IAVWOPSG/4-IP/6
5/8/08
INTERNATIONAL AIRWAYS VOLCANO WATCH OPERATIONS GROUP
(IAVWOPSG)
FOURTH MEETING
Paris, France, 15 to 19 September 2008
Agenda Item 6: Development of the IAVW
6.5: Development of eruptions source parameters (ESP)
FOLLOW-UP ON IAVWOPSG/3 CONCLUSION 3/25:
ASHE INFRASOUND STUDY AT TUNGURAHUA, ECUADOR
(Presented by Canada)
SUMMARY
This information paper reports on the findings of the ASHE Infrasound Project
in Ecuador and presents some case studies. It was prepared by David Fee,
Milton Garces, and Andrea Steffke at the University of Hawaii at Manoa.
SUMMARY
The primary objective of the Acoustic Surveillance for Hazardous Eruptions (ASHE) project was to
demonstrate the possibility of using low frequency acoustic (infrasound) observations over broad regions
for automatic, low-latency volcanic eruption notification to the Washington DC VAAC. Our results
indicate the primary objective can be met for eruptions with a Volcanic Explosivity Index (VEI) of 2 or
greater at distances of 40-250 km. For a station distance of 40 km, it appears possible to identify the
acoustic signature of stratospheric ash injection and notify the VAAC within 5 minutes. At 250 km,
notification latency grows to ~20 minutes. As confirmed during the July 12, 2008 eruption of Okmok
Volcano, for the larger eruptions the results of this study may be extended to the 2000+ km range
coverage provided by the IMS network.
Our eruption detection algorithms were trained using the Vulcanian to Plinian eruptions of Tungurahua
Volcano, Ecuador. For VEI 2 or greater, a sustained volcanic signal was found to be a better indicator of
substantial atmospheric ash injection than transient explosions. The eruptions on 5/12 2006, 7/14-15 2006
and 8/16-17 2006 were used as case studies. The ash-poor eruption of May 06 eruption served as an
interesting example of continuous harmonic tremor punctuated by powerful explosions, but with little
(16 pages)
106738310
IAVWOPSG/4-IP/6
-2-
airborne ash. The July and August 2006 eruption erupted significant ash into the atmosphere which posed
a serious threat to aviation in the region. The detection algorithms developed during the 2006 case studies
were used to automatically notify the VAAC of large explosions and significant ash injection preceding
and accompanying the 2/6/08 Tungurahua eruption, which lofted ash to ~15 km. Extensive work went
into integrating the acoustic records with detailed analyses of satellite imagery for these events.
A very promising result of this project is the recognition that for large eruptions there may be a good
correlation between acoustic energy and ash height. Further, there appears to be a potential to use
coherent acoustic energy in the 0.05-0.25 Hz band as a discriminant for stratospheric ash injection (Figure
4; Figures 10-11). Careful comparisons between satellite and calibrated acoustic data are needed to
confirm this relationship on a broader basis. We recommend this project be continued to assess the
feasibility of using ASHE and IMS arrays to automatically identify eruptions with a VEI of 2 or greater at
regional distances, and possibly smaller eruptions at closer distances.
Brief ASHE Timeline
February 2006:
Arrays deployed in Ecuador
March 2006:
Data received at ISLA (obtained missing data from mid-Feb)
April 2006:
ASHE data product website goes online at: http://www.isla.hawaii.edu/ecuador/. Automatic
explosion detection algorithm running. Website initially updated every 4 hours, then soon after
updated every 1 hour.
March 8th, 2007:
Daily automatic email updates began summarizing Tungurahua volcano infrasound for the
preceding 24 hours.
December 7th, 2007:
Website is updated every five minutes, and explosion and tremor energy
notification emails are sent immediately after detection by algorithm.
