1:45 PM - 3:15 PM
[HTT18-P01] Near-surface Vs structure modeling from H/V spectral ratio of microtremor measurements aided by surface-wave survey data obtained in Ikejiri-gawa depression, Nagano Prefecture
Keywords:Single station microtremor measurement, H/V spectral ratio, Surface-wave survey, Near surface, Ikejiri-gawa depression
We here propose a convenient field measurement method for estimating near-surface S-wave velocity structure, which is one of the most important site characteristics for the earthquake disaster prevention. The method is featured by a combination analysis of HVSR (horizontal to vertical spectral ratio) provided by single point microtremor measurement and synthetic waveform matching to the actual surface wave survey data.
Such after-phase events as SV reflections, converted surface waves hidden in the noise cone are discriminated and utilized for making a velocity model which can explain both a HVSR curve and two-way travel times of after-phase events at a site.
A module incorporated in a commercial comprehensive seismic data processing software named SeisImager® is used for the waveform synthesis.
We conducted a field measurement using the method at Ikejiri-gawa depression nearby the Lake Nojiri, situated in northern part of Nagano Prefecture.
A 3-component seismic data acquisition system (Atom3C) was used for the measurements at a total of 61 points in the depression. Horizontal to vertical spectral ratio (HVSR) was calculated at each measurement point and uploaded to an open geophysical database site named “SeisImager.com”.
Surface wave survey was conducted making use of a 24-channel seismograph with 2Hz-geophones which were set along or on a farm road at 1m intervals. Vertical hitting using wooden hammer (Kakeya) was adopted as the seismic source. 15 short lines were set along Ikejiri-gawa to obtain shot records containing after-phase events along with surface waves. A dispersion curve was reconstructed by means of SASW (spectral analysis of surface waves) method and an optimal 1D S-wave velocity model was derived from the dispersion curve and the mode of occurrence of after-phase events.
Attached figures are the map (a) showing H/V peak frequency at each measurement pint, measured and calculated H/V spectral curves (b) for IKJ_09; the central point of the depression, and optimal S-wave velocity model (c) for IKJ_09. A CPT tip resistance curve is superposed on the velocity model. Both curves are concordant each other. Estimated AVS30 was as quite low as 86 m/s.
Such after-phase events as SV reflections, converted surface waves hidden in the noise cone are discriminated and utilized for making a velocity model which can explain both a HVSR curve and two-way travel times of after-phase events at a site.
A module incorporated in a commercial comprehensive seismic data processing software named SeisImager® is used for the waveform synthesis.
We conducted a field measurement using the method at Ikejiri-gawa depression nearby the Lake Nojiri, situated in northern part of Nagano Prefecture.
A 3-component seismic data acquisition system (Atom3C) was used for the measurements at a total of 61 points in the depression. Horizontal to vertical spectral ratio (HVSR) was calculated at each measurement point and uploaded to an open geophysical database site named “SeisImager.com”.
Surface wave survey was conducted making use of a 24-channel seismograph with 2Hz-geophones which were set along or on a farm road at 1m intervals. Vertical hitting using wooden hammer (Kakeya) was adopted as the seismic source. 15 short lines were set along Ikejiri-gawa to obtain shot records containing after-phase events along with surface waves. A dispersion curve was reconstructed by means of SASW (spectral analysis of surface waves) method and an optimal 1D S-wave velocity model was derived from the dispersion curve and the mode of occurrence of after-phase events.
Attached figures are the map (a) showing H/V peak frequency at each measurement pint, measured and calculated H/V spectral curves (b) for IKJ_09; the central point of the depression, and optimal S-wave velocity model (c) for IKJ_09. A CPT tip resistance curve is superposed on the velocity model. Both curves are concordant each other. Estimated AVS30 was as quite low as 86 m/s.