The changing face of Mount Etna s summit area documented with Lidar technology

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1 GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L09305, doi: /2008gl033740, 2008 The changing face of Mount Etna s summit area documented with Lidar technology M. Neri, 1 F. Mazzarini, 2 S. Tarquini, 2 M. Bisson, 2 I. Isola, 2 B. Behncke, 1 and M. T. Pareschi 2 Received 26 February 2008; revised 3 April 2008; accepted 7 April 2008; published 9 May [1] Morphostructural data derived from Lidar (Light detection and ranging) surveys carried out on Mount Etna in 2005 and 2007 are compared with earlier aerophotogrammetric surveys in 1986 and These data render an unprecedentedly clear and quantitative image of morphostructural and volumetric changes that have affected the summit area of the volcano in the past two decades and permit the production of a new topographic map. The computed volume gain during the period amounts to 112 ± m 3,ata mean annual rate of m 3. The comparison of the various surveys furthermore emphasizes the levels of accuracy and resolution of the different techniques applied. The Lidar technology used in 2007 allows production of high-precision maps in near-real-time, facilitating work concerning environmental hazards such as numerical simulations of, e.g., lava flows. Citation: Neri, M., F. Mazzarini, S. Tarquini, M. Bisson, I. Isola, B. Behncke, and M. T. Pareschi (2008), The changing face of Mount Etna s summit area documented with Lidar technology, Geophys. Res. Lett., 35, L09305, doi: /2008gl Introduction [2] Few locations on Earth change as dramatically and frequently as active volcanoes. They produce deposits that can lead to growth in height of their summits or to destruction and loss in height of portions of their edifices. Mount Etna in Sicily, Italy (Figure 1), is an instructive examples of this type, where the frequent eruptive activity of the last few decades has led to profound changes in the morphology. Since 1986, for example, Etna has produced ten major lava outflows and about 200 paroxysmal events (e.g. lava fountaining episodes [Behncke et al., 2005, 2006]), some of which have led to significant modifications of the summit morphology. [3] The most recent official topography of the Etnean summit area, based on aerophotogrammetry acquired in November 1998, was released in 1999 by the Provincia Regionale di Catania. A previous aerophotogrammetric survey was made in early Obviously there is a strong need for up-to-date volcanological and topographic data not only for hazard assessments, and especially those based on 1 Sezione di Catania, Istituto Nazionale di Geofisica e Vulcanologia, Catania, Italy. 2 Sezione di Pisa, Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy. numerical simulations, but also for anybody working on the volcano. [4] Recently, usage of remote-sensing systems has seen a massive growth in the study of surface morphologies from local to planetary scale. In the case of Etna, the Istituto Nazionale di Geofisica e Vulcanologia (INGV) has commissioned several overflights of the volcano aimed at the acquisition of altimetric data for morphological and volcanological analysis [Mazzarini et al., 2005, 2007]. The applied technology is Lidar (Light detection and ranging), which allows the acquisition of altimetric data at very high resolution. [5] A portion of the acquired Lidar data set is here used along with older conventional topographies to compare four plani-altimetric reliefs of the summit area of Etna covering the period We discuss the significance and quality of the acquired data to assess the usefulness of the different techniques, calculate volumetric changes, and present a 1:10,000 scale topographic map of the Etnean summit area, as of June 2007 (see auxiliary material 1 ) Eruptive Activity [6] During the interval , Etna erupted nearly every year from its summit and frequently from its flanks [Behncke et al., 2005; Allard et al., 2006]. Most of the activity occurred at the Southeast Crater (SEC in Figure 1), particularly in , , and This resulted in a net growth of the SEC cone until 2001, which was followed by partial collapse in , renewed growth in , and minor collapse in the spring of Major eruptive phases occurred also at the Northeast Crater (NEC) in 1986 and , at the Voragine (VOR) in 1989 and , and at the Bocca Nuova (BN) in These led to the filling of those craters and lava overflows, followed by collapse of new pits within the craters. [7] The flank eruptions of , 1989, , 2001, and caused major morphological changes mostly outside the study area, on the middle to upper flanks. 