February 6th, 2008:
Email notification of onset, escalation, and end of significant eruption at Tungurahua
Per month Infrasound Citations by the Washington VAAC
March 07:
April 07:
May 07:
June 07:
July 07: 11
August 07:
September 07:
6
5
3
1
7
4
-3October 07:
November 07:
December 07:
January 08:
February 08:
March 08:
IAVWOPSG/4-IP/6
6
10
22
24
17
4
Number of Tungurahua Explosions Detected at RIOE (37 km) from 2/6/06 to 6/10/08:
20,124
Number of Tungurahua Explosions Detected at LITE (251 km) since 2/6/06*:
>3500
*Note there have been extensive outages at LITE during the deployment that has decreased the number of
detected explosions. LITE operations ceased in April 2008.
Infrasound Detection Methods
A combination of array and signal processing techniques were used to detect explosions and
sustained signal (tremor and/or jetting) from Tungurahua. For the first portion of the experiment the
processing techniques were run every hour, and then eventually shortened to every 5 minutes for station
RIOE. In order to differentiate between signal and noise, the Progressive Multi-Channel Correlation
(PMCC) array processing method was ran for both the RIOE and LITE arrays between 0.5-4 Hz, as this is
the band where the majority of the volcanic signal was concentrated. To maximize signal detection, the
processing was done using 10 second windows with 90% overlap, and 20 frequency bands.
The acoustic source energy (or acoustic energy) was calculated both for explosions and tremor to
determine how energetic the volcanic signal was. The acoustic energy of a far-field source in free space
can be estimated using:
EAcoustic=2πr2/ρc ∫ΔP(t)2dt
where r=source-receiver distance, ρ=air density, c=sound speed, and ΔP=change in pressure. To better
illustrate the volcanoes energetics, acoustic energy was converted to acoustic power by dividing by the
energy value by the time interval. If not otherwise noted, the acoustic power levels quoted in this
document were calculated over 15 minute time intervals. One caveat with calculating the acoustic energy
is that the calculation assumes the change in pressure (ΔP) is entirely produced by the source of interest,
and does not account for contributions from noise or clutter (other coherent sources). These unwanted
contributors can often dominate, particularly in the far-field or during noisy periods. To minimize the
effects of wind noise, the acoustic energy was calculated above 0.5 Hz. In addition, the acoustic energy
was only calculated if the array processing (PMCC) results for that time period corresponded to
significant acoustic signal arriving from within ±7° of Tungurahua (26°-40°). Using this method the
acoustic energy for sustained signal (tremor or jetting) was estimated for a specified interval (Figure 1). If
a high level of acoustic energy was detected over a given time interval, then a notification email was sent
to the VAAC and other interested parties. Thus far, 1x108 J over 1 hour is the selected energy threshold. If
the energy level doubled, another notification was sent out. Once the energy level dropped below the
threshold value, a final email notification was sent.
IAVWOPSG/4-IP/6
-4-
Figure 1. Acoustic energy calculation for the 2/6 eruption. Infrasound array data is high-pass filtered
between above 0.1 Hz and split into 60 sec windows. The top panel shows the correlation value from
PMCC for azimuths within +/-7 degs from Tungurahua. If the correlation value is above 0.6, the data
segment is considered coherent and is colored black. The middle panel shows the filtered waveform. If
the section is red, the signal correlation value is too low or its azimuth is off the target beam. If it is black,
it is coherent and arriving from the Tungurahua azimuth, and the incident acoustic source energy is
estimated. The bottom panel shows the cumulative acoustic source energy for the time interval.
The explosion detection algorithm for Tungurahua initially high-pass filters the data above 0.5 Hz
and runs a STA/LTA (Short term average/Long term average) to determine the onset and end time of any
impulsive signal. Two STA/LTA ratios are used, 2/5 and 3/40 seconds, to ensure both impulsive and
somewhat more emergent explosions are detected. The explosion must also be recorded on all four
channels. The explosion times are then correlated with the PMCC detections to ensure the signal is
arriving from an azimuth within ±7° of Tungurahua. The coinciding PMCC family must have a minimum
RMS amplitude >0.02 Pa and family size >15 pixels. If these conditions are met, then an explosion is
registered and the time, duration, maximum raw amplitude, and acoustic source energy are recorded. The
acoustic source energy is calculated for the explosion duration, and is then normalized by the energy of a
reference event. The reference event had a peak-peak amplitude of ~1 Pa and ~5.65x108 J. By
normalizing the explosion energy, we minimize source geometry, propagation, and topographical effects.
