Duration, Magnitude, and Frequency of Subaerial Volcano Deformation Events: New Results from Latin America Using Insar and a Global Synthesis

Article Geochemistry 3 Volume 11, Number 1 Geophysics 19 January 2010 Geosystems Q01003, doi:10.1029/2009GC002558 G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

Click Here for Full Article Duration, magnitude, and frequency of subaerial volcano deformation events: New results from Latin America using InSAR and a global synthesis

T. J. Fournier, M. E. Pritchard, and S. N. Riddick Department of Earth and Atmospheric Sciences, Cornell University, Snee Hall, Ithaca, New York 14853, USA ([email protected])

[1] We combine new observations of volcano deformation in Latin America with more than 100 previous deformation studies in other areas of the world to constrain the frequency, magnitude, and duration of subaerial volcano deformation events. We discuss implications for eruptive hazards from a given deformation event and the optimum repeat interval for proposed InSAR satellite missions. We use L band (23.6 cm wavelength) satellite-based interferometric synthetic aperture radar (InSAR) to make the first systematic search for deformation in all volcanic arcs of Latin America (including Mexico, Central America, the Caribbean, and the northern and southern Andes), spanning 2006–2008. We combine L and C band (5.6 cm wavelength) InSAR observations over the southern Andes volcanoes to extend the time series from 2002 to 2008 and assess the capabilities of the different radars: L band gives superior results in highly vegetated areas. Our observations reveal 11 areas of volcano deformation, some of them in areas that were thought to be dormant. There is a lack of observed deformation at several erupting volcanoes, probably due to temporal aliasing. The total number of deforming volcanoes in the central and southern Andes now totals 15 (from observations between 1992 and 2008), comparable to the Alaska/Aleutian arc. Globally, volcanoes deform across a variety of time scales (from seconds to centuries) often without eruption and with no apparent critical observation time scale, although observations made every minute are sometimes necessary to see precursors to eruption.

Components: 18,269 words, 19 figures, 3 tables. Keywords: InSAR; Andes; volcano; Caribbean; Central America; deformation. Index Terms: 8485 Volcanology: Remote sensing of volcanoes. Received 15 April 2009; Revised 13 October 2009; Accepted 13 November 2009; Published 19 January 2010.

Fournier, T. J., M. E. Pritchard, and S. N. Riddick (2010), Duration, magnitude, and frequency of subaerial volcano deformation events: New results from Latin America using InSAR and a global synthesis, Geochem. Geophys. Geosyst., 11, Q01003, doi:10.1029/2009GC002558.

1. Introduction imminent eruption [e.g., Swanson et al.,1983; Klein, 1984]. Unfortunately, experience has shown [2] Deformation of the Earth’s surface at volcanoes that volcanoes have different behaviors before provides clues to the myriad processes occurring eruptions. Some volcanoes give little obvious below and above the surface [e.g., Dvorak and warning that they are about to erupt [e.g., Hall Dzurisin, 1997], and might provide warning of an et al., 2004], or may give many indications of

Figure 1. Global map of volcanoes (black triangles) (Smithsonian Institution, Global volcanism report, available at http://www.volcano.si.edu) with areas of observed volcanic deformation shown as red triangles (Table 1). Several areas have not been completely surveyed for deformation (see section 1). impending eruption, but do not actually erupt (e.g., deformation see Table 1) and are due largely to the restless calderas like Long Valley, CA). Because use of satellite-based interferometric synthetic different volcanoes have these different personali- aperture radar (InSAR). Many of the deforming ties, deformation must be monitored at all volca- volcanoes discovered with InSAR were not thought noes, and a history of precursory activity and to be active [e.g., Lu et al., 2000; Amelung et al., eruption should be established for each volcano. 2000a; Lu et al., 2002; Wicks et al., 2002; Pritchard and Simons, 2002]. The hazard from these pre- [3] Of the more than 1500 ‘‘potentially active’’ sumed magma intrusions is unclear: will this mag- volcanoes around the world, the ‘‘past perfor- ma accumulation result in an eruption, or is this a mance’’ of only a few dozen is well documented benign intrusion? [e.g., Simkin and Siebert,1994;Dvorak and Dzurisin, 1997]. Another problem is that the list [4] In an effort to address the question of potential of 1500 potentially active volcanoes is incomplete: hazard from volcano deformation events, we have occasionally, volcanoes that are not believed to be compiled a database of deforming volcanoes from active can erupt (e.g., Mt. Pinatubo, Philippines, the literature (Table 1 and Figure 1). In order to fill 1991) or at least show some sign of seismic or in some regional and temporal gaps of volcano deformation activity [e.g., Pritchard and Simons, deformation, we also add new observations in 2002]. For example, as of 1997, surface deforma- Latin America from 2006 to 2008 using the Japa- tion had been observed at only 44 different volca- nese L band radar instrument on board the ALOS noes using ground-based methods (e.g., traditional satellite. Because 2 years of data are not sufficient surveying, tiltmeters, or the Global Positioning to characterize volcanic activity, we include a System, GPS) [Dvorak and Dzurisin, 1997]. In the longer time series of observations (2002–2008) last decade or so, observations of deformation at for a subsection of Latin America – the Southern volcanoes have more than doubled to 110 (Figure 1; Volcanic Zone of Chile and Argentina. Our new for a complete listing of observations of volcano observations reveal volcanic deformation in 11

2of29 Table 1 (Sample). Deforming Volcanoes Across the Globe Along With Some Information About the Type of Deformation Observeda [The full Table 1 is available in the HTML version of this article] Volcano Observation Magnitude Magnitude Geosystems Geophysics Geochemistry Volcano Latitude Longitude Number Frequency Duration (cm/yr) (mrad/yr) Type Method Aliased Reference Italy Campi Flegrei 37.100 12.700 101.01 1 year >10 years 3 – IE GPS, InSAR yes Gottsmann et al. [2006] Vesuvius 40.821 14.426 101.02 <1 year >10 years 10 – IE InSAR, leveling, yes Lanari et al. [2002] G À tilt, trilateration G Stromboli 38.789 15.213 101.04 continuous 2 days 10 – IE GPS no Mattia et al. [2004] 3 Vulcano 38.404 14.962 101.05 1 year 18 months ±1 – IE leveling yes Ferri et al. [1988] 3

Etna 37.734 15.004 101.06 1 year years 1 – E InSAR, GPS yes Bonforte et al. [2008] deformation volcano of magnitude and duration al.: et fournier 101.06 continuous 6 days 730 – PE GPS, tilt no Bonaccorso et al. [2002] Nisyros 36.580 27.180 102.05 continuous minutes 10 – IE GPS, InSAR no Gottsmann et al. [2007] Africa Gada’ Ale 13.975 40.408 201.05 1 year 3 years 12 – IE InSAR yes Amelung et al. [2000b] Dabbahu 12.600 40.480 201.113 1 year 7 daysÀ 800 – IE InSAR yes Wright et al. [2006] Asal-Ghoubbet 11.700 42.700 201.1251 7 years days 200 – E leveling, yes Ruegg et al. [1979] trilateration Menengai 0.200 36.070 202.06 1 year <3 years 1 – IE InSAR yes Biggs et al. [2009] Longonot À0.914 36.446 202.1 1 year <2 yearsÀ 3.3 – IE InSAR yes Biggs et al. [2009] Suswa À1.175 36.350 202.11 1 year <3 years 1.5 – IE InSAR yes Biggs et al. [2009] Lengai, À2.751 35.902 202.12 1 months 3 monthsÀ 20 – E InSAR yes Baer et al. [2008] Ol Doinyo À Paka 0.920 36.180 202.53 9 months <9 months 25 – IE InSAR yes Biggs et al. [2009] Nyamuragira 1.408 29.200 203.02 1 year 2 months 28 – E InSAR yes Cayol et al. [2007] Nyiragongo À1.520 29.250 203.03 1 year months 15 – E trilateration, yes Poland and Lu [2004] À tilt, InSAR Indian Ocean and Arabia Piton de la 21.23 55.713 303.02 2 months 2 months 30 – E InSAR yes Froger et al. [2004] Fournaise À Harrat Lunayyir 25.17 37.75 301.04 1 year 2 days 14600 – IE InSAR yes Gomez et al. [2009] New Zealand White Island 37.52 177.180 401.04 4 months months-years ±20 – IE leveling yes Peltier et al. [2009] Taupo À38.82 176.000 401.07 1 year years 20 – G InSAR, leveling yes Hole et al. [2007] À À

