Satellite thermal observations of the Bezymianny lava dome : Precursory activity, large explosions, and dome growth

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009jb006966, 2010 Satellite thermal observations of the Bezymianny lava dome : Precursory activity, large explosions, and dome growth S. M. van Manen, 1 J. Dehn, 2 and S. Blake 1 Received 9 September 2009; revised 18 January 2010; accepted 9 March 2010; published 17 August [1] Fifteen years worth of Advanced Very High Resolution Radiometer (AVHRR) data is presented and used to quantitatively assess processes occurring at Bezymianny. This andesitic volcano is one of Kamchatka s most dangerous volcanoes with 16 eruptions in the last decade that have dispersed ash into North Pacific air routes. All known episodes of increased activity for which data were available were detected in band 3 ( mm) AVHRR thermal data. Twenty three peaks can be seen in the data; nineteen peaks correspond to known explosions, while the remaining three peaks correspond to known phases of dome growth that were not believed to have been accompanied by explosive activity. Start and end dates of extrusive phases defined by the thermal data are presented. Repose times between phases of extrusion vary from four months to just over two and half years with an average of just less than a year. Using rank order statistics a maximum time interval between consecutive explosions of 1288 ± 170 days is determined; this could serve as a cut off time for declaring the current dome growth activity over. The calculated cumulative erupted volume (0.28 km 3 ) and time averaged extrusion rate (0.6 m 3 s 1 ) from 1993 to 2008 corresponds to values found at Bezymianny from 1956 to 1976, showing that the satellite based methodology provides a good way of quantitatively assessing dome growth. Three different types of precursors to explosive behavior have been identified at Bezymianny: (1) values that cluster around the mode of the data set prior to explosion, potentially due to endogenous dome growth, (2) upward trends that commence days prior to explosion and reach sensor saturation levels are due to significant extrusion, and (3) a gradual upward trend that starts 5 days prior to explosion, probably due to ramping up of extrusion. This work shows that retrospectively analyzing and modeling of a volcano s thermal signal provides increased insight into its characteristic behavior. The methods used in this paper can be used at other dome building volcanoes around the world. The insights presented here can be used to improve monitoring capabilities to aid in providing early warnings to large explosions at Bezymianny. Citation: van Manen, S. M., J. Dehn, and S. Blake (2010), Satellite thermal observations of the Bezymianny lava dome : Precursory activity, large explosions, and dome growth, J. Geophys. Res., 115,, doi: /2009jb Introduction [2] Bezymianny (Figure 1) has erupted 15 times since 2000 and is one of the four most active volcanoes on the Kamchatka peninsula, one of the most active volcanic regions in the world [Simkin and Siebert, 1994; Gorbatov et al., 1997]. This andesitic volcano has been continually active since a large lateral blast in 1956 [Gorshkov, 1959; Belousov, 1996], exhibiting endogenous and exogenous dome growth, interspersed with explosive activity and dome collapse. Bezymianny has injected ash into the atmosphere up to 1 Volcano Dynamics Group, Department of Earth and Environmental Sciences, The Open University, Milton Keynes, UK. 2 Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, USA. Copyright 2010 by the American Geophysical Union /10/2009JB km above sea level (a.s.l.) within the last decade, presenting a danger to passing aircraft [Kirianov, 1994; Miller and Casadevall, 2000]. [3] Each day millions of dollars of cargo and more than 20,000 passengers pass over the Alaska Aleutian Kamchatka region, which is home to approximately 100 potentially active volcanoes. The region averages 5 eruptions per year and empirical data suggest ash will be present at altitudes of greater than 9 km above sea level at least 4 days per year [Schneider et al., 2000]. Worldwide more than 100 aircraft were damaged by encounters with volcanic ash from 1980 to 1998, costing more than $250 million in repair and replacement [Miller and Casadevall, 2000]. Volcanoes are preferably monitored using multiparameter approaches but approximately only half of the active volcanoes in the North Pacific are seismically instrumented and only a handful are geodetically monitored [Dean et al., 2002]. In most of this region satellite images provide the only regular and reliable 1of20

2 Figure 1. Location of Bezymianny in relation to (a) Kamchatka and (b) the Klyuchevskaya group. (c) Example of AVHRR data over Bezymianny and (d) photo of Bezymianny in 2007, facing north. The small triangles in Figure 1a represent all the volcanoes and volcanic centers recorded on the Kamchatka peninsula. near real time monitoring information on the state of the volcanoes. [4] In 1988 the Alaska Volcano Observatory (AVO, www. avo.alaska.edu) was established to monitor volcanoes in the region and aid in hazard mitigation. Twenty years after the establishment of the AVO, there are now extensive data sets available regarding various aspects of volcanoes in the Alaska Aleutian Kamchatka region, including data derived from the Advanced Very High Resolution Radiometer (AVHRR), which now spans 15 years. Since 1995 the AVO has collaborated with the Kamchatkan Volcanic Eruption Response Team (KVERT, php) based in Petropavlovsk Kamchatsky (Figure 1) to monitor the 29 active Kamchatkan volcanoes using satellite data. [5] Monitoring of volcanoes using satellite data is predominantly done using infrared (IR) wavelengths. It is restricted to two atmospheric transmission windows: 3 5 mm (AVHRR band 3) and 8 14 mm (AVHRR bands 4 and 5). Sources of thermal infrared radiation include active lava with temperatures of C, chilled crust, and other hot material such as pyroclastic deposits and fumaroles emitting steam and gases [Francis and Rothery, 2000]. Remote sensing of active volcanoes in the thermal part of the spectrum contributes to volcano monitoring by: (1) detecting hot sources and measuring their radiant energy, (2) locating new volcanic products on the surface, (3) providing a time series of background level activity with which new activity can be compared [Mouginis Mark et al., 1989]. The ability to detect volcanic activity depends on the spatial extent and 2 of 20

