ERS InSAR Observations of Mt. Etna volcano: Magma inflation and radial spreading

Transcription

1 1 ERS InSAR Observations of Mt. Etna volcano: Magma inflation and radial spreading P. Lundgren 1, F. Casu 2, M. Manzo 2, A. Pepe 2, P. Berardino 2, E. Sansosti 2, R. Lanari 2, and P. Rosen 1 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena 2 Istituto per il Rilevamento Elettromagnetico dell Ambiente, Consiglio Nazionale delle Ricerche, Napoli, Italy Abstract. We present a synthesis of ERS-1 and ERS-2 differential SAR interferometry (InSAR) observations of Mt. Etna volcano over roughly the past ten years from Through this time period Mount Etna underwent a cycle of eruptive activity starting with a large flank eruption that ended in March 1993, followed by two years quiescence, with resumed summit activity starting in the summer of 1995, culminating in the recent large flank eruptions of 2001 and 2002/2003. InSAR observations reveal patterns of surface deformation that result from the changing magma and structural dynamics of the volcano. Individual interferograms spanning each of these major volcanic episodes during the last ten years reveal the essential deformation patterns. Similarities between groups of similar time-span interferograms for both ascending and descending ERS satellite passes show common fringe patterns among each group and distinctive differences between the ascending and descending interferograms are related to true ground deformation and are not dominated by atmospheric effects. In general the deformation patterns are related to a combination of magma chamber inflation/deflation plus nearly radial flank motion to the W, S, and E. To make sense of the large number of interferograms computed and the temporal behavior of Etna s deformation, we compute a time series of ground deformation from We find that during this time interval Mt. Etna experienced magmatic deflation from thew initiation of measurements in 1992 through the spring of 1993, followed by major inflation from , with smaller deflation and inflation episodes from During the entire time period Etna experienced varying amounts of radial spreading to the West, South, and East. Steady relative motion between the West and South flanks, and between the East and North flanks, during this time interval, suggests they are related to gravitational spreading of the volcanic edifice. In contrast, time series analysis shows that growth of a southeastern basal anticline began with the end of magma recharge in 1995, thus showing a direct link between deep-seated magma intrusions and edifice spreading. Together these observations support a complex mode of radial gravitational collapse underlain by deeper magma driven basal spreading, although ultimately the two must be related. The distinct ascending and descending interferograms accompanying the 2001 flank eruption support this model and demonstrate the direct connection between the magmatic and structural components of the volcano. Mt. Etna is a large, highly active volcano that is structurally complex (Fig. 1). It features two well-developed rift systems extending to the north and south from the summit craters through a series of fault systems as its eastern and southern flanks move away from the volcano s center [Rust and Neri, 1996; Borgia et al., 1992, 2000; Froger et al., 2001]. Proc. of FRINGE 2003 Workshop, Frascati, Italy, 1 5 December 2003 (ESA SP-550, June 2004) 114_lundgr

2 2 (a) (b) Figure 1. (a) Shaded relief map of Mt. Etna showing its main faults and rift zones, inset in upper right shows location of study area in eastern Sicily (Italy). Black arrows show general sense of flank motion. Black lines show rift zones and faults. Tick marks indicate normal motion with ticks on the hanging wall. TC is the Trecastagni fault; M is the Mascalucia fault system; SW is the Southwest fault system; WRZ, NRZ, SRZ indicate the Western, Northeastern, and Southern rift zones, respectively. A indicates the anticline crest at the toe of the southern flank. VB is the Valle del Bove. The large solid diamond southeast of the anticline indicates the reference point for the velocity maps shown in Fig. 4. The solid square south of the WRZ and labeled a is the point used in Fig. 5a. Solid and open diamonds adjacent the Mascalucia/anticline, b, Pernicana, c, and SW, d, faults show the locations for the time series displacements shown in Fig. 5b, 5c, and 5d, respectively; with displacements calculated as the difference (solid open). (b) Example of a long time interval differential interferogram spanning the inflation and showing the major structural features observable from the ascending ERS satellite SAR data. A certain amount of debate has centered around whether or not the generally concentric (as observed on ascending ERS data) fringe patterns were in fact surface deformation or whether they represented topographically correlated atmospheric path delays. While we along with a number of groups have sought to address this issue either through comparing independent interferograms or through models of the atmosphere, many of both the large volcano-wide inflationary fringe patterns and the more detailed flank related displacements remain robust when long-time-span (i.e. large signal to noise) interferograms are compared (i.e. the two 6-year interferograms in Figure 2 show minimal differences despite their large signals).

