Journal of Volcanology and Geothermal Research 194 (2010) 201–213
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s
Seismic characterization of the fall 2007 eruptive sequence at
Bezymianny Volcano, Russia
Weston Thelen a,d,⁎, Michael West b, Sergey Senyukov c
a
Pacific Northwest Seismic Network, University of Washington, Johnson, Hall 070, Box 351310, Seattle, WA 98195, USA
Geophysical Institute and Alaska Volcano Observatory, University of Alaska Fairbanks, 903 Koyukuk Dr. Fairbanks, AK 99775, USA
Kamchatka Branch of Geophysical Services, Russian Academy of Sciences, Petropavlosk, Russia
d
Cascade Volcano Observatory, United States Geological Survey, 1300 Cardinal Court, Vancouver, WA, USA
b
c
a r t i c l e
i n f o
Article history:
Received 21 January 2010
Accepted 27 May 2010
Available online 4 June 2010
Keywords:
Bezymianny
earthquakes
conduit
magma chamber
multiplet
a b s t r a c t
We examine an eruptive sequence in late 2007 at Bezymianny Volcano to characterize the magmatic
plumbing system and eruption-related seismicity. Earthquake locations reveal seismicity below and offset to
the north of the volcano along a tectonic fault. Based on historical seismicity, the magma chamber is
postulated to have a top at about 6 km depth. Minor dome explosions, large sub-plinian eruptions and dome
collapses are analyzed using an automated event classification scheme. Low-frequency tremor, interpreted
as gas escape, and low-frequency earthquakes are a dominant proportion of the energy released. We also
examine multiplet earthquakes whose behavior during the study period changed significantly and
systematically before the largest eruption, demonstrating the potential of tracking multiplets to assess
changing conditions with the conduit.
Published by Elsevier B.V.
1. Introduction
In 1955, Bezymianny began an eruption that led to catastrophic
sector collapse and a lateral blast on March 30, 1956 (Gorshkov,
1959). Subsequent volcanic activity at Bezymianny included phases of
discrete dome building and continuous dome building (Tokarev,
1981; Bogoyavlenskaya et al., 1985). Since 1976, 22 years after the
paroxysmal eruption in 1956, long, thick lava flows have accompanied
strong explosions (Bogoyavlenskaya and Kirsanov, 1981). During the
latest phase of activity, eruptions occur 1–2 times each year.
Generally, eruptions since 1976 start by extruding a low-temperature,
fully crystalline spine from the vent (Malyshev, 1995; Belousov et al.,
2002). As the cold spine collapses, rock avalanches are formed off the
sides of the dome. Subsequently an explosion normally occurs over
the course of days to weeks, which is accompanied by pyroclastic
flows. These explosions are often energetic enough to destroy large
parts of the dome that is eventually partial filled by the effusion of a
long andesitic lava flow (Belousov et al., 2002). Since 2000, there have
been at least 11 separate eruptions of this general type at Bezymianny
with a Volcanic Explosivity Index (VEI) between 2 and 3 (Venzke
et al., 2008). Precursory rockfall and large thermal anomalies on the
new dome have allowed the Kamchatkan Branch of Geophysical
⁎ Corresponding author. Current address: Cascade Volcano Observatory, United
States Geological Survey, 1300 Cardinal Court, Vancouver, WA, USA. Tel.: + 1 425 802
2346.
E-mail addresses: [email protected] (W. Thelen), [email protected] (M. West),
[email protected] (S. Senyukov).
0377-0273/$ – see front matter. Published by Elsevier B.V.
doi:10.1016/j.jvolgeores.2010.05.010
Services (KBGS) to issue successful eruption predictions up to 2 weeks
prior to an eruption (Senyukov et al., 2004).
Earthquake locations, often used as a basis for interpreting
magmatic plumbing systems, are available from the earthquake
catalog prepared by the KBGS beginning in 2000 for the entire
Klyuchevskoy Group of volcanoes, including Bezymianny (Senyukov
et al., 2004). The earthquake catalog shows three large clusters of
seismicity. Two clusters are focused under Klyuchevskoy Volcano and
in a deep earthquake cluster located at the base of the crust (∼30 km)
and a shallow cluster (b10 km) under Klyuchevskoy Volcano (Fig. 1;
Senyukov et al., 2004). The third cluster, consisting of seismicity above
7 km depth, is associated with eruptive activity at Bezymianny (Fig. 1;
Senyukov et al., 2004). To better constrain the earthquakes below
Bezymianny Volcano, a project funded by the National Science
Foundation's Partnerships in International Research and Education
(PIRE) installed a temporary network closer to the volcano (Thelen
and Team, 2006). This study utilizes data from both seismic networks
to augment the long time history derived from the permanent array
with the dense coverage afforded by the temporary network.
Multiplets, or earthquake families are commonly observed during
dome-building eruptive sequences at volcanoes such as Mount St.