3 intensity of the hot source, which is controlled by a number of parameters such as variations in activity at the vent, surface temperature fluctuations and changing atmospheric conditions. Another influence is the duration of the activity in comparison to the frequency of satellite overpasses: Strombolian activity is hard to detect as it is unlikely to coincide with a satellite overpass, whereas long lived lava flows and lava domes show up readily and frequently. [6] Bezymianny is characterized by dome building activity. Dome growth is considered to be a cyclic nonlinear processes [Melnik and Sparks, 2005], which results in wide scale variability in the observed phenomena: eruptions can last years or decades [Newhall and Melson, 1983] and the growth of lava domes can be quiescent or violent, with transitions often occurring with little warning. Growth of lava domes can precede or follow major explosive events (Pinatubo, 1999 [Hoblitt et al., 1996]; Mt. St. Helens [Swanson and Holcomb, 1990]), and it can be nearly continuous (Santiaguito [Harris et al., 2002]) or intermittent (Shiveluch [Zharinov and Demyanchuk, 2008]). Behavior depends on eruption rate, magma rheology and thickness of cooling surface [Fink and Griffiths, 1998]. Hazards associated with domes include pyroclastic flows, Vulcanian explosions and lateral blasts. Newhall and Melson [1983] surveyed 156 historic dome eruptions and found that in approximately half of their examples, explosions occurred weeks to months after the onset of extrusive activity. Transitions to explosive activity have been linked to surges in extrusion rate at Unzen, Japan [Nakada et al., 1999] and Soufrière Hills Volcano in Montserrat [Sparks et al., 1998]. On the other hand dome extrusion at Mt. St. Helens was initiated by explosive eruptions [Swanson and Holcomb, 1990]. Lascar (Chile) produced a Plinian explosion after 9 years of dome growth and occasional Vulcanian explosions [Matthews et al., 1997]. [7] The unpredictability of lava domes makes them particularly hazardous, therefore the forecasting of large volcanic explosions remains one of the primary goals of the AVO and volcanology in general [Sparks, 2003]. Understanding the processes involved in producing an explosion at Bezymianny is the first step toward the ability to forecast large explosions and increasing aviation safety in the region. This work presents a 15 year long time series of thermal AVHRR data from the dome at Bezymianny volcano. This is the first time a thermal data series of this duration and volume has been presented for this region. Previous remote sensing work at Bezymianny has focused on single eruptive phases and individual events or shorter time periods [Belousov et al., 2002; Ramsey and Dehn, 2004; Carter et al., 2007, 2008; van Manen and Dehn, 2009]. The 15 years worth of data are described and interpreted to complement the existing understanding of volcanic activity at Bezymianny. The thermal data allow the duration of the phases of dome extrusion to be estimated, and periods of pre and post explosive thermal behavior in AVHRR data to be identified and quantified. In addition, the thermal data are used to calculate extrusion rates and increases in dome volume. Correlations between repose times between explosions and activity occurring at the dome such as intensity of extrusive phases are also investigated in this study. Last, the thermal data are modeled, and the results are compared with visual observations to provide a more robust interpretation of the observed thermal signal. 2. Bezymianny [8] The Kamchatka peninsula is located at a complex tectonic junction: at the southern part of the peninsula the North Pacific plate subducts while at approximately 56 N the junction rotates almost 90 to form a strike slip boundary along the most western edge of the Aleutians [Cormier, 1975; DeMets, 1992; Kogan et al., 2000]. Bezymianny (55.98 N, E, 2900 m; Figure 1) is located in an area known as the Central Kamchatkan Depression (CKD). This large graben was formed by intraarc extension parallel to the subduction trench [Avdeiko et al., 2007], and it is host to the Klyuchevskaya group of volcanoes of which Bezymianny is part. The other active volcanoes in the Klyuchevskaya group are Kliuchevskoi and Tolbachik. The group contains a further 9 volcanoes that have not erupted historically [Melekestsev et al., 1991]. [9] Bezymianny is a young ( year old) active complex andesitic stratovolcano [Malyshev, 2000], characterized by a summit lava dome and pyroclastic flow deposits to the southeast [Bogoyavlenskaya et al., 1991]. Bezymianny s deep magma supply system is thought to be the same as that of Kliuchevskoi (which is basaltic) until a depth of approximately km, there the system splits and part of the magma moves to Bezymianny s magma chamber that is thought to be at a depth of km [Ozerov et al., 1997]. In the past 2500 years Bezymianny has experienced three major cycles of increased activity: (1) years before present (BP), (2) BP and (3) 1955 continuing up to the present day [Bogoyavlenskaya et al., 1991]. In 1955 Bezymianny became seismically active, exploding after a thousand years of quiescence in 1956 with a lateral blast [Gorshkov, 1959; Belousov, 1996]. This created an eastward opening horseshoe shaped crater (1.3 km (north south) by 2.8 km (east west)) in which a dome (termed the Novy dome) has been growing intermittently since After 1977 most dome growth occurred