[U03-P03] Characteristics of ice tremors around Syowa Station, Antarctica, during 2013–2015
キーワード：海氷、微気圧、季節変化、震源決定、スペクトル
Tectonic earthquakes and ice tremors have been observed by seismic stations in Antarctica. Ice tremors are known to be excited by collision of sea ice, crack opening and closing, collapse of icebergs and so on (Kanao et al., 2012). In this study, we classify ice tremors around Syowa Station recorded in 2013–2015 into several types based on the features of waveforms and spectra, and discuss those seasonal variation and causes.
We use the waveform data recorded by STS-1 seismographs at Syowa Station and CMG-40T sensors at several field outcrop stations (i.e, Langhovde, Skallen and Rundvagshetta). The analysis period is from January in 2013 to December in 2015. We define here the tremors, of which P-waves and S-waves are not clear and the duration times are longer than five minutes, as the "ice tremors". We identify the ice tremors by visual inspection of seismograms and spectrograms calculated from velocity waveforms using FFT. We also determine the hypocenters of some ice tremors by grid search using the lag times calculated by the cross-correlation function of the velocity waveform envelopes at different stations.
We recognize that three types of ice tremors (type A–C) and T–phase of tectonic earthquakes (type D) in this study. Type A is the ice tremor which shows a long duration (~67000 s) and broad frequency components in 1–8 Hz. Type B is that of which a dominant frequency changes irregularly over the waveform. Type C is that of which a dominant frequency continuously decreases and overtones are recognized. We find 84 ice tremors in 2013, 148 in 2014 and 198 in 2015. For all the types of ice tremors, the number shows the maximum in autumn (February–April) and the minimum in winter (May–September). In autumn, the cryosphere system starts to melt and the condition of fast sea ice around Syowa Station becomes instability. We consider that the instability of the fast sea ice is necessary to cause ice tremors. From summer to autumn, both the seismic and microbaroms amplitudes are large and the majority of observed ice tremors are type A. From winter to spring, the amplitude of microbaroms is large, while ice tremors are less observed. We, therefore, interpret that type A occurs when broken fast sea ices are rubbed with each other in stormy ocean in summer to autumn. The hypocenters of type C are located in the fast sea ice area around Syowa Station. Thus, we consider that type C occurs when fast sea ices flow out to ocean or ice blocks such as icebergs collide with fast sea ice. On the other hand, we cannot specify a cause of type B tremors.
We use the waveform data recorded by STS-1 seismographs at Syowa Station and CMG-40T sensors at several field outcrop stations (i.e, Langhovde, Skallen and Rundvagshetta). The analysis period is from January in 2013 to December in 2015. We define here the tremors, of which P-waves and S-waves are not clear and the duration times are longer than five minutes, as the "ice tremors". We identify the ice tremors by visual inspection of seismograms and spectrograms calculated from velocity waveforms using FFT. We also determine the hypocenters of some ice tremors by grid search using the lag times calculated by the cross-correlation function of the velocity waveform envelopes at different stations.
We recognize that three types of ice tremors (type A–C) and T–phase of tectonic earthquakes (type D) in this study. Type A is the ice tremor which shows a long duration (~67000 s) and broad frequency components in 1–8 Hz. Type B is that of which a dominant frequency changes irregularly over the waveform. Type C is that of which a dominant frequency continuously decreases and overtones are recognized. We find 84 ice tremors in 2013, 148 in 2014 and 198 in 2015. For all the types of ice tremors, the number shows the maximum in autumn (February–April) and the minimum in winter (May–September). In autumn, the cryosphere system starts to melt and the condition of fast sea ice around Syowa Station becomes instability. We consider that the instability of the fast sea ice is necessary to cause ice tremors. From summer to autumn, both the seismic and microbaroms amplitudes are large and the majority of observed ice tremors are type A. From winter to spring, the amplitude of microbaroms is large, while ice tremors are less observed. We, therefore, interpret that type A occurs when broken fast sea ices are rubbed with each other in stormy ocean in summer to autumn. The hypocenters of type C are located in the fast sea ice area around Syowa Station. Thus, we consider that type C occurs when fast sea ices flow out to ocean or ice blocks such as icebergs collide with fast sea ice. On the other hand, we cannot specify a cause of type B tremors.