Infrasound Signals and Their Source Location Inferred from Array Deployment in the Lützow-Holm Bay Region , East Antarctica : January-June 2015

Characteristic features of infrasound waves observed in the Antarctic represent a physical interaction relating surface environment in the continental margin and surrounding Southern Ocean. Source location of several infrasound events is demonstrated by using combination of two array deployments along a coast of the Lützow-Holm Bay (LHB), East Antarctica, for data retrieving period in January-June 2015. These infrasound arrays being established in January 2013 clearly detected temporal variations in frequency content and propagation direction of the identified seven large events. Many of these sources are assumed to have cryoseismic origins; the ice-quakes associated with calving of glaciers, discharge of sea-ice, collision between sea-ice and icebergs around the LHB. Detail and continuous measurements of infrasound waves in the Antarctic are a proxy for monitoring regional environment as well as climate change in high southern latitude.

isothermal atmosphere) to the lowest frequency of the human audible band (20 Hz). Infrasound waves involving large energy sources can propagate for several thousand kilometers or more along the Earth's surface [1]. As an example, the Sumatra-Andaman great earthquake of 26 December 2004 produced not only tsunami waves but also significant infrasound waves in the atmosphere [2] [3].
Thus, the infrasound can be effectively used for remote sensing of physical aspects of the Earth's atmosphere, compared with seismic waves in the same frequency range recorded by a seismometer.
There are several reports of production of infrasound waves by environmental sources such as volcanic eruptions, ocean waves, earthquakes, and the passage of aircraft [4] [5]. In addition, there have been reports of infrasound waves possibly generated by thunder, sprites, fireballs, meteorite falls, the reentry of artificial vehicles, as well as aurora activities in polar region [6] [7] [8]. In polar region, time-space variations in atmospheric pressure are excited by physical interaction between the atmosphere, oceans, cryosphere, and the surface of solid earth.
These interactive phenomena are involved in surface environmental variations and their generating sources can be measured by using infrasonic waves in polar region.
In April 2008, infrasound observations started at the Japanese station; Syowa (SYO; 69.0˚S, 39.6˚E), in the Lützow-Holm Bay (LHB) of Antarctica. The single infrasound sensor at SYO has been continuously recording the data until the present (as of 2016), and has clearly recorded background contamination from oceanic signals (microbaroms) and their long-term variability of frequency contents and power spectrograms [9] [10].
In austral summer 2013 and 2014, several field stations were established along the eastern coast of the LHB. In particular, two infrasound arrays with different diameters were deployed on the outcrop site at SYO together with the second array on continental ice sheet near the eastern coast of LHB. Observation system of the arrays and initial data recorded in austral summer 2013 are already reported [11]. At the initial observation period, moreover, airburst shock waves emanating from a meteoroid entering the atmosphere over the Russian Republic were identified on 15 February 2013.
In this paper, in addition to the previous study, identification of infrasound sources was tried by using the pair of the arrays deploying at SYO and other sites on continental margin of ice sheet. Source location of several striking infrasound events is determined by combination of two arrays for data period in January-June 2015. Plausible source mechanism of these events is discussed regarding surface environmental change, in particular cryosphere dynamics surrounding the LHB.       The left panels are band-pass-filtered waveforms observed at SYO and S16 arrays, respectively. The upper-right and middle-right panels are the results of PMCC analysis. The diagram shows the back-azimuth (station-to-source) direction and the apparent velocities. The lower-right panel is local area (on Google map) of the estimated event location. Colored lines show the back-azimuth directions from both SYO and S16. Open red circle corresponds to the area of hypocenter of the event.

Analyses and Discussion
2015. On the basis of local geographical information, several crevasses exist over the ice-sheet around the area which might generated the infrasound energy following to be detected at both the arrays of SYO and S16. Figure 4 represents the detection of infrasonic signals generated by the calving event at the western margin of ice sheet on June 4, 2015. The events are exactly located at the continental edge between SYO and S16 arrays. The third example of determined location for the infrasound generating source on June 4, 2015 is shown in Figure 5.  tide-cracks (23 June), respectively. In all the events investigated here, local icequakes, calving of glaciers and/or ice-cliffs, collision of icebergs and sea-ice, as well as the other candidates for generating sources involving cryosphere dynamics might produce distinctive infrasound signals; having high-frequency oscillations ranging between 3 and 8 Hz and amplitudes between 0.01 and 0.1 Pa.
These local signals with high-frequency contents also might be considered to include regional/local earthquakes, however the exact separation for these cryospehre and tectonic origins can be done by future study.  . The left panels are band-passfiltered waveforms observed at SYO and S16 arrays. The upper-right and middle-right panels are the results of PMCC analysis. The lower-right panel is local area (on Google map) of the estimated event location. Colored lines show the back-azimuth directions from both SYO and S16. Open red circle corresponds to the area of hypocenter of the event.

Conclusion
identical events. Many of these sources are assumed to be cryoseismic origins from the comparison with satellite image around the LHB. The cryoseismic events are classified into several categories on the basis of their hypocentral location. That is, they contain ice-quakes associated with calving of glaciers, discharge of sea-ice, collision between sea-ice and icebergs. Detail and continuous measurements of infrasound in Antarctica are a new proxy for monitoring surface environment as well as climate change in high southern latitude.