Once the algorithms were operational, all explosions with an energy ratio greater than 5 caused a
notification email to be sent out.
Infrasound Monitoring Limitations
During periods of low activity or high noise, the infrasound signal-noise levels are such that the
employed algorithms are not nearly as effective. Since the arrays have insufficient wind noise reduction,
the recordings during the middle of the day (~5 hours) are often saturated with wind noise and not useful
for detecting low-level signals. However, during big explosions and the three large eruptions, signal
-5-
IAVWOPSG/4-IP/6
levels were high enough to overcome the ambient wind noise. More regular maintenance of the arrays
would also help raise the detection thresholds by making sure the sensor and site responses did not vary.
False alarms were fairly minimal, as the algorithms employed were correlated extensively with
volcanological observations in case studies before notification services were initiated. Thunderstorms
were one source of false alarms and may be misidentified as explosions, but not as tremor events. The
ASHE project was also greatly assisted by the exhaustive and detailed monitoring of Tungurahua Volcano
by the Ecuadorian Instituto Geofísico (IG), as well as their willingness to assist the ASHE project.
Large explosions can bias the acoustic energy/powere calculations, and a more effective way of
separating explosions and tremor levels would be helpful. However, the relationship between explosions
and tremor is not well understood. For example, large explosions followed by harmonic, gliding tremor
was representative of gas-rich, ash-poor eruptions (e.g. May 06), and multiple episodes of jetting were
preceding (and initiated?) by large explosions (e.g. towards the end of the 2/6/08 eruption, beginning of
7/14 eruption, and around 0435 on 8/17).
Case Study Observations and Correlations
ERUPTION CASE STUDY: 2008/2/6
Main Eruption: 0400-1430 UTC, Total Duration ~10.5 hours, VEI 2-3 estimated from ash heights. Refer
to Figure 2 for a spectrogram, power levels, and ash heights for the 2/6 eruption.
Figure 2. 2/6/08 Eruption Infrasound. Top) Spectrogram between 0.1-10 Hz. Bottom) Black lines denotes
acoustic power calculated every 15 mins. Green lines indicate the max and min inferred ash heights from
satellite data. The red lines show the four infrasound notifications sent out, while the brown lines show
the eight explosion notifications. The cyan diamond is the VAAC estimated ash height derived from
satellite data and coarse atmospheric models. Reanalysis using the G2S atmospheric specifications (in
progress) will reduce height uncertainty.
IAVWOPSG/4-IP/6
-6-
Brief Description of Eruptive Activity
According to news articles, on Feb. 6th, 2008 several hundred to 2,000 people were evacuated as
pyroclastic flows and heavy ash fall inundated the areas surrounding Tungurahua. Based on information
from the IG and satellite imagery evaluation, the Washington VAAC reported that ash plumes rose to
estimated altitudes of 7.3-14.3 km (24,000-47,000 ft) a.s.l. and drifted S and NW. Ashfall was reported in
areas downwind and to the SW and W, including Riobamba (30 km S). Precursory seismicity saturated
local stations and presented similar patterns seen prior to intense episodes in July and August 2006
(source: Smithsonian's Global Volcanism Program).