exogenously and 35 explosions have been recorded since then (Table 1), although due to its remote location and inhospitable weather detailed ground observations are sparse. Twenty explosions (Table 1) have occurred throughout the 15 year time period covered by this study ( ). These eruptions have deposited block and ash flows in the valleys to the east and south. Throughout the 15 years fumarolic plumes rising up to a few hundred meters above the vent were often observed. The dome is currently approximately 1 km in diameter and rises approximately 200 m above the crater rim. [10] Based on incidental observations, cycles of activity at Bezymianny generally consist of the following [Belousov et al., 2002]: they commence with extrusion, sometimes resulting in partial collapse of the dome. Both collapse and extrusion are followed by an explosion that ranges in size from Strombolian (<10 km plume height) to sub Plinian (<20 km plume height) and pyroclastic flow emplacement. Explosions are generally followed by further extrusion of the dome, forming a short lava flow on the surface of the dome [Belousov et al., 2002; Ramsey and Dehn, 2004]. 3of20

4 Table 1. Date and Time for Explosions Recorded at Bezymianny Between 1977 and 2008 a Explosion Date and Time (UTC) Repose Period Since Last Explosion (days) Eruption Column Height (km a.s.l.) VEI Comments 25/03/ Exact time unknown 10/02/1979; 22: Largest explosion since /09/ Exact time unknown 19/04/ Exact time unknown 12/06/ Exact time unknown 20/12/ Exact time unknown 10/06/ Exact time unknown 22/05/ Exact time unknown 16/02/ ? 3 Exact time unknown 12/10/1984; 23: /06/1985; 07: /06/ Exact time unknown 17/12/ Exact time unknown 02/08/ Exact time unknown 03/09/ Exact time unknown 11/03/ Exact time unknown 1 21/10/1993; 04: Largest explosion to date since /10/1995; 17: Health warning issued to the people in Kliuchi regarding high levels of volcanic gas present in the air. 3 08/05/1997; 17: /12/1997; 18: /02/ Exact time unknown 6 13/03/2000; 16: /11/2000; 18: /08/2001; 22: No thermal data available 9 15/12/ Exact time unknown 10 25/12/ Exact time unknown 11 26/07/2003; 09: /01/2004; 22: /06/2004; 20: /01/2005; 08: /11/2005; 12: /05/2006; 09: /12/2006; 11: /05/2007; 15: /10/2007; 14: /08/2008; 10: a Numbered explosions occurred within the time period of the thermal data set presented in this paper. Source: AVO, KVERT, GVN. Dates and times of explosions have been reported by KVERT and the Global Volcanism Network (GVN). Since August 1993, the start of the AVHRR data set, 50% of explosions occurred in May, October and December whereas none occurred in April or September. [11] A time series of photographs and annotated sketches (Figure 2) highlight changes at Bezymianny s growing dome. Although these photographs are not always from the same vantage point and there are variable time periods between them, it is clear that flows were extruded by Bezymianny between successive photos. The flows tend to reach no more than a kilometre in length and they are lobate in shape. In addition it shows that at least since 1993 all lava lobes have originated from the central vent at the top of the dome. An aerial view of the dome overlain by a sketch of extrusion directions of lava flows since 1993 (Figure 3) shows that Bezymianny does not have a preferential direction of extrusion, suggesting that there is no particular part of the Novy dome that is weaker than the rest. Figure 2f shows that lava flows are starting to fill the moat region between the dome and the crater rim, suggesting that eventually Bezymianny will regain a typical stratocone shape. 3. Data [12] The Advanced Very High Resolution Radiometer (AVHRR) is the primary instrument on the polar orbiting weather satellites operated by the National Oceanographic and Atmospheric Administration [Kidwell, 1998]. Currently there are four sensors in orbit at approximately 830 km. It is the main satellite utilized by the AVO and KVERT. Due to polar convergence of the satellite orbits multiple images that include the Klyuchevskaya group are received daily, depending on the distance of the volcano relative to the receiving station and the edge of the station mask. This data has a spatial and temporal resolution sufficient to detect renewed activity at a volcano and can be used to assess the magnitude of that activity [Ramsey and Dehn,2004].AVHRR was first used by the AVO to study the 1989 Redoubt eruption and again during the 1992 eruption of Mt. Spurr. It 4of20

5 Figure 2. A time series of photographs and annotated sketches highlight changes at Bezymianny s growing dome. Extruded lava flows have not been longer than m during Figure 2f shows that Bezymianny is starting to fill the moat between the dome and the crater rim; this means that eventually Bezymianny will regain a typical stratocone shape. Photo credits are indicated on each individual photo. Exact dates on which the photographs were taken are unknown. 5of20

6 Figure 3. An aerial view of the dome with arrows representing directions of lava flows from 1993 to There is no preferential direction, suggesting that there is no specific part of the dome that is weaker. Photo by V. Dvigalo, was not possible to use the data in real time during these eruptions as the data had to be acquired from a receiving station in Florida [Schneider et al., 2000]. After the 1992 eruption the University of Alaska Geophysical Institute obtained its own receiving station, allowing the AVO to do near real time analysis as well as store and catalog the data. [13] The AVHRR sensor acquires swaths in five wavelengths (spanning the visible to the infrared, Table 2), approximately 2600 km wide, by scanning across the Earth from one horizon to the other by continuous 360 rotation of a flat scanning mirror. The scan lines are perpendicular to the satellite s orbit track and the speed of rotation of the scan mirror is selected so that adjacent scan lines are contiguous at nadir. Its instantaneous field of view (IFOV) is 1.3 