Ruapehu 39.28 175.570 410.1 1 year 4 years 2 – trilateration yes Dvorak and Dzurisin [1997, 10.1029/2009GC002558 À and references therein] Indonesia and South Pacific Manam 4.100 145.061 501.02 1 day 6 years – 3.3 E tilt no Mori et al. [1987] Sulu Range À5.500 150.942 502.09 1 year days? 100 – IE InSAR yes Wicks et al. [2007] À a The columns are as follows: volcano name, latitude, longitude,thevolcanonumberassignedbytheSmithsonianInstitution,the relevant (or shortest) observation frequency, duration of deformation event, magnitude of deformation event (in cm/yr or mrad/yr), type of deformation event, observation method, whether or not the observation is aliased, and references. The different types of deformation are broken into

3of29 seven broad categories: E, eruptive; IE, intereruptive; PE, preeruptive; G, geothermal; FD, flow deposit; GW, ground water; F, flank. We do not include the several hundred volcanoes that have robust observations of no deformation. The list of references is not complete. For each volcano, we cited a paper that supports the duration and magnitude of volcano deformation in the table. If multiple techniques have been used, we cite a paper that summarizes the observations or in a few cases, we have cited two papers that use different methods. Many volcanoes have complex temporal deformation patterns. To represent this complexity, we have more than one entry of deformation magnitude and duration for some volcanoes, but our list does not capture the full complexity of variation in types of volcano deformation. Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G different areas (10% of all known), including [8] In areas where the deformation is of likely several areas that were thought to be dormant, subsurface origin, we model the deformation demonstrating the current incompleteness of global data using simple elastic forward models and a volcano monitoring. Furthermore, many volcanic Levenberg-Marquardt algorithm as implemented in areas have still not yet been surveyed for deforma- the Matlab optimization toolbox for inversions. All tion (e.g., Marianas, the Scotia arc, Kurile Islands, inversions contain a deformation source (volcanic many areas of southwest Pacific, and ocean source), a static offset, and a bilinear ramp to islands). In spite of the limitations of global obser- account for interferometric artifacts. We use the vations, patterns of the frequency, magnitude, and following volcanic source models; an opening dis- duration of volcanic deformation events emerge. location [Okada,1992],isotropicpointsource [Mogi, 1958], and a prolate spheroid [Yang et al., [5] In addition to assessing the potential hazard 1988]. The best model is chosen based on misfit from volcano deformation events, another goal of and realistic volcanic source geometries. We know our global compilation is to determine how fre- that variations in source geometry [e.g., Pritchard quently a satellite must image a given volcano to and Simons, 2004a] and 3-D crustal structure [e.g., be able to detect precursory deformation before an Masterlark, 2007] can effect source depths and eruption or to obtain a quality measurement. For volumes, but the simple modeling done here allows example, within the next decade, NASA is plan- us to gain an order of magnitude estimate of these ning to launch an L band radar satellite called important quantities. More sophisticated modeling DESDynI (Deformation, EcoSystems and Dynam- is not justified considering that there are limited ics of Ice) with several observation targets, includ- InSAR acquisitions (we cannot reconstruct enough ing volcanoes [Anthes et al., 2007], and several of the 3-D deformation field necessary to differen- possible repeat intervals have been proposed. Most tiate between models [Dieterich and Decker, InSAR studies to date have used C band radars 1975]), and there is no information on the subsur- (5.6 cm wavelength) over 35 days that are not able face material properties at these volcanoes (neces- to form usable interferograms over vegetated vol- sary to compute deformation in a layered elastic canoes [e.g., Zebker et al.,2000].Weassess medium or in a finite element model). whether the longer radar wavelength of ALOS (23.6 cm) allows coherent interferograms to be formed in Latin America in different seasons and 3. New Results From Latin America over different time scales and investigate how different satellite repeat intervals will affect volcano [9] Latin America includes several distinct volcanic monitoring capabilities. arcs, Mexico, Central America, the Caribbean, and the northern, central, southern, and austral Andes, encompassing about 300 volcanoes in the Smithso- 2. Methods and Data Quality nian catalog of Holocene volcanism (Smithsonian Institution, Global volcanism report, available at [6] We use the JPL/Caltech ROI_PAC software for http://www.volcano.si.edu). Other volcano catalogs data processing [Rosen et al., 2004] and digital indicate 2500 volcanoes in the southern and central elevation models (DEM) from the Shuttle Radar Andes alone, although the majority are dormant Topography Mission with 90 m pixel spacing to [de Silva and Francis, 1991; Gonzalez-Ferran, remove topography from the InSAR phase [Farr et 1995]. While there are several well monitored al., 2007] (Tools for ALOS available at www.roipac. volcanoes in the region, the activity of most is org/ALOS_PALSAR). unknown. Our data will be of value in setting a [7] One advantage of the C band data over the L baseline for assessing whether future bursts of band data is that the shorter radar wavelength is activity are ‘‘normal’’ or may indicate escalating more sensitive to small deformations. We expect unrest that merits further attention. that deformation rates above 3–5 cm/yr within [10] We use the data from ALOS to survey the coherent areas should be detectable with 1 year volcanoes of Central America, the Caribbean and ALOS interferograms. For example at Lonquimay the northern and southern Andes with data that volcano in central Chile, a comparison of C and L spans 2006–2008 (see Tables S1–S4).1 While band data of a subsiding lava flow suggests that the our survey is spatially comprehensive, it is quite nine month ALOS interferogram is at the limit for detecting the subsidence which was clearly seen at C band (see section 3.1.3). 1Auxiliary materials are available in the HTML. doi:10.1029/ 2009GC002558.

4of29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G possible that we have missed deformation that is found active deformation at two of them [Pritchard small in magnitude or spatial scale. We use long and Simons, 2004b]. Our combined ALOS and time periods to maximize sensitivity to small Envisat data analysis allows us to make measure- deformation rates (e.g., 1 year for ALOS), but this ments at about 40 Holocene volcanoes (Table S1) also means that the scattering properties of the and extend observations north of 40.5°S for the ground might have changed between observations first time. Most volcanoes showed no deformation (causing decorrelation), and the observations at within our detection threshold, including no defor- volcanoes with frequent changes in deformation rate mation associated with small eruptions at Nevados and direction might be temporally aliased. Because de Chillań (2003), Villaricca (2003–2007) and data acquisitions are infrequent, data quantity and Llaima (2003, 2007) (Smithsonian Institution, quality are not uniform at all volcanoes. Global volcanism report, available at http://www. volcano.si.edu), or the seismic swarm at Hornopi- [11] For the southern Andes, we combine C and reń (A. Pavez, personal communication, 2008). L band observations in an effort to understand The lack of coeruptive deformation may indicate how effective the different sensors are. By using real constraints on eruptive processes or simply multiple satellites, we compare different time spans reflect poor temporal or spatial sampling, due to (6 years for C band, 1 year for L band) as an decorrelation. We find previously undocumented indication of what our 1 year ALOS derived results volcanic/hydrothermal deformation at Lonquimay, in other parts of Latin America might be missing. Llaima, Laguna del Maule, and Chaiteń volcanoes, We do not present new observations of the central extend deformation measurements at Copahue, and Andes because to date, C band observations pro- illustrate temporal complexity to the previously vide good coverage (1992–2008) of this arid region described deformation at Cerro Hudson and Cor- with little vegetation or population [e.g., Pritchard doń Caulle. We next discuss the deformation in and Simons, 2002, 2004a; Froger et al., 2007; Ruch these seven volcanic areas from north to south, but et al., 2008; Sparks et al., 2008]. We do not present do not discuss earthquake swarm deformation in new observations for the austral Andes, because of the area near Puerto Ayseń which might have a the limited number of ALOS scenes collected in the magmatic component [e.g., Fukushima,2007]. austral summer. Model results are given in Table 2.