The event began at 0420 UTC on February 6th, when tremor increased abruptly at all seismic stations. The
tremor increased steadily until it reaches a maximum at 0611 UTC. The tremor then falls, until 0943 UTC
where it begins again and continues until 1436 UTC, where it drops off rapidly. During these episodes of
tremor great explosions took place, as well as very intense roars, felt earthquakes (including in the OVT),
and pyroclastic flows that descended past the head of the gorges from Juive, Mandur and Cusúa. The
most extensive were further east, and they flowed into the upper part of Cusúa. The emission columns
were continuous; with a significant ash load extending to 10 km of height according to VAAC data. After
this eruption, an activity of continuous emissions with ash contents of moderate-high. Further explosions
were also registered, several of which were of great intensity. (source: IG).
ASHE notification and chronology:
Four activity (acoustic energy) updates were sent on 2/6. Below we summarize the infrasound activity,
why the notifications were issued when they were, and list the relevant notification information.
0200-0400 UTC: Numerous small explosions, followed by larger explosion at ~0420 UTC. Jetting (high
amplitude tremor) begins around 0425 UTC. Tremor between 0.5-1.5 Hz increases prior to 0425 UTC.
0434 UTC, Notification #1
First notification above 1x108 J threshold. Substantial activity in the “MOD” or “Moderate” level,
but still below “HIGH’ energy level of 1x109 J. Notification issued at 2/6 0434 UTC (2/5 18:34 HST, the
local time of the sent email). This notification time is only 4 minutes from the last recorded data segment,
and ~6 mins from the actual time of recorded volcanic activity. Email notification below:
Date: Tue, 5 Feb 2008 18:34:23 -1000 (HST)
From: dfee@isla.hawaii.edu
To: w-vaac@noaa.gov
To: Rene.Servranckx@ec.gc.ca
To: pramon@igepn.edu.ec
To: dfee@isla.hawaii.edu
To: milton@isla.hawaii.edu
Subject: Volcano Infrasound Activity Update for 2008/2/6 03:30-04:30 UTC
DRAFT Infrasound Energy NOTIFICATION, ECUADOR ASHE PROJECT
TUNGURAHUA VOLCANO
DETECTIONS AT STATION RIOE, RANGE OF 37 KM, TIME IN UTC
MOD Energy Level, 1.28e+008 J
Time interval: 03:30-04:30 UTC
0439 UTC, Notification #2
-7-
IAVWOPSG/4-IP/6
Energy level doubles at next 5 minutes calculation, triggering another email alert. Note the energy
level is still in the “moderate” energy range (4.41x108 J, 15 min power level= 1x106 W). Pertinent email
text:
MOD Energy Level, 4.41e+008 J
Time interval: 03:34-04:35 UTC
0440-0500 UTC:
Constant jetting that decreases over time. Signal concentrated between ~0.1-2 Hz. First plume observed in
satellite imagery, with plume detaching from the summit.
0500-0540 UTC:
Some jetting but at lower power levels. New ash plume rising.
0540-0655 UTC:
Jetting increases substantially and becomes more broadband (up to ~9x106 W). Signal is constant, with
notch at ~0.5 Hz. Large explosion at 0626 UTC. Plume continues to increase in size and height.
0700-0830 UTC
Another jetting episode, but spectral structure somewhat different than previous episode. Power levels
peak at ~6x106 W. Signal gradually decreases over time. Harmonics present in higher frequencies around
0800-0830. Ash plume is starting to separate, and does so by 0815 UTC.
0843 UTC
Significant explosion, with amplitude of ~13 Pa and energy ratio of 96. This triggers and email
notification.
Time=09:11:14, Raw Amp=7.99 Pa, Dur=7.0 sec, Energy Ratio=35.64
0911 UTC
Another large explosion with an amplitude of ~8 Pa and energy ratio of 35.64. Meteorological clouds
make it difficult to discern ash cloud…if any.
0945-1115 UTC
Most energetic jetting of entire sequence, with power levels up to 1x107 W. Infrasound signal is
broadband and fairly constant. Shuts off rapidly at 1115 UTC. New plume observed at 0945 and grows in
width and length until 1145 image. This plume appears to stay attached to the vent during the rest of the
sequence.
