Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography ISSN 1598-4850(Print) Vol. 34, No. 4, 357-372, 2016 ISSN 2288-260X(Online) http://dx.doi.org/10.7848/ksgpc.2016.34.4.357 Original article

Sustainable Surface Deformation Related with 2006 Augustine Volcano Eruption in Measured Using GPS and InSAR Techniques

Lee, Seulki1)ㆍKim, Sukyung2)ㆍLee, Changwook3)

Abstract Augustine volcano, located along the Aleutian Arc, is one of the most active volcanoes in Alaska and nearby islands, with seven eruptions occurring between 1812 and 2006. This study monitored the surface displacement before and after the most recent 2006 eruption. For analysis, we conducted a time-series analysis on data observed at the permanent GPS(Global Positioning System) observation stations in Augustine Island between 2005 and 2011. According to the surface displacement analysis results based on GPS data, the movement of the surface inflation at the average speed of 2.3 cm/year three months prior to the eruption has been clearly observed, with the post-eruption surface deflation at the speed of 1.6 cm/year. To compare surface displacements measurement by GPS observation, ENVISAT(Environmental satellite) radar satellite data were collected between 2003 and 2010 and processed the SBAS(Small Baseline Subset) method, one of the time-series analysis techniques using multiple InSAR(Interferometric Synthetic Aperture Radar) data sets. This result represents 0.97 correlation value between GPS and InSAR time-series surface displacements. This research has been completed precise surface deformation using GPS and time-series InSAR methods for a detection of precursor symptom on Augustine volcano.

Keywords : Surface Deformation, GPS, InSAR, SBAS, Volcano Monitoring

1. Introduction important to minimize the damage of a volcanic eruption by broadcasting warnings as early as possible, it is not easy to Of the life-threatening natural disasters such as do so with current technology (Pieri and Abrams, 2005). The , tsunamis, and volcanic activity, those created precursor signs of volcanic activity that have been known to by orogenic movements are harder to predict and cause more us include increases in volcanic frequency and property damage and human casualties. The natural disasters changes in emissions, crust shape, geothermal originating from volcanic activity can devastate cities, heat level, and hydrological conditions (Xu et al., 2012). Of farmlands, and vegetation with flowing and pyroclastic the precursor signs, ground deformation caused by magma flow, with follow-on effects like earthquakes, landslides, chamber movements observed hours or days before the and tsunamis. In addition, the spread of volcanic ash may eruption is one of the most common eruptive activities. It disrupt air travel through a wide area for an extensive period is possible to estimate the time of eruption based on change while causing respiratory problems for residents near the of the shape and form of ground deformation. In general, eruption site (Waythomas and Waitt, 1998). Although it is shallow magma chambers close to the surface, commonly

Received 2016. 07. 21, Revised 2016. 08. 18, Accepted 2016. 08. 26 1) Member, Division of Science Education, Kangwon National University (E-mail : [email protected]) 2) Department of Geoinformation Engineering, Sejong University (E-mail : [email protected]) 3) Corresponding Author, Member, Division of Science Education, Kangwon National University (E-mail : [email protected])

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

357 Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography, Vol. 34, No. 4, 357-372, 2016 around several km under the surface, cause bloating of the information unlike InSAR that provides the LOS(Line-Of- surface due to the mounting pressure within the chamber Sight) directions of the displacement only (Larson et al., (Yazdanparast and Vosooghi, 2013). 2010). As each observational technique has pros and cons, it To monitor and predict volcanic activities, a variety of is advisable to use multiple techniques in a complementary sensors, including seismometer, clinometer, gravimeter, way to ensure a more precise outcome. As the two techniques CGPS(Continuous Global Positioning System), and satellite of InSAR and GPS can offset problems of one approach by image have been used. In particular, high-tech geodetic the other, their simultaneous use improves the precision of survey tools such as the GPS and InSAR have been used surface displacement monitoring (Lee et al., 2010). since the 1980s and have contributed to volcanic monitoring In this study, ENVISAT data and GPS data, collected and research in meaningful ways (Lu, 2005). The recently from June 2003 to August 2010, were simultaneously used developed InSAR technique, which uses two or more to measure the movements before and after the eruption SAR(Synthetic Aperture Radar) images to generate maps of of Augustine volcano in Alaska, which had a record of surface deformation or digital elevation, using differences in discharge in 2006 (Berardino et al., 2002). This research the phase of the waves returning to the satellite, can monitor alert us to important information of symptom for volcanic displacements over several tens of square kilometers with eruption through quantitative analysis from time-series several millimeter- or centimeter-precision, which is ideal surface deformation using GPS and InSAR techniques. for observing earthquake, volcano, glacier, and landslide movements (Jo et al., 2015; Lee et al., 2013; Lee, 2014). The 2. Study Area and Dataset remote-sensing technology like InSAR is widely used in volcano research because of its ability to make high-spatial- 2.1 Study area resolution displacement observations in mountains and other Augustine Island, the main locus of this study, is located difficult to access areas (Jónsson et al., 2002; Pritchard and in the southwestern , about 290 km southwest Simons, 2004). of Anchorage. The island has a land area of 90 km2, with Although GPS was originally developed for military the highest peak reaching 1,260 m (Fig. 1(a)) (Waythomas purposes, it is now used more widely for civilian applications and Waitt, 1998). The Aleutian Islands, of which Augustine including land surveying, navigation, communication, and volcano is a part, belong to a subduction zone at which the weather monitoring. With GPS devices, displacements of Pacific and North American plates meet, with as many as 57 the earth’s crust and faults can be measured with millimeter- volcanoes nearby (Michel et al., 2009). Of these, Augustine precision, which is useful for geophysics research. However, volcano is one of the most active (Beget and Kowalik, 2006; GPS devices that must be installed on the earth’s surface Miller et al., 1998). Since the first recorded eruption in 1812, for observing surface displacements only from the station the volcano has flared up in 1883, 1935, 1964-65, 1976, and where the antenna is set up. Alternatively, InSAR data could 1986. After a 20-year hiatus, it again exploded in January provide the short spatial scale surface deformation such 2006 (Power et al., 2006). as every coherence points within SAR images practically Of the seven eruptions, the most violent 1883 episode lacking in GPS data (Wei et al., 2010). In particular, the use caused a tsunami of 7.5-9.0 m as debris avalanche on the of GPS devices in volcanoes is limited since the antenna northern slope of the island poured onto the sea and caused is easily damaged by eruptions and weather conditions repercussions as far as the in English Bay 80 may affect data collection (Cervelli et al., 2006; Lisowski km away (Miller et al., 1998; Waythomas and Waitt, 1998). et al., 2008). When comparing it to the InSAR technique’s As for the 1976 eruption, which was closest in pattern to the relatively longer data collection cycle, the GPS method is 2006 event, there had been precursor signs nine months before superior in that it can measure surface displacements at the the eruption with a notable increase in earthquake activity, time of eruption and collect three-dimensional locational including 13 volcanic eruptions for three days since January

