10:45 AM - 12:15 PM
[PEM12-P29] Analysis of ionospheric disturbances propagating along multiple paths due to earthquakes by HF Doppler observations
Keywords:HF Doppler, Earthquake, Rayleigh wave, Acoustic wave
It is known that large-scale natural disasters such as earthquakes, tsunamis, and volcanic eruptions generate atmospheric waves, which cause ionospheric disturbances. As concerns the ionospheric disturbances due to earthquakes, their generation mechanisms can be classified into the following two categories.
Rayleigh waves propagating over the earth's surface to the distant epicenter excite sound waves that reach the ionosphere Sound waves generated by ground motion at the epicenter reach the ionosphere directly
Although the characteristics of propagation along paths (a) and (b) have been analyzed in the previous studies , there are few studies that analyzed both types of disturbances generated by the same earthquake with the same observation system. Therefore, the purpose of this study is to identify disturbances propagating along paths (a) and (b) using an HF Doppler (HFD) observation system, and to clarify the frequency characteristics of them.
HFD observation can observe the vertical motions of the ionosphere at the midpoint of transmitter and receives of radio waves at frequencies (5.006 MHz, 6.055 MHz, 8.006 MHz, and 9.595 MHz) based on the difference in transmission and reception frequencies caused by the Doppler effect . Observations are conducted at the University of Electro-Communications and four other institutions. The transmitting station used in this study was the Chofu campus of the University of Electro-Communications, and the receiving stations were Iitate, Kakioka, and Oarai.
In this study, propagation times along paths (a) and (b) were calculated using different methods and compared with HFD data. The propagation time for path (a) was calculated as the sum of the arrival time of the seismic wave just below the reflection point and the propagation time of the sound wave from the ground to the reflection points. The arrival time of seismic waves was determined using seismic data from F-net, a broadband seismic observation network operated by the National Research Institute for Earth Science and Disaster Prevention. The NRLMSISE-00 standard atmospheric model was used to obtain the vertical profiles of atmospheric temperature to calculate the sound wave propagation time. The propagation time in path (b) was calculated by ray tracing for the sound wave to reach each HFD reflection point.
In this study, frequency analysis data of HFD observation data and HFD waveform data were obtained for the Iwate Nairiku earthquake at 8:43 (JST) on June 14, 2008 and the Sanriku-oki earthquake at 11:45 (JST) on March 9, 2011. In these data, there are two types of variations propagating along path (a) and path (b). The intensity of the fluctuations in the former and the latter were dominant in the 20-30 mHz range and in the 60-80 mHz range, respectively.
Therefore, the HFD sounding showed both disturbances by sound waves excited by Rayleigh waves and those by direct waves from the epicenter, with the latter showing dominance at higher frequency components than the former.
Rayleigh waves propagating over the earth's surface to the distant epicenter excite sound waves that reach the ionosphere Sound waves generated by ground motion at the epicenter reach the ionosphere directly
Although the characteristics of propagation along paths (a) and (b) have been analyzed in the previous studies , there are few studies that analyzed both types of disturbances generated by the same earthquake with the same observation system. Therefore, the purpose of this study is to identify disturbances propagating along paths (a) and (b) using an HF Doppler (HFD) observation system, and to clarify the frequency characteristics of them.
HFD observation can observe the vertical motions of the ionosphere at the midpoint of transmitter and receives of radio waves at frequencies (5.006 MHz, 6.055 MHz, 8.006 MHz, and 9.595 MHz) based on the difference in transmission and reception frequencies caused by the Doppler effect . Observations are conducted at the University of Electro-Communications and four other institutions. The transmitting station used in this study was the Chofu campus of the University of Electro-Communications, and the receiving stations were Iitate, Kakioka, and Oarai.
In this study, propagation times along paths (a) and (b) were calculated using different methods and compared with HFD data. The propagation time for path (a) was calculated as the sum of the arrival time of the seismic wave just below the reflection point and the propagation time of the sound wave from the ground to the reflection points. The arrival time of seismic waves was determined using seismic data from F-net, a broadband seismic observation network operated by the National Research Institute for Earth Science and Disaster Prevention. The NRLMSISE-00 standard atmospheric model was used to obtain the vertical profiles of atmospheric temperature to calculate the sound wave propagation time. The propagation time in path (b) was calculated by ray tracing for the sound wave to reach each HFD reflection point.
In this study, frequency analysis data of HFD observation data and HFD waveform data were obtained for the Iwate Nairiku earthquake at 8:43 (JST) on June 14, 2008 and the Sanriku-oki earthquake at 11:45 (JST) on March 9, 2011. In these data, there are two types of variations propagating along path (a) and path (b). The intensity of the fluctuations in the former and the latter were dominant in the 20-30 mHz range and in the 60-80 mHz range, respectively.
Therefore, the HFD sounding showed both disturbances by sound waves excited by Rayleigh waves and those by direct waves from the epicenter, with the latter showing dominance at higher frequency components than the former.