0959 UTC, Notification #2
Over the past 5 hours the hourly energy level has gone up and down, but stayed above 4.41x108 J
(the energy level of the past notification). The “HIGH” threshold has already been attained, but no
notification was issued. In hindsight this was a flaw in the system design and could have been useful. At
2/6 0955 UTC the energy level doubles from its previous value, triggering another update. Also, it would
have been useful to include a phrase related to ‘energy level has double since past update’. Note we are
now in the “HIGH” energy level and have been for awhile, but no alert was triggered since we already
had triggered and the energy level had not doubled since the past update. As mentioned before, a new
plume is observed in satellite imagery and significant eruption is noted by VAAC. Our satellite estimates
put the max plume height at ~9.6 km. Max power level is ~1.1x107 W, highest of this eruption. Pertinent
notification email text:
HIGH Energy Level, 4.69e+009 J
Time interval: 08:55-09:55 UTC
IAVWOPSG/4-IP/6
-8-
1120-1205 UTC
Approximately three shorter, less energetic (but still significant) jetting episodes. Power levels between 26x106 W.
1207-1345 UTC
Five-six jetting episodes that all begin with a large explosion. Duration and frequency content change
somewhat, but fairly similar to each other in spectral shape and duration. Power levels vary between 2-4 x
106 W. Satellite image is cropped and difficult to discern maximum extent of plume. The VAAC reports a
detached plume at 14.3 km moving south (Figure 3), with a smaller plume attached to the summit. We are
in the process of obtaining new satellite data to confirm the estimated VAAC heigh, as the infrasound
data would suggest a higher ash plume from the 1000-1100 UTC infrasonic power levels.
Figure 3. VAAC ash advisory image for the 2/6 Tungurahua eruption.
1345-1430 UTC
Final jetting episode of sequence, power level up to ~8 x 106 W. Signal drops off rapidly.
1529 UTC, Notification #4
After staying at the HIGH level, the volcano finally starts to quiet down and the energy level
drops off rapidly. There is still some activity around 1400-1420 UTC, and this is included in the past hour
of the energy calculation. The VAAC cites decreased seismic and infrasound activity in their advisory at
1633 UTC. Pertinent email text:
-9-
IAVWOPSG/4-IP/6
Activity level decreased
LOW Energy Level, 8.33e+006 J
Time interval: 14:25-15:25 UTC
Pertinent explosion update email text issued during the 2/6 eruption:
Time=06:26:17,
Time=08:43:31,
Time=09:11:14,
Time=09:56:55,
Time=12:07:40,
Time=12:28:17,
Time=13:14:16,
Time=13:37:53,
Raw
Raw
Raw
Raw
Raw
Raw
Raw
Raw
Amp=3.85 Pa, Dur=2.0 sec, Energy Ratio=13.83
Amp=13.16 Pa, Dur=7.0 sec, Energy Ratio=96.04
Amp=7.99 Pa, Dur=7.0 sec, Energy Ratio=35.64
Amp=3.43 Pa, Dur=1.5 sec, Energy Ratio=5.88
Amp=3.84 Pa, Dur=2.5 sec, Energy Ratio=7.38
Amp=8.76 Pa, Dur=6.5 sec, Energy Ratio=30.94
Amp=2.93 Pa, Dur=5.5 sec, Energy Ratio=6.26
Amp=3.21 Pa, Dur=5.6 sec, Energy Ratio=5.69
ERUPTION CASE STUDY: 2006/8/16-17
Main Eruption: 1930-0620 UTC, Total Duration ~10.8 hours, VEI 4 estimated from ash heights.
Refer to Figure 4 for a spectrogram, power levels, and ash heights for the 8/16 eruption.
Figure 4. 8/16/06 Eruption Infrasound. Top) Spectrogram between 0.1-10 Hz. Bottom) Black lines
denotes acoustic power calculated every 15 mins. Green lines indicate the max and min inferred ash
heights from satellite data and G2S high-resolution atmospheric specifications. No satellite data was
available for this study between 0445-0545 UTC.