3. Airborne Lidar Technology and Data Processing [8] Airborne Lidar is a revolutionary technology to produce a high-resolution DEM (Digital Elevation Model) by rapid, accurate and moderate-to-low cost measurements Copyright 2008 by the American Geophysical Union /08/2008GL Auxiliary materials are available in the HTML. doi: / 2008GL L of6

2 Figure 1. Sketch map of Mount Etna showing location of the volcano and study area (dark grey rectangle). The active summit craters are also indicated (VOR = Voragine; NEC = Northeast Crater; BN = Bocca Nuova; SEC = Southeast Crater). of topography by flying over a large area. For these reasons, during the past decade the Lidar technology has expanded in numerous fields of applications, such as in geology, volcanology and geomorphology [Ventura and Vilardo, 2008; Mazzarini et al., 2005, 2007; Glenn et al., 2006] and hydrological modeling and flood prediction [Toyra and Pietroniro, 2005]. The system consists of i) a laser sensor emitting high frequency pulses; ii) a differential GPS receiver determining the location of the laser scanning system in 3-D space and iii) an Inertial Navigation System monitoring position and orientation of the airborne platform. Combining this information with the return time of each pulse the 3D location of objects scanned by the laser is determined [Baltsavias, 1999]. [9] Raw data processing is necessary to separate bareground information from vegetation and man-made features (buildings, etc.). Data processing implies the tuning of filtering procedures mainly based on the progressive densification algorithm [Axelsson, 1999]. Once the final bareground points set is determined, these points are triangulated to obtain a Triangular Irregular Network (TIN) from which an elevation matrix (GRID) is derived and used as DEM in terrain analysis Etna Data Set and DEM Analysis [10] We have considered two DEMs obtained from existing numeric topographic maps (1986, 1998), along with two Lidar-derived DEMs (2005, 2007). The 1986 and 1998 input data sets are based on 1:10,000 numeric cartography derived from aerial photogrammetry. The 10 m equidistance contour lines are triangulated to obtain a TIN refined by the Determination of Earth Surface Structures (DEST) algorithm [Favalli et al., 1999; Favalli and Pareschi, 2004; Tarquini et al., 2007]. The average TIN node density is pt/m 2 for 1986 and pt/m 2 for The comparison amongthefourdemsdeterminesinthe1986and1998models the areas affected by artifacts probably due to the presence of the gas plume (dotted areas in Figure 2a, and 2b). [11] The 2005 and 2007 Lidar data sets consist of (average density: 0.55 pt/m 2 ) and (average density: 0.72 pt/m 2 ) 3D ground points, respectively. The first data set was acquired flying at an average altitude of 3,000 m a.s.l. in September 2005 using an Optech ALTM 3033 operating in the Near Infrared Region (l = mm) and emitting 33,000 pulses per second. The horizontal accuracy is ±1.5 m (1/2000 flight altitude), whereas the vertical accuracy is ±0.4 m. [12] The second data set was acquired flying at a mean altitude of 4,200 m a.s.l. in June 2007 using a Gemini laser altimeter operating in the Near Infrared Region (l = mm) and emitting 110,000 pulses per second. Consequently, the horizontal accuracy increased to ±0.38 m (1/11,000 flight altitude), whereas the vertical accuracy is ± m. According to the data set point density, we derived the two DEMs with a resolution of 2 m (Figures 2c and 2d). The Lidar surveys show uneven gaps inside the craters due to the presence of the gas plume. [13] The surveys results were referred to a GPS station of the Istituto Geografico Militare geodetic network (IGM95) and memorized in the WGS84-UTM 33 coordinate system. To compare the different resolutions of the input DEMs we derived the models in the same TIN format and statistically analyzed the size of the elementary triangles constituting the four TIN meshes. We plotted the percent of model planimetric surface as a