Seismology in Antarctica
The lack of anthropogenic seismic noise makes the Antarctic continent an ideal natural laboratory for seismic studies. In Antarctica, there are many different kinds of seismic sources, most of which are summarised in the following figure:
Kanao et al. (2017)
(a,b) Examples of icequakes, recorded by the vertical component of stations located in Victoria Land. (c) Example of a window of microseism, recorded by the vertical component of a station located in Victoria Land. (d) Sketch summarising the main source processes of icequake (I: stick–slip motion at rock-ice interface; II: crevassing; III: iceberg calving) and microseism (IV: ocean wave energy coupling with the Earth’s ground) (redrawn from Kanao et al., 2017).
In this page, the results of the seismic investigations performed in the framework of the ICEVOLC project are shown. In particular, three different aspects of seismology in Antarctica have been faced:
- Volcano seismicity
Icequakes are defined as coseismic brittle fracture events within the ice (e.g. Podolskiy and Walter, 2016) or, more generally, are seismic events associated with ice dynamics. Many processes can lead to the nucleation of icequakes such as near surface crevassing, iceberg calving, stick-slip motion (e.g. Podolskiy and Walter, 2016). From the analysis of icequakes, new light has been shed on phenomena such as iceberg calving, glacier and sea ice dynamics (e.g. Podolskiy and Walter, 2016, and references therein).
During 27 November 2016 - 10 January 2017 two stations (called STN01, STN02) were installed in Tethys Bay (Victoria Land, Antarctica), close to Mario Zucchelli Station (the Italian base in Antarctica):
(a) Map of Victoria Land. (b) Aerial image of Tethys Bay from Google Earth with the locations of seismic stations (triangles), meteorological stations (circles) and Mario Zucchelli Station (MZS, square). (c,d) Pictures of installations of the seismic stations STN01 and STN02, respectively (from Cannata et al., 2017).
We detected three icequakes, with dominant low frequencies (below 2 Hz), located in the David Glacier area with local magnitude of 2.4-2.6. These events were likely to have been generated at the rock–ice interface under the glacier.
Vertical (a,e,i), E-W (b,f,j), N-S components (c,g,k) of the seismic signals recorded by STN01, and corresponding spectra (d,h,l) of the vertical components (from Cannata et al., 2017).
Aerial image from Google Earth, showing the locations of the 4 seismic stations used for the location analysis (yellow triangles), the epicentres of the 3 seismic events analysed in this study (red circles; event 1: 29 November; event 2: 6 December; event 3: 23 December 2017), and the epicentre of the events analysed by Zoet et al. (2012) (yellow circle). The rectangular insets show the vertical component of the seismic events recorded by the stations used for the location analysis, with the corresponding P and S wave arrival times (red arrows) (from Cannata et al., 2017).
Following the approach of Danesi et al. (2007) and Baumbach and Bormann (2012), we estimated the source parameters. We calculated the log-log spectra of the displacement P-wave phases for the three events, and estimated the amplitude of the flat part (u0), as well as the corner frequency (fc). Assuming a circular fault source model, source radius, area and average dislocation were estimated equal to ~163±54 m, 8.9±6·10 4 m2, and 0.44±0.22 m, respectively. On the other hand, if we assume that the events took place in the ice, source radius, area and average dislocation were ~122±40 m, 5.0±3.4·10 4 m2, and 0.60±0.30 m.
Displacement spectra of the vertical component of the P phases of the three seismic events recorded by STN01 (red line) and STN02 (blue line). u0 and fc indicate the amplitude of the flat part and the corner frequency, respectively (from Cannata et al., 2017).
In the absence of earthquakes and other strong seismic signals, the Earth is not static but constantly vibrating due to many continuous noise sources such as ocean waves, storms and anthropic activities (e.g. Brenguier et al., 2016). The most continuous and ubiquitous seismic signal on Earth is microseism, mainly composed of surface waves and closely related to ocean wave energy coupling with the Earth’s motion (e.g. Aster et al., 2008).
At low and middle latitudes, the maximum microseismic amplitudes are observed during local winter (when nearby oceans are stormier than during local summer). However, such a pattern is reversed in Antarctica, where because of the winter sea ice the oceanic waves are impeded from efficiently exciting seismic energy (e.g. Grob et al, 2011; Anthony et al., 2016).
Short Time Fourier Transform of the seismic signal recorded by the vertical component of VNDA station during 2003-2016.
The following movie shows the relationship between amplitude variations of microseism and sea ice distribution in the Ross Sea. Microseism time series were obtained as STFT and RMS amplitudes calculated on seismic signal recorded by VNDA station (the facilities of IRIS Data Services, and specifically the IRIS Data Management Center, were used for access to VNDA data). Information of sea ice concentration, provided by brightness temperature data within a grid with cell size of 25 x 25 km (Cavalieri et al., 1996), were downloaded from the website http://nsidc.org/data/NSIDC-0051/versions/1.