358 Sustainable Surface Deformation Related with 2006 Augustine Volcano Eruption in Alaska Measured Using GPS and InSAR Techniques

22. Following a 12-day lull, volcanic activity increased slightly between February 6 and 15, which created lithic-rich pyroclastic flows and lava domes in the northern slope. From April 13 to 18, there were final eruptions that accompanied expansion and block-and-ash flows (Coombset al., 2010; Larsen et al., 2010). The 1986 episode showed similar patterns to the 2006 event and the one in 1976, in that large pyroclastic flows gushed out in the beginning of the eruption and moved through the northern slope reaching a point 5 km from the northern coast (Miller et al., 1998). The precursor signs for the eruption in 2006 first appeared in May 2005, with a steady increase in small tremors in and around the crater (Cervelli et al., 2006). Fig. 1(b) represents earthquake epicenters with blue circles and west-east (A-A’) and north-south (B-B’) cross section showing earthquake hypocenters associated with 2006 eruption at Augustine volcano, respectively. Red and blue cross represents depth of hypocenters before and after the 2006 eruption. Aster satellite image displays surface temperature after the 2006 eruption on pyroclastic flow deposit area at northern part of Augustine volcano. Aster satellite data need to be converted to radiance from DNs(Digital Numbers) for thermal bands. Also, Brightness temperature was converted using spectral radiance (Ghulam, 2009). Moreover, Aster satellite image can display surface Fig. 1. Phenomena associated with 2006 eruption on temperature after the 2006 eruption on pyroclastic flow Augustine volcano (a) GPS permanent stations were deposit area at northern part of this island (Fig. 1(b)). After installed on Augustine Island. And AC59, used for reference station, are located near Augustine (b) Seismic a brief lull, the seismic rate began rising rapidly from hypocenters and surface temperature. Blue circles November 17, 2005, with the highest incidence and most represent earthquake epicenters corresponding to seismic severe tremors occurring in early January 2006, when the magnitude (Power et al., 2010) major eruption occurred (Figs. 2(a) and (b)) based on Alaska volcano observatory results. the northern part of the volcano was measured at a relatively On December 2, the seismometer began detecting signs of high 80 °C. This observation matches the preceding studies phreatic explosions, followed by the discovery of a volcanic that the 14 explosions during the 2006 event caused the lava ash dusting mixed with weathered and glassy particles from to flow through the northern slope (Wessels et al., 2010). The December 12 during the active explosions of steam and gas at figure on the upper-left side in Fig. 1(b) is a diagram depicting the peak of the volcano (Mattia et al., 2008). From January 11 the depth of small tremors that occurred before and after the in the following year, the explosion began in earnest, with 14 2006 eruption, with red points indicating pre-eruption and eruptions until January 28 (Michelle et al., 2010). The surface blue points showing post-eruption. The figure indicates that temperature data shown in Fig. 1(b) are collected from images the pre-eruption tremors occurred near the surface while taken from the Aster satellite about ten days after the major post-eruption tremors occurred much deeper in the crater. eruption. As shown in Fig. 1(b), the surface temperature in As the volcanic activity went into a lull from early February

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Fig. 2. (a) Histogram showing the number of earthquakes per day from January 1, 2005 to December 31, 2011 (b) Magnitude of earthquakes from January 1, 2005 to December 31, 2011 (Power et al., 2010)

2006, the frequency of tremors also as a noticeable decline. is located on the North American plate about 24.5 km away During the period between late 2007 and early 2008, there from Augustine Island (Fig. 1(a)). were times when the color-coded level of concern rose to Other GPS observation points near the volcano were “yellow” as the volcanic activity resumed briefly. But it soon installed and operated by the Plate Boundary Observatory returned to a stable state and is continuing to be so as of 2016. for the purpose of volcanic eruption monitoring. Beginning with the installation of the AUGL (renamed AV21 from 2006) 2.2 GPS data observation station in September 2000, the GPS stations In this study, surface displacements were monitored and were installed at 12 points until September 2006. The six analyzed before and after the 2006 event. To that end, GPS stations (AV11, AV16, AV17, AV18, AV19, and AV20) out of data were collected between January 2005 and December the total were installed after the 2006 eruption, which renders 2011 from 13 permanent GPS observation points installed in it impossible to find precursor signs before the explosion Augustine volcano by the PBO(Plate Boundary Observatory). in 2006. As for the GPS stations AV03, AV04, and AV05 All data thus collected are 30-second-interval 24-hour adjacent to the crater, all of them were damaged or burned observations in the RINEX(Receiver Independent Exchange during the 2006 episode. They have stopped collecting data Format) format provided by the FTP server of UNAVCO. ever since except the AV04 point which has been repaired. To all observation points, the AC59 point was added so Fig. 1(b) shows a diagram indicating the permanent GPS that it could be used as a base point when comparing data observation locations used for this study. since the point is located outside the island and therefore is To reduce errors in GPS observation data, we used assumed to be impervious station from the volcanic impact. data provided in file forms by the CODE(Center for Orbit When selecting a reference point, one stable observation Determination in Europe), one of the analysis centers for the point was chosen near the island that had been run for a long IGS(International GNSS Service), and the CDDIS(Crustal time, understanding that the greater its distance from the Dynamics Data Information System) for GPS satellite precise volcano, the higher the margin of error. The AC59 station ephemeris, satellite clock data, the earth’s rotational axis that had been used since September 2004 was selected. It movement, and the earth’s ionosphere model. For the ocean