function of the square root of the area of involved triangles in order to visualize the different DEMs resolutions (Figure 3a). The smaller the triangles the higher the resolution. To compute the volume changes of the studied area we derived the four DEMs with the same extent and cell size and then performed the subtraction on a cell by cell basis to obtain the amounts of volumetric change from one DEM to another. [14] With the aim to evaluate the errors in the volumetric change estimations, we analyzed the Root Mean Square Error (RMSE) of the 1986 and 1998 models, assuming the 2007 Lidar ground points as surface reference because of their low error ( m). The elevation RMSE between the TINs and the adopted reference is evaluated using 132,744 selected points placed in 8 km 2 areas not affected by post-1986 eruptive activity. The resulting global RMSE is 2.69 for the 1986 TIN and 1.98 for the 1998 TIN. Figure 3b compares the positive changes in volume and thicknesses of eruptive products and errors in calculation for the four time windows discussed in the following section. 5. Morphostructural Changes [15] Four difference maps (Figure 4) derived from the comparison of the four DEMs show the accumulation of material in the summit area of Etna during the periods , , and We have added the outlines of eruptive products for each of these periods, as they can be traced from the difference 2of6

3 Figure 2. Shaded images of the four DEMs of the Etnean summit area (7.04 km 2 ) here analyzed, constructed from aerophotogrammetric surveys in (a) 1986 and (b) 1998, and Lidar surveys in (c) 2005 and (d) Dotted patches indicate areas affected by artifacts (see text for explanation). Coordinates in upper left corner are in WGS84-UTM 33 system (meters). maps. A problem arises for the morphological changes affecting the active pits of the NEC, VOR and BN, because the 1986, 1998 and 2005 DEMs fail to represent them correctly probably due to the presence of gas plumes. We have therefore eliminated these areas from our calculations, as well as an area to the south of the BN, which was equally below the gas plume in the 1986 and 1998 DEMs and thus showed abnormally high errors. The new Lidar instrument used in 2007 reveals more detail of the pits interiors, notwithstanding the presence or absence of gas plumes. [16] We have tested whether the differences are due exclusively to eruptive processes (e.g., accumulation of eruptive products, collapse of pit craters) or other factors like volcano deformation might be significant. As shown by Bonaccorso et al. [2006], GPS and InSAR reveal vertical/lateral changes in the summit area of only a few centimeters in and of the same order in the previous years, which fall within the range of error of our Lidar data. We thus consider the influence of volcano deformation negligible. [17] During the >12-yr interval between early 1986 and late-1998, significant accumulation of eruptive products occurred around all four summit craters, amounting to a volume of 26.5 ± m 3, equivalent to m 3 /year (Figure 4a). Most of this accumulation was concentrated at the SEC, whose cone grew from 3120 to 3242 m. This activity occurred in two phases, September 1989 February 1990 and November 1996 July 2001, with most cone growth occurring during sequences of paroxysms [Behncke et al., 2006]. Likewise, lava spreading out from the SEC caused positive morphological changes in areas adjacent to the crater. Accumulation of eruptive products to a few tens of meters occurred around the other summit craters, in particular at the NEC, where a deep breach cutting the NW rim of the crater in 1986 was filled by new eruptive products in [18] Accumulation of new eruptive material during (Figure 4b) largely surpasses that of , which is clearly evident in the volume gain of 74.6 ± m 3, corresponding to m 3 /year four times as high a rate as during the previous time window. Again, much of this material is concentrated around the SEC, which grew by 46 m to an elevation of 3288 m, with a volume gain of m 3 calculated for the cone alone 3of6

