Monitoring the Reawakening of Canary Islands'teide Volcano

Eos, Vol. 87, No. 6, 7 February 2006

VOLUME 87 NUMBER 6 7 FEBRUARY 2006

EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION PAGES 61–72

pyroclastic flows similar to those on Mount Monitoring the Reawakening of Pelée (Martinique) and Mount Vesuvius (Italy).The Teide volcano’s explosive eruptions are the result of magma mixing pro- Canary Islands’Teide Volcano cesses in which basaltic eruptions act as a triggering mechanism. PAGES 61, 65 them of the effusive type, where lava flows In 1990, the International Association of Vol- freely without explosive power.Teide canology and Chemistry of the Earth’s Interior Following more than 30 years of seismic (28.27ºN, 16.6ºW) is a complex stratovolcano, (IAVCEI) identified Teide as being worthy of and volcanic quiescence, the Canary Islands the third-tallest volcano on Earth from base particular study in light of its history of large, region located off the northwestern coast of to tip, reaching an altitude of 3717 meters destructive eruptions and its proximity to pop- Africa started to show signs of seismovolca- above sea level and approximately 7000 ulated areas. For the present high-risk level, nic activity at the end of 2003 (Figure 1). In meters above the adjacent seabed.Teide’s since 1992 Teide has been considered by the spring 2004, there was a significant increase last explosive-type eruptions have been IAVCEI as one of the European Laboratory in the number of seismic events (a mixture dated as having occurred around 1500 years Vo lcanoes, thus receiving special consider- of volcano-tectonic events and regional ago. Future eruptions are considered likely ation from the European Union concerning earthquakes with pure volcanic events such and will include the risk of highly dangerous research proposals. as tremors and long-period signals) located inland on Tenerife Island. The increase ofactivity in 2004 coincided with an increase of fumarolic activity at the Teide volcano on Tenerife Island, an increase in the emission of carbon dioxide in the northwestern part of the island, and changes in the gravimetric field on the northern flank of the volcano.After several seismic events had been felt by the population, the first alert level was declared by the civil protection division of the local government. This apparent reawakening of Teide, which last erupted in 1909, provides an opportunity to study from the initial stages the reactivation of this volcanic area and its related phenomena. This article presents an automatic seismic monitoring system, the Teide Information Seismic Server (TISS), that is monitoring the internal status of the volcano by means of real-time seismic background noise analysis. The system’s main goal is to detect precursors to a potentially dangerous eruptive epi- sode at an early stage.The system, in operation at Teide volcano since November 2004, has proven useful in monitoring changes in the behavior of the volcano’s processes, such as fumarole venting and seismic activity.These external manifestations of the volcano’s processes have been preceded by changes in the monitored parameters (see Figure 2).

A Brief History

Several eruptions have taken place in the Canary Islands in the last 500 years, all of Fig. 1. (top) Signs of activity at the Teide volcano (October 2004). Picture provided by the municipal government of Icod de los Vinos (Tenerife, Canary Islands, Spain). (bottom left) Epicentral B Y A.GARCÍA, J.V ILA, R. ORTIZ, R. MACIA, locations of events from January 2000 to June 2005. Red dots indicate events from May 2004 to R. SLEEMAN, J. M. M ARRERO, N. SÁNCHEZ, July 2005. (bottom right) Evolution of the cumulative seismic energy released by earthquakes M. TÁRRAGA , AND A. M. C ORREIG located inland on Tenerife (data provided by the Spanish Instituto Geográfico Nacional). Eos, Vol. 87, No. 6, 7 February 2006

TEGETEIDE Project

The special topographic characteristics of the Canary Islands, combined with the distri- bution of the islands’ more than two million inhabitants, increase the level of volcanic risk in this area.A Teide eruption similar to the last dated explosive type eruption (with a volcanic explosivity index (VEI) of 3) and comparable with the last eruptions of Unzen Vo lcano in Japan could affect up to 30,000 people; a VEI of 4 could affect more than 400,000 people. The reported increase in seismic activity that has been observed since spring 2004 could represent the beginning of a reactivation of Teide Volcano.An urgent call by the Spanish government to the Spanish National Commission on Seismic and Volcanic Risk Evaluation to organize and manage the available scientific resources to evaluate the possibility of an imminent eruption has been considered.As a result of this call, financial support for research projects on this area was provided, and in the spring of 2005, the Spanish National Research Council (CSIC) initiated the TEGETEIDE project (Geophysi- cal and Geodetic Techniques for the Study of the Teide-Pico Active Volcanic Area) . Fig. 2.Variations in parameters provided by TISS before and after the appearance on 5 December This project will develop and implement 2004 of the fumarole emission in La Orotava Valley: (a) Daily lower envelope of all 60-minute TISS, which is capable of characterizing the PSD curves; (b) Evolution of the amplitude; (c) Predominant frequency; (d) Integrated PSD predominant frequencies and the seismic amplitude. Plots (b), (c), and (d) relates data in four frequency bands. energy released that is observed in the background seismic noise at diverse frequency bands.TISS is capable of monitoring in real presented by [2005], who showed ilar sampling rates, thus resulting in a multidis- time changes of the status of the volcano by Vila et al. that continuous monitoring of the back- ciplinary analysis.The sampling rates of the collecting information in the time and the ground seismic noise levels may provide post-analysis derived time series also allow a spectral domains, using parameters in which signs of activity more than 40 days sooner fast and easy representation of the volcano’s changes due to variations in the recorded than classical seismological methods based seismic activity.The time series also can be signals are easy to observe.The goal is to on earthquake analysis. used as new input for any supplementary detect changes before the expected epi- The adaptation of TISS for the Canary near-real-time analysis.This corresponds to the sodes of activity. Islands uses a power spectral density (PSD) third level of observatory automatic proce- TISS is based on a real-time quality-control estimation to perform spectral analysis of dures [ ESF-EVOP Working Group, 1994]. (RT/QC) analysis of continuously-collected the continuously incoming data [ Welsh, waveform data from seismic stations, and it 1978].TISS computes the PSD of 60-minute Evaluation of TISS During 2004 monitors changes in the behavior of the seis- segments, extracted from the pool of contin- mic signals.These changes reveal variations uous data, and stores selected parameters as in local site effects caused by changes in the Since first becoming operational in Novem- time series.These time series are selected ber 2004,TISS has shown itself to be very use- volcanic activity.The present TISS process frequencies of the PSD, integrated PSD in involves two different stages:The first stage, ful for monitoring changes in the behavior of various frequency bands (energy related the Teide volcano’s processes, such as fuma- termed ‘data preparation,’ and the second parameters), and absolute maximum PSD stage, called ‘data analysis.’The main tasks in roles and seismic activity. For example, on 5 amplitude and its corresponding frequency, December 2004 a new fissure with fumarole data preparation are to build the basic data also in various frequency bands. Moreover, emission appeared in La Orotava valley, on format and to provide visual Live Internet the lower envelope of all the PSD curves the northeastern flank of Teide volcano (see Seismic Server-like outputs (http://www.liss. (minimum value for each frequency) Figure 2).This was preceded by a significant org).The data analysis stage, which is devoted obtained after processing time intervals of increase in low-frequency seismic energy to waveform analysis, selects fixed-length seg- 24 hours is tracked, thus giving an estimation nine days earlier, with a return to the normal ments from the already established database, of all nontransient signals during one full level a few hours after the fissure aperture. assigns time stamps, and applies subroutines day of operation.This envelope contains the During those nine days, the scientific team of spectral analysis. noise that is always present at every fre- remained on alert. The current RT/QC analysis was devel- quency component [ Vila et al., 2005] and is TISS archives the original time series data oped in 2002 to test permanent stations at continuously compared with previous days. in standard formats such as SEED (Standard the Observatori Fabra in Barcelona, Spain TISS provides details of the evolution of this for the Exchange of the Earthquake Data) or [ Llobet et al., 2003]. Its expansion to large information by means of report plots. GSE (Group of Scientific Experts) and is networks was accomplished in 2004 when The post-analysis derived time series have a thus a complete and open system that allows an adaptation of the QC procedure was sampling rate of the order of one sample per the data to be processed by means of stan- developed [ Sleeman and Vila, 2006] to moni- hour, determined by the length of the seg- dard seismological software packages.The tor all stations of the Virtual European Seis- ments analyzed.These time series are then software package runs under Linux, and all mic Network [ Van Eck et al., 2004]. Its off- suitable to be compared directly with many software used can be obtained through GNU line application for monitoring changes in other signals, such as deformation, tempera- and other free software sites.There are no background noise near active volcanoes is ture, and gas emission, that are sampled at sim- intrinsic limitations for the number of chan- Eos, Vol. 87, No. 6, 7 February 2006 nels or computations.The performance analy- with the Institut d’Estudis Catalans and the on time averaging over short modified periodo- sis of the system reveals that a 1.2 GHz Pen- Observatories and Research Facilities for grams,IEEE Trans.Audio Electroacoust.,AU-15, 70–73. tium III personal computer can handle more European Seismology. than 400 channels at 100 samples per second. Author Information Sources of data and programming facilities References can be found on the TISS Web pages (http:// Alicia García, Ramon Ortiz, Jose M. Marrero, www.sistegeteide.info/tiss and http://sismic. ESF-EVOP Working Group (1994),Automated Systems Nieves Sánchez, and Marta Tárraga, Departamento for Volcano Monitoring, Eur.Volcanol. Proj. Monogr., de Volcanología, National Museum of Natural Sci- am.ub.es/tiss). Examples of RT/QC implemen- vol. 2, 20 pp., Eur. Sci. Found., Strasbourg, France. tations are available at http://sismic.am.ub.es/ ences–CSIC, Madrid; Josep Vila and Antoni M. Cor- Llobet,A., J.Vila, O. Fors, and R. Macià (2003), Linux reig, Departament d’Astronomia i Meteorologia, ISEV/ and http://www.orfeus-eu.org/data-info/ based LISS systems:The ISEV and SISLook exam- Universitat de Barcelona and Laboratori d’Estudis dataquality.htm. ples,ORFEUS Electron. Newsl., 5(1), 5. Sleeman, R., and J.Vila (2006),Towards an automated Geofísics “Eduard Fontserè,” Institut d’Estudis Cata- uniform quality control of the Virtual European lans, Barcelona, Spain; Ramon Macià, Departament Acknowledgments Broadband Seismograph Network (VEBSN),ORFE- de Matemàtica Aplicada 2, Universitat Politècnica US Electron. Newsl.,. In Press. de Catalunya, Barcelona, Spain; Reinoud Sleeman, TEGETEIDE (Tenerife) is a project funded Van Eck,T. , C.Trabant, B. Dost,W. Hanka, and Koninklijk Nederlands Meteorologisch Instituut, De D. Giardini (2004), Setting up a virtual broadband by the Spanish Ministry of Education and Bilt, Netherlands. seimograph network across Europe, Eos Trans.AGU, For further information, contact J.Vila; E-mail: Science (contract CGL2003-21643-E).TISS is 85(13), 125, 129. hosted by the Departament d’Astronomia Vila, J., R. Macià, D. Kumar, R. Ortiz, and A. M. Correig [email protected] i Meteorologia of the Universitat de Barce- (2005), Real time analysis of the unrest of active volcanoes using variations of the base level noise seis- lona, Spain, and by the municipal govern- mic spectrum,J.Volcanol. Geotherm. Res., in press. ment of Icod de los Vinos (Tenerife). Soft- Welsh, P. D. (1978),The use of fast Fourier transform for ware developments are performed together the estimation of power spectra:A method based