Helens (Fremont and Malone, 1987; Thelen et al., 2008), Soufriere
Hills (Rowe et al., 2004; Green and Neuberg, 2006) and Mount
Redoubt (Stephens and Chouet, 2001). Because of their repetitive
nature, these earthquakes suggest a stationary, non-destructive
source. The sensitivity of earthquake families to changes in eruption
dynamics has been poorly explored. Bezymianny is an excellent
candidate for testing the hypothesis that changes in eruption
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In this paper, we aim to characterize and describe the seismic
activity occurring during a 3-month time period that included a
diverse range of volcanic activity including small explosions, dome
failure and characteristic explosive eruptions. We used data from the
PIRE temporary seismic network deployed on the edifice of the
volcano that has allowed us more detailed results than has been
possible previously. This new dataset also permits a new interpretation for the plumbing system beneath this volcano through improved
earthquake locations and interpretation of existing seismic data in the
context of our new results. The new dataset also allows us to
characterize the behavior of multiplets in several different eruptive
phenomena and assess the utility of tracking multiplets in a
monitoring environment.
1.1. Regional setting
Fig. 1. North–South cross-section of earthquakes in the Klyuchevskoy group. Earthquakes are from the KBGS earthquake catalog between 1999 and 2007.
dynamics will result in changes in characteristics of earthquake
families as Bezymianny often has multiple eruptions in a given year,
and at least one previous occurrence of multiplets (West et al., 2007).
Bezymianny Volcano is located on the Kamchatka Peninsula,
Russia within the Klyuchevskoy Group of volcanoes (Fig. 2, inset). The
Klyuchevskoy Group includes several large active volcanic centers
including Tolbachik, Klyuchevskoy, Zimina, Bezymianny, and Ushkovsky. The volcanoes are a product of subduction of the northern
edge of the Pacific Plate under the Okhotsk Plate. Several authors have
speculated that the volume and composition of material erupted
within the Klyuchevskoy Group is largely influenced by a slab tear of
the Pacific Plate, and the subsequent mantle flow around the edge of
the tear (e.g. Gorbatov et al., 2000; Levin et al., 2002). Bezymianny is
overshadowed by nearby Kamen and Klyuchevskoy volcanoes, which
tower over a kilometer higher than Bezymianny's summit dome. The
edifice of Bezymianny is built on the shoulder of the extinct Kamen
volcano. Kamen was active into the Holocene (b12,000 years)
however, the exact date of the last eruption is unknown (Braitseva
et al., 1991).
Fig. 2. Seismometer network map at Bezymianny Volcano. Labels without triangles are volcanic edifices in the immediate area. Other stations in the KBGS network exist outside the
map limits, providing good azimuthal coverage for larger events. Inset map: regional map of Kamchatka, Russia. Bezymianny Volcano is shown with a black star.
W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
1.2. Eruption chronology
The analyses in this paper largely focus on a 3-month sequence of
eruptive activity between September 1, 2007 and December 1, 2007
for which high quality seismic data are available through the PIRE
network (Fig. 2). There were three clear increases in seismic
amplitude (Sept. 25, Oct. 14 and Nov. 5) that correlated with observed
eruptive activity (Fig. 3). The September 25 eruption had minor preeruptive seismicity, no post-eruptive seismicity beneath the edifice
and a very small plume (KBGS, 2007a). No pyroclastic flow or ash
deposit was preserved and thus it is interpreted that no juvenile
material was involved in this small event (P. Izbekov, pers. comm.,
2008). The October 14 eruption was more characteristic of the twiceyearly eruptions that have occurred at Bezymianny since 2000. The
October eruption had significant pre-eruptive seismicity, large
thermal anomalies, a pyroclastic flow that included juvenile material
(T. Kayzar, pers. comm, 2009), and a 7 km high eruption plume (KBGS,
2007b). Additionally, significant post-eruptive seismicity extended to
4 km depth. The November 5 eruption had no pre-eruptive seismicity
(KBGS, 2007c), and a pyroclastic flow that contained no juvenile
material.
2. Earthquake locations
2.1. Methods
We present improved earthquake locations using a new temporary
network around Bezymianny. The network, comprising 8 threecomponent broadband sensors and 24-bit digitizers, was installed in
stages between October 2007 and August 2009 (Thelen, 2009). To
calculate absolute earthquake locations, we use the program SPONG,
an adaption of FASTHYPO (Herrmann, 1979), which is benchmarked
against HYPOINVERSE (Klein, 1985), and is currently used for routine
earthquake locations at the Pacific Northwest Seismograph Network
(PNSN). SPONG assumes a flat earth for hypocenter solutions,
however allows stations to be embedded within the shallowest
layer of the velocity model. Our datum is the elevation of our highest
station (BESA, 2400 m elevation). Stations with lower elevations than
the datum, in excess of the thickness of the top layer of the velocity
model, are fixed to the base of the top layer. The layered 1-D velocity
model used to calculate earthquake locations was developed by
Senyukov (2004). The KBGS network earthquake locations using the
1-D velocity model allow solutions at elevations in excess of the
summit of the volcano. We are only confident in earthquake locations
below our highest station, and thus we adjusted the 1-D velocity
model to fit our datum.
We use the combined networks to derive improved earthquake
locations. To obtain new locations, each earthquake in the KBGS
catalog in the vicinity of Bezymianny is recalculated with phase picks
from the PIRE network and the velocity model discussed above.
