| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 103 === Seismic stations including the Broadband Array in Taiwan for Seismology (BATS) were incorporated within the Central Weather Bureau Seismic Network (CWBSN) since 2012. However, site effects of seismic stations have not been applied for M_L determination that it may affect the magnitude estimation. Moreover, the appearance of relatively low seismicity is observed in 2012 from monthly event counts for〖 M〗_L >2.0. Thus, we would like to examine whether there is a relationship between the updated seismic network and low seismicity or not. In this study, we first collect the earthquake catalog data during the period from 1994/01/01 to 2012/12/31 to calculate the station correction in order to reduce site effect in estimating magnitude. There is a strong correlation between station correction and near-surface geology beneath the station. Stations located on soil sites have high amplifications with negative station corrections. In contrast, stations located on hard rock sites have low amplifications with positive station corrections. After applied the station corrections to revise〖 M〗_L estimation, the monthly event-occurrence rates return inside the range of one standard deviation in 2012 and revealed no low or high anomaly. Therefore, the reason low seismicity in 2012 is that new seismic stations which are installed on rock sites lead to underestimate of the magnitude determination. Next, we investigate the spectral decay parameter kappa (κ) to get more information for site effects of seismic stations. We used 133 earthquakes recorded by 30 borehole seismic array deployed by Central Weather Bureau (CWB) to estimate the κ value. Each vertical array includes two force balance accelerometers at the surface and the borehole. Based on the regression analysis between κ value and hypocentral distance for each surface-borehole station pairs, most of resulting κ0 derived from surface stations are higher than results of borehole stations. These valuable dataset of κ0 values offer an excellent opportunity for us to evaluate the quality factor (Qs) at shallow depths. In comparison with a study of site corrections, these higher κ0 values associated with negative station corrections can correspond to effect of soil conditions. In order to determine the linear relationship between station corrections and κ0 values, we find the co-site stations of TSMIP and CWBSN to estimate κ0 and the regression line. In this study, the station corrections of magnitude and κ0 values both can investigate the site effects beneath the stations. Furthermore, the regression line of these two can be the information for the estimation of earthquake magnitude including borehole stations in the future.
|
|---|