3 3 Figure 2. Six year interferograms from the ascending ERS satellite data. The two images at the left and center each span similar time intervals (i.e. dominated by large inflation). Their difference (right panel) does not show significant large-scale signal. The results of Fig. 2 are borne out in other similar time span interferograms from ascending tracks, yet they differ significantly from the ascending data over similar time intervals (Fig. 3). The time series analysis of more than 200 interferograms are shown in Fig. 4 [Lundgren et al., 2004]. The relative average velocitie shown in Fig. 4. The top row (Fig 4a, b) show the ascending and descending track velocities over the entire time period (1992 to 2001) and thus are dominated by flank inflation over the volcano plus the superposition of radial flank motion. If we subtract the two average velocities we are left with the E-W component of the horizontal velocities (as projected into the two radar line of sight directions), Fig 4c. The E-W motion shows outward expansion of the volcano, partly due to inflation and also due to flank motion as demonstrated by the creeping flank faults. The sum of the two time series velocities gives mostly vertical motion (with less than 10% north contamination). Here (Fig. 4d) the signal is dominated by inflation with evidence for basal anticline growth to the SE near Catania (southern coast in these images). We can look at the time evolution of distinct points from the time series analysis to try and better understand the dynamics of the volcano (Fig. 5). From the motion of individual points we see the large scale deflation-inflation of the edifice from 1992 through 1996 followed by significant, though lower amplitude, inflation and deflation episodes, though the long-term trend from 1996 onward is relatively flat (Fig. 5a). In Fig. 5b we see the relative motion of the east flank and the growing anticline. Here we see that relative motion does not begin until 1995, after the magmatic system has recharged and summit activity had resumed. In contrast, the motions across the two other major flank faults (Fig. 5c, d) show steady motion. The non-steady motion of the anticline suggests that it is linked to deeper seated magma chamber inflation (i.e. the 5

4 4 km b.s.l depth of the magma chamber during the inflation; Lundgren et al., 2003; Patanè et al., 2003), whereas the steady flank motions of the SW and Pernicana slank faults may be linked to shallower edifice collapse. Together these observations support a structural spreading model similar to that of Merle and Borgia [1996] and Tibaldi and Groppelli [2002]. Figure 3. Long time interval differential interferograms showing the difference between ascending and descending InSAR fringe patterns. The left images are from ascending data (compare also with Fig. 2) while the right hand images are from descending data. The time series analysis velocities show a distinct jump in velocities toward the west across the southwest (SW) trending flank fault (Fig. 4c). This is confirmed by individual interferograms that show either reduction in inflationary (Fig. 6, top) fringes over the west flank (due to the subtracting effect of westward motion on descending ERS interferograms) or individual isolated fringes confined to the west flank and in each case terminated by the SW fault trace (Fig. 6, bottom). Low (or complex) signal interferograms such as those shown in Fig. 6 were easier interpreted after interferograms for the 2001 flank eruption (Fig. 7) suggested that assuming the interferograms represented a single event, spreading of Etna occurred over both its east and west flanks in response to the event [Lundgren and Rosen, 2003].

5 5 Figure 4: Time series inversion velocity maps. Average linear velocities for each pixel are shown with an average coherence greater than or equal to 0.6, Figure 5: InSAR time series inversion solutions for selected locations shown in Fig. 1. Line of sight (satellite range) displacements in panels (a-c) are from the ascending time series, while panel (d) is from the descending time series solution. (a) Motion of the area of maximum deflation/inflation, showing the deflation following the analysis technique of Berardino et al., [2002]. Zero velocity is referenced to a point indicated by the large solid diamond in Fig. 1. (a) Velocities for the ascending ERS satellite track 129, frame (b) Velocities for the descending track 222, frame (c) Difference between the ascending and descending velocity maps for pixels in common. (d) Sum of the ascending and descending velocity maps. Solid lines indicate faults shown in Fig. 1. See Lundgren et al. [2004] for complete explanation of these results. accompanying the flank eruption that ended in 1993, and the subsequent rapid inflation and period of summit activity from late 1995 to (b) Relative motion across the TrecastagniMascalucia fault and the anticline on the SE flank of Mt. Etna. (c) Relative motion across the Pernicana fault. (d) Relative motion across the SW flank fault system.

