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[SSS13-13] Factors of large amplitude ground motions excited by small to moderate earthquakes
- Examination of KiK-net Kuriyama-nishi of the 2013 Tochigi-ken Hokubu Earthquake (Mj6.3) -
Keywords:2013 Tochigi-ken Hokubu Earthquake, KiK-net Kuriyama-nishi, Large Amplitude, Attenuation Relationship
In KiK-net Kuriyama-nishi (TCGH07), a large amplitude ground motion with peak acceleration of about 1.2G was observed in the 2013 Tochigi-ken Hokubu Earthquake (Mj6.3). Both borehole and surface records are characterized as being predominant about 6Hz. Hikima (2015) divided small to moderated earthquakes into five group based on the focal region, and examined the difference in site amplifications of TCGH07 using spectral inversion method. Predominant frequency around 6Hz are limited characteristics found only in earthquakes that occurred in the west and vicinity of TCGH07. He guessed that the cause was the influence of site amplification factor and path characteristics.
In this study, in order to investigate the factor of large amplitude earthquake ground motion of TCGH07 from another point of view, we estimated attenuation relationship of the peak acceleration of 5-8Hz ground motion using observation records of small to moderate earthquakes, and evaluated the amplification factor of TCGH07. Futhermore, we investigated the spatial distribution of the residuals for the attenuation relationship in TCGH07, and compared with slip distribution of the main shock.
The attenuation relationship was estimated using borehole records of 17 KiK-net observatories around TCGH07 for inland crustal earthquakes that occurred in the focal region of the main shock. The database used for this study consists of the 1147 records of 178 earthquakes in the range of the hypocentral distance about 3 to 100 km, the focal depth within about 11 km and Mj 2 to 5.2. Maximum acceleration amplitude of the larger of the two horizontal components was used. The attenuation relationship was estimated in a model obtained by adding the term of the site amplification factors to the regression model by Si and Midorikawa (1999). The coefficients for the hypocentral distance representing the viscous damping were given 0.002 with reference to Si and Midorikawa (1999). For the constraint of the solution, KiK-net Fujiwara-2 (TCGH17) with the highest S-wave velocity was taken as the reference of the site amplification factors.
Fig.(a) shows the site amplification factor evaluated at each observatory as the distribution for the S-wave velocity of the observation depth. It tends to be proportional to the reciprocal of S-wave velocity. If the relationship between them is approximated by a linear equation, the amplification factor evaluated in TCGH07 greatly exceeds this average relation. It can be said that TCGH07 is easy to shake at frequency 5-8Hz compared with surrounding observatories. The residual of the maximum acceleration amplitude (observation / attenuation relationship) was evaluated for each earthquake in TCGH07. Fig.(b) shows their spatial distribution. In this figure, the center of the color contour is adjusted to be the average value of the residuals. The gray contour denotes the slip distribution of the main shock by the Hikima (2014). Regional characteristics are found in the distribution of residuals, such as small in the earthquakes that occurred in the south and east, and large in the earthquakes that occurred in the west. Regions where the residuals are large roughly correspond to the slip region of the main shock. Fig.(c) shows the dependence of the residual on the back azimuth. Although there was a bias in the number of earthquakes, the average and standard deviation calculated by grouping at 30 degree intervals were shown in red circles and lines respectively. Obvious orientation dependence was found. The residual is large in the case of the earthquake occurring in the direction of N225-300deg.E corresponding to the slip region of the main shock. In addition to the shakeability of TCGH07 around 6Hz, the amplitude of the earthquake ground motion is further increased in the case of the earthquake occurred around the slip region of the main shock.
At the time of the main shock, as one of the reasons why the large amplitude ground motion was observed in TCGH07, it can be considered that the site amplification factor is large around 6Hz as pointed by Hikima (2015). However, the regional characteristics of the residuals of the small to moderate earthquakes shown in this study may indicate differences in the excitation characteristics of earthquakes. In that case, even at the time of the main shock, short-period ground motion strongly excited in the region where the residuals were large further increased the amplitude of TCGH07. We are investigating the stress drop of small to moderate earthquakes that occurred in the focal region of main shock.
In this study, in order to investigate the factor of large amplitude earthquake ground motion of TCGH07 from another point of view, we estimated attenuation relationship of the peak acceleration of 5-8Hz ground motion using observation records of small to moderate earthquakes, and evaluated the amplification factor of TCGH07. Futhermore, we investigated the spatial distribution of the residuals for the attenuation relationship in TCGH07, and compared with slip distribution of the main shock.
The attenuation relationship was estimated using borehole records of 17 KiK-net observatories around TCGH07 for inland crustal earthquakes that occurred in the focal region of the main shock. The database used for this study consists of the 1147 records of 178 earthquakes in the range of the hypocentral distance about 3 to 100 km, the focal depth within about 11 km and Mj 2 to 5.2. Maximum acceleration amplitude of the larger of the two horizontal components was used. The attenuation relationship was estimated in a model obtained by adding the term of the site amplification factors to the regression model by Si and Midorikawa (1999). The coefficients for the hypocentral distance representing the viscous damping were given 0.002 with reference to Si and Midorikawa (1999). For the constraint of the solution, KiK-net Fujiwara-2 (TCGH17) with the highest S-wave velocity was taken as the reference of the site amplification factors.
Fig.(a) shows the site amplification factor evaluated at each observatory as the distribution for the S-wave velocity of the observation depth. It tends to be proportional to the reciprocal of S-wave velocity. If the relationship between them is approximated by a linear equation, the amplification factor evaluated in TCGH07 greatly exceeds this average relation. It can be said that TCGH07 is easy to shake at frequency 5-8Hz compared with surrounding observatories. The residual of the maximum acceleration amplitude (observation / attenuation relationship) was evaluated for each earthquake in TCGH07. Fig.(b) shows their spatial distribution. In this figure, the center of the color contour is adjusted to be the average value of the residuals. The gray contour denotes the slip distribution of the main shock by the Hikima (2014). Regional characteristics are found in the distribution of residuals, such as small in the earthquakes that occurred in the south and east, and large in the earthquakes that occurred in the west. Regions where the residuals are large roughly correspond to the slip region of the main shock. Fig.(c) shows the dependence of the residual on the back azimuth. Although there was a bias in the number of earthquakes, the average and standard deviation calculated by grouping at 30 degree intervals were shown in red circles and lines respectively. Obvious orientation dependence was found. The residual is large in the case of the earthquake occurring in the direction of N225-300deg.E corresponding to the slip region of the main shock. In addition to the shakeability of TCGH07 around 6Hz, the amplitude of the earthquake ground motion is further increased in the case of the earthquake occurred around the slip region of the main shock.
At the time of the main shock, as one of the reasons why the large amplitude ground motion was observed in TCGH07, it can be considered that the site amplification factor is large around 6Hz as pointed by Hikima (2015). However, the regional characteristics of the residuals of the small to moderate earthquakes shown in this study may indicate differences in the excitation characteristics of earthquakes. In that case, even at the time of the main shock, short-period ground motion strongly excited in the region where the residuals were large further increased the amplitude of TCGH07. We are investigating the stress drop of small to moderate earthquakes that occurred in the focal region of main shock.