3.1. Southern Volcanic Zone 3.1.1. Laguna del Maule of South America [14] Laguna del Maule is a caldera on the Chile- [12] The Southern Volcanic Zone (SVZ) of South Argentina border encompassing pyroclastic cones, America, (33°S–46°S) includes about 1300 volca- stratovolcanoes, and lava domes of Pleistocene noes, with approximately 60 considered potentially through Holocene age [e.g., Gonzalez-Ferran, active (having eruptions within the Holocene or 1995; Smithsonian Institution, Global volcanism last 10,000 years) [Gonzalez-Ferran, 1995; Smith- report, available at http://www.volcano.si.edu], but sonian Institution, Global volcanism report, avail- with no known historic activity. The observed able at http://www.volcano.si.edu] (Figure 2c). We deformation field (Figures 3b and 3c) has a max- compare results from the European Space imum inflation rate of about 18.5 cm/yr in the radar Agency’s ERS and Envisat C band radar satellites line of sight from ALOS spanning January 2007 to between the years 2002–2008, with the ALOS L January 2008. Using C band Envisat between band radar satellite from 2007 to 2008 (spanning March 2003 and February 2004, the interferogram January–March 2007 to January–March 2008). is coherent but shows no deformation (Figure 3a), Since observations are limited to the austral sum- indicating the deformation rate is variable in time. mer, comprehensive coverage of all volcanoes is We assume that the deformation is the result of a not possible. While a few volcanoes are monitored volume change caused by the inflation of a magma by the Southern Andes Volcano Observatory chamber or injection of hydrothermal fluids. A (OVDAS, administered by the Chilean Servicio shallowly dipping sill is the best fit model located Nacional de Geologı´a y Mineria), the activity at 5 km depth with a dip of 20° and an approx- of the majority of these volcanoes is not well imate opening of 60 cm/yr (Table 2). The model known. residuals along with the model predictions are [13] A previous satellite-based InSAR study made shown in Figures 3d–3f. The root mean squared observations of most volcanoes south of 40.5°S error (RMSE) difference between the data and the during the austral summer using C band radar, and model is 1–2 cm for most interferograms we 5of29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G

Figure 2. Interferometric coherence from ALOS for the volcanic arcs of Latin America draped over shaded topography. (a) Central America and Me´xico, (b) the northern Andes, (c) the southern Andes, and (d) the Caribbean. The time period for most interferograms is 1 year, and the baseline is about 1 km, although in a few cases (especially the Caribbean), only shorter time periods are available. For a complete list of interferograms used, see Tables S1–S4. Coherence is calculated in a 5 5 pixel moving box. Large triangles with black outlines are volcanoes from the Smithsonian Institution (GlobalÂ volcanism report, available at http://www.volcano.si.edu), and yellow triangles show volcanoes mentioned in the text. (e) A reference map of the study area. have studied, probably within the range of the a similar rate of deformation up to January 2008 inherent InSAR noise [e.g., Pritchard and Simons, (Figure 4). A 1 year ALOS interferogram (Figure 4d) 2004a]. does not show any discernable signal, most likely because the deformation rate is too slow for this 3.1.2. Copahue short time period. Further, we model the data shown in Figures 4b and 4c with an isotropic [15] Copahue is an active stratovolcano along the Chile-Argentina border with an active hydrothermal source (Figure 4e). The models for the 2 interfero- system, a VEI 2 eruption in 2000 [Gonzalez-Ferran, grams indicate a relatively constant contraction rate 3/yr (Table 2). The shallow depth 1995; Smithsonian Institution, Global volcanism of about 0.001 km of the source, 4 km, is consistent with the expected report, available at http://www.volcano.si.edu] and depth of hydrothermal activity. volcanic tremor [Iba´nez et al., 2008]. Euillades et al. [2008] combined 16 scenes of an ascending track of Envisat data (spanning December 2002 and 3.1.3. Lonquimay April 2006) with a time series approach to infer [16] Lonquimay is a dominantly andesitic strato- subsidence at a rate of 2 cm/yr. We confirm their volcano in central Chile with 5 historical eruptions result with the ascending data, and in addition, we [Gonzalez-Ferran, 1995], the most recent being use a descending track of Envisat data to document from 1988 through 1990 when about 0.2 km3 of

6of29 Geosystems Geophysics Geochemistry Table 2. Model Results From Deforming Volcanoes in the Andesa Chaiten Cordon Caulle Maule Copahue Tungurahua

Dates Apr 2007 Apr 2007 Apr 2007 Apr 2007 Jan 2007 Jan 2007 Jan 2007 Feb 2007 joint Mar 2003 Feb 2004 joint Dec 2007 G to Jul to Jul to Jul to Jul to Feb to Feb to Jan to Dec to Feb to Jan to May G 3

2008 2008 2008 2008 2008 2008 2008 2007 2005 2008 2008 3 Model form dike normal fault Mogi first second sill sill sill Mogi Mogi Mogi dike/crack

fault Mogi Mogi deformation volcano of magnitude and duration al.: et fournier MSE 4.54 9.04 5.7b 3.85c 1.03 1.40 1.39 0.20 1.02 0.50 0.05 Longituded 72.606 72.66 72.653 72.706 72.239 72.173 70.492 70.504 70.556 71.143 71.151 71.15 78.46 (deg) À À À À À À À À À À À À À Latituded 42.822 42.843 42.839 42.854 40.492 40.543 36.081 36.079 36.072 37.841 37.832 37.834 1.4687 (deg) À À À À À À À À À À À À À X positiond 3.2 1.1 0.5 4.9 2.3 3.2 8.3 7.2 2.5 2.4 1.7 1.8 1.2 (km) À À À À À Y positiond 1.3 1.1 0.7 2.3 2.6 3.0 6.7 6.5 5.8 1.0 2.0 1.8 0.1 (km) À À À À À À À À Depthd (km) 15.9 0.8 0.9 5.1 6.9 3.8 5.3 8.5 5 5.6 3.4 4.0 0.55 DV ––– 25 43 16 – – – 4.6 3.3 2.8, 4.1 – (106 m3) À À À À À Length (km) 2.5 3.4 5.0 – – c 9.4 6.8 8.3 – – – 7.1 Width (km) 16.2 12.8 4.8 – – c 2.8 6.3 5.7 – – – 0.4 Dip (deg) 106 66 54 – – c 21 31 17 – – – 85 Strike (deg) 148 À50 À91 – – c 19À 11 À172 – – – 90 Open/slip 4.2 1.5À 1.4À – – c 0.9 0.8À 0.6, 0.5 – – – 0.5 (m) À Static shift 29 28.5 28.4b 3.6c 2.4 14 1.7, 14 0.6 6.4 0.4, 6.5 2.9 (cm) À À À À À À À À x ramp 0.119 0 0b 0.20c 0.02 0.23 0.01, 0.24 0.07 0.1 0.06, 0.1 0.05 (cm/km) À À À À À y ramp 0.065 0 0b 0.02c 0.01 0.23 0.01, 0.24 0.02 0.0 0.02, 0.01 0.01 (cm/km) À À À À À À À À 10.1029/2009GC002558

a The time span of each interferogram is shown in the date row, and ‘‘joint’’ refers to an inversion that uses both available interferograms. All models include a static shift and bilinear ramp for each interferogram. b Values are for both fault and Mogi models. c Values are for both first and second Mogi models. d For fault and sill models the longitude, latitude, and depth values indicate the center of the updip edge of the plane. 7of29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G