360 Sustainable Surface Deformation Related with 2006 Augustine Volcano Eruption in Alaska Measured Using GPS and InSAR Techniques

Table 1. ENVISAT data and baseline information

No. SAR data Image Swath Date Perpendicular Baseline (m) 1 ENVISAT IS2 2003-06-18 0 2 ENVISAT IS2 2003-07-23 -85.2763 3 ENVISAT IS2 2004-07-07 162.8891 4 ENVISAT IS2 2004-09-15 1097.2572 5 ENVISAT IS2 2004-10-20 610.4680 6 ENVISAT IS2 2005-06-22 674.8073 7 ENVISAT IS2 2005-07-27 748.2596 8 ENVISAT IS2 2005-08-31 1064.3222 9 ENVISAT IS2 2005-10-05 -113.1489 10 ENVISAT IS2 2006-07-12 904.9796 11 ENVISAT IS2 2006-08-16 1024.1759 12 ENVISAT IS2 2006-09-20 -705.3840 13 ENVISAT IS2 2006-10-25 -37.4735 14 ENVISAT IS2 2007-06-27 289.7287 15 ENVISAT IS2 2007-09-05 755.1095 16 ENVISAT IS2 2007-10-10 59.3813 17 ENVISAT IS2 2008-07-16 400.3948 18 ENVISAT IS2 2008-08-20 650.2457 19 ENVISAT IS2 2009-08-05 257.1193 20 ENVISAT IS2 2009-09-09 879.9638 21 ENVISAT IS2 2010-07-21 0.5192 22 ENVISAT IS2 2010-08-25 271.5063 tidal model, the FES2004 file was provided by OSO(Onsala the other observation point with millimeter-precision based Space Observatory). on an observation point whose precise coordinates are already well known. In contrast, the absolute positioning 2.3 InSAR data method determines the coordinates independently by using To measure the degree of surface displacement at signals received in the observation point from GPS satellites Augustine volcano through InSAR, ENVISAT data were that can measure coordinates with centimeter-precision. To acquired, the radar satellite operated by the ESA(European secure surface displacement data with millimeter-precision, Space Agency). The data were collected include 22 images in this study adopted the relative method using constraints. 2229 tracks dated from June 2003 to August 2010 (Table 1). For relative measurement, the earlier-mentioned AC59 observation point was used, with the coordinates for the 3. Methodology observation point based on IGS08. For precision baseline interpretation, Bernese GPS 3.1 GPS processing Software V5.0 developed by the Astronomical Institute of In general, GPS data processing can be done either the University of Bern, Switzerland, has been used. Bernese through the relative or absolute positioning method. The is a software program widely used by scientists across the relative positioning method determines the coordinates of world for GPS network adjustment calculation, the earth’s

361 Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography, Vol. 34, No. 4, 357-372, 2016 ionosphere and troposphere modeling, and satellite orbit al., 2006). The snow and ice covering the devices in winter estimation. It is capable of determining precise integer lead to delays in GPS signal transmission, which resulted ambiguity resolution even for extremely long baselines as far in errors as large as 15 cm vertically from the observation as 2,000 km (Dach et al., 2007). point AV04. To precisely monitor surface displacement Data processing using Bernese was performed in the order data, which is the main purpose of this study, therefore, we specified in Fig. 3 and includes the following processes. Firstly, avoided using abrupt coordinate changes in the winter season formation of an optimal baseline through the OBS-MAX by separating data with wide margins of error. In this case, method that selects a set of baselines including maximum the separation process was applied to data after the eruption common observation data from all possible combinations. with an assumption that the data observed just before the Secondly, estimation of relative tropospheric errors by fixing eruption would not be affected by the error as there would be a point among observations to Niell model values to keep the no problem with snow and ice after the temperature rose. All stability of estimated values from deteriorating abruptly due data values with large margins of error have been discarded to high correlations arising from the relatively short baseline from the analysis. distance around 10 km, when estimating tropospheric errors. Thirdly, determination of integer ambiguity numbers by way 3.2 InSAR processing of the Quasi Ionosphere-free method that makes use of the L1 To avoid the coherence of interferogram due to snow and and L2 carrier signals. ice, SAR data were collected between mid-June and mid- But the observation points AV04 (altitude of 915.95 m) and October. We calculated the perpendicular baseline distance AV05 (altitude of 1,036.61 m) installed near the peak of the of all images as a way to create interferograms with a high volcano experienced difficulties due to their antennas and coherence and ultimately created 25 interferograms, with mounts becoming covered with snow and ice (Cervelli et the vertical baseline less than 400 m. All interferograms were created with the complex multi-look operation, with two looks in range and ten looks in azimuth directions. As a result, each pixel has an area of around 40 m by 40 m. The SRTM(Shuttle Radar Topography Mission) DEM(Digital Elevation Model) was used with about 30m resolution per second to remove the topographic phase contribution in the original interferogram. Of the differential SAR interferometry images created from ENVISAT, only 16 interferograms selected as those with high positional errors because of a low coherence were excluded. As an initial reference point for time-series analysis, a point near the AV02 observation point was selected. Finally, the 16 interferograms were selected for more effective investigation of surface displacement patterns on the area were applied with the SBAS time-series surface displacement observation algorithm. With this, compensation for errors such as the positional errors due to the atmosphere, DEM errors and errors due to satellite orbits was possible. In general, in cases where abrupt surface displacements occur such as with volcanic eruptions, it is difficult to create Fig. 3. Diagram of data processing using Bernese software from GPS data differential interferometry images because the coherence of