It is evident how the fluctuatons of microseism amplitudes coincide with variations in the sea ice distribution in Ross Sea. In particular, Antarctic winters, characterised by extended sea ice, show the weakest microseismic amplitudes.
Seismic activity on Mt. Melbourne has been discontinuously monitored since 1990, when a research program on physical volcanology on Mt. Melbourne was funded within the framework of the Italian “Programma Nazionale di Ricerche in Antartide” (Privitera et al., 1992; Gambino and Privitera, 1994, 1996). From 1990 to 1994, four seismic stations (two one-component and two three-component) discontinuously recorded seismic signals on Mt. Melbourne.
In 2017, the seismic investigation of the Mt. Melbourne volcano resumed in the framework of the ICEVOLC project, funded by the “Programma Nazionale di Ricerche in Antartide”.
By putting together all the old and new observations, it can be noted that different kinds of seismic signals have been identified on Mt. Melbourne volcano: i) icequakes; ii) local seismic events; iii) tremor.
As for the former, icequakes are defined as coseismic brittle fracture events within the ice (e.g. Podolskiy and Walter, 2016) or, more generally, are seismic events associated with ice dynamics. Icequakes, recorded on Mt. Melbourne, are generally characterized by short duration (less than 10 sec), high spectral content (>10 Hz), sharp amplitude decrease with distance, and no clear P and S phases. On the basis of their features, these events, representing most of the seismic events recorded on Mt. Melbourne, are probably associated with ice faulting forming crevassing (e.g. Gambino and Privitera, 1994; Walter et al., 2009; Roosli et al., 2014).
Concerning the local seismic events, Gambino and Privitera (1994, 1996) identified seismic events characterized by low frequencies (0.7-6.0 Hz), duration longer than 20 sec, emergent first arrivals, and S waves difficult to be identified. The sources were located on the volcano eastern flank at depth of 2.8 km (error equal to 4.4 km). Gambino and Privitera (1996) formulated two different hypotheses on the source of these events: i) long period events related to fluid dynamics inside the plumbing system; ii) volcano-tectonic earthquakes due to fracturing processes taking place within the volcano edifice. In both cases, the Authors stated that the identification of these events is a clue on the continuous and active internal dynamics of Mt. Melbourne volcano.
Finally, during January 2017 a long-lasting tremor-like signal, with frequencies between 2 and 8 Hz, was recorded simultaneously at two seismic stations located on the Melbourne summit (close to the fumarolic areas) 200 m apart from each other. Since it is impossible to locate the source of this signal (recorded by only two stations), it is hard to define the source nature of this tremor. On the basis of the higher spectral content, we can exclude oceanic-related sources (similar to microseism). Hence, the possible sources candidates are: i) dynamics of fluids within the volcano plumbing system, and in this case this tremor could be named volcanic tremor; ii) subglacial discharge of water melted by the volcanic heat release in the fumarolic areas. Glaciohydraulic tremor tremor-like seismic signals have been recently observed in glacial regions (e.g. Bartholomaus et al., 2015).
Examples of waveforms and spectrograms of a high-frequency icequake (a, recorded on 21-Jan-2017 at 13:46:00), a low-frequency icequake (b, recorded on 29-Nov-2016 at 07:33), and a local seismic events (c, recorded on 10-Dec-1990 at 05:14), acquired on Mt. Melbourne.
Spectrograms of the vertical component of the seismic signals recorded by MEL1 and MEL2 stations during 20-24 January 2017. Three different kinds of signals are evidenced: microseism, a teleseism, tremor.
At the time of this writing, the only seismic recordings ever acquired on Mt. Rittmann were collected in January 2017 in the framework of the ICEVOLC project. Such data were recorded by two broadband seismic stations, temporarily installed on the rim of the fumarolic wall of Mt. Rittmann at about 130 m apart from each other, from 11 to 19 January 2017. Approximately we detected 100 seismic events, that on the basis of several waveform and spectral parameters were classified into two different classes: i) high-frequency events; ii) low-frequency events.
The former class contains events characterized by frequencies higher than 8-10 Hz, short duration (less than 10 s), sharp amplitude decrease with distance. Similar to the seismic type identified also on Mt. Melbourne, these events are probably associated with ice faulting forming crevassing.
As for the latter events, the low-frequency events show spectral content lower than 8-10 Hz, duration of about 10 s, emergent onset, and no clear P and S phases. Hence, they resemble to the long period events, that on volcanoes are generally associated with fluid dynamics inside plumbing system (e.g. Chouet and Matoza, 2013). However, to shed a light into the nature of these low-frequency events, recordings by a higher number of stations will be necessary, to constrain epicentral coordinates and focal depth.
Examples of waveforms and spectrograms of a low-frequency event (a,b, recorded on 19-Jan-2017 at 12:05) and high-frequency events (c,d, recorded on 11-Jan-2017 at 07:09), acquired on Mt. Rittmann.
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