Figure 3. At Laguna del Maule in southern Chile, (a) an Envisat image spanning March 2003 to February 2004 shows no deformation, but (b and c) recent ALOS images show inflation. Both of the ALOS interferograms are used to model the deformation source as an inflating sill like structure (Table 2). (d) The residual from the image spanning January 2007 to January 2008 and (e) the model prediction for that image. The surface projection of the model is drawn as a black rectangle with the updip edge of the sill shown in bold (Figure 3e). (f) The model prediction for February 2007 to December 2007. The black line shows the Chile-Argentina border. White triangles show Holocene volcanoes (Smithsonian Institution, Global volcanism report, available at http://www.volcano.si.edu), while black triangles are from the catalog of [Gonzalez-Ferran, 1995]. The line of sight (LOS) between the satellite and the ground is shown by the arrow. The interferograms have been unwrapped and then rewrapped at different intervals to highlight different magnitudes of deformation. blocky andesitic lavas erupted from the NE flank 3.1.4. Llaima vents [Naranjo et al., 1992; Smithsonian Institu- [17] Llaima is a compound stratovolcano in Chile tion, Global volcanism report, available at http:// and is one of its largest and most active volcanoes www.volcano.si.edu]. An Envisat interferogram [Gonzalez-Ferran, 1995; Smithsonian Institution, spanning December 2002 to January 2008 shows Global volcanism report, available at http://www. deflation with a maximum rate of about 2 cm/yr volcano.si.edu]. InSAR shows temporally complex (Figure 5). The deformation is likely due to the deformation of uplift and subsidence up to 11 cm subsiding lava flows from the 1988–1989 erup- on the eastern flank (Figure 6). The reason for the tion, which have subsided by as much as 20 m in deformation is unknown, but appears to begin in some places (H. Moreno, personal communication, December 2007 and be related to the January/ 2008). February 2008 eruption. The 2008 eruption produced lahars and/or pyroclastic flows (Smithsonian

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Figure 4. Subsidence at Copahue, located in southern Chile, is too slow to be seen in single year interferograms: (a) December 2002 to February 2004 and (d) January 2007 to February 2008. Multiyear interferograms show approximately 2 cm/yr of maximum subsidence: (b) February 2003 to February 2005 and (c) February 2004 to January 2008. Interferograms shown in Figures 4b and 4c are used to model the deformation source as an isotropic point force (Table 2). (e) The model prediction for February 2004 to January 2008. The source location is drawn as a black dot. (f) The residual of the model and data in Figures 4e and 4c, respectively. The black line shows the Chile- Argentina border. Symbols are the same as in Figure 3.

Institution, Global volcanism report, available at and (2) the area of deformation shows high corre- http://www.volcano.si.edu) that traveled down the lation, while decorrelation would be expected on eastern flank in the general location of the defor- freshly deposited surfaces. We suspect that the mation. These deposits seem to be an unlikely deformation is related to an observed ‘‘sector col- cause of the deformation, however, because lapse and a kind of creep movement’’ (H. Moreno, (1) the flows did not travel very far down the flank personal communication, 2008).

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Figure 5. Interferograms showing subsidence of the 1988–1990 lava flows from Navidad crater near Lonquimay volcano from 2002 to 2008. (a and b) Subsidence in several areas of the lava flow are seen in Envisat data. We have interpolated the SRTM 90 m DEM to 30 m here to reveal details of the deformation field. (c) An ALOS interferogram that spans a shorter time interval only shows deformation in a limited part of the lava flow, which appears to be subsiding the fastest. Symbols are the same as in Figure 3.

3.1.5. Cordoń Caulle September 2008 and off and on alert status prior to that (http://www.sernageomin.cl). [18] We observe varying deformation at Cordoń Caulle, a 17 km long by 2.5 km wide zone of volcanic fissures, domes and craters between 3.1.6. Chaiteń Cordillera Nevada caldera and Puyehue volcano [20] Chaiteń is located in Southern Chile and is [Gerlach et al., 1988] in southern Chile. The most composed of a small caldera, 3 km in diameter, recent eruption occurred in 1960 following 2 days with a growing lava dome [e.g., Gonzalez-Ferran, after the M9.5 Chilean earthquake [Gonzalez- 1995; Smithsonian Institution, Global volcanism Ferran,1995;Lara et al.,2004;Walter and report, available at http://www.volcano.si.edu]. Amelung, 2007] (called Cordon Gaulle by Walter Thirty-eight hours of intense seismic activity and Amelung [2007]). Previous work by Pritchard started on 30 April 2008, and culminated in the and Simons [2004b] showed a possible deflation first eruption in 9400 years [Lara et al., 2008; rate of 3 cm/yr between February 1996 and Smithsonian Institution, Global volcanism report, February 1999 (Figure 7a), but at the time only available at http://www.volcano.si.edu]. The vol- a single interferogram could be made of this area. ume of the ash deposits is between 1 and 5 km3 and Our new analysis of Envisat data (Figures 7b and the dome volume, which continues to grow, was 7c) shows that from 2003 to 2005, there was a about 0.5 km3 by December 2008 [Lara et al., mean inflation rate of 1 cm/yr, and from 2004 to 2008]. In the previous study by Pritchard and 2006 the rate increased to 3 cm/yr. An ALOS Simons [2004b], the InSAR phase at Chaiteń was interferogram made from January 2007 to February only coherent on the young lava within the caldera, 2008 (Figure 7d) exhibited a marked increase in but no deformation was observed (upper limit of inflation rate, to 19.8 cm/yr. about 3 cm/yr). Also, no deformation was observed in several ALOS interferograms between April [19] The complex deformation pattern at Cordoń 2007 and 16 April 2008 (Figure 8b), suggesting Caulle was fit using two isotropic point sources that any precursory deformation above about (Figure 7f), one at 7 km depth and the other at 3 cm/yr occurred less than 2 weeks before the 4 km depth (Table 2). The cumulative volume eruption, perhaps coincident with the seismicity change of the two sources is 0.06 km3. The 3 38 h before the eruption. This suggests rapid magma shallower source is 0.02 km and the deeper accent at this long dormant volcano. source is 0.04 km3. The deformation may be a result of hydrothermal activity (Cordoń Caulle is [21] Unfortunately, the earliest posteruptive ALOS the largest active hydrothermal area of the SVZ scene is from July 2008 and includes a combina- [e.g., Gonzalez-Ferran, 1995; Smithsonian Institu- tion of preeruptive, coeruptive and posteruptive tion, Global volcanism report, available at http:// deformation. Between April 2007 and July 2008, www.volcano.si.edu]) and/or an inflating magma Chaiteń showed 22 cm of subsidence (Figure 8a). chamber. The area has been under yellow alert due We have made only one successful ERS interfero- to abnormal seismic swarms from May 2008 to gram because of few acquisitions, slow delivery

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Figure 6

11 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G

Figure 7. Cordo´n Caulle is located in southern Chile and shows temporal variation in its deformation. (a) Pritchard and Simons [2004b] reported subsidence. (b and c) Envisat interferograms between 2003 and 2006 are mostly incoherent but suggest that deformation might have occurred. (d) An ALOS interferogram between January 2007 and February 2008 shows uplift. Note that the wrap rate is different in each interferogram. The recent deformation (Figure 7d) is modeled using two isotropic sources (Table 2). (e) The residuals and (f) the model prediction (the source locations are shown as black dots (Figure 7f)). Symbols are the same as in Figure 3. from the ground station, and problems with the successful Envisat interferograms are possible Doppler ambiguity. The ERS interferogram does because of a lack of acquisitions. Further compli- not reveal any deep deformation during 35 days cating interpretation of the deformation at Chaite´n of the dome building phase (track 468 spanning is that the eastern part of the ALOS interferogram 6 September to 2 August 2008), although it is is incoherent, probably a result of ash deposition incoherent in the summit area (Figure 8c). No

Figure 6. During several periods of volcanic activity between 2002 and 2007, InSAR shows no obvious deformation at Llaima, located in southern Chile: (a) February 2003 to January 2005 and (c) February 2007 to November 2007. (d, e, g, and h) Multiple interferograms show subsidence along the east flank related to activity in late 2007 and early 2008 (circled region). (b and f) Less obvious, the possible deformation on the east flank is interpreted to be related to a slow landslide. Symbols are the same as in Figure 3. 12 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G