land during the upcoming Polar Year. Den- Adam Paulsen, a Pioneer mark, as part of an international consortium that had pooled their resources to study the Arctic, was to conduct meteorological, geo- in Auroral Research magnetic, and auroral observations (including the shape, color, strength, movement, and PAGES 61, 66 position of aurorae). Paulsen, along with five other men, sailed from Copenhagen aboard The 20 to 30 years following the first Inter- the Royal Greenland Trade Department’s national Polar Year in 1882–1883 was a period small three-masted sailing ship Ceres on 17 of quickly advancing knowledge and under- May 1882, bound for Godthaab (64º10’N, standing of auroral phenomena.This was the 51º40’W), a small town on the west coast of time when hypotheses of aurora being due to, Greenland.After a four-week voyage they for example, reflections of fires from the inte- arrived at their destination. rior of the Earth or sunlight from ice particles The location was found favorable for auro- were abandoned and replaced by the mecha- ral observations. Godthaab is located just on nism of precipitating electrons. the poleward side of the belt of maximum One of the auroral researchers at that time auroral activity, an area advantageous for the was the Dane Adam Frederik Wivet Paulsen study of the periodic variations of auorae. (1833–1907). However, when reading litera- The team also planned to investigate rela- ture about auroral history, his ideas and tionships between the aurora and magnetic work do not seem to have attracted much disturbances, and to attempt to measure the interest outside his own and neighboring height above the ground of the aurora, countries. For example, in his sweeping his- because such knowledge might help to dis- torical account Majestic Lights:The Aurora in close its causal mechanisms. Science, History, and the Arts [1980], author Fig. 1 Adam Paulsen, age 67. The pencil The height measurements were not suc- Robert Eather only referred to Paulsen in a drawing (25 x 30 cm) was made by Harold cessful, because the distance of about five couple of lines. Moltke on 10 February 1900 during Paulsen’s kilometers between two observation sites on Eather did not mention any of Paulsen’s expedition to Akureuyi, Iceland. the ground was much too short for a good original and still valid ideas, namely, that elec- quality triangulation to be made. However, trical currents flowing parallel to the Earth’s the study of the relation between aurora and magnetic field exist and move upward within officer in the Danish army, and from 1849 magnetic disturbances inspired Paulsen to the auroral bands, that the aurora is pro- to 1851 he participated in a war against suggest that field-aligned currents exist.This duced by cathode rays (electrons), and that Germany. However, an interest in physics was a milestone in auroral research. the source of the cathode rays is located in changed his career path.Adam Paulsen The expedition team returned to Denmark the upper regions of the Earth’s atmosphere. earned a master’s degree in physics in in the autumn of 1883, and the following The purpose of this article is to draw atten- 1866 from the University of Copenhagen, year Paulsen became the director of the tion to Adam Paulsen and his contributions Denmark, and two years later he was Danish Meteorological Institute, a position to the understanding of the aurora. awarded a gold medal by the same for a he held until his death in 1907. prize essay on the different theories of Paulsen and the 1882–83 Expedition galvanism (electricity produced by to Greenland chemical processes). Paulsen’s Discoveries After completing his university studies, Paulsen was born on 2 January 1833 in Paulsen became a high school teacher in In a comprehensive report about observa- Nyborg, Denmark. He had planned to be an Copenhagen. In 1880, at the age of 47, he tions during the International Polar Year and quit teaching after he was asked by the the following years in Greenland, Paulsen director of the Danish Meteorological Insti- [1893] presented, among other discoveries, B Y T. S. J ØRGENSEN AND O. R ASMUSSEN tute to head a Danish expedition to Green- observations indicating the existence and Eos, Vol. 87, No. 43, 24 October 2006

up the 3718-meter-high Teide stratovolcano, which is nested in the collapse embayment. Recent Unrest at Canary Islands’ The main phase of construction of Teide concluded 30,000 years ago. Since then, the Teide Volcano? stratovolcano has erupted just once: the eighth-century summit eruption, during PAGE 462, 465 into a stratigraphic sequence, particularly which the vent system and lavas increased those eruptions that have occurred in the the elevation of the volcano from about When a volcano that has been dormant past 10,000 years (the Holocene). 3600 to 3718 meters. for many centuries begins to show possible The results of this study indicated that the This recent eruptive history is in stark con- signs of reawakening, scientists and civil central differentiated (phonolitic) Teide vol- trast to the much higher explosivity, with fre- authorities rightly should be concerned canic complex was the direct consequence quent plinian eruptions, of the precollapse about the possibility that the volcanic unrest of the activity of the rifts. The rifts developed activity of the Cañadas volcano (>200,000 might culminate in renewed eruptive activity. a progressively steepening and unstable vol- years ago). In the past 30,000 years, erup- Such was the situation for Teide volcano, cano at the center of Tenerife, and about tions have occurred at a rate of only four to located on Tenerife in the Canary Islands, 180,000–200,000 years ago rift activity trig- six per millennium, with a predominance when a mild seismic swarm during April– gered a massive landslide that generated an (70%) of very low hazard, basaltic eruptions July 2004 garnered much attention and impressive horseshoe-shaped collapse from fissures and cones on the rift zones, caused public concern. However, that atten- embayment. The embayment’s headwall is and the remaining eruptions from phonolitic tion completely ignored the fact that the the 17 by 10 kilometer Las Cañadas caldera. lava domes with only localized explosive seismic recordings of the swarm were due to Subsequent rift and central activity progres- activity (e.g., Mña Blanca, ~2000 years ago) a much improved monitoring system rather sively filled the embayment, finally building at the basal perimeter of the main stratovol- than due to an actual event of alarming magnitude or extent. It is important in any effective program of volcano-risk mitigation that the response to an apparent change in the status of a volcano should include the immediate imple- mentation or augmentation of monitoring studies to better anticipate possible outcomes of the volcanic unrest. Equally important, emergency-management officials, using available scientific information and judgment, must take appropriate precautionary measures— including information of the populations at potential risk—while not creating unjust anxiety or alarm. This article briefly reviews the circumstances and unfortunate societal consequences of the scientific, governmental, and media response to the seemingly heightened seismic activity on Tenerife in 2004. This article describes a series of misinterpretations of geophysical and geological data that led to raised levels of alarm on the island, although being of little actual volcanological significance.