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Uncertainties in PIRE phase picks are generally less than 0.1 s. Phase
pick errors of existing KBGS picks were 0.3 s, which is what is assumed
by the KBGS. The sample rates are similar (PIRE: 100 samples/s, KBGS:
124 samples/s) between the two networks. We also produce a second
earthquake dataset using the PIRE network based on phase detections
from the edifice stations. Though these events are less constrained in
the north without the KBGS stations, the detection threshold is much
lower because of the proximity of the PIRE stations to the edifice.
To quantify the errors introduced by the PIRE network geometry,
we run a simulation of earthquake locations around the network and
calculated the difference between the original earthquake locations
and the recalculated earthquake locations. First, we build a 3-D grid
with 0.2-degree increment in latitude and longitude, and a 1 km
increment in depth. For each grid point, synthetic travel times are
calculated to each station in the PIRE network. A hypocenter is
calculated for each set of synthetic travel times and differenced from
the original location (Fig. 4). As expected, the errors are lowest
directly under the network, extending in some cases to approximately
6.5 km depth (− 4 km above sea level). Errors remain acceptable a
few kilometers north of the summit, where seismic activity is high. In
most cases the vertical error dominates, however further from the
perimeter of the network, the horizontal errors become comparable to
the vertical errors.
To further refine the absolute earthquake locations, we relocate
a subset of earthquakes using the double difference algorithm
(Waldhauser and Ellsworth, 2000) using only the manually-picked
phase differences. Using phase pick differences instead of absolute
phase arrivals reduces the error introduced from inaccuracies in the
velocity model. Linear artifacts in phase-differenced earthquake
locations can exist from a poor distribution of phase picks around
the epicenter. To quantify this effect, we use a jackknife test (Efron
and Gong, 1983), which evaluates changes in the distribution of phase
picks around an earthquake by removing one station at a time and
relocating all of the earthquakes using the remaining phase picks from
the reduced station set. For each station that is removed, a new
earthquake location is calculated and the distance between the full
network location and the jackknife location is used as an estimate of
error due to network configuration.
2.2. Hypocenter solutions
The KBGS catalog between September 2006 and August 2007
contains 342 earthquakes at Bezymianny Volcano. Fig. 5 shows these
earthquakes located with the addition of PIRE phase picks, at least 1
edifice station arrival, 6 total phases and a Root Mean Square (RMS)
residual of less than 0.5 s. The catalog contains both high- and lowfrequency events. No S-waves were used from the KBGS catalog
because the S-wave phase picks could not be reliably corroborated on
PIRE seismograms when PIRE and KBGS seismometers were collocated. Adding the additional PIRE stations resulted in a 35% improvement
Fig. 3. Eruption chronology between September 1, 2007 and December 1, 2007. The rectified seismic amplitude (counts) is shown as black and gray lines for stations BELO and BESA,
respectively. Dark gray dots are the size (number of pixels) of the thermal anomaly on the dome of Bezymianny and light gray dots are the maximum temperature of the anomaly.
Thermal information is from the AVHRR platform on the NOAA 17 and NOAA 18 satellites (KBGS, 2007a,b,c). Each pixel is 1 km across.
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arrival are shown in Fig. 5. The resulting 73 earthquakes in the PIRE
catalog have an average error of 0.11 s, an average of six phase picks
and an average azimuthal gap of 185°.
Like the KBGS earthquake locations, the earthquake locations
using only PIRE network stations showed a north–northeast trend.
Similar trends between the two earthquake datasets are reliable
because the datasets use station configurations that are significantly
different. In addition to the north–northeast trend of epicenters, a
cluster of seismicity to the east of the main cluster, between stations
BELO and BERG, was detected at 4.5 km depth.
To reduce relative earthquake location errors discussed above,
we relocated both earthquake datasets with manually-picked phase
differences using the HypoDD program (Waldhauser and Ellsworth,
2000). Earthquakes were selected that had 5 or more phase picks.
Earthquake locations of the 121 relocated earthquakes are shown in
Fig. 6. Of the 121 earthquakes, 75% have 7 or more phase picks in the
solution. As in the other datasets, there was a general orientation of
epicenters to the north–northeast. These earthquake locations show
more clustering than the other datasets, a characteristic often seen
when comparing absolute locations to phase-differenced locations.
An apparent aseismic zone exists above 2 km elevation, which is
present because the shallowest earthquakes often have solutions
with depths above the top of the velocity datum and are
subsequently discarded from further analysis. Jackknife ellipses
encompassing 95% of the errors due to a poor distribution of phase
picks were 0.72 km, 0.76 km and 0.77 km in X, Y and Z directions,
respectively. Jackknife errors considering only earthquakes with 7 or
more picks were not significantly better than the earthquakes with 5
or more picks.
2.3. Tectonic and volcanic interpretation
Fig. 4. Results of sensitivity tests of PIRE network. Contours represent the distance in
kilometers between the initial and final synthetic earthquake location. Contours above
5 km are not shown. The dark black line is the elevation profile along the cross-section.
A. Cross-section of location errors looking west. Bezymianny is located at 55.97°
latitude. The highest point in the cross-section is the summit of Kamen Volcano.
B. Cross-section looking north.
in the average RMS of the residual of the earthquakes in the relocated
catalog (0.32 s to 0.21 s). The average number of P-wave phase picks
was 7.8 and the average azimuthal gap was 146°. Absolute earthquake
locations trend north–northeast, sub-parallel to the trend between
Kamen and Klyuchevskoy volcanoes with most depths between −2
and 2.5 km elevation above sea level (0 to 4.5 km below the summit).