ASHE chronology:
0516 UTC: M4.7 Regional Earthquake near Palora occurred (IG).
1930-2130 UTC
Emergent onset of jetting. The signal is fairly band limited and increases gradually. Steam-rich, ash-poor
plume observed by IG. Eight km continuous plume is evident in satellite imagery.
2150 - 2400 UTC
IAVWOPSG/4-IP/6
- 10 -
First pyroclastic flow’s (PF) observed along with small lava fountain. Infrasonic jetting continuing at
fairly constant level, as is ash cloud.
0000-0145 UTC
Jetting decreases slightly, then increases again around 0100-0130 UTC (infrasound power levels around
8x106 W). Explosions and increased “roars” are observed and accompanied by more PFs and an 800 m
lava fountain. Plume height (8.5 km), width, length, and intensity all increase coincident with increased
infrasound activity.
0145-0330 UTC
Slight decrease followed by significant increase in activity (power level>1.4x108 W at ~0300 UTC).
Broadband jetting and some explosions apparent. Notch in infrasound power spectrum also at ~0.6 Hz.
Incandescent blocks, ash cloud, roars, and explosions were observed in nearby towns of Riobamba and
Ambato. Ashfall is similar to 7/14 paroxysmal (climactic) phase. Multiple dangerous PFs occurred.
Plume intensity still increasing and rises up to ~9.5 km.
0330-0430 UTC
Elevated activity continues and jetting levels continue between 0.7-1.0 x107 W. New pulse in ash cloud at
0415 UTC up to 14 km, although thi is not apparent in the infrasound or observatory records. Activity
elevated, but power levels not significantly unlike earlier periods. Lava fountain ~1.5 km high, with
continued PFs. Activity declines substantially around 0430 for a brief period.
0436-0530 UTC
Heightened activity begins again with an energetic explosion at 0436 UTC. Jetting continues for the next
~1 hr similar to previous levels of ~0.5x107 W. IG reports renewed activity and explosion at ~0502 UTC,
which does not agree with the infrasound records. Unfortunately, no satellite data is available to infer ash
heights.
0530-0620 UTC
Paroxysmal and Plinian phase of eruption (Figure 5). Power levels increases to highest level (up to 3x107
W) and jetting shifts to lower frequency (dominant frequency <0.1 Hz). Significant infrasound below 0.1
Hz for first time. Very large, dark ash cloud ejected up to 26 km, with significant width and volume.
Plume is moving in two separate directions and is clearly stratospheric for the first time. The lava fountain
is now up to 6 km above the vent and ballistic fragments are later found up to 13 km away. The most
destructive and extensive PFs occur, killing six people.. Infrasound energy calculations are not completely
accurate of actual pressure release, as a significant portion of the signal is below the flat response of the
sensor (0.1 Hz). However, the shift to lower frequency, more energetic jetting is unmistakable (Figures
4,5).
- 11 -
IAVWOPSG/4-IP/6
Figure 5. Spectrogram for the Plinian phase of the 8/16 eruption. Note the increase in energy and
decrease in frequency of the signal around 0530 UTC. The frequency axis is plotted on a logarithmic
scale.
ERUPTION CASE STUDY: 2006/7/14-15
Main Eruption: 2234-0315 UTC, Total Duration ~4.75 hours, VEI 3 estimated from ash heights.
Refer to Figure 6 for a spectrogram, power levels, and ash heights for the 7/14 eruption.
Figure 6. 7/14/06 Eruption Infrasound. Top) Spectrogram between 0.1-10 Hz. Bottom) Black lines
denotes acoustic power calculated every 15 mins. Green lines indicate the max and min inferred ash
heights from satellite data.