Volcanological and Structural Evolution of Mount Teide

At 3718 meters, Mount Teide volcano is the third-highest volcanic structure on the planet and the highest peak in the Atlantic Ocean. Conversely to the intensely studied and geologically well-understood Hawaiian volcanoes, many crucial geological aspects of Teide volcano were insufficiently addressed until quite recently. As an example, until 2001, only a single radiometric age was available for the most recent activity of the Teide volcanic complex. A joint Spanish-French project in 2001–2005 produced a detailed geological map of Teide and its northwest and northeast rifts, and provided 28 new radiocarbon ages and 26 Fig. 1. Seismicity within the island of Tenerife: (a) From 1 January 1985 to 31 December 1999; val- K/Ar (potassium/argon) ages. This project ues listed are the local ‘Richter’ magnitudes; (b) From 1 January 2000 to 1 April 2005 (modified completely changed the understanding of from Instituto Geográfico Nacional, Spain); MbLg is the body wave magnitude using the Lg wave the geological and structural evolution of (a surface wave). Note that the MbLg values in (b) are essentially equivalent to the local mag- the Teide volcanic complex, allowing the nitudes in (a), suggesting that seismic intensity between these two time periods has remained majority of the eruptions to be separated constant. Eos, Vol. 87, No. 43, 24 October 2006 cano. The recent eruptive record, combined with the available petrological and radiometric data, provides a rather optimistic outlook on major volcanic hazards related to Teide and its rift zones, posing only very localized threats to the one million inhabitants of Tenerife and the 4.5 million annual visitors to Teide National Park.

Unrest at Teide Volcano?

The prediction of an explosive eruption in 2004 was based on reports of significantly increased earthquake activity and volcanic gas emissions. However, no major eruption followed and questions remain whether the available evidence was used in a correct and sensible way. From 22 April to 28 July 2004, about 50 low-magnitude (M = 1–3) earthquakes were recorded in Tenerife, with most of the epicenters localized at the northwest rift zone, in the area of the Icod Valley. Only three were felt by residents. Earthquakes of this type are normal in volcanic oceanic islands (e.g., Hawaiian Islands, Reunión). In addition, low-magnitude earthquakes (generally M <

3) have been reported in all of the Canary Fig. 2. (a) Long-term variation in CO2 emission in Tenerife [Martín, 1999]. (b) Short-term varia- 222 Islands (Figure 1). In May 1998, a bigger tion of radon-222 ( Rn) and carbon dioxide (CO2) in a deep borehole in Las Cañadas caldera earthquake (M = 5.3) hit Tenerife, but television [from Soler et al., 2004]. (c) Frequent spectacular ‘plumes’ in the summit area of Teide known and newspaper sources notified the public locally as ‘la Toca del Teide,’ or ‘Teide’s headdress’ (Photo by J. C. Carracedo). (d) Model of the that the seismic activity was not hazardous, formation of clouds at the summit of Teide volcano by local orographic convergence (see http:// and the event was promptly forgotten. Prior www.acanmet.org.es/menu_archivos/documentos/FTLCTeide.doc). to 1998, smaller earthquakes were frequently recorded throughout the archipelago, but no addition of new seismic stations immediately al., 2004] and linked to systematic changes volcanic activity took place since the 1971 led to low-magnitude events within the island in barometric pressure (Figure 2b). There- Teneguía eruption on La Palma island. being recorded for the first time since sur- fore, if discrete measurements are taken In 2004, however, the local and interna- veillance work began in 1985 (Figure 1). and reported, apparent ‘significant’ tional mass media were bombarded with The focal depths of the majority of these increases in gas emissions can be obtained, persistent reports of unrest at Teide volcano events were not determined as part of the which simply correspond to barometric with little to no mention of more conserva- routine seismic monitoring, and therefore pressure changes. tive views, and predictions were made of an one of the most powerful constraints in Nevertheless, a recent article [García imminent, large-scale explosive eruption that locating the source of the seismicity was et al., 2006] insists on the reawakening of Teide was dubbed ‘El Volcán de Octubre’ (the lacking. An initial interpretation of the seis- based on the aforementioned seismicity and ‘October Volcano’). The tourist island of micity suggested dyke emplacement at a volcanic gas emissions. In support of the pre- Tenerife was renamed ‘Terrorife’ in the inter- depth of three to four kilometers. However, diction of increasing volcanic activity, these national press [Christie, 2004], and residents the majority of the epicenters were located authors have cited two apparently new fea- along the northern coast towns began sleeping far from the rift zone, in an area in the Icod tures: fumaroles at the summit crater of fully dressed and panic-buying food and Valley that satellite interferometry has shown Teide and a new fumarole inside the Oro- other household supplies. to have subsided by up to 10 centimeters tava Valley. Spectacular ‘plumes’ in the sum- Instead of providing clarifying and reas- [Fernández et al., 2003]. Groundwater has mit area of Teide (known locally as ‘la Toca suring press communiqués, the authorities been continuously and intensively extracted del Teide,’ or ‘Teide’s headdress’) are caused decided to raise the level of alert on the in this area since the 1960s, depleting the aqui- by strong winds and other atmospheric con- basis of a reported increase in seismicity fer and causing the ground to sink. Micro- ditions as well as by increased fumarolic and the emission of volcanic gases. At no faulting associated with the subsidence may activity related to barometric pressure changes, time was there universally accepted scien- well have caused seismicity. It is noteworthy and have frequently been cited over the cen- tific evidence of volcanic activity on the that according to the interferometry study, turies in ships’ logs (Figures 2c and 2d). The island, but the ‘volcanic crisis’ alert was offi- this is the only area of recent ground defor- new ‘fumarole’ of the Orotava Valley, in turn, cially maintained until February 2005, with mation in Tenerife. The putative eruptive which gave a clear meteoric water isotopic frequent reports in the media of enormous regions of Teide and Las Cañadas are com- signature, is located 50 meters from an emissions of volcanic gases, boiling of the pletely stable. unlined 40-meter-deep well used for the dis- island aquifer, movements of magma under- But if the source of the seismicity is non- posal of high-temperature wastes from a neath the volcano, and impending explosive volcanic, what about the reported enor- nearby cheese factory, suggesting a rather eruptions of Teide forecast for October 2004. mous increase in gas emission? Continuous, more simple explanation for the origin of Was there any convincing evidence for real-time monitoring of gas emissions at this particular vapor exhalation site [Car- this at all? The official institution responsible Teide and the rifts [Martín, 1999] provided racedo et al., 2006]. for monitoring seismic and volcanic activity evidence of the nearly constant total gas It would thus seem that the prediction of an in Spain—the Instituto Geográfico Nacional emission of the volcanic system (Figure 2a). imminent volcanic eruption at Teide was and (IGN)—improved the seismic network of Significant diurnal and seasonal variations is lacking hard scientific evidence, and its dra- Tenerife in 2000. According to the IGN, the in gas emission rates are observed [Soler et matic presentation by the media was not only Eos, Vol. 87, No. 43, 24 October 2006 unnecessarily damaging to the tourism-based References Author Information economy of the island industry, but also caused undue anxiety and hardships for citi- Carracedo, J. C., F. J. Pérez Torrado, E. Rodríguez Juan Carlos Carracedo, Estación Volcanológica Badiola, A. Hansen, R. Paris, H. Guillou, and S. Scail- de Canarias, Consejo Superior de Investigaciones zens coping with the alarmist forecasts (Note, let (2006), Análisis de los riesgos geológicos en Científicas (CSIC), La Laguna, Tenerife, Spain; none of the authors is in any form associated el Archipiélago Canario: Origen, características, E-mail: [email protected]; Valentin R. Troll, with the tourism industry anywhere). probabilidades y tratamiento, Anu. Estud. Atl., 51, Department of Geology, Trinity College, Dublin, Effective and reliable communications 513–574. Ireland; Francisco J. Pérez Torrado, Departamento Christie, M. (2004), Welcome to Terrorife, Daily Record, must be established among scientists, gov- Física-Geología, Universidad de Las Palmas de 16 June. Gran Canaria, Spain; Eduardo Rodríguez Badiola, ernment officials, the news media, and the Fernández, J., T. T. Yu, G. Rodríguez-Velasco, and Departamento de Geología, Museo Nacional de populace affected, and must be tried and J. González-Matesanz (2003), New geodetic moni- Ciencias Naturales, CSIC, Madrid, Spain; Alex Han- tested before an actual crisis. This is an toring system in the volcanic island of Tenerife, sen Machín, Departamento de Geografía, Universi- Canaries, Spain: Combination of InSAR and GPS dad de Las Palmas de Gran Canaria; Raphael Paris, essential step in restoring the credibility of techniques, J. Volcanol. Geotherm. Res., 124, 241–253. Maison de la Recherche, Centre Nationale de the scientists and civil authorities, so that García, A., J. Vila, R. Ortiz, R. Macia, R. Sleeman, J. M. Recherche Scientifique (CNRS), Clermont-Ferrand, Marrero, N. Sánchez, M. Tárraga, and M. Correig they, working together, will be much better France; and Hervé Guillou and Stéphane Scaillet, positioned to respond adequately to a potential (2006), Monitoring the reawakening of Canary Island´s Teide volcano, Eos Trans. AGU, 87(6), 61, 65. Laboratoire des Sciences du Climat et de genuine volcanic crisis at Teide. Martín, M. C. (1999), Variación espacio-temporal del l’Environnement, Comision Energie Atomique, nivel de emisión de radón en una zona volcánica CNRS, Paris, France. Acknowledgments activa: Tenerife (Islas Canarias), Ph.D. dissertation, 270 pp., Univ. of La Laguna, Laguna, Spain.