The trend in earthquake locations shows a near vertical structure
possibly deepening to the north of Bezymianny.
To locate smaller and shallower seismicity than is possible with
the sparser and more distal KBGS network, we analyze a subset of
seismicity during November 2007 using event detections from the
PIRE network. Earthquakes were chosen based on their amplitude and
impulsive phase arrivals regardless of the frequency content. 138
earthquakes were initially located. Earthquake locations with 5 or
more phase picks, residuals less than 0.5, and one edifice station
Different earthquake datasets, station configurations and earthquake location techniques all reveal earthquake locations in a north–
northeast trending lineament with a vertical or steeply east-dipping
orientation (Figs. 5 and 6). We suggest that the earthquakes are
occurring along a preexisting fracture or fault structure. Magma
often utilizes existing weaknesses in the crust to get to the surface,
and the relative ease of moving magma along a fault structure as
opposed to homogeneous crust with distributed fractures could help
explain the frequency and size of eruptions at Bezymianny (Costa et
al., 2007).
Fault zones have been previously identified within the Klyuchevskoy
Group of volcanoes with a combination of magnetotellurics and gravity
(Balesta et al., 1976). A large north–northeast trending topographic high
exists in the Cretaceous-age and crystalline bedrock centered about
5 km to the west of Bezymianny, which has been interpreted as a deep
seated fault that assists magma to the surface (Balesta et al., 1976). To
the south, the same fault, or a sub-parallel fault, was utilized in the
1975–1976-fissure eruption of Tolbachik (Fedotov et al., 1980; Zobin,
1990). Outside of seismicity associated with volcanic activity, the fault
has not had any large historical earthquakes.
The presence of seismicity that extends very clearly from
Bezymianny toward Kamen volcano is an intriguing result considering
Kamen has had no historic eruptions. It was active in the Holocene,
though its dissected morphology suggests that it has been quiescent
for several 100 years. If the earthquakes are responding to magma
movement, then it is possible that Bezymianny is using part of the
same plumbing system that previously fed eruptions at Kamen
Volcano.
There exists a small earthquake-free (aseismic) zone directly
underneath Bezymianny Volcano in the high-resolution earthquake
locations at 1 to 1.5 km depth below the crater (Fig. 6, inset). Aseismic
zones have been previously interpreted at other volcanoes as indicating
the presence of magma, fluids or gases below the surface (Weaver et al.,
1987). The presence of an aseismic zone alone cannot distinguish the
W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
205
Fig. 5. Absolute earthquake locations from Bezymianny Volcano. Upper left: map view of absolute earthquake locations using events indentified within the KBGS catalog with added
PIRE picks (gray circles), and earthquake locations using only the PIRE network (white circles). Dark contours are at 1 km elevation intervals, and labels are in kilometers. Black
triangles are stations within the PIRE network. Lower left: east–west cross-section through edifice. Dark line is elevation profile along cross-sectional line. Upper right: north–south
cross-section through edifice. No vertical exaggeration.
Fig. 6. Earthquake locations at Bezymianny using differential travel times. Symbols and axes are the same as Fig. 5. The dashed box in the upper right plot shows the boundaries of the
inset plot. The inset at the bottom right shows the detail of the shallow aseismic zone (dashed circle).
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W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
type of fluid or gas present in that zone. Fluid inclusions in the rims of
plagioclases erupted from recent eruptions require the storage of
magma at depths that roughly coincide with the depth of the
earthquake-free zone (P. Izbekov, pers. comm., 2009). It is not clear if
the proposed shallow magma chamber is transient, or a permanent
feature. The geometry of the earthquake-free zone is speculative given
the potential errors in the earthquake locations, and the short time
period of the analysis.
Earthquakes during eruptions at Bezymianny since 1999 characteristically extend from the surface to a depth of approximately 6 km
(Fig. 7). This is clear evidence of stress changes either due to
pressurization or depressurization of the conduit feeding the eruption
(Barker and Malone, 1991). Even minor (∼1 m) inflation of a conduit
and pressurization of the surrounding crust can lead to large changes
in Coulomb stress many kilometers away from the conduit, which can
lead to earthquakes on a variety of fault planes around the conduit
(Roman, 2005). Given this scenario, a magma chamber or magma
storage area may exist where the seismicity ceases at depth (Fig. 8).
Depressurization of the conduit and magma chamber can also be
an efficient source of seismicity after an eruption (Scandone and
Malone, 1985). The May 25 and June 12, 1980 eruptions of Mount St.
Helens had similar eruptive volumes to the bi-annual eruptions at
Bezymianny (∼ 106–107 m3; Belousov et al., 2002; Scandone and
Malone, 1985; Belousova, pers. comm.. 2009) and possessed posteruptive seismicity between the surface and the magma chamber.
Assuming a similar relationship at Bezymianny as exists at Mount St.
Helens between depressurization seismicity and magma withdrawal,
the deepest extent of post-eruptive seismicity should mark the top of
the magma chamber (Fig. 8). The regional gravity field supports the
presence of a magma chamber with a center of mass at 10 km deep,
which is consistent with the interpretation of a magma chamber top
at approximately 6 km deep (Balesta et al., 1976).
