9:00 AM - 9:15 AM
[SSS10-11] Estimation of Nagao Fault using multi-linear-array microtremor
Keywords:microtremors, linear array, Nagao Fault, autocorrelation, single-station cross-correlation, phase velocity dispersion curve
The Nagao Fault, located north end of the Sanuki Mountains with the length of 30 km, was investigated by Kagawa Prefecture (1997). The seismic reflection survey was applied along the two survey lines, Nagao and Konan lines and revealed the exact location and the shape of the fault. The seismic reflection survey is possible to estimate the location and the shape of the fault, but it cost much and it is not easy to apply to many faults existing in Japan Islands. For the seismic disaster mitigation, the microtremor exploration is often used to estimate the subsurface structure of the basin, because of cost effectiveness. We therefore try to apply the microtremor exploration to estimate the fault location and shape by revealing the high-resolution subsurface structure model.
We performed the multiple linear array microtremor measurement along the Nagao and Konan survey lines where the seismic survey has been conducted. We used 6 sets of seismometer JEP-6A3 (10V/g) with the data logger LS8800 which were deployed 10 m interval and made 50 m linear array. After the measurement of the array, we moved them 5 m south along the survey line. Then, the next array was moved 60 m along the survey line from the previous array. We performed 550 m survey line with 18 linear arrays in total at the Nagao line. At the Konan line, we performed measurement at the 1.2 km survey line with 17 linear arrays but these arrays were not overlapped. The microtremors were measured 15 or 10 minutes. Both horizontal and vertical components of microtremors were measured.
We computed H/V, autocorrelation and single-station cross-correlation functions for the measured microtremors. The peak amplitude and frequency of H/V curve at the range of several Hz band varied across the fault location, which was estimated by the seismic survey method, at both Nagao and Konan survey lines. The autocorrelation and single-station cross-correlation functions also varied across the fault. The reflection profile is similar with that estimated from microtremors.
We further estimated the Rayleigh-wave dispersion-curve of phase velocity from cross correlation of the vertical component using zero-crossing method (Ekstrom et al., 2009) by seismic interferometry. Because the linear array was conducted using 6 or 7 sensors with the interval of 10 m, the combination of 10, 20, 30, 40, 50 and 60 m length pair is available. The dispersion curves estimated from these pairs were similar each other. We observed a phase velocity of 500 m/s at several ten Hz and we observed a dispersion curve up to 40 Hz. The dispersion curve significantly varied across the fault at both Nagao and Konan survey lines.
I thank to the people who supported the microtremor measurements.
We performed the multiple linear array microtremor measurement along the Nagao and Konan survey lines where the seismic survey has been conducted. We used 6 sets of seismometer JEP-6A3 (10V/g) with the data logger LS8800 which were deployed 10 m interval and made 50 m linear array. After the measurement of the array, we moved them 5 m south along the survey line. Then, the next array was moved 60 m along the survey line from the previous array. We performed 550 m survey line with 18 linear arrays in total at the Nagao line. At the Konan line, we performed measurement at the 1.2 km survey line with 17 linear arrays but these arrays were not overlapped. The microtremors were measured 15 or 10 minutes. Both horizontal and vertical components of microtremors were measured.
We computed H/V, autocorrelation and single-station cross-correlation functions for the measured microtremors. The peak amplitude and frequency of H/V curve at the range of several Hz band varied across the fault location, which was estimated by the seismic survey method, at both Nagao and Konan survey lines. The autocorrelation and single-station cross-correlation functions also varied across the fault. The reflection profile is similar with that estimated from microtremors.
We further estimated the Rayleigh-wave dispersion-curve of phase velocity from cross correlation of the vertical component using zero-crossing method (Ekstrom et al., 2009) by seismic interferometry. Because the linear array was conducted using 6 or 7 sensors with the interval of 10 m, the combination of 10, 20, 30, 40, 50 and 60 m length pair is available. The dispersion curves estimated from these pairs were similar each other. We observed a phase velocity of 500 m/s at several ten Hz and we observed a dispersion curve up to 40 Hz. The dispersion curve significantly varied across the fault at both Nagao and Konan survey lines.
I thank to the people who supported the microtremor measurements.