RESEARCH/REVIEW ARTICLE Tilt signals at Mount Melbourne, Antarctica: evidence of a shallow volcanic source Salvatore Gambino, Marco Aloisi, Giuseppe Falzone & Angelo Ferro Catania Department, National Institute of Geophysics and Volcanology, Piazza Roma 2, IT-95123 Catania, Italy Keywords Abstract Tilt monitoring; volcanic dynamics; 8 8 physics volcanology; ground deformation; Mount Melbourne (74 21? S, 164 43? E) is a quiescent volcano located in Victoria Land. northern Victoria Land, Antarctica. Tilt signals have been recorded on Mount Melbourne since early 1989 by a permanent shallow borehole tiltmeter Correspondence network comprising five stations. An overall picture of tilt, air and permafrost Salvatore Gambino, Catania Department, temperatures over 15 years of continuous recording data is reported. We National Institute of Geophysics and focused our observations on long-term tilt trends that at the end of 1997 Volcanology, Piazza Roma 2, IT-95123 showed coherent changes at the three highest altitude stations, suggesting the Catania, Italy. E-mail: salvatore.gambino@ presence of a ground deformation source whose effects are restricted to the ingv.it summit area of Mount Melbourne. We inverted these data using a finite spherical body source, thereby obtaining a shallow deflation volume source located under the summit area. The ground deformation observed corroborates the hypothesis that the volcanic edifice of Mount Melbourne is active and should be monitored multidisciplinarily. To access the supplementary material for this article, please see the supplementary files under Article Tools, online. Ground deformation studies of volcanoes are fundamen- (Dzurisin 1992, 2003; Ferro et al. 2011). These changes tal for forecasting eruptions and better understanding range from slow (monthsÁyears) to fast (minutesÁdays), magmatic processes (Dzurisin 2003). Spatial and tempo- highlighting different processes occurring at volcanoes ral (short- and long-term) patterns of ground deforma- (Gambino et al. 2014). In particular, slow variations tion are detected by using different techniques such indicate the inflation/deflation of the edifice caused by InSAR, GPS, tiltmeters, strainmeters, EDM and levelling volume changes in magma reservoirs (e.g., de Zeeuw-van and results are usually related to changes in the volume Dalfsen et al. 2005) or movements of geothermal fluids and location of magma. Especially at volcanoes with (e.g., Nakaboh et al. 2003). Tilt measurements have steam vents, ground deformation can also originate from revealed, in different monitored volcanic areas, middle movements of geothermal fluids, where the fluid dy- and long-term ground deformation related to edifice namic may be caused by heating of the geothermal inflation/deflation, such as at Piton de la Fournaise reservoir (e.g., Nakaboh et al. 2003; Battaglia et al. 2006). (Fontaine et al. 2014), Mount Etna (Bonaccorso et al. The frequent occurrence of long-term ground deforma- 2004), Stromboli (Bonaccorso et al. 2008; Bonaccorso tion (monthsÁyears), in open and closed volcanic systems, et al. 2009), Vulcano (Gambino et al. 2007) and Vesuvius has recently been shown using satellite techniques (e.g., (Ricco et al. 2013). Chaussard et al. 2013; Henderson & Pritchard 2013), Located in northern Victoria Land, Mount Melbourne levelling (e.g., Murase et al. 2014), EDM (Battaglia et al. (2732 m a.s.l.) is a large stratovolcano (Fig. 1a) belonging 2006) and, more rarely, tilt (Bonaccorso et al. 2015). to the McMurdo Volcanic Group (Kyle 1989) and is one of Shallow borehole tiltmeters are used to monitor the the closed conduit volcanoes in Antarctica with the po- ground deformation of volcanoes throughout the world tential for large-scale explosive eruptions. Physical volca- since they are able to measure precise tilt variations nological studies aimed at studying Mount Melbourne’s Polar Research 2016. # 2016 S. Gambino et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 1 International License (http://creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Citation: Polar Research 2016, 35, 28269, http://dx.doi.org/10.3402/polar.v35.28269 (page number not for citation purpose) Tilt signals at Mount Melbourne S. Gambino et al. Mount Melbourne volcano Abbreviations in this article EDM: electronic distance measurements Mount Melbourne is a volcanic edifice situated between GA: genetic algorithm Terra Nova and Wood bays in northern Victoria Land, GPS: global positioning system with a summit caldera of about 1 km in diameter and InSAR: interferometric synthetic aperture radar PS: pattern search characterized by fumarolic activity (Nathan & Schulte VLNDEF: victoria land