362 Sustainable Surface Deformation Related with 2006 Augustine Volcano Eruption in Alaska Measured Using GPS and InSAR Techniques interferograms before and after the surface displacement is in surface displacement to the southwest, the opposite lowered. Even when interferograms can be created, it is easy direction from the crater. To examine this change in more for the irregularly occurring displacements in wide-ranging detail, the coordinate change values for each direction (north, areas like a volcanic eruption to be excluded from analysis. east, and up) were amplified at the moment of the eruption, For this reason, this study conducted a time-series analysis by which is indicated below the figure of AV01, AV02, and dividing it into pre-eruption and post-eruption, with separate AV04. From AV05, which is closest from the crater, the displacement distribution diagrams for the entire observation displacement rate from the northeast was 15 cm. The amount period including both pre- and post-eruption. of bloating was 5 cm, which was 5-10 cm higher horizontally and 5 cm lower vertically when compared to the AV04 4. Results observation point. This implies that the surface displacement amounts 4.1 GPS data analysis between east and west differ, with the horizontal element For the 2,556 days from January 1, 2005 to December 31, higher in the east and the vertical element superior in the 2011, data have been estimated by baseline interpretation, west. In contrast, the observation value at AV03 located and a three-dimensional time-series data set has been in the north has a higher displacement value to the north created with the 12 noon observation time for each day at horizontally. Comparison of the observation values for the 12 permanent observation points in and around Augustine observation points from AV11 to AV20 that were installed volcano as the observation standard. The daily three- in mid-2006 after the eruption shows that the displacement dimensional geocentric coordinates thus created from the values observed in AV18 and AV19 had values in opposite baseline interpretation were then converted into local plane directions before the eruption. This may be because of the coordinates, which are indicated in Fig. 4 with north and east contraction of magma chambers as well as the compaction of and up divided into two parts. AV01 in Fig. 4 was the closest the pyroclastic flow deposits. observation point among those that collected data before Fig. 5 shows the time-series displacement values collected and after the eruption, which shows surface displacements at AV04 and AV05, the observation points located closest to relatively clearly. the crater of Augustine volcano. The two observation points As can be understood from the figure, the change in were damaged on January 17 and 13, 2006, respectively, coordinates is seen visibly before the main eruption, which when the volcano erupted, which rendered it impossible to seems to be precursor signs. In particular, four months measure displacements. As they are located very close to before the eruption a phenomenon of surface elevation of the crater, however, it is possible to detect precursor signs about 3 cm with an accompanying coordinate change of 3 more clearly than others. From the time-series graph, we cm to the south was observed. When Augustine volcano can see that the gray line changes markedly from November began erupting on January 11, 2006, the time-series data at 11, 2005. A day before the observation day, the time-series the AV01 observation point showed an abrupt subsidence. coordinates that exhibited a very slow linear pattern abruptly Beginning in the second half of 2006, however, subsidence changed into very dynamic coordinates. The time series at at the rate of 0.14 cm/year was observed without a clear AV05 exhibited a slight offset on November 17, 2005, which horizontal coordinate change, which implies that the volcanic can be interpreted as a physical phenomenon when the crust activity entered a stable phase. broke off near the observation point ahead of the eruption In the case of the AV02 observation point, the surface (Fig. 5). Indeed, the earthquake activity histogram in Fig. 2 displacement was almost identical to that of AV01, with a shows that the earthquake frequency increased significantly smaller amount of displacement as it was farther from the from mid-November. After this, the AV04 observation point crater. For AV04, the bloating of the areas around the crater showed a horizontal coordinate change to the southwest with immediately before the eruption created a significant change an accompanying surface lifting of up to 15 cm while AV05

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Fig. 4. Time series of Augustine’s permanent stations. Light gray dots are outlier and the pink box is shown eruption period. Black line means trend of volcano behavior after eruption

364 Sustainable Surface Deformation Related with 2006 Augustine Volcano Eruption in Alaska Measured Using GPS and InSAR Techniques showed a horizontal coordinate change to the northeast with indicator of the degree of volcanic activity. As a result, we 10 cm of surface lifting (Fig. 5). can see that each baseline changes in a similar pattern. The Fig. 6 shows the change in baseline length as time passed in whole research period can be divided into five sub-periods, the shape of a triangle (AV01-AV02, AV02-AV-03, and AV03- with separate analysis for each period. AV01) across north and south as well as to the west with The end results are listed in Table 2. Stage 1 is a time the volcano’s crater in the center. Beginning in September period in which the baseline length has not been changed by 2005, the change in baseline length is clearly visible. When volcanic activity. A time-series analysis on the same period this is compared with the cumulative graph (Fig. 2(a)) for also reveals that there were few signs of volcanic activity earthquakes that occurred in Augustine volcano since 2005 that preceded the 2006 eruption. At Stage 2, in which especially end of this year, the time period for the increase steady lengthening of the baseline occurred for five months, in earthquake activity and the change in baseline length in surface lifting accompanied by a radial-shaped expansion the two observation points corresponds quite closely. The horizontally with the crater at the center is noted. The speed increase in baseline length may be due to the expansion of the of lifting is estimated at 9.7 cm/year, with that for AV04 volcano as magma rose up just below the surface. The change and AV05 that are close to the crater showing rates of 8 and in baseline length that cut across the crater can be used as an 11 cm, respectively. The reason the displacement rates are higher in observation points closer to the crater may have to do with the fact that the observation points are more subject to magma sources. For the relatively short subsequent two months at Stage 3, a full-fledged eruption occurred. Unlike the preceding stage, the third stage shows polar opposite results both vertically and horizontally, which includes an abrupt shortening of the baseline length and a rapid sink rate of 9.2 cm/year, as well as horizontal displacements around the crater (Table 2). The surface displacement that includes the subsidence and horizontal movement was caused by the contraction of the volcano after the eruption. Stage 4 shows a pattern of

Fig. 6. Time series of distance change between stations AV01-AV02, AV02-AV03, and AV03-AV01. Trend of Fig. 5. Time series analysis of surface deformation distance are changed after August 2005. And the changing at AV04 and AV05 stations points agree with inflation point of earthquakes