Soler, V., et al. (2004), High CO2 levels in boreholes Insightful reviews by Robert I. Tilling and at El Teide volcano complex (Tenerife, Canary Chris Stillman greatly improved this contri- Islands): Implications for volcanic activity moni- bution and are gratefully acknowledged. toring, Pure Appl. Geophys., 161, 1519–1532.

Another issue that could be addressed by the NAC in February, following initial discus- news sion at the October meeting, is the future availability of rockets for small- and medium- sized science missions. Science committee member Neil DeGrasse Refocusing NASA Planetary Science Funding Tyson, director of the Hayden Planetarium at the American Museum of Natural History in PAGE 463 the beginning. As a comparison, in the field of astrophysics, researchers who are awarded New York, N.Y., noted that the marketplace for these rockets has diminished, and that NASA should invest more money in data observing time with the Hubble Space Tele- this could threaten the ability of NASA to analysis for its planetary science missions, scope also are provided funds for analyzing obtain them on time and in needed quanti- even if it means delaying or canceling a the data and publishing results. In planetary ties. He suggested that the NASA administra- future mission, members of the science com- missions, the data usually are certified and tor should discuss with other federal agencies mittee of the NASA Advisory Council (NAC) archived, but very little of it is properly ana- the potential for using other types of rockets suggested at a 12 October meeting. lyzed and transformed into scientific results, as launch vehicles for NASA science missions. Science committee member Mark Robinson, he said. Science Committee Chair Edward David, director of the Center for Planetary Sciences “This is really an issue of getting our value Jr., president of EED, Inc., and a former presi- at Northwestern University in Chicago, Ill., for the dollar. We spend hundreds of millions, dential science advisor, said that “it is impor- said that large amounts of data from NASA sometimes billions, [of dollars] on these tant that the whole NAC continues to monitor planetary science missions are accumulating, planetary missions, and very little of the data the situation and make suggestions how to and the funds for data analysis often are is ever looked at,” Stern said. “I think that it is address the shortage of access.” Schmidtt noted inadequate to properly analyze all of it. For the sense of the planetary [science] commu- that there are many other potential launch example, there is a large amount of currently nity that skipping a future mission to solve systems available and in use by other federal unanalyzed Mars data that could be used in this problem would be worth it,” he said. agencies, particularly the U.S. Department of planning for the Mars Science Laboratory Mary Cleave, associate administrator for Defense. (expected to launch in 2009), and particu- NASA’s Science Mission Directorate, said that David also told the NAC that the science larly for site selection. This includes data the agency has been trying to maintain an committee needed a member with expertise from the recently arrived Mars Reconnais- opportunity for launching a mission to Mars in Earth science, citing the departure of former sance Orbiter, which is likely to send more every 26 months. The agency could skip one committee chair Charles Kennel. Schmidtt data to Earth in its first year than has been of these launch opportunities and put more said that he hoped such an appointment collected by all previous Mars missions money into R&A, but it needs guidance from would be made by the NASA administrator combined. the NAC, she said. within the next few weeks. Kennel was one Science committee member Alan Stern, NAC Chair Harrison Schmidtt asked Robin- of three science committee members who executive director of the Space Science and son to draw up a formal recommendation resigned in August and were replaced the Engineering Division of the Southwest Research emphasizing the need for NASA to provide next month (see Eos 87(40), 2006). Institute in San Antonio, Tex., explained that funds for data analysis for these missions. missions need to have more research and The council could consider the recommen- —SARAH ZIELINSKI, Staff Writer analysis (R&A) funds attached to them from dation at its next meeting in February 2007. Eos, Vol. 88, No. 28, 10 July 2007