mrad resulting in a resolution at nadir of 1.1 km. Every sample corresponds to a rotation of 0.95 milliradians by the scan mirror. Therefore, there are samples per IFOV and these analog data are digitized on board at a rate of 39,936 samples per second per channel [Kidwell, 1998]. The amplitude of all channels is a measure of the scene s radiance, which is acquired from the same place on Earth in all channels at the same time. There are two different generations of AVHRR instruments that provided data for this study, they were flown on NOAA 11 through NOAA 18 (Table 3). The AVHRR\2 was flown on NOAA 11 through NOAA 14 and had five channels whereas the newer version, AVHRR\3, flown on NOAA 15 18, has six channels. The additional channel (3A) is used interchangeably with channel 3B during the day and night. This study uses two (band 3 and 4) of the three (band 3, 4 and 5) thermal channels of the AVHRR sensor. These channels are calibrated onboard by using an internal calibration target (ICT) and deep space [Cracknell, 1997; Kidwell, 1998]. The thermal state of the ICT is measured by four platinum resistance thermometers. Uncertainty in pixel integrated radiant temperatures is directly related to uncertainties in the temperature of the ICT [Trishchenko et al., 2002]. NOAA 12 had the noisiest of the sensors, with errors of up to 4 K associated with it [Trishchenko et al., 2002]. In general noise of the AVHRR sensors located on NOAA 11 to NOAA 16 varies from 0.03 K to 0.3 K at a temperature of 300 K, increasing significantly with decreasing temperatures. NOAA 14 has errors up to 3 K while NOAA 15 and 16 show inconsistencies of less than 0.5 K [Trishchenko et al., 2002]. Data on uncertainties are not available for NOAA 17 and 18 as they were launched after Trishchenko et al. completed their research and no more recent investigation has been done. [14] Once received from the satellite by the AVO, the digital number (DN) is converted into radiance. A DN is the converted electrical pulse of the energy received by the radiometer and it is proportional to the strength of the signal. The radiance received will be affected by atmospheric attenuation and instrumental effects for which no correction is applied, as well as surface spectral emissivity. Using the Planck function the DN is converted into pixel integrated temperature. Images are georeferenced and projected on a Lambert azimuthal grid. Data are not topographically corrected because of the large geolocation errors in AVHRR data (up to 20 km). [15] This study utilizes nighttime data over Bezymianny as this eliminates any solar effects that could be mistaken for thermally elevated pixels. Images with a zenith angle greater than 55 have been discarded as data contained in these are too geometrically distorted [Harris et al., 1997]. This left almost 8000 nighttime images (e.g., Figure 1c) over Bezymianny between August 18, 1993 and August 31, These images were examined for thermally elevated pixels, defined as a pixel at least 5 C hotter than its surrounding 8 pixels. Gaps in this data set include a large time span without data due to problems with the AVO receiving station from April until August 20, In addition Table 2. AVHRR Instrument Specifications a Channel Wavelength Range (mm) Spectrum Range Spatial Resolution (km) IFOV (mrad) Approximate Repeat Time (h) VNIR VNIR A (only AVHRR\3) VNIR B SWIR TIR TIR a Band 3 (3B on AVHRR\3 instruments) is used to assess volcanic activity at the Bezymianny dome. Band 3 saturates at 49.5 C for AVHRR\2 models and C for AVHRR\3. Background temperatures are derived from band 4. 6of20

7 Table 3. Details of the NOAA Satellites That Carry the AVHRR Sensors Used in This Study Satellite AVHRR Model Launch Date Decommission Date Altitude (km) Period (min) Inclination (deg) NOAA /09/88 16/06/ NOAA /05/91 10/08/ NOAA /12/94 23/05/ NOAA /05/ NOAA /09/ NOAA /06/ NOAA /05/ there is a lack of summer nighttime data in 2000, 2001 and 2002 and between late February and early June Consequently, there are no data surrounding the August 6, 2001 explosion and there were only 10 images preceding the June 18, 2004 explosion. For 2002 the predominant data source is the AVHRR on NOAA 12, the noisiest sensor. The AVHRR sensor on NOAA 16 also provided noisy data from February 2004 until September Data Analysis [16] This study examines trends of the pixel integrated radiant temperature (T int ) from AVHRR band 3 ( mm) for the single hottest thermally anomalous pixel in each pixel AVHRR subsection centered over Bezymianny. AVHRR band 3 lies within the mid infrared and therefore it is the closest band to the peak spectral emissions of bodies radiating at C. The radiant temperature detected by AVHRR is integrated over the entire pixel area, it does not directly reflect the temperature of the hottest volcanic material, unless the hot material homogenously occupies the whole pixel. However, magmatic temperatures can t be recorded in AVHRR data as the sensor saturates at much lower temperatures: AVHRR\2 saturates at 49.5 C and AVHRR\3 saturates at C. For a pixel to be classified as thermally anomalous (i.e., T int 5 C above the surrounding pixels, this is one standard deviation larger than potential background variability) a body radiating at 950 C against a background temperature of 0 C, only % of a pixel needs to be occupied. This is equivalent to 6 m 2 in a 1.1 km 2 AVHRR pixel. The amount of radiated energy required to raise a pixel 5 C above background temperatures at Bezymianny, and thus be detected by the AVHRR sensor, depends on the season: the lowest amount of radiating energy from the summit area that has been detected at Bezymianny is approximately 10 7 W ( C T int in a C background). The largest amount of radiated energy from the summit has been over W, this has also been in winter as this is when the greatest temperature contrasts occurs (56.85 C T int in a background of C). This value is an underestimate as the pixel was saturated. Although the dome