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Table 2. Step of surface deformation of the 2006 Augustine volcano eruption

Period Step Deformation type Amount of Mean of Start End deformation up Velocity Step1 January 1, 2005 August 14, 2005 No deformation -0.1 cm -0.2 cm/yr Step2 August 15, 2005 January 10, 2006 Inflation +3.9 cm +9.7 cm/yr Step3 January 11, 2006 March 16, 2006 Rapid deflation -1.6 cm -9.2 cm/yr Step4 March 17, 2006 October 15, 2006 Slightly inflation +0.2 cm +0.3 cm/yr Step5 October 16, 2006 December 31, 2011 Very slow deflation -1.1 cm -0.2 cm/yr increasing baseline length, with slight surface lifting and Therefore, the surface displacement patterns observed in radial-shaped displacement around the crater. Compared to Stage 5 do not seem to suggest additional eruptions after the the second and third stages, the speed of surface displacement 2006 episode. slowed significantly. Finally, at Stage 5 between October 16, 2006 and the end of 2011, a very slow sink rate was 4.2 InSAR data analysis witnessed, with a few instances of large subsidence observed Fig. 7 shows the average speed of surface displacements at some points. In particular, points AV04, AV18, and AV19 occurring between 2003 and 2006 just prior to the eruption exhibited sink rates of 2.1, 4.0, and 3.3 cm, respectively (Fig. (Fig. 7(a)) and that between 2006 right after the eruption and 4). According to a study by Wessels et al. (2010) that made 2010 (Fig. 7(b)). According to the measurement by SBAS use of thermal infrared ray aerial photos and Aster satellite method prior to the eruption in 2006, the northern slope thermal band images on the 2006 eruption, the northern exhibited a fast sink rate of 3.5 cm/year while all other slopes slope where the AV18 observation point was located was the showed a slow uplift rate of 0.5-1.0 cm/year. The area with a locale for most pyroclastic flows, just like in 1986 (Miller et high sink rate on the northern slope was due to the hardening al., 1998). According to “Deposits from the 2006 eruption of pyroclastic flows just like the surface subsidence observed of Augustine volcano” produced by Coombs and Michelle in AV03 and AV18 GPS observation points (Fig. 1(a)) (Larsen et al., 2010), AV19 was also located in an area with differently with a radial subsidence pattern from crater sediments of pyroclastic flows. according to magma chamber’s shrink. The northern slope Given these facts, the subsidence of the area is likely to be showed subsidence in the area with sediments of pyroclastic due to the compacting of the sedimentary pyroclastic flows flows during the eruption in 1986, with continuous subsidence rather than the outcome of magma-caused displacements. observed between 1992 and 2005 for a total of 40 cm (Lee et

Fig. 7. Surface deformation averaging maps before (a) and after 2006 (b) eruption

366 Sustainable Surface Deformation Related with 2006 Augustine Volcano Eruption in Alaska Measured Using GPS and InSAR Techniques

Fig. 8. Time series surface deformation maps from June 18 2003 to August 25 2010 associated with 2006 eruption , , , , 𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙 𝑁𝑁0 𝐸𝐸0 𝑈𝑈0 , , al., 2010). The uplifting observed in all other slopes may, be , angles, respectively. 𝑙𝑙𝑙𝑙𝑙𝑙 0 0 0 𝐿𝐿 𝑇𝑇 𝑉𝑉 𝑁𝑁 𝐸𝐸 𝑈𝑈 𝑙𝑙𝑙𝑙𝑙𝑙 𝜃𝜃 0 𝜃𝜃 0 0 interpreted as a sign of an impending eruption. 𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙 𝑁𝑁0 𝐸𝐸0 𝑈𝑈0 𝑉𝑉 𝑁𝑁 𝐸𝐸 𝑈𝑈 𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇 InSAR measures the degree of displacement of 𝐿𝐿LOS𝑇𝑇 = [ ] = [sin( ) sin( ) sin( ) cos( ) cos ( )] (1) 𝜃𝜃𝜃𝜃𝐿𝐿 𝜃𝜃𝜃𝜃𝑇𝑇 = [ ] = [sin( ) sin( ) sin( ) cos( ) cos ( )] directions from the satellite to the surface from 2003 to 𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙 𝑁𝑁0 𝐸𝐸0 𝑈𝑈0 𝑙𝑙𝑙𝑙𝑙𝑙 𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇 𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇 𝜃𝜃𝐿𝐿 𝑙𝑙𝑙𝑙𝑙𝑙 ==[[ ]] ==[sin[sin(( ))sinsin(( )) sin sin(( ))coscos(( )) cos cos ( ( ))]] 2010 in this study without dividing it into pre-eruption and As the inner product of the unit 𝑙𝑙𝑙𝑙𝑙𝑙vector is0 equivalent0 0 𝑙𝑙𝑙𝑙𝑙𝑙 to the 𝐿𝐿 𝑇𝑇 𝐿𝐿 𝑇𝑇 𝐿𝐿 𝑙𝑙𝑙𝑙𝑙𝑙 𝑙𝑙𝑙𝑙𝑙𝑙 0 0 0 𝑙𝑙𝑙𝑙𝑙𝑙 𝐿𝐿 𝑇𝑇 𝐿𝐿𝑉𝑉 𝑇𝑇 𝑁𝑁 𝐸𝐸 𝑈𝑈𝐿𝐿 𝑙𝑙𝑙𝑙𝑙𝑙 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙 𝑁𝑁0 𝐸𝐸0 𝑈𝑈0 𝑙𝑙𝑙𝑙𝑙𝑙 𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇 𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇 𝜃𝜃𝐿𝐿 𝑙𝑙𝑙𝑙𝑙𝑙 post-eruption for measuring time-series surface deformation 𝑉𝑉 size𝑁𝑁 𝐸𝐸 of𝑈𝑈 the projected𝜃𝜃 vector,𝜃𝜃 we𝜃𝜃 can project𝜃𝜃 the𝜃𝜃 3D surface (Fig. 8). This time-series processing is appropriate to linear 𝑔𝑔𝑔𝑔𝑙𝑙 displacement vector observed by GPS to the LOS 𝑉𝑉 surface deformation measurements at high coherence 𝑔𝑔𝑔𝑔areas.𝑙𝑙 direction using Eq. (2) as𝑔𝑔𝑔𝑔 𝑙𝑙follows: 𝑔𝑔𝑔𝑔𝑉𝑉 𝑙𝑙 𝑉𝑉 𝑉𝑉 LOS = = [ ] [ ]