public. As the media pressed for news, scien- forum tists involved in the official committee, and some who were not involved, volunteered their views on potential scenarios, including improbable catastrophic ones that made headlines. This information contradicted the Communication Between Professionals official ‘call to calm’ and caused confusion among the public. Although the head of the During Volcanic Emergencies official committee tried to clarify the situation, the official scientific team was continually crit- PAGe 288 groups and individuals involved in the crisis, icized by scientists not involved in the man- and being responsible and honest with media agement of the crisis, some of whom had Successful communication between scien- by keeping them informed of all facts and any rejected the offer to become a part of the offi- tists, officials, media, and the public is impera- changes in the situation. cial team. tive during a volcanic crisis. Misunderstanding However, during the 2004–2005 seismic crisis While the crisis in Tenerife did not result in can lead to confusion and distrust, and it ulti- on the Canary Island of Tenerife (see “Monitor- an eruption, it is vital to learn from it lessons mately can transform an emergency into a ing the reawakening of Canary Islands’ Teide about scientific communications and relations, disaster. volcano,” by A. García et al., Eos Trans AGU, specifically during volcanic crises. While vol- Experience developed during volcanic cri- 87(6), 61, 2006), scientists did not always follow canic areas host many competent experts, dur- ses in the Caribbean has helped identify ‘good this protocol, and confusion and occasional ing times of crisis responsible scientists should practice’ guidelines for communication by sci- panic within the population ensued (see never volunteer their personal forecasts and entists during volcanic emergencies (see “Recent unrest at Canary Islands’ Teide vol- opinions to the media and should support the “Communication during volcanic emergen- cano?,” by J. C. Carracedo et al., Eos Trans. AGU, decisions taken by the civil authorities and the cies: An operations manual for the Caribbean, 87(43), 2006). The island is volcanically active, official scientific group. Differences of opinion by C. Solana et al., Benfield Greig Haz. Res. and the most recent eruption took place in among scientists are expected, and dialogue Cent., Univ. Coll. London, 2001; available at 1909. In 2004, scientists working on the island and discussion should be encouraged. How- http://www.bghrc.com). Key actions include interpreted observed changes in geochemistry ever, this should be conducted out of the pub- the following: and seismicity as being of volcanic origin, lic eye. Before a crisis, encouraging open dia- • Ensuring effective communication and which led to the creation of an official scien- logue between the parties involved to agree to appropriate understanding of information tific committee to forecast the likely evolution a protocol of communication can prevent a within and between groups managing the cri- of the crisis and to assess different possible sce- future recurrence of the situation encountered sis. narios. on Tenerife. • Disseminating only one clear and consis- At the beginning of the crisis, the flow of tent message—the one established by the offi- information was constrained to the monitor- —Carmen Solana and Claire Spiller, School cials in charge. ing scientists and civil authorities. When the of Earth and Environmental Sciences, University of • Building trust among participants and the general population felt three earthquakes, the Portsmouth, Portsmouth, UK; E-mail: Carmen.solana@ public through unanimous support of official scientific committee deemed it necessary to port.ac.uk decisions, maintaining the credibility of all communicate the nature of the crisis to the FEATURE Feature Seismicity and gas emissions on Tenerife: a real cause for alarm? Juan Carlos In recent months the media has been drawing attention to the possibility of 1 a dangerous eruption of the Teide Volcano on Tenerife in the Canary Islands. Carracedo & 2 Before accepting this prediction, which may well be detrimental to the Valentin R. Troll 1Estación Volcanológica de tourist-based economy of the island, it would be wise to examine the Canarias, IPNA-CSIC, La evidence on which it is based. Laguna, Tenerife, Spain, [email protected]; Teide has long been recognized as a significant eruption’ was the final summit eruption of Teide, but 2Department of Geology, volcano. At 3718 m above sea level, it is the third the new radiometric data now shows that eruption to Trinity College Dublin, highest volcanic structure on the planet (Fig. 1), after have been much older, 1140 ± 60 years before Dublin 2, Ireland, Mauna Loa and Mauna Kea in the Hawaiian Islands, present, i.e. in the eighth century AD. [email protected] and early drew the attention of renowned scientists One of the interesting features this study has such as Leopold von Buch and Alexander von revealed is that the Teide central volcanic complex Humboldt. Regrettably this interest was not sustained was a direct consequence of the activity of the rifts. into the twentieth century; whilst Hawaii attracted The rifts developed a progressively steepening, intense study, geological investigations of Teide’s unstable volcano at the centre of the island, and history languished. For example, hundreds of about 180–200 000 years ago triggered a massive radiocarbon age determinations have allowed for a landslide that generated the impressive horseshoe- Fig. 1. View of Teide volcano precise reconstruction of the Hawaiian volcanic shaped collapse embayment, whose headwall is the from the north east. history, but until 2001 only one single radiometric age was available for Teide volcano. With no regular recordings, evidence of historic seismic and volcanic activity has only been by word of mouth. The situation changed in 2001 when a five-year, joint Spanish–French research project began; a project which produced a detailed geological map of Teide and the NW and NE rifts (Fig. 2), petrographic analysis of numerous samples, and radiometric studies, which produced 28 new radiocarbon ages and 26 new K/Ar ages. This project completely changed our understanding of the geological and structural evolution of the Teide volcanic complex, and identified the sequence of eruptions that had occurred in the last 10 000 years. For example, one eruption of historic significance was that of 24 August 1492, which Christopher Columbus reported as ‘a big fire in the sierra of Tenerife... similar to those of Mount Etna in Sicily’. It seems it was not a summit eruption of Teide; a radiocarbon age from an eruption of the Boca Cangrejo cinder cone (Crab’s Mouth cone), located in the NW Rift, is in close agreement with this historic date. It had been believed that the ‘Columbus

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Fig. 2. Geological map of Teide and the caldera, showing in colour the most recent eruptions (last 30 000 years).

famous 17 × 10 km Las Cañadas caldera (Fig. 3). have occurred at a rate of only 4–6 per millennium, Following this, further rift and central activity the majority (70 per cent) being low-hazard basaltic progressively filled the embayment and built up the eruptions from fissure and cones on the rift zones, 3718 m-high Teide stratovolcano, nested in the and the others producing phonolitic lava-domes with collapse embayment (Fig. 4). The main phase of only very localized explosive activity (Fig. 5), as for construction ended 30 000 years ago, since when example Mña Blanca, around 2000 years old, which the stratovolcano has erupted only once. That is situated at the foot of the main stratavolcano. eruption was the eighth century AD summit eruption, It can be seen from the results of the study that in which raised the volcano from a height of 3600 m to the present phase of the island’s volcanic activity its present 3718 m. there appears to be little risk of major volcanic This recent eruptive history is in stark contrast to hazards, with any eruptions likely to pose only very the pre-collapse activity of the Cañadas volcano, localized threats to the inhabitants of Tenerife or to which was much more explosive, with frequent visitors to the Teide National Park (about 4.5 million plinian eruptions that generated extensive ash flows annually). Why then are there dire predictions of and ash falls. In the last 30 000 years, eruptions volcanic catastrophe (Fig. 6)?

Fig. 3. Western view of Las Cañadas caldera.

along the northern coast in the towns of Icod, Garachico, etc., began sleeping fully dressed and panic-buying food and other household supplies. Some even sold their houses! Evacuation instructions were given in schools and hospitals, and gas masks, power generators and other supplies were stockpiled in public buildings. The situation was made even worse by the authorities who, instead of clarifying the situation, raised the level of alert apparently only on the basis of what was reported as an increase in seismicity and the emission of volcanic gases. However, at no time was there any universally accepted scientific evidence of actual volcanic activity on the island. The ‘volcanic crisis’ alert was officially maintained until February 2005, with frequent reports in the media of enormous emissions of volcanic gases, boiling of the island aquifer, presumably from raised volcanic temperatures, movements of magma underneath the volcano and an explosive eruption forecast for Unrest at Teide volcano? Fig. 4. Teide volcano nested in the Las Cañadas caldera. Image October 2004. The prediction of an explosive eruption seems to be by NASA. based on reports of significantly increased earthquake activity and volcanic gas emissions. But is this Was there really any convincing evidence for evidence correct, and does it imply an imminent any of this? violent eruption? Firstly, consider the seismicity. Prior to 2000 there From 22 April to 28 July 2004, about 50 low- were only two seismic stations on Tenerife, both at magnitude (1 to 3) earthquakes were recorded in the far north-eastern end of the island. In that year a Tenerife, with most of the epicentres localized on the third station was deployed inside the Caldera de Las NW rift zone in the area of the Icod Valley. Only three Cañadas, and immediately low-magnitude events were actually felt by residents. This frequency is within the island began to be recorded for the first nothing special, such earthquakes are normal in time since surveillance began in 1985. Unfortunately volcanic oceanic islands; about 10 000 are recorded the focal depths of these events were not determined, annually in Hawaii, of which only 1200 are greater thus depriving observers of one of the most powerful than magnitude 3. Low magnitude earthquakes constraints on locating the source of the seismicity. (generally less than magnitude 3) have been recorded In the initial reports, the activity was interpreted as in all the Canary Islands. In May 1998 a bigger being due to dyke emplacement at a depth of 3–4 km. quake (magnitude 5.3) hit Tenerife, but a hazard- However, it is possible that a number of the events disclaiming statement was may have had a non-volcanic origin, such as traffic published in the media or explosions made during the excavation of water and the event promptly tunnels for groundwater mining. forgotten. The majority of the epicentres were located far So why, in 2004, were from the rift zone, in an area in the Icod Valley which the local and international satellite interferometry has shown to have subsided mass media bombarded by up to 10 cm. Groundwater has been continuously with persistent reports of and intensively extracted in this area since the unrest at Teide volcano, 1960s, depleting the aquifer and causing the ground and predictions made of an to sink. Microfaulting associated with the subsidence imminent, large-scale may well have caused seismicity. It is noteworthy explosive eruption that that according to the interferometry study this is the became dubbed ‘El Volcán only area of recent ground deformation in Tenerife. de Octubre’ (October The putative eruptive region of Teide and Las Volcano)? The publicity Cañadas is completely stable. had such a bad effect that Tenerife was nick-named ‘Terrorife’ and residents Fig. 5. Geological map of Teide’s peripheral phonolitic domes.

