Volcanic Tremor and Its Use in Estimating Eruption Parameters

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Conference Proceeding

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Volcanic tremor, a continuous seismic signal, accompanies virtually all eruptions. Several published studies have examined the relation between tremor reduced displacement (DR, a normalized amplitude measure) and the Volcanic Explosivity Index (VEI) or ash plume height. The goal of these studies is to determine the physical relationship between tremor and eruptions and to use DR time histories or peak values to provide real-time estimates of eruption parameters. This study examines tremor for 50 eruptions from 31 volcanoes. Several systematic trends are observed: 1) large eruptions produce stronger tremor than small ones; 2) fissure eruptions produce stronger tremor than circular vents for the same fountain height; 3) eruptions with higher gas content produce stronger tremor than those with low gas content at the same volcano; and 4) phreatic eruptions produce stronger tremor than magmatic eruptions for the same VEI. The task of using tremor DR to estimate eruption parameters is fundamentally a statistical problem with several factors contributing to uncertainties. First, tremor occurs when volcanoes do not erupt as well as when they do. Based on a worldwide sample, 60 to 80 percent of tremor episodes accompany eruptions, while 20-40 percent do not. Thus there is a significant chance that no eruption is occurring. Second, for each VEI, there is a range of DR, so it is possible to overestimate or underestimate the VEI from the DR. Hence there will always be a false alarm rate of about 10 percent. Improvements can be made in the estimates if the types of eruptions, shapes of vents, and gas contents are known. These can be estimated in advance from previous eruptions or measured near-real-time from independent data. However, adding additional information takes time, thus delaying forecasts. A primary benefit of seismic data is that it is real-time, it is not affected by darkness, and is usable during poor weather, although the signal-to-noise ratio can be worsened. Monitoring tremor DR is thus an effective way to characterize eruptions in progress.

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Citation / Publisher Attribution

Proceedings of the 2nd International Conference on Volcanic Ash and Aviation Safety, p. 49-50