Figure 8. Chaite´n volcano, located in southern Chile, erupted violently in early May 2008. (a) Coeruptive subsidence is captured in an ALOS interferogram that spans the onset of the eruption and several months following. (b) A preeruptive interferogram shows no precursory deformation up to 2 weeks prior to the first explosive eruption. (c) A posteruptive ERS interferogram is not coherent enough to show any significant deformation. (d) The coeruptive deformation is modeled with a collapsing dike (Table 2), drawn as a rectangle with the updip edge in bold, along with the model prediction. (e) The residual. Symbols are the same as in Figure 3. during the time span (Figure 8a). This leaves an might have contributed to the observed deforma- ambiguity of the cause of deformation. tion field and reflects the complicated tectonic region surrounding Chaite´n [Lange et al., 2008]. [22] We are able to match the deformation pattern The volume changes required by the geodetic data, with either a collapsing dike, dip-slip faulting, or a 0.03–0.1 km3 cannot account for the large volume combination of faulting and magma chamber de- of erupted material, 1.5–3.5 km3 [Lara et al., flation. The model prediction from the best fit 2008]. Even accounting for the compressibility of model (a dike) is shown in Figure 8d. The residual the magma it is difficult to reconcile these differ- between the data and models is rather large ences. Given the short accent time and long time ( 4–5 cm; the residuals for the best fit model span of the observation it is conceivable that many are shown in Figure 8e), perhaps reflecting addi- deformation processes are super imposed in the tional unmodeled deformation sources. A swarm single observation. It is also possible that some of of moderate sized (Mw 3.0–5.0) earthquakes that the magma rose from depth and passed through the occurred in April–May 2008 (NEIC catalog) magmatic system without leaving a geodetic signal. 13 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G

Figure 9. Located in southern Chile, Cerro Hudson has been inflating since at least 1993 [Pritchard and Simons, 2004b]. (a) An Envisat interferogram spanning April 2004 to April 2007 is largely decorrelated but suggests that inflation continues. (b) An ALOS interferogram spanning March 2007 to December 2007 is decorrelated due to the large baseline (1.7 km) and steep topography in this area. Symbols are the same as in Figure 3.

3.1.7. Cerro Hudson (Figure 2b), we find two volcanoes that have deformation during this time period; Tungurahua [23] Cerro Hudson is an ice-filled caldera in south- is deflating and Reventador shows subsiding lava ern Chile that produced one of the largest eruptions flows. Mothes et al. [2008] used ALSO InSAR of the 20th century in 1991 (Smithsonian Institu- data to observe deformation at these volcanoes plus tion, Global volcanism report, available at http:// Antisana, but our data for Antisana were not www.volcano.si.edu). Pritchard and Simons definitive regarding the existence of deformation. [2004b] found inflation of 5 cm/yr (perhaps de- The SAR scenes processed for this study are creasing in time between 1993 and 1999) which described in Table S2. they modeled with a spherical point source at 5 km depth. Deformation at Cerro Hudson is difficult to observe because of decorrelation in both the C and 3.2.1. Tungurahua L band data (Figure 9), but Envisat data spanning [25] Inflation at Tungurahua is observed in several 2004–2008 shows about 2 cm/yr of inflation interferograms spanning from December 2006 to (Figure 9a). The rate and shape of deformation is August 2008. Loss of coherence at the volcano consistent with the declining rate of deformation summit obscures most of the deformation except in and the depth of the magma chamber from one image from December 2007 to May 2008 Pritchard and Simons [2004b]. (Figure 10b). During this period eruptive activity occurred continually, characterized by strombolian 3.2. Northern Volcanic Zone eruptions that reached maximum altitudes of 5– of South America 14 km (Smithsonian Institution, Global volcanism report, available at http://www.volcano.si.edu). [24]PreviousstudiesoftheNorthernVolcanic The inflation is likely associated with the injection Zone (NVZ) of South America were not possible of magma into the volcanic edifice. There are with C band SAR satellites [e.g., Zebker et al., several interferograms that partially constrain the 2000; Stevens and Wadge, 2004], although Bonva- evolution of this deformation episode. An image lot et al. [2005] found deformation at Reventador, spanning from December 2006 to December 2007 Ecuador. Ground surveys have also revealed de- shows no significant deformation (Figure 10a). Two formation at two volcanoes in Colombia [Dvorak partially overlapping interferograms, December and Dzurisin, 1997], and Guagua Pichincha, Ecua- 2007 to May 2008 and March 2008 to August dor [Garcia-Aristizabal et al., 2007] (Table 1). 2008, show a decreasing trend in the magnitude of Using data that covers the entire volcanic arc deformation (Figures 10b and 10c). The earlier

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Figure 10. In central Ecuador, Tungurahua volcano is uplifting on its western flank. (a) An ALOS interferogram spanning December 2006 to December 2007 shows very little, if any, deformation, but (b) in the following 5 months (December 2007 to May 2008) 12 cm of uplift has occurred. (c) In an overlapping interferogram (March 2008 to August 2008) the magnitude and area of uplift has decreased by approximately half. A shallow dike (Table 2, see section 3.2.1 for details) is used to model the deformation in Figure 10b. (d) The model prediction and projection of the model (black line). (e) The residual of the model (model – data). Symbols are the same as in Figure 3. images show 12 cm line of sight inflation and the deposits (Smithsonian Institution, Global volca- later image shows roughly half that amount. The nism report, available at http://www.volcano. areal extent of the deformation field is also smaller si.edu). An interferogram that spans January 2008 in the latter image. to January 2009 shows as much as 20 cm/yr of subsidence of these deposits, likely associated with [26] The deformation pattern is elongated in the cooling and compaction (Figure 11). east-west direction and is modeled with a shallow dike that extends from the summit to the west 7 km (Figure 10d and Table 2). The opening 3.3. Central America and Mexico dislocation is nearly vertical and reaches to within [28]TherehavebeenseveralpreviousInSAR 100 m of the surface and extends to approximately studies of selected volcanoes in Central America 500 m depth. Without additional information about and Mexico. Null deformation was observed at a the seismicity or eruptive chronology it is difficult few volcanoes with C band data [Zebker et al., to speculate how realistic a dike model is for 2000], deformation due to surface loads was seen at describing the current activity. Colima, Mexico, with persistent scatterer InSAR [Pinel et al.,2008],anddeformationhasbeen 3.2.2. Reventador suggested at San Miguel Volcano, El Salvador by Schiek et al. [2008], but our data regarding defor- [27]AsectorcollapseatReventadorformeda mation at this volcano is inconclusive. Ground horseshoe shaped crater which is now filled with surveys revealed deformation at three different an andesitic cone and pyroclastic and lava flow volcanoes (Table 1) [Dvorak and Dzurisin, 1997]. 15 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G

Figure 11. Reventador is located in northern Ecuador. An ALOS interferogram is coherent on lava flows inside the horseshoe shaped crater and reveals subsidence of some of those flows (circled). Symbols are the same as in Figure 3.

[29] Our data between 2007 and 2008 reveal subsidence of a lava flow at the base of Parıćutin cinder cone, but show no obvious subsurface Figure 12. An ALOS interferogram spanning August volcanic processes occurring at any of the 113 2007 to May 2008 shows subsidence of the lava flow at volcanic centers. The lack of volcano deformation Parıćutin in central Mexico. The lava was erupted in Central America is somewhat surprising consid- between 1943 and 1952 and continues to show ering the level of eruptive activity (Table 3), but subsidence in the thickest part of the flow. Symbols may be related to the short time window of the are the same as in Figure 3. observations and the relatively poor coherence in the heavily vegetated region. The ALOS scenes processed for this study are described in Table S3. area most likely indicates the thickest part of the flow (Figure 12). [30] Parıćutin is the famous volcano born from a corn field [Pioli et al., 2008] in central Mexico in [31] Lu et al. [2005] explored subsiding lava 1943. The eruption lasted nearly 10 years and flows inside Okmok Caldera and concluded that produced voluminous lava deposits around the thermoelastic contraction of the flows was the central vent. A total eruptive volume of roughly largest contributor to the deformation. The sub- 1.4 km3 [Stasiuk et al., 1993] is deposited over a siding flows at Okmok are comparable in age to circular area of 2.5 km radius, giving an average the Parıćutin deposits, the Okmok flows were flow thickness of 70 m. The nonuniform thick- emplaced in 1945 and 1958. The subsidence rate ness of the flows means that the deposits are likely at Okmok, 1.5 cm/yr, is considerably less than the to be much thicker in some areas. The lava flow 4–4.5 cm/yr observed at Parıćutin. The increased deposits are believed to be subsiding due to ther- subsidence rate at Parıćutin is likely the result mal contraction and compaction, and the subsiding of differences in the deposit thicknesses. At