3. Earthquake classification
3.1. Automatic event classification methods
An initial set of earthquakes is detected using triggers in the
continuous seismic data on stations BELO, BESA and BERG. We use a
short-term average to long-term average ratio (STA/LTA) to determine the initial triggers. Only edifice stations are used to emphasize
the activity occurring at Bezymianny. The continuous data is filtered
between 2 and 12 Hz to accentuate local event arrivals. When triggers
at three stations are within 8 s of one another, an event is declared.
Fig. 8. Interpretation of seismicity underneath Bezymianny Volcano.
Volcanic earthquakes are often classified based on frequency
(Chouet, 1996). In this study, we divide earthquakes into high- and
low-frequency earthquakes based on the relative proportion of
spectral energy below and above 5 Hz (Fig. 9). By taking the ratio of
the average of two short frequency bands, an event can be accurately
characterized (Buurman and West, in press). In this study, we use a
low-frequency band between 1 and 5 Hz, and a high-frequency band
between 6 Hz and 10 Hz. For each event, we calculate the spectra of a
13 second window around the trigger (3 s before, 10 s after) and
calculate the ratio of the averages of the high- and low-frequency
band. Frequency ratios are then averaged across edifice stations (BELO,
BERG and BESA) to get the final frequency ratio for a given event.
To insure that the background noise in the signal is not contaminating the spectra, the frequency ratio is discarded if the maximum signal to
noise ratio (SNR) of the event is less than 3. The SNR cutoff is chosen
based on visual inspection of spectra. The SNR at a given time is
determined by the center time of the signal average over a 3 second
window divided by the average noise in a window 3 s before the first
arrival.
Fig. 7. Time–depth plot of shallow seismicity underneath Bezymianny Volcano. Earthquakes are plotted as circles sized based on their magnitude. KBGS-confirmed eruptions are
denoted by vertical gray lines. An apparent deepening of earthquakes after 2004 corresponds to the addition of new stations closer to the volcano that better constrain the
earthquake depths. The summit of Bezymianny lies at approximately 3 km elevation. A relative increase in the number of earthquakes after 2004 is likely due to the presence of
closer stations as well.
W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
207
Fig. 9. Waveform definitions used for ratio analysis at Bezymianny Volcano. For each panel, the waveform is shown on the left and the power spectrum is shown on the right. All
waveforms have been filtered between 1 and 20 Hz.
The cutoff between high- and low-frequency events is chosen near
a shoulder in the histogram of frequency ratio distribution (Fig. 10),
which emphasizes events with noticeably more low-frequency
energy than high-frequency energy, similar to Power et al. (1994)
and Lahr et al. (1994). The boundary between high-frequency and
low-frequency events is not particularly striking. This may reflect a
continuum in source processes between brittle fracture and fluid
resonance (Lahr et al., 1994; Neuberg, 2000). Observationally, the
cutoff could be chosen to be 0.1 higher or lower and still be similar to
other event classification schemes (i.e. Lahr et al., 1994). Changing the
cutoff within these boundaries does not significantly affect the results
of the event classification. The parameters for classifying events are
shown in Table 1.
The duration for each event on each station is calculated by finding
the first time past the maximum amplitude of the event when the SNR
drops below 1.5, similar to durations that are calculated for codawave magnitudes of local earthquakes (Dewberry and Crosson, 1995).
The durations of events are highly sensitive to station noise and site
effects, thus we used the median duration of the three edifice stations
to determine the final event duration.
By analogy with signals seen at Mount St. Helens (Malone et al.,
1981) and Soufriere Hills (Neuberg, 2000; Calder et al., 2005), high-
frequency signals with long durations are classified as rockfall. Most of
these events also have emergent onsets and a cigar-shaped envelope
(Fig. 9). Few, if any, alternative seismic sources exist in the literature
to describe such events. No direct visual observations of rockfall have
been correlated with seismic signals at Bezymianny though continuous visual observations are rare, particularly during eruptive periods.
The presence of a large dacite dome within the crater of Bezymianny,
which is as large or larger than the domes at Mount St. Helens and
Soufriere Hills, suggests that rockfall must be present. The duration
delimiting rockfall from regular events is chosen deliberately so that
only small- and intermediate-sized dome events are classified as
high- or low-frequency events. Larger, locatable events from
Bezymianny also may be classified as either rockfall or low-frequency
tremor (LFT) if their length exceeds 25 s. Most of the longer duration
events that are locatable by the KBGS occur around the time of
eruptions, when the counts of rockfall and LFTs are already high. Thus
the effect of misidentified Bezymianny earthquakes is proportionally
small.
3.2. Automated event classification results
Using ratios to classify event types, we analyzed the sequence of
seismicity between September 1, 2007 and November 1, 2007
(Fig. 11). Increased rates of low- and high-frequency earthquakes
appeared prior to the September and October eruptions. The
September and October eruptions both had peaks in rockfall activity,
while the November eruption was relatively quiet. LFTs maintained a
steady background rate through the September eruption, but
increased dramatically prior to the October eruption with a peak of
occurrence after the October eruption. Large variations in the
occurrence of low-frequency earthquakes and LFTs in November
2007 coincided with increased activity at Klyuchevskoy, and thus may
Table 1
Parameters for classification of events at Bezymianny Volcano.