network for deformation control 1967; Keys et al. 1983; Kyle 1989; Wo¨rner & Viereck 1989). Mount Melbourne is one of the stratovolcanoes in the McMurdo Volcanic Group, which has developed internal dynamics began in 1988 within the framework of along the western margin of the Ross Sea in Victoria the Italian Antarctic Research Programme. A tilt network Land, and parallel to the Transantarctic Mountains (Kyle of five permanent continuously recording stations was 1989). This area (Fig. 1a) was recently affected by installed in 1988Á89 (Figs. 1b, 2) and a seismic network in distensive tectonics on the north-westÁsouth-east struc- 1990 (Bonaccorso et al. 1996; Bonaccorso, Gambino, tural system forming the Rennick Graben to the north Falzone et al. 1997). Gambino & Privitera (1996) identified and on the NNEÁSSE system to the south, evidenced by and characterized local seismicity at Mount Melbourne. the Nansen Graben (Skinner 1983; Carmignani et al. Several seismic analyses, performed on a data set of 17 1989; Lanzafame & Villari 1991). The volcanic field seems micro-earthquakes, suggested these events were either to have formed at the intersection of these systems and ‘‘long-period events’’ (e.g., Chouet 1992), indicating the the development of the volcanic activity is generally active presence of fluids in source processes, or were linked related to the apparent discontinuity of the two structural to fracturing processes in a medium in transition between systems (Lanzafame & Villari 1991). the brittle and plastic states (Gambino & Privitera 1996). Volcanism in the Mount Melbourne area started during On the whole, seismic and tilt data indicate that Mount the upper Pliocene with the formation of the Cape Melbourne is a quiescent volcano with its own slow, but Washington shield volcano. During the lower and middle continual, internal dynamics (Bonaccorso et al. 1995; Pleistocene, volcanism migrated towards the Transantarc- Bonaccorso, Gambino & Privitera 1997). tic Mountains, while in the upper Pleistocene/Holocene Here we report observations and analyses of 15 years of the Mount Melbourne stratovolcano was associated with continuous ground tilt collected at Mount Melbourne the establishment of a crustal magma chamber (Giordano that highlight changes and suggest a possible internal et al. 2012). The volcanic activity at Mount Melbourne is volcanic source. testified by blankets of pyroclastic pumice and scoria fall (a) (b) s C. Adare Tilt station in 600 ta n VL06 GPS benchmark u o Fumaroles M Baker c Rocks ti Admiralty Mts. 00 c r –72° 10 –74°15' a t n Rennick C. Hallet a s n a FAL 800 r G Malta Plateau Edmonson Point T raben Mt. Melbourne FAL1 VIL1 VIL Mt. Overlord Wood Bay –73° 1400 Culman I. Mt. Rittmann Sea Miller Nunatak CONT 680 Northern Victoria Land Ross Willows Nunatak 70° 380 –74°30' –74° C 80° 400 a m Mt. Melbourne Antarctica p 300 b Mt. Nansen e Shield Nunatak l l 400 G west east l a c Graben i e r Terra Nova Bay T –75° o n 273 Nansen Main tectonic lines g Cape Washington 5km u Tectonic lines e 100 km M. Zucchelli Italian Base 165° 170° 164° 165° Fig. 1 Tectonic sketch map of (a) northern Victoria Land (redrawn from Carmignani et al. 1989). (b) Map of the permanent tilt network on Mount Melbourne. 2 Citation: Polar Research 2016, 35, 28269, http://dx.doi.org/10.3402/polar.v35.28269 (page number not for citation purpose) S. Gambino et al. Tilt signals at Mount Melbourne Tilt components direction tangentially (Tilt Y) with respect to the Mount Melbourne summit. In particular, increases in the radial component (+Y + = up) (+X + = up) indicate inflation of the crater area, while positive varia- tions of the tangential component indicate inflation at 908 Data logger counter-clockwise with respect the radial component. The RX - T X Battery boreholes also have two thermocouples (T1, T2) (copperÁ Thermocouple constantan type, 0.18C precision) shaft (mean depths 1.0 and 1.9 m, respectively) and a tilt temperature sensor at the ends of the instrument that records the permafrost temperature. The control data-logger was programmed for 48 data/day sampling (one sample every 30 min) and T 1 includes acquisition of the two tilt components, air (Tbox) and ground temperatures and instrumental control para- meters, such as battery and DC/DC voltage. Each station is Quartz sand equipped with solid state memories which guarantee data acquisition through the winter months, while high capacity batteries power the station. T 2 2.5 metres Data The tilt network data set spans the beginning of 1989 to the end of 2003. Partial interruptions were caused by Tiltmeter body with data-logger auto-protection, which activates under a internal electonics