Table 3. Interarc Comparison of Volcano Deformation Number Number With Number of Volcanic Arc of Volcanoesa Historic Eruptions InSAR Time Span Volcanoes Deforming

Central America 113b 37 2006–2008 1 Caribbean 16 8 1998–2000,c 2006–2008 1, 1 Northern Andes 35b 16 2006–2008 8 Central Andes 69 13 1992–2008 3 Southern Andes 63b 27 1993–1999,c 2002–2008 2, 7 Austral Andes 8 6 1996–1999 0 Alaska/Aleutiansd 91 42 1992–2008c 15

a Smithsonian Institution (Global volcanism report, available at http://www.volcano.si.edu). b Only includes mainland volcanoes. c Only a portion of the arc is covered in this time interval. d Based on Lu et al. [2007, and references therein].

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similar to that found during an earlier time interval with ERS data [Wadge et al., 2006].

[33] Caution should be taken at volcanic centers with active deposition of material or dome growth. Changes to the topography from these processes can manifest as apparent ground deformation if care is not taken to use up-to-date DEMs. For instance the interferogram in Figure 13 shows 7 cm of line of sight uplift of the recent deposits from SHV, but also a large amount of apparent motion at the summit of the volcano (circled region), likely associated with changes in the topography of the active dome.

4. Global Compilation

[34]Wehaveattemptedtocompileallknown deforming volcanoes and some of the properties of their volcano deforming events (duration, magnitude, and relation to eruptive processes) in Table 1 and Figure 14. We have not attempted to catalog every volcano deformation event (Kilauea only would consume the entire table), but try to sample the range of different types of deformation events. We have mixed together many processes that can Figure 13. Soufrie`re Hills Volcano on Montserrat cause surface deformation, because it is not simple island in the northern part of the Caribbean Islands has been erupting for more than a decade. A 46 day ALOS to separate these into different categories. The types interferogram shows deformation of recent pyroclastic of phenomena that can cause ground displacements deposits on the east flank, similar to the deformation include: hydrothermal circulation, subsidence of observed in a 35 day ERS interferogram in 1999 [Wadge volcanic deposits, loading by deposits, loading et al., 2006]. Apparent deformation at the summit and unloading by snow and ice, eruptive conduit (circled) may be the result of DEM errors associated processes, degassing, thermal and phase changes in with topography changes caused by dome growth. magma, dome growth/collapse, dike emplacement Symbols are the same as in Figure 3. and others. We categorize the observed deformation into seven general categories; eruptive, intereruptive, preeruptive, deformation of a volcanic deposit, Okmok the flow deposits are 25 m thick, while at geothermal, groundwater and flank deformation. Parı´cutin, although the thickness is not well con- We also indicate whether the observation is aliased strained, it likely exceeds 70 m. or not. Any observation that does not show temporal evolution of the event is considered to be 3.4. Caribbean aliased. Figure 14 shows that these processes occur [32] The ALOS data availability for this region is at a spectrum of different time scales and many much sparser than the other areas in Latin America. occur at multiple time scales (from seconds to We process scenes from 2006 to 2008, but some centuries), and can occur either with or without interferograms only span a few months because of eruption. It should be noted that tilt events are the limited data (Table S4). Temporal decorrelation included in Figure 14 for completeness and that in the Caribbean may be the largest obstacle to the tilt magnitude (on the right axis) is not meant using InSAR in this region. Images with a temporal to correspond to an equivalent displacement rate baseline of about 1 year and longer are almost (given on the right axis), instead tilt is scaled to completely decorrelated. We only find volcano fit within Figure 14. deformation associated with Soufrie`re Hills Volca- [35] On a world wide basis, it is not clear if the no (SHV) on Monserrat Island. Several images duration or magnitude of deformation events show potential deformation associated with volca- provide an indication of the potential for future nic deposits on the flanks of SHV (Figure 13),

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Figure 14. Duration of deformation versus rate calculated for a variety of volcanoes around the world using the information from Table 1. Even if deformation only lasts a few minutes, the rate is calculated in units per year. Tilt observations use the scale on the right side of the graph, while other types of deformation observations (GPS, InSAR, EDM, etc.) use the scale on the left side. Observations that include coeruptive deformation have open symbols. Very few observations are taken frequently enough to be considered nontemporally aliased. eruption. The only obvious trend to be drawn from Figure 2c to Figure 1 of Pritchard and Simons Figure 14 is that the magnitude of deformation [2004b]). However, it is hard to directly compare appears to decrease with the duration of the defor- the C and L band coherence because the baselines mation event. This is not a surprising trend to find and time periods are not equal. At a few volcanoes in this type of figure and it should not be inter- in the southern Andes, we have processed both C preted as having any significant implication for and L band data so a qualitative comparison can be volcano deformation. At the short time scale end of made. In some areas, coherence at C and L bands is Figure 14, we lack observations of small magni- similar, for example; Laguna del Maule (Figure 3), tude deformation because current technology does Copahue (Figure 4), Lonquimay (Figure 5), and not allow for measurements of such minute changes. Llaima (Figure 6). Regions of coherence are larger At the long time scale end of Figure 14 there is an in L band than C band in interferograms at Cordo´n upper limit for how long any deformation rate can Caulle and Chaite´n (Figures 7 and 8). At Cerro be sustained. Hudson, the C band interferogram is more coherent, probably because the large baseline of the 5. Discussion ALOS interferogram causes topographic decorrelation in the areas of high relief (Figure 9). This 5.1. Comparison of C and L Band decorrelation can be reduced by using a DEM during image coregistration [Yun et al., 2008]. [36] Because of the longer wavelength, we expect L band data to have higher coherence and fringe [37]Atsomevolcanoes,deformationisonly visibility than C band, and on a regional scale this detected with C band data (Copahue, Hudson, seems to be true in the southern Andes (compare maybe Lonquimay), because of the short time span

