Computing Site Effects via Using Blank Noise Records in Strong Motion Stations, Middle Taiwan

Abstract:

Site response frequency could be obtained through using ambient noise and earthquake data. Measuring microseismic in situ, could collect better stable signals and higher density data arrays but costing more labors and hard to get data in time series, in contrast, earthquake stations could record data automatically during wide range of time scales but causing lager error in calculating results from different seismic source conditions. Based on these pros and cons, this study aims on assessing the applicability of computing site response from background noise in earthquake stations. Data were collected from Central Weather Bureau free field strong motion system, the instruments would be triggered when vibration amplitudes exceeding a specific threshold (about 3.9 gal), and the 20 seconds data before trigger point would be retained. The blank records before P wave arrival from this 20 seconds were considered as ambient noise, and Nakaruma horizontal to vertical spectral ratio (HVSR) technique was involved to analysis the site response. Results were compare with response frequency from S waves in middle Taiwan, showing that the background noise from strong motion records also could reflect peak frequency as S wave band, and had well consistency with previous studies used microseismic data. This study provides a new vision for strong motion station data applying to site effect analysis, the blank record is a worth considering material which could assist in understanding site response.