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[SSS10-P14] Estimation of Site amplification factors among seismic networks around Aomori prefecture by means of coda wave amplitude
Keywords:site amplification factor, strong ground motion, AS-net
The site amplification factor indicates the degree of amplification of seismic waves in the surface layer, which is essential for estimating the damage and understanding the ground motion of a large earthquake. Understandings of the site amplification characteristics of each station is important from the perspective of disaster prevention.
Coda waves, the long-lasting seismic waves that follow the direct wave arrivals, consist of waves scattered by the heterogeneous structure of the lithosphere. It is well known that the coda wave amplitude becomes homogeneous among stations after a long elapsed time from the earthquake, and thus only the site effect emerged and the path effect is eliminated from the coda wave amplitude. At elapsed times after twice of S-wave arrival, the difference of amplitude among stations can be regarded the difference of relative site amplification factors. Based on this idea, Takemoto et al. (2012) estimated the spatial distribution of broadband site amplification factors in Japan using inversion analysis based on coda wave amplitude. They showed larger site amplification factors were distributed in the major sedimentary basins in the low-frequency band of 0.5-1 Hz.
In this study, we estimated the spatial distribution of site amplification factors in a frequency band of 0.5-1 Hz, 1-2 Hz, 2-4 Hz, 4-8 Hz, and 8-16 Hz, respectively, around Aomori Prefecture, northern Honshu Island, Japan, by inversion analysis based on the coda wave amplitude of a large number of seismograms obtained by K-NET, KiK-net, and AS-net, the dense seismic network installed covering northern Aomori and southwestern Hokkaido. Notice that we used surface and borehole seismometers of KiK-net as independent stations, which enabled us to estimate bottom-to-surface relative amplification in the borehole.
The results show that in the 0.5-1, 1-2, and 2-4 Hz, the estimated site amplification factors vary greatly among locations. In particular, the factors are small in the Kitakami Mountains and large in the Tsugaru peninsula and southeastern Aomori Prefecture. On the other hand, at higher frequency bands of the 4-8 and 8-16 Hz, the distribution of site amplification was quite different; the site amplification factors vary over a relatively narrow range. Smaller factors were obtained in the west and larger in the east along the volcanic front. In this study, the site amplification factors of the borehole stations are also estimated. At 0.5-1 and 1-2 Hz, the distribution of site amplification factors is similar to that of the ground surface, but at higher frequencies, there is no correspondence with that of the surface. The spatial distribution of site amplification factors of the AS-net station is similar to that of the KiK-net borehole stations in 4-8 and 8-16 Hz.
In the low frequency, there is a good correlation between the estimated site amplification factors and the average S-wave velocity of upper 30 m near the ground surface and the depth of seismic bedrock. However, little correlation was found in the high frequencies. Therefore it is suggested that site amplification in low frequency is affected by deep structure, and site amplification in high frequency is affected by the shallow structure.
Coda waves, the long-lasting seismic waves that follow the direct wave arrivals, consist of waves scattered by the heterogeneous structure of the lithosphere. It is well known that the coda wave amplitude becomes homogeneous among stations after a long elapsed time from the earthquake, and thus only the site effect emerged and the path effect is eliminated from the coda wave amplitude. At elapsed times after twice of S-wave arrival, the difference of amplitude among stations can be regarded the difference of relative site amplification factors. Based on this idea, Takemoto et al. (2012) estimated the spatial distribution of broadband site amplification factors in Japan using inversion analysis based on coda wave amplitude. They showed larger site amplification factors were distributed in the major sedimentary basins in the low-frequency band of 0.5-1 Hz.
In this study, we estimated the spatial distribution of site amplification factors in a frequency band of 0.5-1 Hz, 1-2 Hz, 2-4 Hz, 4-8 Hz, and 8-16 Hz, respectively, around Aomori Prefecture, northern Honshu Island, Japan, by inversion analysis based on the coda wave amplitude of a large number of seismograms obtained by K-NET, KiK-net, and AS-net, the dense seismic network installed covering northern Aomori and southwestern Hokkaido. Notice that we used surface and borehole seismometers of KiK-net as independent stations, which enabled us to estimate bottom-to-surface relative amplification in the borehole.
The results show that in the 0.5-1, 1-2, and 2-4 Hz, the estimated site amplification factors vary greatly among locations. In particular, the factors are small in the Kitakami Mountains and large in the Tsugaru peninsula and southeastern Aomori Prefecture. On the other hand, at higher frequency bands of the 4-8 and 8-16 Hz, the distribution of site amplification was quite different; the site amplification factors vary over a relatively narrow range. Smaller factors were obtained in the west and larger in the east along the volcanic front. In this study, the site amplification factors of the borehole stations are also estimated. At 0.5-1 and 1-2 Hz, the distribution of site amplification factors is similar to that of the ground surface, but at higher frequencies, there is no correspondence with that of the surface. The spatial distribution of site amplification factors of the AS-net station is similar to that of the KiK-net borehole stations in 4-8 and 8-16 Hz.
In the low frequency, there is a good correlation between the estimated site amplification factors and the average S-wave velocity of upper 30 m near the ground surface and the depth of seismic bedrock. However, little correlation was found in the high frequencies. Therefore it is suggested that site amplification in low frequency is affected by deep structure, and site amplification in high frequency is affected by the shallow structure.