場址效應研究：地震規模之測站修正量與利用井下地震儀陣列資料量測衰減參數

Abstract

中央氣象局地震觀測網(Central Weather Bureau Seismic Network, CWBSN)於2012年新增測站加入地震觀測網中，但是還未將新增地震測站的場址效應用以修正規模，其會影響地震芮氏規模的估算；另外，從芮氏規模大於2.0的地震個數統計中，發現在2012年有低地震活動度之現象，因此，我們想要了解新增測站與2012年的低地震活動度，這兩者之間有無關聯性。本研究第一部分使用CWBSN地震目錄中，1994/01/01至2012/12/31所有地震事件的芮氏規模與單一測站所估算的地震規模來得到測站修正量，為了要降低測站的場址效應，將使用測站修正量來修正芮氏規模。此測站修正量充分的反映測站下方之地質條件，測站位於鬆軟的土壤層，其修正量為負值，相反地，測站位在堅硬的地層上，則為正值。透過修正地震規模後，在2012年芮氏規模大於2.0的地震個數明顯的增加，並沒有出現異常的現象，因此2012年並沒有出現低地震活動度之現象，而是中央氣象局新增的測站大多分布於山區，造成地震規模低估之問題。除了測站修正量，本研究第二部分進行分析振幅頻譜中高頻衰減參數kappa(κ)，進一步獲得更多台灣地震測站場址效應的資訊，我們使用氣象局30個井下地震儀陣列，選取芮氏規模大於4.0且震源深度小於35公里的133個地震事件去計算κ值，每個地震儀陣列於地表和井下的位置各有一部強震儀，利用地表與井下測站算出的κ值與震源距進行迴歸分析後，可以得到κ0值，該值代表了該測站的近地表衰減特性，從結果中可以發現，井下地震儀陣列中地表站的κ0值大於井下站的κ0值。進一步，我們可以使用在本研究中所求出的井下地震儀陣列的κ0值，研究淺層的衰減參數Qs值。另外，將臺灣κ0值分布圖與測站修正量做比較，高κ0值出現在測站修正量為負值的地區，再次驗證與測站所在的地質狀況產生的場址效應有直接相關，為了找出測站修正量與κ0值的關係式，我們將台灣強地動觀測計畫(Taiwan Strong Motion Instrumentation Program, TSMIP)的自由場強震站與CWBSN共站之測站，計算其κ0值，並得出迴歸公式。在本研究所得到的測站修正量與κ0值可以用來討論測站的場址特性，並且透過井下地震儀陣列的κ0值與測站修正量的迴歸式，可提供在未來將井下地震儀加入地震規模估算做為參考。

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.