is continually thermally active, the magnitude of the signal is variable, depending on whether activity is purely fumarolic ( 10 3 W) or if active lava is present on the surface (10 6 W and greater). Based on the numbers presented above, the signal produced by fumaroles is too small to be detected in AVHRR satellite data as a thermal anomaly. Therefore the signal detected in the AVHRR data is of magmatic origin. [17] Activity is assumed to only occur at the single hottest pixel in each AVHRR image as this is the area of maximum extrusion, regardless of whether this is at the vent or at the toe of the lava flow. Also, due to the way in which the AVHRR data are acquired, pixels may overlap, particularly at greater zenith angles, causing multiple pixels to be recorded as thermally anomalous. In addition the size of the vent on the top of the lava dome ( 0.01 km 2 ) is approximately two orders of magnitude smaller than the size of the pixel (1.1 km 2 ), locating the vent within a single pixel. The pixel integrated radiant temperature (T int ) is also used to calculate extrusion rates, which can be compared to extrusion rates derived from other methods and those found at other similar volcanoes. In addition extrusion rates can be used to calculate the volume of magma that has been extruded at the dome. [18] To calculate extrusion rates the two component method [Dozier, 1981; Rothery et al.,1988;crisp and Baloga, 1990] was used. This method assumes that a pixel with an elevated temperature (T int ) is composed of subpixel hot spots at temperature (T h ), occupying fractional area (A h ) while the remainder is at background temperature (T bg ) occupying (1 A h ). Although the two component model is a simplification of an actual lava flow and its surroundings, the results can be used to calculate radiative thermal flux and extrusion rates. Using: Lð; T int Þ ¼ A h Lð; T h Þþð1 A h ÞL ; T bg where L is the Planck function for a blackbody at wavelength l, the temperature and the area covered by each component can be estimated. There are three unknowns in equation (1): the fractional area occupied by the hot component (A h ), the temperature of the hot component (T h ) and the temperature of the background (T bg ). Background temperatures (T bg ) were estimated using the mean temperature of the 40 by 40 subsection in band 4 ( mm), excluding pixels marked as thermally anomalous. Band 4 was used for background temperatures as it is closer to the peak emittance of the background than any of the other available bands. By assuming either the area occupied by the hot component (A h ) or the temperature of the hot component (T h ), the remaining parameter can be calculated. In this paper, the value of T h ranged from sub solidus 950 K to 1223 K and the radius (r) ofa h was varied between 90 and 150 m based on visual observations of the vent area. Maximum values of Th are obtained when A h is small, and minimum values of Th are obtained when A h is large and vice versa. ð1þ 7of20

8 Table 4. Physical Properties Used in the Calculation of Extrusion Rates at Bezymianny Parameter Value Source Solidus ( C) 850 a Hanson and Glazner [1995] Bulk density (kg m 3 ) 2660 a Waples and Waples [2004] Specific heat capacity 815 a Waples and Waples [2004] (J kg 1 K 1 ) Crystal fraction 0.5 Belousov et al. [2002] Latent heat of crystallization (J kg 1 ) a Hanson and Glazner [1995] a When not available generic values andesite have been chosen. [19] Consequently, heat lost through radiation (Q r ) and convection (Q c ) can be calculated, based on the calculated value of A h, to account for the surface heat losses: Q r ¼ "A h Th 4 T bg 4 ð2þ Q c ¼ A h h c T h T bg where t is the atmospheric transmissivity, is a shape factor, s is the Stefan Boltzmann constant, " is the emissivity of the lava, h c is the convective heat transfer coefficient. The convective heat transfer coefficient used was 11 W m 2 K 1 [Keszthelyi and Denlinger, 1996]. The shape factor takes account of the geometry of the radiating body, in other words it is an estimate of the proportion of the amount of radiation leaving the lava flow compared to what actually arrives at the sensor. As the precise geometry of the volcanic edifice changes and the viewing geometry of the satellite is hard to constrain, a shape factor of 8 = 1 has been applied. Atmospheric transmissivity is also difficult to account for accurately as it is affected by the total transmissivity of the atmosphere as well as the concentration of volcanic gases above the vent [Dehn et al., 2002], both of which can change, therefore t = 1 has been used. [20] To provide a practical reference frame to evaluate Bezymianny s eruptions, time averaged discharge rates for Bezymianny were calculated [Harris et al., 2003]. As stated before, these are based on the single hottest pixel in each image: ð3þ ðq r þ Q c Þ E r ¼ ð4þ c p T þ 8c L where r is the lava density, c p its specific heat capacity, 8 is the mass fraction of crystallization, dt is difference between the lava temperature and the solidus and c L is the latent heat of crystallization. These values have been obtained from the literature and are listed in Table 4. As this method equates the amount of heat lost by a lava flow to the amount of heat produced by a cooling lava flow and the temperature of the lava flow is assumed to be stationary, changes in the extrusion rate will reflect changes in the area covered by the lava flow at the time of acquisition. This reflects the average extrusion rate (volume) with time, therefore providing a time averaged extrusion rate as opposed to an instantaneous extrusion rate [Wright et al., 2001; Harris et al., 2007]. As time averaged discharge rates are calculated for Bezymianny, a viscous dome forming system, following Harris et al. [2007] the term extrusion is used. [21] Time averaged extrusion rates can be used to calculate the volume of magma extruded at the dome by integration. It is assumed that between two consecutive data points