Fig. 10. Histogram of ratios from all analyzed events. The black arrow shows the
division between our classification of “low-frequency” and “high-frequency” events.
Classification
Frequency ratio (high/low)
Duration
(s)
Low-frequency
High-frequency
Rockfall
Low-frequency tremor
≤0.6
N0.6
N0.6
≤0.6
b25
b25
N25
N25
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Fig. 11. Event occurrence with time at Bezymianny Volcano. Black bars represent the occurrence of a given event type calculated every 4 h. Gray arrows are explosion times. Gaps in
the occurrence of all event types after eruptions reflects continuous tremor.
reflect contamination of the seismic record (S. Senyukov, pers. comm.,
2008). The LFT signals are a combination of spasmodic tremor, and
low-frequency events that immediately precede rockfall. In the latter
case, a low-frequency event occurs at the beginning of a rockfall
signal, suggesting a coupled relationship. Such coupled events have
been identified at Soufriere Hills, Montserrat where the long period
portion was identified as violent gas escape from the dome that
accompanied rockfall (Neuberg, 2000; Luckett et al., 2008).
Tracking the amplitude evolution of events as a function of the
type of event reveals temporal changes or trends in processes that
may not be detected using all of the undifferentiated events together.
We used the cumulative maximum amplitude as a proxy for energy
release between September and November 2007 (Fig. 12). At
Bezymianny, the LFTs and low-frequency earthquakes have a
significantly higher energy release than the rockfall or high-frequency
events (Table 2).
Fig. 12. Cumulative amplitude release of different event types at Bezymianny Volcano. Vertical gray lines are eruption times.
W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
209
Table 2
Table summarizing observations of ratio analysis at Bezymianny Volcano. A “#” sign means number, “amp.” stands for amplitude, “Cum.” stands for cumulative and “n/c” stands for
no change.
Eruption
Sept. 25, 2007
Oct. 14, 2007
Observation
Pre-eruption
Post-eruption
# of low-frequency
n/c
# of high-frequency
n/c
Minor peak 2 days after eruption, Up, 12 h prior
slow decay
n/c
Up, 24 h prior
Up, 14 h prior,
Rapid decrease
peak at eruption
# of LFT
n/c
n/c
Cumulative amp. low-frequency n/c
n/c
Pre-eruption
# of rockfall
Up 4 days prior
Cumulative amp. high-frequency Minor increase
n/c
Cumulative amp. rockfall
Cumulative amp. LFT
n/c
n/c
Up 3.5 days prior
Broad increase starting Oct.1,
apid increase 1+ days prior
Broad increase starting Oct.1,
rapid increase with eruption
Increase 3 days prior
Increase 2.5 days prior
Small increase
Small increase
4. Multiplet analysis
4.1. Methods
As in the automated event classification, we consider all detected
events within the continuous record when detecting and classifying
multiplets. For each detection, we cut a nine-second window
encompassing the event, including 2 s before the earliest trigger.
We use a relatively large window prior to the automatic pick because
triggers are often observed to be later than the actual first arrival,
specifically in cases where the first arrival was emergent. Nine-second
windows are also used to increase the stability of the cross-correlation
values and prevent cycle skipping, which is common in smaller time
windows (Schaff et al., 2004). After all of the events are selected for a
certain day, each nine-second waveform is normalized and crosscorrelated on each individual station. Cross-correlation is a measure of
similarity commonly used to compare two waveforms (e.g. Poupinet
et al., 1984; Dodge et al., 1995; Rowe et al., 2004).
After cross-correlation of every event against every other event,
the result is an N × N matrix of cross-correlation values for each
station where N is the number of events. Following the method of
Petersen (2007), we then find the mean cross-correlation value of
each row of the correlation matrix at a single station (BELO). The row
with the highest average cross-correlation value is then searched for
events that exceeded the normalized cross-correlation value threshold. We chose a cross-correlation threshold of 0.7 based on visual
observations of similarity. Below that value, events cannot be visually
distinguished as being similar and higher values of the crosscorrelation threshold divide the events into smaller multiplets that
appeared similar.
The events that exceed the cross-correlation threshold with the
selected event are then checked for similarity on the other stations
(BESA and BERG). Any event within the row that exceeds the crosscorrelation value threshold on 2 out of 3 stations is considered part of
the same multiplet and removed from the pool of remaining
waveforms. After removal, the row with the highest average is again
selected, and the same method as above is followed to extract
subsequent multiplets until there are no events left.
To produce a continuous catalog, the day-long multiplet catalogs
must be merged. In order to distill the day-long catalogs into a
manageable number of traces, we stack each event within each
multiplet into a single representative waveform for each multiplet. All
stacks are amplitude-normalized and cross-correlated, creating an
Nov. 5, 2007
Post-eruption
Peak 12 h after eruption, slow decay
Contaminated
Peak 3 days after eruption, slow decay Minor increase
after eruption
4 day decrease to background
n/c
Peak at eruption, slow decay after
Rapid decrease 2 days after eruption,
broad decrease until Nov. 1
Rapid decrease after eruption,
slow decrease until Nov. 1
Decrease to background 4 days after
Rapid decrease after eruption,
slow decrease until Nov. 1
Contaminated
Contaminated
n/c
n/c
Contaminated
M × M matrix of cross-correlation values for each station where M is
the total number of daily stacks. The M × M matrix includes all event
stacks from each day during the entire study period. We use the same
technique to identify similar stacks in the cross-correlation matrix
that we used to identify similar events in the day-long individual
event analysis. Stacks are considered similar and combined if the
normalized cross-correlation threshold is 0.8 on two or more stations.