4 Figure 3. (a) Plot of the model surface percent versus the square root of triangle area of the four TINs. Note the improvement in the resolution from 1986 to 2007 TINs. (b) Volume growth (m 3 ) versus thickness of accretional areas. The yellow portion of the plot highlights thickness >20 m that account for the SE Crater growth during an early phase ( ), the main phase ( ) and the full period ( ). by Behncke et al. [2006]. The NEC also shows a slight increase in height on its W rim from m in 1998 to 3330 m in The upper portion of the October November 1999 BN lava field is clearly visible in the western part of the area. The remaining areas of accumulation represent tephra emitted from the BN and VOR in 1999, and from the SEC in , as well as a certain amount of pyroclastics of the 2001 and flank eruptions [Neri et al., 2005], especially in the SE portion of the area. [19] Volume loss in this time window (apart from the areas not considered for their excessive errors appearing as striped patches in Figure 4b) accounts for 2.2 ± m 3 and mostly corresponds to the formation of collapse pits in the SE part of the BN in late-1999 and on the ESE flank of the SEC during the flank eruption [Neri and Acocella, 2006]. [20] Accumulation between the 2005 and 2007 surveys (Figure 4c) amounts to a volume of 14.9 ± m 3 ( m 3 /year), and is virtually entirely confined to the SEC and its satellite vents active between July 2006 and May The summit height of the SEC remained at 3288 m, in spite of this significant eruptive activity. Morphological changes at that crater include 1) the filling of its summit pit, 2) the obliteration of the collapse pit on the ESE flank of its cone, 3) the formation of a new, smaller pit in a somewhat lower position on the same flank, 4) the carving out of a deep trench on the SE flank of the cone, and 5) minor subsidence associated with a small graben on the W flank of the cone as well as the opening of new vents on the S flank of the BN. Lava piled up to thicknesses of several tens of meters to the ESE of the SEC. Minor accumulation (by up to 5 m) is seen in much of the summit area. Apart from a continuous sheet of tephra erupted from the SEC in , which extends from that crater to the E and NE, accumulation appears to be accentuated in depressed areas, likely due to aeolian redistribution of recently erupted pyroclastics. [21] The volume loss in Figure 4c amounts to 4.3 ± m 3. This consists of the destruction of the septum between the VOR and the BN by a phreatic explosion on 12 January 2006 [Giammanco et al., 2007]. The combined difference map spanning the full interval of 21 years (Figure 4d) highlights the observations made in the aforementioned sections. Most notably it underscores the growth of the SEC (whose summit elevation increased by nearly 170 m) and its surrounding lava fields as well as the accumulation of eruptive products around the other summit craters. The total volume gain is 112 ± m 3.We cannot give a reasonably correct estimate of volume loss during the same period since this supposedly occurred mostly in the areas affected by errors and thus not included in our quantitative analysis. 6. Conclusions [22] We have tested the latest Lidar instrument applied in 2007 at Mount Etna against previous Lidar and earlier aerophotogrammetry-based surveys to show that (a) the Lidar technology is by far more accurate than the earlier methods, and (b) earlier surveys show a persistent disturbance in areas covered by volcanic gas plumes, such as the interior of the summit craters and adjacent flanks. The latest Lidar instrument capability of viewing the crater interiors even in the presence of a gas plume has significantly improved compared to earlier techniques. This technique can be carried out at comparatively low cost and quasiinstantaneously whenever the need arises, and thus allows up-to-date mapping at unprecedentedly high-precision in volcanic areas subject to frequent eruption-related morphological changes. [23] The products of our study are: (1) a 1:10,000 topographic map of the summit area of Mount Etna (auxiliary material), up-to-date as of June 2007, which will be useful for anyone working in this hazardous area; (2) a sequence of difference maps highlighting the morphostructural changes in Etna s summit area during the past two decades, which also permit to quantify the volumetric changes affecting that area in different time windows, and neatly underscore the varying degrees of accuracy of the different survey techniques. Our study on Etna serves as a 4of6