18 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G or large baseline of the ALOS data. Although a ably because one scene was acquired right after a direct comparison at Chaite´n is not possible rain event (S. Hensley, personal communication, because C band data spanning the eruption does 2009). R. Ahmed et al. (A survey of temporal not exist, we get a sense of the data quality from decorrelation from spaceborne L-band repeat-pass previous C band interferograms that show decorre- InSAR, submitted to Remote Sensing of Environ- lation over 1 year. Deformation is detected with ment, 2009) show that wind/rain causes loss of both C and L band at Llaima and Cordon Caulle, coherence at L band in 1 day repeats with SIR-C but L band has superior coherence, revealing more data (from the Space Shuttle) over the eastern of the deformation pattern. U.S. An unanswered question is how quickly coherence is recovered after a rain/wind storm. [38] L band is more sensitive to ionospheric effects than C band, and we observe such effects in [41] We know that 46 day interferograms do not the Northern, Central and Southern Andes (see work in most snow or ice areas (e.g., the southern Appendix A). The L band data has higher spatial and austral Andes during austral winter). A signif- resolution than the available C band data, and seems icant fraction of volcanoes around the world have to reveal errors in the DEM (see Appendix A). snow cover for at least part of the year. A short repeat time will maximize the chance that there [39] In the northern Andes, coherence seems to be will not be a precipitation or melting event that highest at high elevations where there is no snow might diminish coherence. We do not know how and there is less vegetation (Figure 2b). A 46 day this coherence diminishes with time (e.g., it depends repeat provides coherent interferograms in the on the quantity and properties of the snow), but vegetated lowlands when the spatial baseline is a 7 day repeat interferogram at Mt. St. Helens from small and/or when spatial averaging is applied. For UAVSAR was incoherent when there had been longer time spans the lower-elevation regions a snow event between acquisitions (S. Hensley, become decorrelated. In the southern Andes, it is personal communication, 2009). necessary to avoid austral winter, and even then coherence is the lowest in the foothills–western side of Cordillera (Figure 2c) which receives the 5.2. Interarc Comparison most precipitation and has the most vegetation. The and Global Synthesis coherence in the vegetated southern Andes depends [42] Throughout Latin America, we find that most on baseline length, even in low-lying areas; for deforming volcanoes are not erupting and we do example, compare the region west of Cordo´n Caulle not observe deformation at most of the erupting in Figure 2c (path 119, perpendicular baseline volcanoes. How representative are these results? A 1.8 km) which is more coherent than the regions comparison between Latin America and the at similar longitude to the north (path 118, Alaska/Aleutian arc (almost completely surveyed perpendicular baseline 2.6 km), even though both by InSAR, [e.g., Lu et al., 2007]) is illustrative of interferograms span 1 year. In the Caribbean interarc variations (Table 3). The Aleutian arc has (Figure 2d), the coherence is the lowest of any about the same number of historic eruptions (about region we studied, and short time period interfero- 40) and actively deforming volcanoes (about 15) as grams with small baselines are essential. Vegeta- the central and southern Andes combined, although tion and terrain cause the most problems, with the there are many more volcanoes in the central and inland areas of most of the islands becoming southern Andes (91 in the Aleutians and 132 in the decorrelated the fastest. In the northern Andes, central and southern Andes of Holocene age Central America, and the Caribbean the L band (Smithsonian Institution, Global volcanism report, coherence is clearly superior to C band results available at http://www.volcano.si.edu) and about from these areas [e.g., Zebker et al., 2000; Wadge 2500 total [e.g., Gonzalez-Ferran, 1995]). It is et al., 2006]. perhaps not fair to group the central and southern Andes together as these arcs have many composi- [40]Withgoodbaselinecontrol(<250m),we tional and structural differences (e.g., the crustal conclude that at L band sufficient coherence is thickness approaches 70 km in the CVZ but is maintained for solid earth applications in vegetated about 40 km in the SVZ; and volcanism is more areas at 46 days, most of the time. We do not have basaltic in the SVZ (Smithsonian Institution, Global enough data yet to quantify ‘‘most of the time,’’ volcanism report, available at http://www.volcano. but we know it is not 100%. A 7 day repeat si.edu)). For example, the magnitude of deforma- interferogram from UAVSAR (L band) in San tion (of magmatic and/or hydrothermal origin) is Francisco is incoherent in vegetated areas presum-

19 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G larger in the SVZ than in the CVZ. While the there might be an absence of deformation imme- magnitude of deformation observed in the SVZ diately before an eruption as a magma chamber for 5 of the 7 deformation areas is greater than asymptotically approaches some fixed pressure 5 cm/yr, in the CVZ no volcanic deformation has yet [Dvorak and Okamura, 1987]. Yet, while a decrease been observed to exceed 5 cm/yr. Deformation in deformation rate was observed before the 2008 caused by surface processes, such as the subsidence eruption of Okmok volcano, Alaska, similar to that of recent volcanic deposits at Lonquimay, Parıćutin, at Kilauea, continuous GPS measurements still and Reventador, is not unusual in volcanic arcs found precursory deformation in the weeks/months [e.g., Pritchard and Simons, 2004a; Lu et al., before the eruption [Larsen et al., 2009]. The hours 2005; Masterlark et al., 2006; Whelley et al., to weeks time scale for precursors also seems to hold 2008], but the activity at Llaima defies simple at Piton de la Fournaise [Collombet et al., 2003], explanation. Sakurajima, Suwanosejima and Semeru [Iguchi et al., 2008], Etna [Bonaccorso et al., 2002], Soufrie`re [43] One goal of this study is to assess the range Hills [Voight et al.,1999]andMt.St.Helens and variability of deformation styles observed [Swanson et al., 1983]. worldwide. It is clear that volcano deformation events occur frequently without eruption and are [46] Seismic precursors before eruptions have been usually short lived. In the CVZ and SVZ alone, we studied at more volcanoes than deformation pre- have observed deformation to start or cease at 7 of cursors. For example, a global compilation of the 15 areas of deformation (Chaiteń, Cordoń 191 swarms associated with eruptions have a mean Caulle, Llaima, Laguna del Maule, Ticsani, Hualca time scale of 8 days although this time scale Hualca, and Lazufre) during the few years of includes precursory, coeruptive and posteruptive InSAR observation. On the other hand, some areas swarms [Benoit and McNutt, 1996]. There is an- have been continuously deforming for decades, and ecdotal evidence, at Long Valley, California, for even millennia, without eruption [e.g., Newhall example, that [Sorey et al., 2003, p. 171] ‘‘episodes and Dzurisin, 1988]. Centuries long patterns of of accelerated deformation generally precede deformation alternating between uplift and subsi- increases in earthquake activity by several weeks dence at Campi Flegrei [Cinque et al.,1985], to months.’’ Therefore, seismic information seems Yellowstone [Pierce et al., 2002], and possibly Long to indicate that deformation observations on a time Valley [Reid, 1992], have yielded net uplift of only scale of days to weeks could be useful for eruption 15–40 m. Uplift at Socorro has been continuous precursors. between at least 1912–2008, but proposed evidence for continuous uplift lasting 10,000 years or more [47] Sometimes there is an uptick in deformation is dubious [Fialko et al., 2001; Finnegan and starting months to years before an eruption (e.g., Pritchard, 2009]. Sierra Negra, Galapagos [Chadwick et al., 2006], and Okmok, AK [Fournier et al.,2009]),but eruptions that are triggered by a dike intrusion 5.3. Optimum Observation Interval may only start deforming 24 h or less before an for Volcano Deformation eruption (e.g., Hekla, Iceland, 2000 [Sigmundsson, [44] There are two key questions related to the 2006], and Sierra Negra, 2005 [Geist et al., 2008]; optimum interval for making deformation observa- Chaiteń, Chile, was dormant for 9400 years until a tions, which translates into a satellite repeat over- few days before the 2008 eruption (this work)). flight time interval: Thus, some basaltic and more silicic volcanoes require deformation observations every minute [45] 1. How frequently do observations need to be (or hour) to catch dike intrusions. made to observe deformation precursors before eruptions? There is incomplete data to answer this [48] 2. How frequently do observations need to be question in a global sense (and different volcanic made to understand the physical processes occur- systems seem to have different behaviors), but ring at a particular volcano, independent of erup- there is anecdotal evidence that precursors occur tion? Another way of asking this question is what before eruptions on time scales of hours to weeks. is the time scale for variations in volcano defor- Klein [1984, p. 3059] notes that at Kilauea time mation? In the Aleutians, more than 60% of scales of less than 20 days are important: ‘‘Tilt eruptions lasted less than 24 days during the past level is an eruption precursor significant to better decade (Z. Lu, personal communication, 2009), than 99.9% when averaged over any interval from which implies that observations on a daily to weekly 1 to 20 days.’’ For basaltic systems like Kilauea, basis are necessary in order to monitor deformation