4.3 Comparison of GPS and InSAR LOSLOS== 𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙==[∙[𝑉𝑉𝑔𝑔𝑔𝑔 𝑙𝑙 𝑁𝑁]]0 𝐸𝐸0[𝑈𝑈[0 𝑙𝑙𝑙𝑙𝑙𝑙 ∙ ∆𝑁𝑁]]∆𝐸𝐸LOS∆𝑈𝑈 𝑔𝑔𝑔𝑔=𝑙𝑙 = [ ] [ ] = + + In contrast, GPS observes three-dimensional displacements 𝑙𝑙𝑙𝑙𝑙𝑙 𝑔𝑔𝑔𝑔𝑙𝑙 0 0 0 𝑙𝑙𝑙𝑙𝑙𝑙 𝑔𝑔𝑔𝑔𝑙𝑙 𝑙𝑙𝑙𝑙𝑙𝑙 𝑔𝑔𝑔𝑔(2)𝑙𝑙 0 0 0 𝑙𝑙𝑙𝑙𝑙𝑙 𝑔𝑔𝑔𝑔𝑙𝑙 𝑉𝑉𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙 ∙ ∙𝑉𝑉𝑔𝑔𝑔𝑔𝑉𝑉 𝑙𝑙 𝑁𝑁𝑁𝑁0 𝐸𝐸𝐸𝐸0 𝑈𝑈𝑈𝑈0 𝑙𝑙𝑙𝑙𝑙𝑙 ∙ ∙ ∆∆𝑁𝑁𝑁𝑁∆∆𝐸𝐸𝐸𝐸∆∆𝑈𝑈𝑈𝑈𝑔𝑔𝑔𝑔𝑙𝑙 𝑉𝑉 ∙ 𝑉𝑉 𝑁𝑁 𝐸𝐸 𝑈𝑈 ∙ ∆𝑁𝑁 ∆𝐸𝐸 ∆𝑈𝑈 = + + from the antenna location from 2005 to 2011 (Fig. 4), which = +𝑁𝑁0 ∙ ∆𝑁𝑁 +𝐸𝐸0 ∙ ∆𝐸𝐸 𝑈𝑈0 ∙ ∆𝑈𝑈 = + + 0 0 0 makes it impossible to compare the observation values of 𝑁𝑁𝑁𝑁0 ∙ ∙∆∆𝑁𝑁𝑁𝑁 𝐸𝐸𝐸𝐸0 ∙ ∙∆∆𝐸𝐸𝐸𝐸 𝑈𝑈𝑈𝑈0 ∙ ∙∆∆𝑈𝑈𝑈𝑈 0 0 0 𝑁𝑁 ∙ ∆𝑁𝑁 𝐸𝐸 ∙ ∆𝐸𝐸 𝑈𝑈 ∙ ∆𝑈𝑈 SAR and GPS. To compare the displacement values of the In addition, the deformation derived by interferometric two observation methods, it is necessary to recalculate techniques is relative to a reference area in the image, not the 3D (north, east, and up) displacements observed by an absolute value. Hence, an additional process that derived GPS by projecting them onto the LOS directions of the the value from GPS time-series analysis was added to InSAR satellite. Suppose that the unit vector of LOS direction as results pre- eruption and post-eruption, respectively.

, each element , of the unit vector can be expressed as in Fig. 9 compares the GPS-based surface displacement

𝑙𝑙𝑙𝑙𝑙𝑙 , 0, 0 0, , projected to the LOS direction with SBAS time-series result. 𝑉𝑉Eq. (1) below,𝑁𝑁 where𝐸𝐸 𝑈𝑈 and are initial value of 3D , , , , displacements𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 by projecting0 0 0 them0 0onto0 the LOS directions of Especially, we cannot match directly GPS and InSAR time- 𝐿𝐿 𝑇𝑇𝑉𝑉 𝑉𝑉 𝑁𝑁𝑁𝑁𝐸𝐸 𝐸𝐸 𝑈𝑈 𝑈𝑈 𝜃𝜃 𝜃𝜃 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 0 0 0 0 0 0 the satellite,𝑉𝑉 𝑉𝑉 and denote𝑁𝑁𝑁𝑁𝐸𝐸 𝐸𝐸 the𝑈𝑈 satellite𝑈𝑈 ’s sight and track series results in case of station AV 04 in Fig. 9 because of 𝜃𝜃𝐿𝐿𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇𝜃𝜃𝑇𝑇 𝐿𝐿=𝐿𝐿 [ 𝑇𝑇 𝑇𝑇 ] = [sin( ) sin( ) sin( ) cos( ) cos ( )] 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 ==[ [ ] ] ==[sin[sin( ( ) sin) sin( ( ) )sin sin( ( ) cos) cos( ( ) )cos cos ( ( )])] 𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙 𝑁𝑁0 𝐸𝐸0 𝑈𝑈0 𝑙𝑙𝑙𝑙𝑙𝑙 𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇 𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇 𝜃𝜃𝐿𝐿 𝑙𝑙𝑙𝑙𝑙𝑙 ==[ [ ] ] ==[sin[sin( ( ) sin) sin( ( ) )sin sin( ( ) cos) cos( ( ) ) cos cos ( ( )])] 𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙 𝑁𝑁0𝑁𝑁𝐸𝐸00𝐸𝐸𝑈𝑈00𝑈𝑈𝑙𝑙𝑙𝑙𝑙𝑙0 𝑙𝑙𝑙𝑙𝑙𝑙 𝜃𝜃𝐿𝐿𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇𝜃𝜃𝑇𝑇 𝜃𝜃𝐿𝐿𝜃𝜃𝐿𝐿 𝜃𝜃𝑇𝑇𝜃𝜃𝑇𝑇 𝜃𝜃𝐿𝐿𝜃𝜃𝐿𝐿𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 367 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 0 00 0 0 0𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝐿𝐿 𝐿𝐿 𝑇𝑇 𝑇𝑇 𝐿𝐿 𝐿𝐿 𝑇𝑇 𝑇𝑇 𝐿𝐿 𝐿𝐿𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑉𝑉 𝑉𝑉 𝑁𝑁𝑁𝑁𝐸𝐸 𝐸𝐸𝑈𝑈𝑈𝑈 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃 𝜃𝜃