the extrusion rate remains constant at the rate calculated at the earlier time. Dividing the total volume extruded by duration of an eruption provides a mean eruption output rate [Harris et al., 2007] while a mean annual output rate was calculated by dividing the entire volume effused from 1993 to 2008 by the time in years between the first and the last data point. [22] All energy flux, extrusion rate and volume calculations most likely underestimate the actual volcanic output, even when t and have been set equal to 1. This is due to the irregular data sampling, sensor saturation in certain images, potential cloud interference, and the fact that other sources of heat loss such as rainfall and hydrothermal circulation are ignored [Francis and Rothery, 2000]. Errors in the calculated values can arise from uncertainties associated with the chosen physical properties of the magma (Table 4). Physical properties for Bezymianny magma have been used when available, but otherwise generic values appropriate for andesite have been used. 5. Results [23] The AVHRR data stored at AVO provides a 15 year thermal time series, during which Bezymianny underwent twenty explosive eruptions (Table 1). This section describes observations from the radiant temperature data and it presents the results from extrusion rate and volume calculations Presence and Character of Thermal Anomalies [24] Thermally elevated pixels were identified in 2007 of the 7942 nighttime images, 2.5% of these images showed saturated pixels. Of the twenty explosive eruptions that occurred, nineteen can be analyzed; there were no satellite data available for the August 2001 explosion. On average Bezymianny shows a thermal anomaly in 25% of the data set (Figure 4a), though this ratio varies by year, depending on the activity at the volcano and whether the receiving station was operational. It can be assumed that the absolute number of thermal anomalies detected is a function of the number of available images in combination with the level of activity at the volcano. A third of all thermal anomalies were detected in January, February and December, whereas 15% occurred in June, July and August combined (Figure 4b). A seasonal influence is visible in Figure 4b, although May departs from this trend due to the larger number of explosions that have taken place in May (Figure 4c). [25] The majority of the thermal anomalies detected at Bezymianny are between 20 C and 0 C (Figure 5a), which corresponds to 5 15 C above background temperature (Figure 5b). The line at 49.1 C in Figure 5a corresponds to the saturation temperature of the AVHRR\2 suite of instruments while the line at C represents the saturation level of AVHRR\3. Sensor saturation temperatures are reached during 14 of the 22 peaks, 13 of these were associated with explosive events. When only AVHRR\2 data are available saturation following an explosive event always occurs while saturation did not occur in all of the AVHRR\3 data associated with explosive events, therefore a second peak at C is not visible. 8of20

9 Figure 4. Data availability per year and the number of thermal anomalies detected per month. Please note that the 1993 and 2008 numbers do not comprise a full year and 1994, contain data gaps. (a) Number of nighttime satellite images available with zenith angles less than 55 that show a thermal anomaly as a percentage of all available nighttime images with zenith angles 55. (b) Number of thermal anomalies per month from 1993 to Most thermal anomalies occur in winter as the cold background temperatures enable low energy volcanic activity to be detected. (c) Number of explosions per month from 1993 to [26] The fact that most of the thermal anomalies have been detected in winter can be explained by an argument proposed by Dehn et al. [2000]: the colder background temperatures enable detection of weaker thermal activity. Although an alternative hypothesis, that there is a seasonal influence on the timing of explosions [Matthews et al., 2002; Mason et al., 2004] is possible Thermal Anomalies in a Time Series Context Defining Phases of Dome Growth [27] Based on the radiant temperature data (Figure 5a) and changes in slope of the cumulative volume plot (Figure 6) twenty three peaks in the data can be defined between 1993 and Nineteen of the twenty three peaks in the time series of pixel integrated radiant temperature are surrounding recorded times of explosions (Table 5). The remaining three peaks (1994, 1996 and 1998) were not accompanied by a known explosion (P. Webley, personal communications, 2009). The 1994 peak could be the tail end of the October 1993 peak, but due to the absence of data between these two peaks this cannot be verified thermally. [28] Figure 7 illustrates the phase surrounding the December 2006 (number 17) explosion. The start of the phase is chosen when the thermal values rose significantly above the background values. What is significant differs between phases and the amount of data available. In the example shown it is three consecutive points that rise above the mode (most frequently occurring value, 8.5 C) and the end is exemplified by 3 consecutive points below the mode. In some phases the start and end are more easily observed in the cumulative extruded volume data as breaks in slope. For the example shown, Carter et al. [2008] published high resolution thermal ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data and some seismic and visual observations. ASTER data have a pixel resolution of 90 m at nadir. It is interesting to note that AVHRR pixelintegrated radiant temperatures above background start rising above the mode of the data set early November, while low frequency seismic events that are thought to represent rockfalls or extrusive activity were not observed until December 9, 2006 [Carter et al., 2008]. No increase in temperature was observed in the ASTER data. On December 27, four days after the initial explosion, visual observations indicated that the dome had been