20 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G during an eruptive crisis. Globally (Table 1), it is Voight et al.,1999;Bonaccorso et al.,2002; clear that some volcanoes change their rate and Collombet et al.,2003;Iguchi et al.,2008]. pattern of deformation on a daily to weekly time Deformation precursors before eruption might scale. It is worth noting that observations that span occur only days before an eruption at a volcano months-years might measure some deformation, that has been dormant for thousands of years but are sometimes temporally aliased and not (e.g., Chaite´n). capturing all physical processes involved. [51] 2. In terms of data quality, both C band and L [49] The danger of aliasing the deformation signal band observations have proved useful when com- is exemplified very well in the case of the 2006 bined together to maximize spatial and temporal eruption of Augustine Volcano, Alaska. The com- coverage of the SVZ. Given previous studies mon mode for InSAR analysis in Alaska (and other [Rosen et al., 1996], it is not surprising that we snowy areas) is to use images acquired only in the conclude that L band data quality is superior to summer months [e.g., Lu et al., 2007] due to snow available C band data in highly vegetated areas cover and the long repeat intervals of the existing (especially Central America, the northern Andes SAR satellites. The cumulative deformation at and the Caribbean). The repeat interval necessary Augustine from the summers bracketing the erup- to maintain L band coherence is regionally variable tion would show no deformation of the volcano, in Latin America depending on distance to the even though the GPS record shows a dynamic ocean, elevation, and type of vegetation, but for deformation history during the yearlong interval volcanic applications, a 46 day repeat seems to [Cervelli et al., 2010]. Since deformation associ- maintain coherence as long as there is good base- ated with various volcanic processes occurs at line control. different temporal and spatial scales the problem of aliasing should always be considered in [52]3.Fromourcomprehensivesurveyofall regions where very little is known about the volcanoes in Latin America (minus the central volcanoes. The lack of deformation observed Andes), we document 11 centers of volcanic during several volcanic eruptions (for example, deformation, including discovery of deformation Nevados de Chilla´n, Villaricca and Llaima (this in 4 locations. It seems that there are significant work); Shishaldin [Moran et al., 2006]; Korovin, variations in deformation characteristics between Pavlof, Cleveland, Chiginagak, Augustine, and volcanic arcs. For example, the few deforming Veniaminof, Alaska [Lu et al., 2007]; Kliuchevskoi, volcanoes found in the northern Andes, the Carib- Sheveluch, and Bezymianny, Kamchatka [Pritchard bean and Central America seems contradictory to and Simons, 2004c]; Lascar, Irruputuncu, Aracar, the high level of eruptive activity in these regions and Sabancaya, central Andes [Pritchard and (Table 3). There also seems to be a low number Simons,2004a])isatleastpartlyexplainedby of deforming volcanoes relative to eruptions in temporal aliasing. Kamchatka (P. R. Lundgren, personal communication, 2009). While 1 year of observations may not be sufficient to find all deforming volcanoes 6. Conclusions in these arcs, our study in the southern Andes over a similar time from ALOS found four areas [50] 1. Our global compilation (and previous work of deformation. It is possible that no deformation [e.g., Dzurisin, 2003]) indicates that the deforma- is recorded because magma may not refill every tion episodes at dormant volcanoes can be short chamber before or after eruption, or might not lived (lasting weeks to months), and commonly even be stored in a shallow chamber before occur without eruption. Geochemical analysis also erupting [Dzurisin, 2003]. It seems that the nature infers that there might be several small intrusions of the magma plumbing in the northern Andes, spanning the decades to centuries before an erup- Kamchatka, and Central America is different from tion [e.g., Zellmer et al., 2003], so that any eruption the central and southern Andes, perhaps because of related to current volcano deformation might be the tectonic regime, magmatic flux, magma or many years from now. On the other hand, there are crustal composition or some other factor. Nonethe- several examples where volcano deformation has less, based on continuous GPS observations at a been used to indicate that an eruption might occur few Alaskan volcanoes during eruptions (Okmok within a few months [e.g., Larsen et al., 2009] or and Augustine), we suspect that the frequency of even predict an eruption a few minutes to days InSAR observations has not been sufficient to before eruption [Swanson et al., 1983; Klein, 1984; measure coeruptive deformation.

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geodetic stations) are an important complement to InSAR, especially in cases where magma movements are undetectable by the satellite (because the magma moves without deforming the surface, the deformation time scale is short, or the deformation is too small). Veniaminof, Alaska and Suwanose-jima, Japan are examples where deformation was seen with GPS instruments [Iguchi et al., 2008; Fournier et al., 2009], but not with InSAR [Lu et al., 2007; Aoki et al., 2008].

[54] 4. Globally, volcanoes deform across a variety of time scales (from seconds to centuries) with no apparent critical observation time scale, other than to note that observations should be made as frequently as possible. For dike intrusions, the necessary time scale is seconds to minutes, which is

Figure A1. ALOS interferogram from ascending path 109 (acquired around 2200 local time) spanning December 2006 to December 2007. While the fringes could be caused by errors in the orbital parameters, we have only seen similar distortions in this area. Through pair-wise logic of making multiple interferograms with each date, we have determined that the data on 6 December 2006 were corrupted, possible by a large phase change in the ionosphere. Unlike other examples in the central and southern Andes, this phase distortion does not have associated streaks of decorrelation. Symbols are the same as in Figure 3.

[53] Even when we do observe coeruptive deformation, the currently available repeat intervals leave significant gaps in observation that leave many processes poorly understood. For example, InSAR observations of the 2008 Chaite´n eruption are ambiguous as to the nature of deformation (diking, faulting, or some complex combination) and cannot account for the observed eruptive volume, probably because of temporal aliasing of the deformation signal. Future InSAR satellites with more frequent revisits (like NASA’s DESDynI) or Figure A2. ALOS interferogram from ascending path constellations of satellites are necessary to truly 105 (acquired around 2200 local time) spanning August determine the existence or nonexistence of coerup- 2007 to February 2008. Fringes in the northern part of tive deformation (as well as the nature of this the interferogram are from the Mw 7.7 earthquake on deformation and the existence or nonexistence of 14 November 2007, but the fringes in the southern part precursory deformation). In any case, ground-based of the image are associated with streaks of decorrelation sensors (particularly gravimeters, and continuous and are presumably of ionospheric origin. 22 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G

characteristics seem to vary across the regions, with streaks of decorrelation and phase perturba- tion in the central and southern Andes (similar to those seen in polar regions [e.g., Gray et al., 2000]) and only long spatial wavelength phase perturba- tions observed in the northern Andes. No ionospheric artifacts have been observed in Central America or the Caribbean arcs. These potential ionospheric disturbances affect a small proportion of the acquired SAR images. For example, in the northern Andes only 2 of more than 30 processed images are negatively affected by the ionosphere. While ionospheric effects are thought to be more important at L band than C band, in the central Andes, we have seen phase distortions with a similar orientation to those in Figure A2 with C band radar data collected during a similar time of day [Pritchard, 2003].

[56] We have several examples where we think that errors in the 3 arcsec (90 m/pixel) SRTM DEM create a phase change in the ALOS interferograms, particularly in Ecuador (Figures A4 and A5). There are several potential error sources when using the 3 arcsec SRTM DEM with ALOS data. First, the ALOS fine beam pixel size at full resolution is about 10 m, so to get the interferogram resolution Figure A3. ALOS interferogram from ascending path 114 (acquired around 2200 local time) spanning December 2007 to March 2008. While some fringes in this interferogram may be caused by errors in the orbital parameters, the fringes in the southern part of the image are associated with streaks of decorrelation and are presumed to be of ionospheric origin. The black line shows the Chile-Argentina border. Symbols are the same as in Figure 3. outside the realm of currently available or near- future satellite InSAR observations.

Appendix A: Quality of L Band Interferograms

[55] We see linear phase distortions oblique to the satellite track in a few interferograms in the northern, central and southern Andes that we attribute to ionospheric effects (Figures A1–A3). Because of the timing of acquisitions (between 2000 and 0200 Figure A4. ALOS interferogram from ascending path local time, when ionospheric scintillation is most 109 spanning 46 days (8 June 2007 to 24 July 2007). The phase change on the west side of Imbabura volcano, intense), the location within about 20° of the Ecuador, is seen in several interferograms, but we magnetic equator (where these scintillations are suspect that the signature is due to a DEM error. The most intense outside of the polar regions), and perpendicular baseline is about 300 m, which means that the fact that the distortions are only associated with a DEM error of 200 m is necessary to cause 11.8 cm of single SAR images, we suspect that the distortions phase change (corresponding to 2p radians); that is, the are of ionospheric origin [e.g., Xu et al., 2004]. The ambiguity height is about 200 m. 23 of 29 Geochemistry Geophysics 3 fournier et al.: duration and magnitude of volcano deformation 10.1029/2009GC002558 Geosystems G

program. Thanks to F. Amelung and an anonymous reviewer for helpful comments that greatly improved the manuscript. The GMT program was used to create Figures 1–13 and A1–A5 [Wessel and Smith, 1998].

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