𝑔𝑔𝑔𝑔𝑙𝑙 𝑉𝑉 𝑉𝑉𝑔𝑔𝑔𝑔𝑉𝑉𝑔𝑔𝑔𝑔𝑙𝑙 𝑙𝑙 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑙𝑙 𝑙𝑙 𝑉𝑉 𝑉𝑉 LOS = = [ ] [ ]

LOS𝑙𝑙𝑙𝑙𝑙𝑙LOS=𝑔𝑔𝑔𝑔= 𝑙𝑙 0 0==0[ 𝑙𝑙𝑙𝑙𝑙𝑙[ ] ] [ [ 𝑔𝑔𝑔𝑔 𝑙𝑙 ] ] 𝑉𝑉 LOS∙ 𝑉𝑉LOS== 𝑁𝑁 𝐸𝐸 𝑈𝑈 ==[ ∙[ ∆𝑁𝑁 ∆ 𝐸𝐸] ∆]𝑈𝑈 [ [ ] ] = + + 𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙𝑉𝑉𝑙𝑙𝑙𝑙𝑙𝑙∙ 𝑉𝑉∙𝑔𝑔𝑔𝑔𝑉𝑉𝑔𝑔𝑔𝑔𝑙𝑙 𝑙𝑙 𝑁𝑁0𝑁𝑁𝐸𝐸00𝐸𝐸𝑈𝑈00𝑈𝑈𝑙𝑙𝑙𝑙𝑙𝑙0 𝑙𝑙𝑙𝑙𝑙𝑙∙ ∆∙ 𝑁𝑁∆𝑁𝑁∆𝐸𝐸∆𝐸𝐸∆𝑈𝑈∆𝑈𝑈𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑙𝑙 𝑙𝑙 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑙𝑙 𝑙𝑙 0 00 0 0 0𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑙𝑙 𝑙𝑙 0 == 0 𝑉𝑉 𝑉𝑉+∙ 𝑉𝑉+∙ 𝑉𝑉0 𝑁𝑁+𝑁𝑁+𝐸𝐸 𝐸𝐸𝑈𝑈𝑈𝑈 ∙ ∙∆𝑁𝑁∆𝑁𝑁∆𝐸𝐸∆𝐸𝐸∆𝑈𝑈∆𝑈𝑈 𝑁𝑁 ∙ ∆𝑁𝑁 =𝐸𝐸= ∙ ∆𝐸𝐸 +𝑈𝑈+∙ ∆𝑈𝑈 ++

𝑁𝑁0𝑁𝑁∙0∆∙𝑁𝑁∆𝑁𝑁 𝐸𝐸0𝐸𝐸∙0∆∙𝐸𝐸∆𝐸𝐸 𝑈𝑈0𝑈𝑈∙0∆∙𝑈𝑈∆𝑈𝑈 0 0 0 0 0 0 𝑁𝑁𝑁𝑁∙ ∆∙𝑁𝑁∆𝑁𝑁 𝐸𝐸 𝐸𝐸∙ ∆∙𝐸𝐸∆𝐸𝐸 𝑈𝑈𝑈𝑈∙ ∆∙𝑈𝑈∆𝑈𝑈

Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography, Vol. 34, No. 4, 357-372, 2016

Fig. 9. Time series analysis of crustal deformation detected by GPS and InSAR on the LOS component

of displacement at Augustine volcano is within a reasonably reliable range.

5. Discussion

Fig. 4 shows a list of coordinate displacements from relative positioning through GPS data converted into local NEU (north, east, and up) coordinates. As can be seen from the time series, there were coordinate changes two months before the eruption in six observation points (AV01, AV02, AV03, AV04, AV05, and AUGL (renamed AV21 from 2006)) installed before the 2006 explosion. On November 11, 2005, Fig. 10. Correlation analysis between GPS and InSAR there was a sudden offset in the time series of AV04 and AV05, with a visible increase in seismic activity. Since the offset, low coherence interferograms by 2006 volcanic eruption. the speed at which coordinates changed increased rapidly at So we used time-series processing result from dividing it AV04 and AV05, with the detection of a weak change in the pre and post-eruption and reference point from GPS result deformation rate in observation points far from the magma at eruption time. From this, we can see that the degree of source. Since the eruption in 2006, the time series in each displacement through the two observation methods matches observation point shows a more stable state of displacement at the level of sub-centimeter scale for pre- and post-eruption compared with the one before. At observation points AV18 with 0.97 correlation value (Fig. 10). It implies that the degree and AV19, simultaneous horizontal movements and surface