partially destroyed. Consecutive visual observations in March 2007 showed that craters in the dome that had been created by the May 2006 (number 16) explosion had been infilled and produced a lava lobe, providing evidence of extrusion. In the AVHRR data this is represented by high pixel integrated radiant temperature values above background following the explosion and a steep increase in the calculated extruded volume (Figure 7). [29] The peak associated with the 1993 explosion, the largest eruption since the 1956 paroxysm, has by far the largest cumulative thermal output (Table 5). As sampling of Bezymianny s thermal state is highly irregular because of its dependence on timing of satellite overpasses, number of sensors available and weather conditions, data has been compared on an average radiated energy per day basis as well as for the total radiated energy per phase. The cumulative radiated energy per phase (Figure 8a) is highest for the 1993 paroxysm that was followed by months of dome growth. No specific trend is visible for the cumulative radiated energy per phase (Figure 8a) or for the average amount of radiated energy per day during a particular phase (Figure 8b). There is a positive correlation between repose time between phases and the amount of energy radiated during the following phase (Figure 8c). 9of20

10 Figure 5. Time series and temperature distribution of the thermal anomalies. (a) Pixel integrated temperature in C. The top line represents saturation temperatures for AVHRR\3 instruments while the line below it represents saturation values for AVHRR\2 instruments. (b) Pixel integrated radiant temperatures above background. (c) Distribution of radiant temperature of the thermal anomaly in C. (d) Distribution of radiant temperature above background of thermal anomalies in C Calculated Extrusion Rates and Cumulative Extruded Volume [30] Single pixel time averaged extrusion rates (Figure 6) follow the pixel integrated radiant temperature pattern, peaking at the same times, reaching a maximum of 4 18 m 3 s 1, just prior to the February 1999 explosion. Until 1999, single pixel time averaged extrusion rates of the activity associated with explosive events peak around 5 10 m 3 s 1, whereas the 10 of 20

11 Figure 6. Time averaged extrusion rates and cumulative erupted volume from mid 1993 to mid Twenty two peaks can be observed in the data. Maximum (gray) and minimum (blue) extrusion rates and cumulative extruded volumes are shown. The red symbols indicate extrusion rates and cumulative extruded volumes for a time averaged extrusion rate of 0.6 m 3 s 1 between 1993 and The stepwise increases in cumulative erupted volume get progressively smaller after three episodes not linked with explosive activity peak at 4 8m 3 s 1. After 2002, maximum single pixel time averaged extrusion rates of extrusive activity penecontemporaneous with explosive ash plume forming events decline to a maximum of 4 12 m 3 s 1 for the December 2002 event, not exceeding 2 11 m 3 s 1 for the next seven eruptions until the December 2006 episode, when single pixel time averaged extrusion rates peaked at 4 14 m 3 s 1. After this they decline again. [31] During the fifteen years, cumulative extruded volume derived from calculated extrusion rates for the Bezymianny dome (Figure 6) shows a relatively smooth linear trend, although some stepwise increases are visible, particularly for the tail end of the 1993/1994 peak. Due to the manner in which the cumulative extruded volume is calculated some stepwise increases (such as the one in 2001) cannot be verified to be real. It is known, however, that an explosion occurred during this time (Table 1), presumably accompanied by extrusion. If this was the case, the step shown is potentially an underestimate of the amount extruded as the steps before and after the 2001 explosion were larger. By assuming constant extrusion at Bezymianny, the total extruded amount could be overestimated, however, this assumption compensates to some unknown degree for cloudy days on which no thermal anomalies were observed even though extrusion was occurring on the ground. Unfortunately there are currently no estimates from visual or other data that provide an independent estimate of extruded volume. Most of the stepwise increases correspond to the peaks associated with the explosive behavior as well as the peaks that were not accompanied by explosions. Taken as a whole, the 15 years show a linear trend in volume extruded, with the exception of the large extruded volume associated with the 1993/1994 eruption and a period between early 1998 and mid 2004 where the erupted volumes are slightly greater. Extruded volumes from 2004 onward show a smooth curve, with fewer stepwise increases. The cumulative extruded volume for mid 1993 to mid 2008 amounts to km 3. This suggests a mean annual extrusion rate of million tons per year ( m 3 s 1 ) over 15 years. This is similar to the value of 0.6 m 3 s 1 reported by Belousov et al. [2002] for the period Assuming Bezymianny s time averaged extrusion rate has remained at 0.6 m 3 s 1 for the period , results in a cumulative extruded volume of 0.28 km 3, and a maximum extrusion rate of 13 m 3 s 1. Unfortunately there are no estimates of extrusion rates obtained by other methods available for Bezymianny for the time period , 11 of 20

