Volcanic plume height measured by seismic waves based on a mechanical model

Abstract

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, B01306, doi:10.1029/2010JB007620, 2011 Volcanic plume height measured by seismic waves based on a mechanical model Stephanie G. Prejean 1 and Emily E. Brodsky 2 Received 4 April 2010; revised 3 November 2010; accepted 23 November 2010; published 26 January 2011. [ 1 ] In August 2008 an unmonitored, largely unstudied Aleutian volcano, Kasatochi, erupted catastrophically. Here we use seismic data to infer the height of large eruptive columns such as those of Kasatochi based on a combination of existing fluid and solid mechanical models. In so doing, we propose a connection between a common, observable, short‐period seismic wave amplitude to the physics of an eruptive column. To construct a combined model, we estimate the mass ejection rate of material from the vent on the basis of the plume height, assuming that the height is controlled by thermal buoyancy for a continuous plume. Using the estimated mass ejection rate, we then derive the equivalent vertical force on the Earth through a momentum balance. Finally, we calculate the far‐field surface waves resulting from the vertical force. The model performs well for recent eruptions of Kasatochi and Augustine volcanoes if v, the velocity of material exiting the vent, is 120–230 m s −1 . The consistency between the seismically inferred and measured plume heights indicates that in these cases the far‐field ∼1 s seismic energy radiated by fluctuating flow in the volcanic jet during the eruption is a useful indicator of overall mass ejection rates. Thus, use of the model holds promise for characterizing eruptions and evaluating ash hazards to aircraft in real time on the basis of far‐field short‐period seismic data. This study emphasizes the need for better measurements of eruptive plume heights and a more detailed understanding of the full spectrum of seismic energy radiated coeruptively. Citation: Prejean, S. G., and E. E. Brodsky (2011), Volcanic plume height measured by seismic waves based on a mechanical model, J. Geophys. Res., 116, B01306, doi:10.1029/2010JB007620. 1. Introduction and Motivation [ 2 ] Empirical studies have suggested that the amplitude of high‐frequency or broadband seismic waves radiated during large volcanic eruptions generally scales with the height of an eruption column [McNutt, 1994a]. However, a direct calculation of the expected seismic wave amplitude based on physical models has not yet been successful. Connecting commonly observable data such as seismic wave amplitudes to a model of the flow in the eruptive jet would provide a new tool to test and improve our understanding of eruptive physics. For instance, small‐scale turbulence is thought to play a major role in the entrainment of hot particles and gases and hence the buoyancy of eruptive columns, yet there are few measurements of the strength or distribution of small‐scale features in real eruptive columns [Andrews and Gardner, 2009]. Using seismic data to provide observational constraints on eruption column flow processes is particularly attractive as seismic data are often available even in remote U.S. Geological Survey, Alaska Volcano Observatory, Anchorage, Alaska, USA. Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Copyright 2011 by the American Geophysical Union. 0148‐0227/11/2010JB007620 settings. Thus use of the seismic database would greatly increase the number and type of eruptions amenable to study. [ 3 ] Pragmatically, a physical model connecting plume height and seismic data would allow the use of seismology as a remote sensing technology to infer volcanic plume height. It would be a particularly useful tool for exploring eruption dynamics in remote environments where direct observation is not possible, such as the volcanoes of the northern Pacific Ocean. Although many volcano observa- tories worldwide are gradually replacing short‐period seismometers with broadband seismometers, we still largely rely on short‐period instruments for forecasting, monitoring, and analyzing eruptions. These realities motivate the development of the model described below. [ 4 ] In this study, we build on previous work to develop a physical model for the expected amplitude of seismic waves from an eruption that generates a plume of a given height. As a cautionary note, we explore the potential errors in this formulation and describe situations where the model is not applicable, such as small eruptions or eruptions where most mass ejected is not entrained in the plume. After reviewing previous work on coeruptive seismology and the char- acteristics of the 2008 Kasatochi and 2006 Augustine eruptions, we use the connection between plume height and B01306 1 of 13