The individual events of similar stacks are then combined to create
one multiplet. Because the individual events within a stack do not
perfectly correlate with the stack itself, we choose a higher crosscorrelation threshold than in the individual event analysis. The
intention is to keep a cross-correlation value between individual
events of combined stacks at or above the cross-correlation threshold
of the day-long individual analyses.
The analysis attempts to produce a complete catalog, however
during long time periods it is often unreasonable, given computing
resources, to consider every event that occurs every day for several
months. To reduce the computing load in combining multiplets across
individual days, we only consider events that have occurred at least
twice on a given day. Though this approach will skip events that occur
only once per day, they do not offer sufficient time resolution to be of
significant use for our purposes.
4.2. Multiplet results
Multiplets continued unaffected through the September and
November eruptions, while no multiplet that occurred prior to the
October eruption reoccurs after the eruption (Fig. 13). This suggests a
fundamental change occurring in the multiplet source area during the
October eruption. Whatever multiplet source, or sources, that existed
prior to the eruption were destroyed during the explosion. During the
September and November eruptions, either the eruptions were not
strong enough to destroy the multiplet source areas, or the multiplet
source area was deeper than the source depth of the eruption. Stacks
of each multiplet show the tremendous variety of sources and
dominant frequencies (Fig. 14). No obvious change in dominant
frequency of the multiplets was observed around any explosion.
The observation of most multiplets on only edifice stations implies
a location in the upper 1–2 km of the edifice. A subset of located
multiplets that were recorded with the full PIRE network reveals a
range of depths between 0.5 and 6 km in regions consistent with
seismicity in Fig. 5.
210
W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
Fig. 13. Multiplet timeline from September 1, 2007 to December 1, 2007. Each circle is an earthquake; black dots that lie on a line are part of a single multiplet. The gray line is the
rectified seismic amplitude on station BELO. Vertical black lines mark volcanic events. Only multiplets with 5 or more events are included.
5. Discussion
5.1. Conceptual model of pre-eruptive seismicity and multiplet
development
Multiplets provide insight into the conditions in and around the
region in which they occur. Here we assume that earthquakes,
multiplets or not, are happening at the edges of a conduit, or along
cracks peripheral to the conduit. Between eruptions there is
measurable SO2 emitted from the dome (Clark et al., 2007). During
background periods, magma and the associated gases move slowly
through the conduit. Steady gas escape implies a relatively stable
source region for the production of repeating earthquakes. This
environment results in relatively long-lived multiplets since large
Fig. 14. Multiplet stacks of the earthquakes recorded on station BESA. The month and day of the start of the multiplet is shown on to the left of the respective stack. The number of
events within the multiplet is shown to the right and above the respective trace. Columns provide stacks from multiplets identifiers 1–18, 19–36, and 37–54 (corresponding to
Fig. 13) in the first, second and third columns, respectively.
W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
211
stresses are not reorienting or irreversibly altering sources. Multiplet
events during background periods will be small and of similar size,
reflecting the relatively low and consistent stresses within the
conduit. Prior to a large eruption, pressure builds in the conduit
system, forcing faster and more chaotic gas and/or magma transport.
Higher stresses and pressure gradients allow more gas escape into the
surrounding crust, producing precursory seismicity. In our conceptual
model, multiplet sequences leading up to eruptions have shorter
durations with fewer contemporary multiplets, resulting from
constantly changing volatile pathways.
5.2. Application of the conceptual model to event observations
We apply this conceptual model to the September, October and
November 2007 eruptions (Fig. 15). Since the multiplets make up less
than 30% of the overall seismicity, we treat the multiplets and event
types as different observations. During the September eruption, there
was no obvious change in multiplet parameters. Only a minor amount
of pre-eruptive seismicity was present in the form of rockfall and LFTs,
and no deep post-eruption seismicity was present. There is no
geologic evidence for extrusion during the September event, however
it is possible that the precursory rockfall activity could reflect
increased deformation from endogenous dome growth. Through
50 years of extrusion, the dome is at or near the angle of repose over
much of its surface and thus a minor steepening of the surface from
inflation of the dome will result in increased rockfall. The rapid
decrease in rockfall after the eruption in September thus suggests that
the deformation source was immediately removed after the small
plume was erupted (KBGS, 2007a). The eruption was not strong
enough to significantly alter the multiplet source area, since many
multiplets continued unchanged through the eruption (Fig. 16). Given
its seismic characteristics and small size, it is clear that the eruption
involved only the very shallowest part of the conduit and dome, above
any active multiplet source area.