12 Table 5. Phases of Dome Growth, Explosions, and Precursory Activity a Number Start End Duration (days) Time Since Last Phase (days) Cumulative Volume (10 6 m 3 ) Cumulative Radiated Energy (x10 11 J) Average Radiated Energy (x10 9 J/day) Images in Phase Explosion Date and Time Precursory Type Images in 30 Days Preceding Explosion 1 10/4/1993 4/7/ /21/1993; 4: /2/1994 3/21/ NA NA 2 9/17/1995 1/8/ /5/1995; 17: /21/1996 9/1/ NA NA 3 4/6/1997 7/6/ /8/1997; 17: /8/1997 2/21/ /4/1997; 18: /22/1998 9/6/ NA NA 5 1/22/1999 5/5/ /25/ /23/1999 4/29/ /13/2000; 16: /19/2000 3/17/ /1/2000; 18: ND ND ND ND ND ND ND ND 8/6/2001; 22:28 ND ND 9 11/17/2001 1/11/ /15/ /23/2002 2/6/ /25/ /23/2003 8/16/ /26/2003; 9: /11/2003 2/20/ /12/2004; 22: /16/2004 8/12/ /18/2004; 20: /6/2004 5/17/ /11/2005; 8: /22/ /24/ /30/2005; 12: /2/2006 6/22/ /9/2006; 9: /8/2006 1/21/ /23/2006; 11: /4/2007 6/5/ /10/2007; 15: /7/ /30/ /14/2007; 14: /30/2008 8/31/ /19/2008; 10: a Quiescent dome growth is indicated by the year in which it occurred while phases accompanied by explosions have been numbered. NA = not applicable, ND = no data, this explosion occurred during a period when no data were received due to problems with the receiving station. Cumulative volume estimates based on a time averaged extrusion rate of 0.6 m 3 s 1 for however time averaged extrusion rates calculated here fall within ranges observed at other domes [Nakada et al., 1999; Calder et al., 2002; Harris et al., 2003] Precursory Thermal Patterns [32] All of the explosions that occurred from mid 1993 until mid 2008 for which data were available showed thermal anomalies in the preceding thirty days. Increased numbers of thermally anomalous pixels in the 30 days prior to explosion were observed in years for which more images are available (Table 5). Although the mere presence of a thermal anomaly would often be taken as a precursor to more violent volcanic activity this is not the case at Bezymianny, which shows an almost continuous thermal presence at temperatures 5 10 C above the background (Figure 5b). Therefore we here use the word precursor to indicate a specific pattern within the thermal data that precedes an explosion. [33] Three different precursory patterns prior to explosive eruptions can be distinguished: Type (1) No observed changes in the thermal activity occurred twice (Figure 9a); thermal anomalies are present but their values hover around the mode of the data set and they do not show a significant trend. [34] Type (2) Large increases in pixel integrated radiant temperatures occurred on three occasions (Figure 9b); this type of precursory behavior is characterized by pixels that reach the saturation temperatures of the sensor and remain at or close to these levels prior to explosion. In addition, the thermal anomalies show an increase in temperature above background starting days before the explosion. [35] Type (3) precursory activity patterns are characterized by gradual minor changes in the precursory pixelintegrated radiant temperatures (e.g., Figure 9c); this is the commonest category with fourteen eruptions. These are characterized by a marked increase in temperature 1 5 days prior to explosion, but sensor saturation levels are not reached apart from in the case of the November 30, 2005 explosion, where a single saturated pixel was recorded less than two hours prior to the recorded explosion time Repose Times [36] Repose times between explosions were examined starting with the March 25, 1977 explosion when Bezymianny started to display more exogenous behavior. In total 35 explosions have been reported during this time by AVO and KVERT (Table 1). Although these provide a good indication of Bezymianny s activity, it is a possibility that smaller explosions may have gone unnoticed at this remote volcano before the advent of regular satellite monitoring. Repose times are 327 days on average, with a minimum of 131 days and a maximum of 959 days between explosions (Table 1). Generally they range from approximately days, with the period from 1987 to 1997 being the exception with significantly longer repose times of days. Repose times since 1993 have varied by a factor of approximately 3 but they have tended to decrease from 1993 to [37] Applying rank order statistics to this data set, where t is the time between explosions in days and ranking these so that t 1 t 2. t N, and then plotting this using a log log scale (Figure 10) shows a reasonable fit (R 2 = 0.947) to a power law in the form: y ¼ ax b where, for the line shown in Figure 10, the coefficients a and b are and respectively. Following Pyle ð5þ 12 of 20

13 Figure 7. Defining a phase of dome growth. The gray band defines the phase surrounding the December 23, 2006 explosion. (a) Based on the pixel integrated radiant temperature data, in this case, 3 consecutive points above the mode signal the beginning (08/10/06) while three consecutive points below the mode signal the end (21/01/07) of the phase. (b) Defining a phase can also be done by examining breaks in slope of the cumulative extruded volume plot. [1998] this can then be used to determine what the next largest interval between explosions will be using t next ¼ ð2 þ 1Þ 1 t1 ð6þ ð þ 1Þ where pffiffiffiffi m =1/b and the relative error in m is approximated by 1/ N where N is the number of data points. For small sample sizes 1/b deviates from the actual value of m [Sornette et al., 1996] this can be corrected for using: ¼ 1 2 b 1 þ 1 þ 8 1 ð7þ 2 N making m = ± 0.28 and therefore the next largest interval between explosions is 1288 ± 170 days, or approximately three and half years. As the line does not provide a good fit to the curved end of the data, this number is a conservative estimate. This number could be taken, as in the work of Pyle [1998], as an upper limit, which, when exceeded without an explosion happening could signal the end of the dome building period that commenced in [38] Applying rank order statistics to the timing between consecutive phases (Table 5 and Figure 10b) results in a power law equation where a = and b = (R 2 = 0.85). Based on this equation the next largest interval between successive phases is 371 ± 9 days (m = 1.9 ± 0.22). 13 of 20

14 Figure 8. Radiated energy of the eruptive phases versus time and repose periods between successive phases. (a) Total radiated energy per phase. There is a decrease in the radiated energy per phase from mid 1993 to mid (b) Average radiated energy per phase per day, this plot also shows a decrease in the radiated energy. (c) Repose time versus amount of energy radiated during the subsequent phase of dome growth, a positive correlation can be observed. 14 of 20