5 L09305 NERI ET AL.: MT. ETNA S SUMMIT AREA L09305 Figure 4. Difference maps obtained from the DEMs cover the periods (a) , (b) , (c) and (d) The outlines of eruptive products (lavas = solid lines; pyroclastics = dashed lines) are drawn from limits recognizable in the maps. TdF = Torre del Filosofo. Striped patches represent areas affected by artifacts (see text for explanation). Coordinates in upper left corner are in WGS84-UTM 33 system (meters). prime example for similar surveys in any part of the world affected by rapid morphostructural changes. [24] Acknowledgments. We thank A. Gudmundsson for his helpful review. References Allard, P., B. Behncke, S. D Amico, M. Neri, and S. Gambino (2006), Mount Etna : Anatomy of an evolving eruptive cycle, Earth Sci. Rev., 78, , doi: /j.earscirev Axelsson, P. (1999), Processing of laser scanner data Algorithms and applications, ISPRS J. Photogramm. Remote Sens., 54, , doi: /s (99) Baltsavias, E. P. (1999), Airborne laser scanning: Basic relations and formulas, ISPRS J. Photogramm. Remote Sens., 54, , doi: / S (99) Behncke, B., M. Neri, and A. Nagay (2005), Lava flow hazard at Mount Etna (Italy): New data from a GIS-based study, in Kinematics and Dynamics of Lava Flows, edited by M. Manga and G. Ventura, Geol. Soc. Am. Spec. Pap., 396, , doi: / (13). Behncke, B., M. Neri, E. Pecora, and V. Zanon (2006), The exceptional activity and growth of the Southeast Crater, Mount Etna (Italy), between 1996 and 2001, Bull. Volcanol., 69, , doi: / s x. Bonaccorso, A., A. Bonforte, F. Guglielmino, M. Palano, and G. Puglisi (2006), Composite ground deformation pattern forerunning the Mount Etna eruption, J. Geophys. Res., 111, B12207, doi: / 2005JB Favalli, M., and M. T. Pareschi (2004), Digital elevation model construction from structured topographic data: The DEST algorithm, J. Geophys. Res., 109, F04004, doi: /2004jf Favalli, M., F. Innocenti, M. T. Pareschi, G. Pasquare, F. Mazzarini, S. Branca, L. Cavarra, and A. Tibaldi (1999), The DEM of Mt. Etna: Geomorphological and structural implications, Geodin. Acta, 12(5), Giammanco, S., K. W. W. Sims, and M. Neri (2007), Measurements of 220 Rn and 222Rn and CO2 emissions in soil and fumarole gases on Mt. Etna volcano (Italy): Implications for gas transport and shallow ground fracture, Geochem. Geophys. Geosyst., 8, Q10001, doi: / 2007GC Glenn, N. F., D. R. Streutker, D. J. Chadwick, G. D. Thackray, and S. J. Dorsch (2006), Analysis of Lidar-derived topographic information for characterizing and differentiating landslide morphology and activity, Geomorphology, 73, , doi: /j.geomorph Mazzarini, F., M. T. Pareschi, M. Favalli, I. Isola, S. Tarquini, and E. Boschi (2005), Morphology of basaltic lava channel during the Mt. Etna September 2004 eruption from airborne laser altimeter data, Geophys. Res. Lett., 32, L04305, doi: /2004gl Mazzarini, F., M. Pareschi, M. Favalli, I. Isola, S. Tarquini, and E. Boschi (2007), Lava flow identification and aging by means of Lidar intensity: 5 of 6

6 Mount Etna case, J. Geophys. Res., 112, B02201, doi: / 2005JB Neri, M., and V. Acocella (2006), The Etna eruption: Implications for flank deformation and structural behaviour of the volcano, J. Volcanol. Geotherm. Res., 158, , doi: /j.jvolgeores Neri, M., V. Acocella, B. Behncke, V. Maiolino, A. Ursino, and R. Velardita (2005), Contrasting triggering mechanisms of the 2001 and eruptions of Mount Etna (Italy), J. Volcanol. Geotherm. Res., 144, , doi: /j.jvolgeores Tarquini, S., I. Isola, M. Favalli, F. Mazzarini, M. Bisson, M. T. Pareschi, and E. Boschi (2007), TINITALY/01: A new Triangular Irregular Network of Italy, Ann. Geophys., 50, Toyra, J., and A. Pietroniro (2005), Towards operational monitoring of a northern wetland using geomatics-based techniques, Remote Sens. Environ., 97, , doi: /j.rse Ventura, G., and G. Vilardo (2008), Emplacement mechanism of gravity flows inferred from high resolution Lidar data: The 1944 Somma- Vesuvius lava flow (Italy), Geomorphology, 95, , doi: / j.geomorph B. Behncke and M. Neri, Sezione di Catania, Istituto Nazionale di Geofisica e Vulcanologia, Piazza Roma, 2; I Catania, Italy. (neri@ct.ingv.it) M. Bisson, I. Isola, F. Mazzarini, M. T. Pareschi, and S. Tarquini, Sezione di Pisa, Istituto Nazionale di Geofisica e Vulcanologia, via della Faggiola, 32; I Pisa, Italy. 6of6