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unusual for the volcano. The new ‘fumarole’ in the Orotava Valley probably has a quite different explanation. Its water isotope signature is clearly phreatic, not volcanic, and it is located 50 m from an unlined 40 m-deep well used for the disposal of high- temperature wastes from a nearby cheese factory. It would thus seem that the prediction of an imminent volcanic eruption at Teide is based on distinctly flimsy and dubious evidence, and its dramatic presentation by the media is not only unnecessarily damaging to the tourism-based economy of the island, but also to the credibility of responsible scientists, the true value of which will be What is the scientific evidence for the Fig. 6. ‘Scientists’ predictions’ crucial for the correct management of any future reported increase in gas emissions? in the local press. genuine volcanic crisis. Continuous real-time monitoring of gas emissions at Teide and in the rifts, reported in 1999, gave Suggestions for further reading evidence of a nearly constant total gas emission from Carracedo, J.C. 1994. The Canary Islands: an the volcanic system. On a day-to-day basis, local example of structural control on the growth of emissions may vary and significant diurnal and large oceanic island volcanoes. Journal of seasonal variations in gas emission rates do Volcanological and Geothermal Research, v.60, apparently relate directly to systematic changes in pp.225–242. barometric pressure. If spot measurements are taken Carracedo, J.C. 1999. Growth, structure, instability in periods of low barometric pressure, they can show and collapse of Canarian volcanoes and an apparent ‘significant’ increase in emissions. In comparisons with Hawaiian volcanoes Journal of 2004, shortly before the predicted October eruption, Volcanological and Geothermal Research, Special a visit to the summit crater of Teide, (in which both of Issue, v.94, pp.1–19. us took part), did not reveal any fumarolic activity at Carracedo, J.C. & Day, S.J. 2002. Geological Guide of all. the Canary Islands. Classic Geology in Europe. Terra In support of the prediction of increasing volcanic Publishing, London. activity, observers have cited two apparently new Guillou, H., Carracedo, J.C., Paris, R. & Pérez features; fumaroles at the summit crater of Teide and Torrado, F.J. 2004. Implications for the early a new fumarole inside the Orotrava Valley. In fact shield-stage evolution of Tenerife from K/Ar ages spectacular ‘plumes’ are commonly seen in the and magnetic stratigraphy. Earth and Planetary summit area of Teide and are known locally as ‘la Science Letters, v.222, pp.599–614. Toca del Teide’ or ‘Teide’s head-dress’ (Fig. 7). These Walter T.R., Troll V.R., Caileau B., Schmincke H.-U., are caused by atmospheric conditions as well as by Amelung F. & van den Bogaard, P. 2005. Rift increases in fumarolic activity related to barometric zone reorganization through flank instability on pressure changes and have frequently been cited over ocean islands – an example from Tenerife, Canary the centuries in ship’s logs. They are in no way Islands. Bulletin of Volcanology, v.65, pp.281–291.

Fig. 7. A. Frequent spectacular ‘plumes’ in the summit area are locally known as ‘Teide’s headdress’ and are usually clouds caused by strong winds and pressure-dependant changes in fumarole activity and are not unusual for the volcano (picture by J.C. Carracedo). B. Model of cloud formation at Teide volcano (Alvarez and Hernandez 2006, Canary Meteorological Service, see http:// www.acanmet.org.es/ menu_archivos/documentos/ FTLCTeide.doc).

New evidence for the reawakening of Teide volcano J. Gottsmann,1,2 L. Wooller,3 J. Martı´,1 J. Ferna´ndez,4 A. G. Camacho,4 P. J. Gonzalez,4,5 A. Garcia,6 and H. Rymer3 Received 17 July 2006; revised 13 September 2006; accepted 19 September 2006; published 25 October 2006.

[1] Geophysical signals accompanying the reactivation of central volcanic complex (CVC), the third-highest volcanic a volcano after a period of quiescence must be evaluated as complex on Earth rising almost 7000 m from the surround- potential precursors to impending eruption. Here we report ing seafloor [Garcıá et al., 2006]. The increase in onshore on the reactivation of the central volcanic complex of seismicity, including five felt earthquakes, coincided with Tenerife, Spain, in spring 2004 and present gravity change both an increase in diffuse emission of carbon dioxide along maps constructed by time-lapse microgravity measurements a zone known as the Santiago Rift [Pe´rez et al., 2005] and taken between May 2004 and July 2005. The gravity increased fumarolic activity at the summit of the 3718 m changes indicate that the recent reactivation after almost a high Teide volcano [Garcıá et al., 2006]. century of inactivity was accompanied by a sub-surface mass addition, yet we did not detect widespread surface 2. Integrated Geodetic Network on Tenerife deformation. We find that the causative source was evolving in space and time and infer fluid migration at depth as [3] As a reaction to the developing crises, we installed the most likely cause for mass increase. Our results the first joint ground deformation/microgravity network on demonstrate that, even in the absence of previous baseline the island in early May 2004, two weeks after the start of data and ground deformation, microgravity measurements increased seismicity. The network consists of 14 bench- early in developing crises provide crucial insight into the marks, which were positioned to provide coverage of a 2 dynamic changes beneath a volcano. Citation: Gottsmann, J., rather large area (>500 km ) of the CVC, including the Pico L. Wooller, J. Martı´, J. Fernańdez, A. G. Camacho, P. J. Gonzalez, Viejo-Pico Teide complex (PV-PT), the Las Canãdas caldera A. Garcia, and H. Rymer (2006), New evidence for the (LCC) as well as the Santiago Rift (SR) (Figures 1 and 2). reawakening of Teide volcano, Geophys. Res. Lett., 33, L20311, The network was designed to meet rapid response require- doi:10.1029/2006GL027523. ments, i.e., the network can be fully occupied to a precision of less than 0.01 mGals of individual gravity readings and less than 0.04 m in positioning errors within six working 1. Introduction days despite the frequently rugged terrain. The first reoc- [2] Anomalous geophysical signals at dormant volca- cupation of the network was performed in July 2004, noes, or those undergoing a period of quiescence, need to followed by campaigns in April 2005 and July 2005. be evaluated as potential precursors to reawakening and Benchmark locations and cumulative ground deformation possible eruption [White, 1996]. There are several recent and gravity changes between May 2004 and July 2005 are examples of volcanic re-activation after long repose inter- given in Table 1 in the auxiliary material.1 All results are vals culminating in explosive eruption [Nakada and Fujii, given with respect to a reference located south of the LCC 1993; Robertson et al., 2000], but non-eruptive behaviour is (benchmark LAJA). Within the average precision of bench- equally documented [De Natale et al., 1991; Newhall and mark elevation measurements (±0.03 m), using two dual- Dzurisin, 1988]. The dilemma scientists are confronted with frequency GPS receivers during each campaign, we did not is how to assess future behaviour and to forecast the observe widespread ground deformation. However, between likelihood of an eruption at a reawakening volcano, when May 2004 and July 2005, four benchmarks, two located in critical geophysical data from previous activity is missing the eastern sector of the LCC (MAJU and RAJA), one due to long repose periods. In Spring 2004, almost a century marking the northern-most end of the network and also the after the last eruption on the island, a significant increase in lowest elevation (766 m; CLV1) and finally a benchmark the number of seismic events located inland on the volcanic located on an isolated rock spur on the western LCC rim island of Tenerife (Figure 1) marked the reawakening of the (UCAN, supporting online material) did show ground uplift above measurement precision. Residual gravity changes (corrected for the theoretical Free-Air effect), observed 1Institute of Earth Sciences ‘‘Jaume Almera’’, Consejo Superior de Investigaciones Cientificas, Barcelona, Spain. during the May–July 2004, May 2004–April 2005, and 2Department of Earth Sciences, University of Bristol, Bristol, UK. May 2004–July 2005 periods are listed in the supporting 3Department of Earth Sciences, Open University, Milton Keynes, online material and shown in Figure 2. UK. 4Institute of Astronomy and Geodesy, Consejo Superior de Investiga- ciones Cientificas, Universidad Complutense de Madrid, Madrid, Spain. 3. Results 5Environmental Research Division, Institute of Technology and Renewable Energies, Tenerife, Spain. [4] The observed gravity changes do not fit a simple 6Department of Volcanology, Museo Nacional de Ciencias Naturales, symmetrical pattern as observed, for example, during cal- Consejo Superior de Investigaciones Cientificas, Madrid, Spain.

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July 2005 (Figure 2c). During the same time, a marked gravity decrease was recorded at the intersection of the Orotava Valley (OV) and the LCC (Figure 2c). [5] Except for two benchmarks (MAJU and RAJA) where observed gravity changes can be explained by free- air effects (gravity changes due to elevation changes), mass/ density changes in the sub-surface appear to cause the major part of the perturbation of the gravity field.