Leading up to the October eruption, the multiplets changed
behavior systematically. The durations and number of concurrent
multiplets decreased significantly. Multiplets occurring before the
eruption did not continue through the eruption nor did they
reactivate after the eruption, suggesting an irreversible change in
the multiplet source area during the explosive phase. The eruption
was also preceded by significant increases in non-multiplet seismicity
including low- and high-frequency earthquakes, rockfall and LFTs
(Fig. 11). From the perspective of cumulative moment release, the
rockfall and LFTs were particularly productive prior to the October
eruption (Fig. 12). Prior to the October eruption precursory increases
in the number and average amplitude of LFTs suggest a rapid influx of
gas into the dome prior to the eruption, presumably ahead of rising
juvenile material. If LFTs are used as a proxy for gas, then the October
eruption included more gas than the September eruption, an
observation supported by the relative heights of the observed
eruption plumes. There were no direct observations of the dome
immediately prior to the October eruption, thus it is not clear if
increases in precursory rockfall were the result of dome deformation
or the degradation of a cold and degassed plug extruded at the surface,
as described in the 1997 Bezymianny eruption (Belousov et al., 2002).
Deep post-eruptive seismicity was also present, suggesting that the
October explosion and subsequent lava flow sourced a deeper part of
the conduit than the September eruption. This deep and energetic
source permanently changed the conduit and, hence, the behavior of
the multiplets.
Multiplet behavior and characteristics of event occurrence during
the October eruption provided insight into the seismic signature of
lava flow emplacement. The timing and duration of the lava flow is
speculative as no observations of the lava flow emplacement were
made until days after the October 2007 eruption. Peaks in the rates of
low- and high-frequency events occurred several hours after the
Fig. 15. Diagrammatic description of the three eruptive events shown with cutaway
cross-sections. Light gray within the cutaway represents magma, circles are earthquakes and the white stars represent the multiplet source area. Diagrams are not to
scale.
October eruption (Fig. 11). In contrast the September eruption had no
lava flow, and the rates of high- and low-frequency earthquakes
remained unchanged. Thus the high amplitude low- and highfrequency events are likely recording the ascent and extrusion of
magma during the October eruption. The peak in numbers of low- and
high-frequency events after the initial explosive eruption in October
2007 may coincide with the opening of the volcanic system and
beginning of the lava flow. The slow decay of several parameters after
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W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
Fig. 16. Multiplet behavior during September and November eruptions. A.) Record section of 62 events from multiplet 6 (Fig. 13) recorded on station BELO. Dates reflect the start
time of the record. The black arrow shows the approximate time of the eruption. B.) Cross-correlation coefficient of the first event in multiplet 6 to every other event with time. Gray
vertical line represents the time of the September eruption. C.) Record section of 562 events from multiplet 26 on station BESA. Labels and arrows are same as A. D.) Cross-correlation
coefficient of third event in multiplet 26 compared with all other events on station BESA.
the eruption including the numbers of events and cumulative
amplitude, paired with increases in multiplet durations and number
of concurrent multiplets are a reflection of the gradually declining
overpressure after the opening of the volcanic conduit. The surface
expression, if observed, would likely have been a slowing of the rate of
extrusion of lava at the surface.
The November eruption did not obviously affect the multiplet
characteristics and no individual multiplet was affected by the
eruption (Figs. 13 and 16). There was a lack of significant preeruptive seismicity, no juvenile material (T. Kayzar, pers. comm.,
2008) and no ash cloud was reported despite a large thermal anomaly
(KBGS, 2007b). Local scientists interpret the November event as a
dome collapse (P. Izbekov, pers. comm., 2008). This interpretation is
consistent with the unchanged patterns in multiplet behavior if the
dome collapse was strictly a shallow process and did not extend
deeper into the conduit and through the multiplet source region or if
the perturbation in the shallow conduit was not strong enough to
disrupt the multiplet source region.
magma accumulation zone at approximately 7 km deep, offset 1–
2 km toward Kamen volcano. Earthquake lineations suggest that
the magmatic plumbing of Bezymianny may be controlled by a
large crustal fault that is sub-parallel to the trend between
Bezymianny and Klyuchevskoy. We studied a sequence of three
volcanic events to understand the seismic signatures of different
types of eruptive phenomenon. The most energetic signals are lowfrequency earthquakes and LFTs. Precursory increases in high- and
low-frequency earthquakes are also present before the most
powerful eruption. Multiplet behavior only appears to be affected
by the largest eruptions, which are sourcing deeper material than
surficial explosions and dome collapses. We also present a model
assuming that changes in multiplet behavior are tied to conditions
within the conduit. Changes in the multiplet duration, number of
concurrent multiplet and average multiplet amplitudes prior to the
October eruption demonstrate how multiplets can be used
to assess the conditions with the conduit prior to large explosive
eruptions.
6. Conclusions
Acknowledgments
Using composite seismic network around Bezymianny Volcano,
Russia, we established the geometry and location of a shallow
volatile zone 1–1.5 km below the dome, and an additional deeper
We wish to acknowledge the PIRE program for funding the
logistics and research that went into this paper. We also give thanks to
the Institute of Volcanology and Seismology, Kamchatka and the
W. Thelen et al. / Journal of Volcanology and Geothermal Research 194 (2010) 201–213
Kamchatkan Branch of Geophysical Services. Without their logistical
support, the seismic network at Bezymianny would not have been
installed. Matthew Haney and an anonymous reviewer contributed to
the clarity of this manuscript greatly.
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