IAVWOPSG/4-IP/6
- 12 -
2130-2230 UTC
IG reported an increase in tremor at 2130 UTC. No significant increase in infrasound records, although
wind-induced noise is still somewhat present. No plumes in satellite imagery.
2234-0000 UTC
Moderate-strong explosion at 2234 UTC signals onset of jetting. Power levels gradually rise and fall
between 1-3.5 x106 W. IG ash height estimates initially obscured by cloud cover, but at 2300 UTC they
report a 6 km high plume of low-moderate ash content. The first PF occurs between 2300-2315 UTC.
Satellite imagery shows the plume’s intensity increasing and it appears to be bifurcated with the top
around 11.9 km.
0000-0200 UTC
Infrasound power levels increase rapidly between 0000-0100 UTC (from ~3-9.5 x 106 W). The jetting is
broadband and satellite imagery shows a large plume increasing over time, reaching maximum height by
around 0045 UTC (~14.2 km) and obscuring the vent until 0315 UTC. Between 0100-0200 UTC the
jetting energy is focused at lower frequencies (<1-2 Hz) and infrasound power levels drop slightly, but no
discernible change in plume height is evident. Over 14 PF flows were recorded between 0000-0400 UTC.
0200-0400 UTC
After a slight decrease in power, the jetting rises and peaks around 0215-0245 UTC (1 x 107 W). The
plume does not appear to change height during this period. At 0315 UTC the main plume has moved off
the vent and a secondary plume is now apparent. The infrasound power levels drop off rapidly around
0300 UTC, coincident with the main plume detaching from the vent. No significant infrasound activity
associated with secondary plume. Steady 1.4 Hz tremor resumes soon after, and large explosions occur
around 0600-0700 UTC.
- 13 -
IAVWOPSG/4-IP/6
ERUPTION CASE STUDY: 2006/5/12
Continuous activity throughout day, focus on 0000-1600 UTC
Refer to Figure 7 for a spectrogram, power levels, and ash heights for the 8/16 eruption.
Figure 7. 5/12/06 Eruption Infrasound. Top) Spectrogram between 0.1-10 Hz. Bottom) Black lines
denotes acoustic power calculated every 15 mins. No ash emissions were detected in the satellite imagery.
Eruption Summary/Observations:
Moderate-strong explosions occurred regularly throughout the day, and 15 minute power levels
ranged between ~0-1 x 106 W. Between 0000-1600 UTC, 37 explosions were detected with an energy
ratio above 0.3. No ash plumes were detected in satellite imagery. The IG reported nearly all of the
aforementioned explosions, but the associated emissions either had low ash contents or were comprised of
only steam and gas. A few of the explosions ejected incandescent blocks. The 1.4 Hz background tremor
was at low levels. However, the most prominent tremor is frequently preceded by large explosions. This
tremor often has a dominant peak below 1.5 Hz, exhibits gliding (frequency shifting of the spectral
peaks), and is unique in the dataset (Figure 8).
Figure 8. Spectrogram showing typical explosion and tremor signals for 5/12/06.
IAVWOPSG/4-IP/6
- 14 -
The type of activity mentioned above was unusual in that the energetic explosions and tremor
produced little ash. The spectral content and waveform features for the 5/12 explosions were fairly
similar. Most of the explosions are characterized by an impulsive compressional phase, somewhat long
durations (>6 sec), and an often complex rarefaction phase. In order to evaluate the similarity of the
explosions, waveform cross-correlation was performed on all 37 selected explosions for this day (Figure
9). All of the explosions have cross-correlation values above 0.75, and most are above 0.9.
Figure 9. Cross-correlation between 5/11-5/15 2006. The values show the cross-correlation coefficient of
all the explosions to a selected “master event.” Most of the waveforms exhibit a high degree of similarity
over each day short term, but tend to vary over a larger scale. This could be due to propagation effects or
changes in the explosion dynamics.
