Page 21 - Spring 2018
P. 21

 from depth to the surface and depressurizes. For example, at depth in the Earth, a 1-m3 volume of a common magma (rhyolite) at 900°C may contain about 5 percent water by weight. As this magma approaches the surface, this quantity of water would expand enormously to about 670 m3 of vapor (Sparks et al., 1997). The degree of explosivity of a volcano is thus controlled to the first order by two main factors: volatile content and magma viscosity (low-viscosity magmas allow volatiles to escape more easily; high-viscosity magmas resist volatile escape and allow larger pressures to build).
Low-viscosity magmas characteristically exhibit long-lived effusive eruptions punctuated by short explosive bursts, along with occasional gas-rich lava fountaining episodes. On the other end of the spectrum, high-viscosity magmas and magmas containing a high concentration of volatiles typically erupt explosively. It is these eruptions, most com- monly associated with the tall, steep-sided volcanoes termed stratovolcanoes (e.g., Mount Fuji, Mount St. Helens, Mount Vesuvius) that send up the tall eruptive columns of ash into the atmosphere that can endanger aircraft (these eruption columns are called plinian eruption columns). Interactions of magma with subsurface groundwater systems at volca- noes can also have explosive results.
Seismo-Acoustic Signatures of Explosive Volcanic Eruptions
Explosive eruptions are seismo-acoustic phenomena, gen- erating large-amplitude acoustic (infrasound) and seismic waves, with the infrasound commonly recorded out to great- er distances (Figure 3). Volcano-seismic disturbances that precede eruptions (e.g., Figure 3a) are thought to be pro- duced by a variety of processes, including interactions be- tween subsurface magma and water, magma degassing, the brittle behavior of the magma itself, and ordinary earthquake faulting processes in solid rock related to magma movement and pressure changes (Chouet and Matoza, 2013). Seismic signals are also produced by the explosive eruption itself, as magma fragments (breaks into small pieces) and ash col- umns are ejected through volcanic vents and craters. The typical seismic expression of a sustained explosive erup- tion is a broadband signal (~0.1 to 20 Hz) called eruption tremor. Seismic signals that precede and accompany erup- tions usually have a limited propagation distance, typically a few tens of kilometers or up to a few hundred kilometers for larger eruptions. In contrast, similar-sized explosive eruptions produce powerful broadband (~0.01 to 20 Hz) in- frasound signals (Figure 3b) that can be ducted efficiently
Figure 3. Seismic vertical component velocity (a) and infrasonic pressure (b) recordings of the March 9, 2005 (Julian day 68), phreatic (steam-driven) explosion of Mount St. Helens, Washington, USA. Forty-eight hours of waveform data are shown, beginning at 00:00 on March 8, 2005 (day 67). The broadband seismometer was colo- cated with the central element of a 4-element infrasound array in a quiet forest site ~13.4 km from the source (Matoza et al., 2007). In each case, we show data from ±1 day spanning the event on March 9, 2005 (day 68), at ~01:26:17 UTC (blue arrows). The seismic tremor accompanying the phreatic explosion has a similar amplitude to seis- micity before and after. A large unambiguous infrasound signal de- lineates the explosive eruption timing (Matoza et al., 2007). See text for further discussion.
over long ranges (thousands of kilometers) in atmospheric waveguides. These infrasound signals are now routinely detected on sparse ground-based infrasound networks such as the IMS. In remote volcanic regions, infrasound is sometimes the only ground-based technology to record an explosive eruption and can therefore provide vital informa- tion to complement satellite data and estimate ash-release parameters for improving aviation safety. At seismically in- strumented volcanoes, infrasound data reduce ambiguity in explosion detection by clearly delineating the timing and duration of the explosive eruption. This is particularly useful when the volcano is visually obscured by cloud cover.
An example of how seismic and infrasound recordings com- plement one another is shown in Figure 3, which displays seismic and infrasound waveforms from a colocated seismo- acoustic station deployed 13.4 km from Mount St. Helens volcano, Washington, USA (Matoza et al., 2007). This is called a helicorder plot and mimics traditional seismograph recordings that were originally made on a piece of paper wrapped around a rotating drum. Time progresses from the upper left in the figure to the lower right, with each 60-min- ute waveform progressing from left to right in the figure and the next 60-minutes plotted directly below. Mount St. Helens underwent continuous eruptive activity from 2004 to 2008.
The figure shows a 2-day record around a phreatic (steam- driven) explosion that occurred on March 9, 2005 (all times
 Spring 2018 | Acoustics Today | 19

   19   20   21   22   23