquakes’⋯” (written by Karl Ludoph Griesbach, Austrian co-founder of seismotectonics, in the year 1869).

Manuscript submitted end of 1988.