368 Sustainable Surface Deformation Related with 2006 Augustine Volcano Eruption in Alaska Measured Using GPS and InSAR Techniques subsidence were recorded (Fig. 4). been calculated by a straight fitting by means of least-squares To analyze surface displacement patterns more precisely adjustment by each interval. The map on the left side of Fig. at the time of eruption, changes in baseline lengths between 12 represents a horizontal displacement vector while the one observation points AV01, AV02, and AV03 were diagramed on the right side represents a vertical displacement vector. (Fig. 6). This figure shows clearly the change in baseline In Stage 2 (Figs. 12 (a) and (a’)), in which a pattern of length before and after August 2005. When compared with gradually increasing baseline length has been shown, there the seismic histogram (Fig. 2(a)) that occurred at Augustine was a radial-shaped horizontal pattern with the crater at the from April 2005 to March 2006, the time of increased center, with an accompanying vertical surface uplifting. seismic activity and changes in baseline length between the This surface deformation is believed to have been caused two points matches closely, with both seismic activity and by the mounting pressure within the volcano as magma baseline length recording their highest values in January moved closer to the surface. It is possible here to interpret 2006 when the main eruption occurred. It implies that the this surface change as a precursor sign of the eruption in increase in baseline length is a precursor sign of a major January 2006. A surface lift at the average rate of 9.7 cm/year eruption, which can be interpreted as the distance between was detected, with an average of 3.9 cm of vertical surface the two points being moved farther apart due to the mounting displacement recorded during the same time. In particular, pressure from magma that bloated the volcano. the observation points AV04 and AV05, which are close to The three-component time series in Fig. 11 shows changes the crater, experienced significant lifting (8 cm and 11 cm, i n baseli ne d ist a nces (AV01-AV02, AV0 4 -AV05, AV18-AV19, respectively), as well as an average horizontal displacement and AV01-AUGL (renamed AV21 from 2006)) among the of 9.5 cm. The difference may have been due to the fact that observation points across the crater. From this, we can see the two points adjacent to the crater were influenced more by that each baseline had similar patterns and the entire time the magma source. During the two months in Stage 3 (Figs. series can be divided into the following five time periods: 12 (b) and (b’)), an abrupt shortening of baseline length and (1) 1/1/2005 – 8/14/2005; (2) 8/15/2005 – 1/10/2006; (3) rapid sink rate of 9.2 cm/year occurred (Table 1). As there 1/11/2006 – 3/16/2006; (4) 3/17/2006 – 10/15/2006; and (5) was an eruption in earnest during this stage, the observed 10/16/2006 – 12/31/2011. subsidence may be largely due to the contraction of internal Except for Stage 1, in which no clear baseline length pressure within the volcano after the eruption. A negative change was detected, surface displacement vectors are vertical coordinate change as well as a horizontal surface calculated and diagramed for stages 2 to 5 (Fig. 12). The displacement movement toward the crater was detected. surface displacement vector for each observation point has During Stage 4 (Figs. 12 (c) and (c’)), a slight baseline length increase was detected after an abrupt baseline length shrinkage in Stage 3. Just like the second stage, there was a radial-shaped displacement toward the crater detected at this stage. Comparing to earlier stages 2 and 3, the uplift rate of 0.3 cm/year in Stage 4 is much slower than before (Table 1). Finally, in Stage 5 (Figs. 12 (d) and (d’)) from the time of the 2006 eruption to the present, a very slow sink rate has been noted, with relatively large episodes of surface subsidence reported at some observation points. Most instances of subsidence occurred near the crater, with Fig. 11. Time series of distance change among the each AV18 and AV19 observed to have sunk by 4.0 cm and 3.3 cm, station. Each pair of stations spanning the Augustine’s respectively. The AV18 observation point’s location is where summit. And the section was separated based on distance change trends (Unit: cm) most pyroclastic flows occurred in the 2006 event. The area

369 Journal of the Korean Society of Surveying, Geodesy, Photogrammetry and Cartography, Vol. 34, No. 4, 357-372, 2016

6. Summary and Conclusions

Surface displacement patterns were analyzed before and after the 2006 eruption of Augustine volcano of Alaska as a way to determine precursor signs before an eruption, post-eruption earth crust changes, and prediction of a major eruption. To complement the limitations of one approach with another, GPS and InSAR techniques were used and observed surface displacements occurring between 2003 and 2011. To that end, the permanent GPS data were collected between the years 2004 and 2011, as well as the ENVISAT SAR image data between 2003 and 2010 for time-series analysis of the observed data. The GPS observation data were then applied with the SBAS algorithm for a time-series analysis based on SAR images. By taking into account the features of SAR images that are not capable of creating differential interferometry images when large displacements such as volcanic eruptions occur suddenly, the data were analyzed by breaking down the period into pre and post- eruption stages. The results are summarized as follows. First, we found that an average of 2.3 cm/year of surface uplift before the eruption were observed to be a precursor sign of the 2006 eruption. In particular, we confirmed an abrupt uplift and subsidence of the surface through GPS observation. By observing the change in baseline distance across the crater, we divided the observation period into five stages, Fig. 12. Comparison surface deformation between each period. The left side figure means horizontal deformations with each stage showing a different surface displacement and the right side means vertical deformations (a) pattern. Second, the surface displacement patterns observed 15.08.05-10.01.06 (b) 11.01.06-16.03.06 (c) 17.03.06-15.10.06 by GPS and SAR imaging have in general stabilized after (d) 16.10.06-31.12.11 the 2006 eruption. But in some areas a continuing trend of subsidence at the rate of 1.6 cm/year has been reported. In was also the locus of pyroclastic flows in volcano eruptions some areas with confirmed cases of subsidence, this may be before 2006 (Miller et al., 1998). According to “Deposits due to hardening of the sedimentary pyroclastic flows from from the 2006 eruption of Augustine volcano” produced previous volcanic activities instead of the result of the current by Coombs and Michelle (Larsen et al., 2010), AV19 was volcano eruption. We can draw a conclusion from this that the also located near the area where there were sedimentary displacement patterns from 2006 to the present suggest that pyroclastic flows. Therefore, the subsidence was most likely there are no precursor signs as of yet to suggest additional caused by the hardening of the sedimentary pyroclastic flows eruptions. Third, the CGPS displacements were projected to rather than the outcome of magma-caused displacements. the LOS vector for comparing SBAS InSAR and GPS time Therefore, the surface displacement patterns observed in series. As a result, the degree of surface displacement by Stage 5 do not seem to suggest additional eruptions after means of the two observation methods matches the level of the 2006 episode. sub-centimeter scale. A correlation test indicated that the two

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