5:15 PM - 6:45 PM
[SSS07-P17] Attenuation of ambient noise autocorrelation function in Hachijojima Island
Keywords:Autocorrelation function, Ambient noise, Hachijoijima Island, Attenuation
Seismic waves are scattered and attenuated by heterogeneities when they propagate in the earth's crust. The degree of scattering and attenuation is important information for understanding the heterogeneities in the crust. A lot of studies have been conducted to estimate the parameter Qc, which indicates the attenuation of coda waves. Coda waves are composed of multiple-scattering waves of S-wave arriving after the S-wave's first arrival.
The autocorrelation function (ACF) of ambient noise can be regarded as a backscattered wave assuming a co-located source and receiver (Claerbout, 1968). ACF shows an attenuation similar to the coda wave (Weglar and Sens-Schönfelder, 2006). However, there are few studies on the parameter Qacf, which indicates attenuation of ACF.
In this study, we estimated Qacf and Qc from the seismic record in Hachijojima Island using the records of temporary seismic observations conducted during 2019-2020 and stationary seismic stations. We considered the spatial distribution of Qacf and Qc on Hachijojima Island. Qacf and Qc were compared. We discussed their relationship with the structure of Hachijojima Island.
Qacf and Qc could be approximated by the power function of frequency Qacf=9.70f1.37 and Qc=32.8f1.12, respectively. Qacf was slightly smaller than Qc. These values showed good correspondence with shallow crust and volcanic values in comparison with Qc at various locations in the world by Sato and Fehler (1998). The spatial distribution of Qacf and Qc was compared between the Nishi-yama and Higashi-yama, and the Nishi-yama volcano tended to have a slightly smaller Qacf at some frequency although the trend was not clear.
In general, Qc is known to have relatively small values in the shallow crust. Since the ambient noise is mainly composed of surface waves, Qacf is expected to show attenuation of shallow regions. Therefore, Qacf is considered to be smaller than Qc. The active periods of Nishi-yama and Higashi-yama are from about 100,000 years ago to about 4,000 years ago, and from about 10,000 years ago onward, respectively. Since the Nishi-yama is more recent, the results of this study indicate that the attenuation of seismic waves is larger in the Nishi-yama volcano. It may be related to the formation age and activity of the volcano.
The autocorrelation function (ACF) of ambient noise can be regarded as a backscattered wave assuming a co-located source and receiver (Claerbout, 1968). ACF shows an attenuation similar to the coda wave (Weglar and Sens-Schönfelder, 2006). However, there are few studies on the parameter Qacf, which indicates attenuation of ACF.
In this study, we estimated Qacf and Qc from the seismic record in Hachijojima Island using the records of temporary seismic observations conducted during 2019-2020 and stationary seismic stations. We considered the spatial distribution of Qacf and Qc on Hachijojima Island. Qacf and Qc were compared. We discussed their relationship with the structure of Hachijojima Island.
Qacf and Qc could be approximated by the power function of frequency Qacf=9.70f1.37 and Qc=32.8f1.12, respectively. Qacf was slightly smaller than Qc. These values showed good correspondence with shallow crust and volcanic values in comparison with Qc at various locations in the world by Sato and Fehler (1998). The spatial distribution of Qacf and Qc was compared between the Nishi-yama and Higashi-yama, and the Nishi-yama volcano tended to have a slightly smaller Qacf at some frequency although the trend was not clear.
In general, Qc is known to have relatively small values in the shallow crust. Since the ambient noise is mainly composed of surface waves, Qacf is expected to show attenuation of shallow regions. Therefore, Qacf is considered to be smaller than Qc. The active periods of Nishi-yama and Higashi-yama are from about 100,000 years ago to about 4,000 years ago, and from about 10,000 years ago onward, respectively. Since the Nishi-yama is more recent, the results of this study indicate that the attenuation of seismic waves is larger in the Nishi-yama volcano. It may be related to the formation age and activity of the volcano.