6 6 Figure 6. Descending track ERS interferograms showing the effects of west flank motion either through a subtraction of inflationary: positive surface motion as projected into the radar line of sight (LOS), top. Or as fringes associated with west flank motion itself giving negative LOS fringes, bottom. Figure 7. Ascending (a) and descending (b) interferograms spanning the 2001 flank eruption of Mt. Etna. See Lundgren and Rosen [2003] for details. Despite these observations, consensus over the structural mechanisms for the observed deformation of Mt. Etna remains distant. Problems with gaps in the InSAR record allow

7 7 different interpretations as to whether the 2001 interferograms represent the same event (see Bonforte et al., this issue). Further interpretation based on more thorough numerical models may help constrain the suite of models that can explain the observations. Future work must include point measurements, such as tilt and GPS, to derive comprehensive models for Mt. Etna s complex structural and magmatic system. Acknowledgments. ERS raw SAR data were provided by the European Space Agency. This work has been partially supported by the Italian Space Agency and by the Italian National Group of Volcanology (GNV). The precise ERS-1/2 satellite orbit state vectors have been provided by the University of Delft, The Netherlands. The digital elevation model was from the NASA SRTM mission. The authors would like to thank S. Guarino, M. Manunta and G. Zeni for their help. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. References Berardino, P., G. Fornaro, R. Lanari, and E. Sansosti, A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms, IEEE Trans. Geosci. Remote Sensing, 40, , Borgia, A., L. Ferrari, and G. Pasquare, Importance of gravitational spreading in the tectonic and volcanic evolution of Mount Etna, Nature, 357, , Borgia, A., R. Lanari, E. Sansosti, M. Tesauro, P. Bernardino, G. Fornaro, M. Neri, and J. B. Murray, Actively growing anticlines beneath Catania from the distal motion of Mount Etna s decollement measured by SAR interferometry and GPS, Geophys. Res. Lett., 27, , Froger, J.-L., O. Merle, and P. Briole, Active spreading and regional extension of Mount Etna imaged by SAR interferometry, Earth Planet. Sc. Lett., 187, , Lundgren, P., P. Berardino, M. Coltelli, G. Fornaro, R. Lanari, G. Puglisi, E. Sansosti, and M. Tesauro, Coupled magma chamber inflation and sector collapse slip observed with synthetic aperture radar interferometry on Mt. Etna volcano, J. Geophys. Res., 108(B5), 2247, doi: /2001jb000657, Lundgren, P., F. Casu, M. Manzo, A. Pepe, P. Berardino, E. Sansosti, and R. Lanari, Gravity and magma spreading of Mount Etna volcano revealed by radar interferometry, Geophys. Res. Lett., in press, Lundgren, P., and P. A. Rosen, Source model for the 2001 flank eruption of Mt. Etna volcano, Geophys. Res. Lett., 30(7), 1388, doi: /2002gl016774, Merle, O., and A. Borgia, Scaled experiments of volcano spreading, J. Geophys. Res., 101, 13,805-13,817, Patanè, D., P. De Gori, C. Chiarabba, and A. Bonaccorso, Pressurization of Mount Etna s volcanic system, Science, 299, , Rust, D., and M. Neri, The boundaries of large-scale collapse on the flanks of Mt. Etna, Sicily, in W.J. McGuire, A.P. Jones, J. Neuberg (Eds.), Volcano Instability on the Earth and other Planets, Geol. Soc. Lond. Spec. Pub. 110, , Tibaldi, A., and G. Groppelli, Volcano-tectonic activity along structures of the unstable NE flank of Mt. Etna (Italy) and their possible origin, J. Volcanol. Geotherm. Res., 115, , 2002.