It is conceivable that a “master” waveform representative of this unique type of explosion could
be constructed and used in a detection algorithm. For example, the explosion detection algorithm could
cross-correlate the newly detected waveform against a “master” ash-poor explosion waveform to
determine if the new explosion constituted an ash hazard. However, before this could be done a
comprehensive waveform cross-correlation study would have to be performed to determine the reliability
of this type of test. One possible direction of study would be to correlate explosions during times of
similar atmospheric conditions to minimize propagation effects.
General Observations from the Case Studies
Rather than just exception-based notifications, regular (0.5 hour?) notifications of continued
activity would have been useful for the February 08 event. A persistent notifications of eruption status has
now been implimented. The system was originally configured to only notify of large changes in activity,
and did not do an entirely satisfactory job of continuous updates. In a perfect world the notifications
would alert the VAAC of a heightened eruption and update them of any ongoing activity or significant
changes. The VAAC could then search for more information, such as referring to the more
comprehensive ASHE website.
Although two notifications were issued at the onset of the 2/6/08 eruption, the VAAC was not
notified of the HIGH threshold being reached because the power levels had not doubled. This has been
remedied for future notifications, which trigger when the HIGH level thresholds are reached. Further
refinement of the volcano infrasound notification levels has also been implemented.
- 15 -
IAVWOPSG/4-IP/6
The prototype ASHE system did a decent job of notifying the VAAC of the onset of the two main
eruption pulses for the 2/6 eruption (~0600 and 1000 UTC). In their 2/6 advisories, the VAAC states two
large eruptions occurred at 0545 and 1000 UTC. Infrasound notifications of both of these pulses were
sent. The end of the eruption was also clear from the final notification email.
Rather than energy levels being calculated over the past hour, acoustic power calculated at a
shorter interval (15 minutes?) seem to be a more effective monitoring parameter. However, large and/or
multiple explosions in a short time period can bias the energy calculation. Perhaps a better indicator of
high-energy tremor or jetting could be constructed by subtracting the energy of the explosions.
The three main eruptions all have very similar spectra (Figure 10). No other periods of infrasonic
activity during the experiment had a spectrum that resembled these three cases. Another possible means
of identifying large, ash producing eruptions of VEI >2 could be to identify infrasonic signals with a
spectra similar to those in Figure 10. This is the subject of a separate ongoing study.
Figure 10. Power Spectral Density. The black line shows a typical background spectrum at the RIOE
array, while the red, tan, and turquoise lines are representative of energetic volcanic jetting. The spectral
structure of the jetting is unique in the dataset to the three main eruptions. The dark blue line shows the
spectra for the Plinian stage of the 8/17/06 eruption sequence.
As shown in Figure 10, the Plinian stage of the 8/17/06 eruption of Tungurahua is enriched in very low
infrasonic energy (<0.5 Hz). Figure 11 divides the acoustic energy into octave bands, and shows the
relative enrichment of acoustic power below 0.1 Hz with increasing ash injection height. The peak
acoustic power of 9 GigaWatts in the 0.0626-0.125 octave band during the climactic stage of the eruption
8/17 is extraordinary, and could serve as a clear discriminant for stratospheric ash injection.
IAVWOPSG/4-IP/6
- 16 -
Figure 11. Acoustic power per octave, in Watts, for the August 16-17, 2006 eruption of Tungurahua
Volcano, Ecuador.
Potential for Deriving Source Parameters
Based on the acoustic records captured during the Vulcanian to Plinian eruptions of Tungurahua volcano,
source parameters that may be estimated during large eruptions include (but may not be limited to) the
height probability, start time, and duration of an ash cloud injection that could pose a hazard to
international carriers at cruising altitudes. The possibility of inferring these source parameters even at
telesonic distances was suggested by the recognizable acoustic fingerprint of the July 12-13, 2008
eruption of Okmok Volcano recorded by infrasound arrays in Fairbanks, AK (1700 km away),
Washington (3540 km), Hawaii (3900 km), and Japan (4435 km).
— END —