4. Effect of Water Table Fluctuations

[6] Data from two drill holes, located in the eastern half of the LCC (Figure 2d), provide information on water table fluctuations during the period of interest. A drop of ca. 5 cm/month between surveys 1 and 4 was recorded in one drill hole located close to benchmarks 3RDB and MAJU, which is similar to the average monthly drop in water level due anthropogenic extraction over the past Figure 1. Perspective view of Tenerife island located in 3 years [Farrujia et al., 2004]. Water levels decreased by the Canarian Archipelago off the coast of northwest Africa 22 cm/month on average between February 2000 and (inset), using a colour-coded digital elevation model January 2004 in a drill hole located close to benchmark (DEM; elevation in meters). Highest point is Teide volcano MIRA. The gravity decrease of 0.025 mGal recorded (3718 m a.s.l.) located at 28.27°N and 16.60°W. Black between May 2004 and July 2005 at benchmark MIRA, open squares indicate epicentres of seismic events recorded located at the intersection of the Las Canãdas caldera and between May 2004 and July 2005 by the National the Orotava valley, can be explained by a net water Geographic Institute (available at http://www.ign.es). table decrease (dh) of 3 m, consistent with this earlier Black rectangle identifies the area covered by the joint trend, assuming a permeable rock void space (8) of 20% GPS/gravity network. LCC indicates the location of the 3 and a water density (r) of 1000 kg/m (Dgw =2pGrfdh) Las Canãdas caldera. [Battaglia et al., 2003]. Following the same rationale, gravity changes at 3RDB and MAJU are corrected by À0.008 mGal to account for the recorded water table fall dera unrest at the Campi Flegrei [Gottsmann et al., 2003] or in the nearby borehole. Hence, any gravity change observed at Long Valley [Battaglia et al., 2003]. The spatial distri- within the central and eastern parts of the LCC (3RDB, bution of gravity changes across the area under investiga- MAJU, RAJA, MIRA) can be fully attributed to changes in tion is asymmetrical. The smallest gravity changes were (shallow) groundwater levels and we treat the net mass observed in the central and eastern depression of the LCC, change as zero for this area in the computation of overall where cumulative changes over the 14-month period where mass changes in the following sections (Figure 2d). only slightly higher than the precision level (±0.015 mGal 2 [7] Outside the LCC, comprehensive monitoring data on on average; 1 mGal = 10 mm/s ). A marked positive gravity groundwater level is lacking and correction for groundwater anomaly, with a maximum amplitude of around 0.04 mGal, level variations is difficult. Groundwater is collected and developed in the Northwest of the covered area between extracted along several hundred (sub)horizontal tunnels May and July 2004, while a negative anomaly was found to (galerias) protruding into the upper slopes of the CVC the east, centered on station MIRA. The gravity increase [Marrero et al., 2005]. Since 1925, a decrease of several noted between the first two campaigns (benchmarks C774 hundred meters in the groundwater level has been noted for and CLV1) was followed by a decrease sometime between the area covered by the northern and western slopes of the July 2004 and April 2005. During the same period, a N-S CVC (available at http://www.aguastenerife.org). We there- trending positive anomaly appears northwest of the PV-PT fore consider it very unlikely that the gravity increase noted summit area between, reaching the western part of the LCC in the north and west of the CVC is related to an increase in (Figures 2a–2b). In addition, gravity increased significantly the groundwater table, and hence infer deeper processes to along the northern slopes of Pico Teide, including bench- be the most probable cause of gravity change in this region. marks TORR and FUEN located close to the La Orotava valley between July 2004 and April 2005, adding to the impression of a spatio-temporal evolution of the causative 5. Interpretation source. It is interesting to note that on 5 December 2004 a [8] The coincidence of earthquake epicenter concentra- new fissure with fumarole emission appeared in the Orotava tion (a mixture of volcano-tectonic events and regional valley (further information available at http://www.iter.es). earthquakes with pure volcanic events such as tremors A gas plume emanating from the summit fumaroles of Pico and long-period signals) in the area of gravity increase over Teide was particularly noticeable during October 2004 the same time period (Figure 2d), suggests that both signals [Garcıá et al., 2006], between surveys 2 and 3. In summary, are related to the same or linked phenomena. Unfortunately, significant gravity changes occurred mainly across the precise data on earthquake hypocentres are not available, northern flanks of the PV-PT and along a ca. 6 km wide but a semi-qualitative analysis suggests a depth of several zone along the western side of the volcanic complex into kilometres (R. Ortiz, personal communication, 2005). The the westernmost parts of the LCC between May 2004 and

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Figure 2. Residual gravity changes between (a) May and July 2004, (b) May 2004 and April 2005, and (c) May 2004 and July 2005. (d) Same as Figure 2c but corrected for the effect of water table changes. Gravity changes are draped over a DEM of the central volcanic complex (CVC) of Tenerife. Black line in Figure 2a delineates the Las Can˜adas caldera (LCC) wall. Benchmark locations (crosses) and identification are shown as well as the prominent topographic features of the Santiago Rift, Teide volcano, and the Orotava Valley (OV). Uncertainty in gravity changes are on average ±0.015 mGal (1 mGal = 10 mm/s2). In Figure 2c the area to the east of the CVC, where a gravity decrease was detected, coincides with the intersection of the Las Can˜adas caldera with the collapse scar of the Orotava valley. This zone represents a major hydrological outlet of the caldera. In Figure 2d stars represent epicentres of seismic events recorded between May 2004 and July 2005. Both gravity increase and seismicity appear to be spatially and temporally correlated. Line A-B represents datum for profile shown in Figure 4. spatial coverage of the benchmarks does not allow the the PV-PT complex believed to host chemically evolved wavelength of the May 2004–July 2005 gravity anomaly magma [Ablay et al., 1998]. However, since the positive to be assessed precisely. In particular, the lower limit of the anomaly is only defined by four benchmarks (CLV1, C774, wavelength along the northern slopes of the PV-PT complex CRUC, and TORR) its actual wavelength could be smaller cannot be unambiguously retrieved on the basis of the than 17 km and the source depth could be shallower than available data. The maximum wavelength of the gravity inferred above. Furthermore, ambiguities remain on the anomaly is on the order of 17 km if defined by both actual amplitude of the anomaly, which is defined only by observed and interpolated (kriging) data (Figure 2d) on data observed at CRUC. The continuation of the positive the northern slopes of the PV-PT complex, which implies anomaly in the western part of the LCC (Figure 2c) shows a a maximum source depth of between 2.5 to 5.2 km below shorter wavelength indicating a shallow (few km deep) the surface, assuming simple axisymmetrical source geom- source. etries [Telford et al., 1990]. This would place the source to [9] Due to the spatial separation of benchmarks an within the depth of the shallow magma reservoirs beneath assessment of sub-surface mass addition is greatly biased

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Figure 3. (a) Predicted and (b) residuals between observed and predicted gravity changes (mGal) for the period May 2004 to July 2005. Predicted values are derived from inversion for an infinite horizontal cylinder as an approximation of the zone undergoing a mass/density increase at the northern and western slopes of the PV-PT complex. Observed gravity changes were corrected for the effect of water table fluctuations in the central and eastern part of the LCC prior to inversion. Red colours indicate that the model is predicting higher gravity changes than observed, blue colours indicate the opposite. Green colours indicate match between predictions and observations. on the selection of the area affected by gravity increases. We the number of earthquakes recorded by the National Geo- define a maximum area by a kriging-based interpolation of graphic Institute (available at http://www.ign.es). Dykes the gravity changes between May 2004 and July 2005 in the along the Santiago Rift are on average less than 1 m wide. northern and western parts of the CVC. A Gaussian Quad- Ground deformation caused by an individual dyke of this rature integration over this area gives a mass addition of size a few km below the surface would be below the 1.1*1011 kg, with lower and upper 95% confidence bounds precision of our GPS measurements. Thus, a magma injec- of 8.4*109 kg and 2.0*1011 kg, respectively. These values tion into a conjugated fault system, perhaps at some angle to should be regarded as maximum values. the Santiago Rift, cannot be unambiguously ruled out as [10] In theory, subsurface volume changes derived from the trigger for the reawakening of the volcanic complex in ground deformation data can be correlated to sub-surface May 2004. There is, however, little evidence to support the mass changes from gravity data to infer the density of the idea that the mass increase observed during campaigns 2 causative source. However, in the absence of significant and 3 is caused solely by magma movement. surface deformation, the source density cannot be deter- [11] An alternative explanation for the observed gravity mined directly and the nature of the source remains ambig- increase is fluid migration through the CVC. Volcano- uous. However, three scenarios are worth considering when tectonic events detected in the seismic record [Garcı´a et assessing causative processes for the observed gravity al., 2006; Ta´rraga et al., 2006] may have triggered the increase: (1) arrival of new magma at depth, (2) migration release and upward migration of hydrothermal fluids from a of hydrothermal fluids, and (3) a hybrid of both. Volcanic deep magma reservoir. Alternatively, fluid migration may eruptions of the CVC over the past few centuries were have resulted from (1) the perturbation of an existing deep dominantly fed by basic and intermediate magmas in the hydrothermal reservoir and resultant upward movement of form of fissure eruptions along the Santiago Rift [Ablay and fluids due to magma injection or (2) from pressurising Marti, 2000], implying shallow dyke emplacement along seawater saturated rocks. this NW-SE trending extension zone. The observed gravity [12] Migration of hydrothermal fluids through a perme- increase between May 2004 and April 2005 (Figure 2) able medium causes little surface deformation, but the appears to denote a zone at a 45° angle to the strike of filling of pore space increases the bulk density of the the rift. The wavelength of the anomaly in the western and material resulting in a gravity increase at the ground surface. central parts of the LCC (Figure 2d) is not consistent To explore this scenario, and as a first order approximation, with shallow dyke emplacement to perhaps within a few we performed a inversion of the gravity change recorded tens or hundred meters depth. There is also no other between May 2004 and July 2005 along the northern and direct geophysical or geochemical evidence in support of western slopes of the PV-PT complex for a source repre- magma emplacement in the form of a shallow dyke over the sented by a N-S striking infinite cylindrical horizontal body 14-month observation time. However, dyke emplacement at [Telford et al., 1990]. The approximation of an infinite body greater depth (a few km below the surface) into the Santiago is valid as long as the radius of the cylinder is far smaller Rift (with partial contribution to the gravity increases at than its length. The model results depend linearly on density benchmarks CLAV1, C774 and CRUC), perhaps recharging change but non-linearly on both the radius and depth of the an existing reservoir, cannot be unambiguously excluded body. Using a global optimization iterative method [Sen and for the period May–July 2004, coinciding with the peak in Stoffa, 1995] with various initial values for depth and

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fluids inside the complex triggering the observed gravity changes. [14] We demonstrate that time-lapse microgravity monitoring of active volcanoes can provide vital insights into their sub-surface dynamics, particularly where structural complexities and heterogeneous mechanical properties of the subsurface do not obey a simple linearly elastic rela- tionship of stress generation and resultant ground deformation [Dvorak and Dzurisin, 1997]. Arrival of a small batch of magma at depth and the release and upward migration of Figure 4. Cross-section through the CVC along the profile hot fluids may be a common trigger of reactivation after A-B shown in Figure 2d, including a conceptual model of long repose periods and may be quantifiable by perturba- events between May 2004 and July 2005. (1) Likely tions in the gravity field but may not be accompanied by injection of magma during peak of onshore seismic activity ground deformation. Quantification of sub-surface mass/ two weeks before installation of network (May 2004). density changes must be regarded as essential for the (2) Release of fluids or perturbation of existing hydro- detection of potential pre-eruptive signals at reawakening thermal system causing migration of fluids from NW to SE volcanoes before ground deformation or other geophysical (May–July 2004; Figure 2a) and later (July 2004–April signals become quantifiable [Rymer, 1994]. 2005 and further into July 2005) along a N-S striking zone (Figures 2b–2d). (3) Upward migration of fluids along the [15] Acknowledgments. This research is part of the TEGETEIDE upper surface of the high density/low permeability Boca project funded by the Spanish Ministry of Education and Science (MEC) Tauce magmatic body situated beneath the western caldera under contract CGL2003-21643-E. J.G. also acknowledges support from a [Ablay and Kearey, 2000]. (4) Fluid migration into an ‘‘Ramon y Cajal’’ grant by the MEC and a Royal Society University Fellowship. L.W. acknowledges the support of an Open University Re- overlying aquifer, located at a depth greater than 900 m search Development Fund fellowship. J.F., A.G.C., and P.J.G. also ac- beneath the LCC floor and thought to feed the PT summit knowledge support from EU projects ALERTA (MAC/2.3/C56) of the fumaroles [Aranã et al., 2000], can explain the increased INTERREG III B Program and from Spanish MEC projects REN2002- 12406-E/RIES and GEOMOD (CGL2005-05500-C02). The authors thank fumarolic activity of Teide in 2004. The western caldera the Consejo Insular de Aguas de Tenerife (CIATFE) and I. Farrujia for boundary fault may act as a pathway for fluids to shallower granting access to water table data on Tenerife and N. Perez for providing depth. cGPS data. References radius, we find convergence of the inversion results at a Ablay, G. J., and P. Kearey (2000), Gravity constraints on the structure and volcanic evolution of Tenerife, Canary Islands, J. Geophys. Res., 105, depth of 1990 ± 120 m below the surface using residual 5783–5796. gravity data from all benchmarks. While depth is insensitive Ablay, G. J., and J. Marti (2000), Stratigraphy, structure, and volcanic to the assumed source density change, the radius scales to evolution of the Pico Teide-Pico Viejo formation, Tenerife, Canary Islands, J. Volcanol. Geotherm. Res., 103(1–4), 175–208. the inverse of density. Assuming a volume fraction of 30% Ablay, G. J., M. R. Carrol, M. R. Palmer, J. Marti, and R. S. J. Sparks which is fully permeable, filling this void space with (1998), Basanite-phonolite lineages of the Teide Pico Viejo volcanic (hydrothermal) fluids of density 1000 kg/m3 would produce complex, Tenerife, Canary Islands, J. Petrol., 39(5), 905–936. a bulk density increase of 0.3 kg/m3. The resultant source Aranã,V.,A.G.Camacho,A.Garcia,F.G.Montesinos,I.Blanco, R. Vieira, and A. Felpeto (2000), Internal structure of Tenerife (Canary radius is around 80 ± 20 m. Although the fit to the data is Islands) based on gravity, aeromagnetic, and volcanological data, J. Vol- within errors very good (Figure 3), we find that the positive canol. Geotherm. Res., 103(1–4), 43–64. anomaly in the eastern part of the LCC cannot be satisfy- Battaglia, M., P. Segall, and C. Roberts (2003), The mechanics of unrest at Long Valley caldera, California. 2. Constraining the nature of the source ingly modelled. For this area, we conclude on either a local using geodetic and micro-gravity data, J. Volcanol. Geotherm. Res., effect or, more likely, an error in the GPS measurements 127(3–4), 219–245. during the installation of benchmark RAJA, since the De Natale, G., F. Pingue, P. Allarde, and A. Zollo (1991), Geophysical and geochemical modelling of the 1982–1984 unrest phenomena at Campi reported gravity increase results from the free-air effect of Flegrei caldera (southern Italy), J. Volcanol. Geotherm. Res., 48(1–2), the 7 ± 4 cm inflation detected over the 14 months period. 199–222. Ignoring the potentially erroneous GPS measurement, the Dvorak, J. J., and D. Dzurisin (1997), Volcano geodesy: Search for magma gravity residual for RAJA matches those of neighbouring reservoirs and the formation of eruptive vents, Rev. Geophys., 35, 343– 384. benchmarks MAJU and 3RDB. Combining all available Farrujia, I., J. L. Velasco, J. Fernandez, and M. C. Martin (2004), Evolucioń geophysical information, we conclude that migration of del nivel frea´tico en la mitad oriental del acuı´fero de Las Canãdas del hydrothermal fluids along a permeable N-S striking zone Teide: Cuantificacioń de para´metros hidrogeolo´gicos, paper presented at VIII Simposio de Hidrogeologıá, Assoc. Esp. de Hidrogeol., Zaragoza, is the most likely cause of the observed perturbation of the Spain. gravity field. A conceptual model of mass migration cov- Garcıá, A., J. Vila, R. Ortiz, R. Macia, R. Sleeman, J. M. Marrero, ering the 14-month observation period is shown in Figure 4. N. Sańchez, M. Ta´rraga, and A. M. Correig (2006), Monitoring the reawakening of Canary Islands’ Teide Volcano, Eos Trans. AGU, 87(6), 61. 6. Conclusions Gottsmann, J., G. Berrino, H. Rymer, and G. Williams-Jones (2003), Hazard assessment during caldera unrest at the Campi Flegrei, Italy: A [13] While magma recharge at depth into the northwest- contribution from gravity-height gradients, Earth Planet. Sci. Lett., ern rift zone of Tenerife is likely to have triggered the 211(3–4), 295–309. Marrero, R., P. Salazar, P. A. Hernańdez, N. M. Pe´rez, and D. Lo´pez (2005), reawakening of the CVC, the cause of the 14-month pertur- Hydrogeochemical monitoring for volcanic surveillance at Tenerife, bation of the gravity field is most probably not related to Canary Islands, Geophys. Res. Abstr., 7, Abstract 09928, sref-id: magma flow. A more likely scenario is the migration of EGU05–A-09928.

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