宜蘭地區最大地動加速度之估算

Abstract

本研究提出完整的模式以推算震源頻率函數及強震站的場址效應。使用中央氣象局強地動觀測計畫1996~2009年強震站觀測紀錄，先利用複數解載解析強震站地震紀錄，找出PGA瞬時的頻率函數；利用波線追跡法及能量衰減構造及平均法推算震源函數；再推算出各強震站的場址效應。最後利用已知震源函數、路徑效應及場址效應估算宜蘭地區各強震站的最大加速度峰值。得到以下幾點結論： (1) 各強震站PGA瞬時的頻率函數與全時段地震紀錄的頻率函數有極大的相似性。 (2) 宜蘭地區放大頻率範圍在1Hz~5Hz為主，且靠近山區頻率有變大趨勢。宜蘭地區第一主頻的場址效應，放大倍率大多落於0.9~2.5倍之間。而且靠近蘭陽平原西北部強震站放大倍率較低(1.2~1.8倍)，東部平原放大率較高(2.4~3.0倍)。 (3) 利用已知震源函數、路徑效應及場址效應推估宜蘭地區的最大地動加速度峰值，除了南澳地區強震站有明顯低估外，其餘地區有相當的準確度。In this study we develop a procedure to estimate the peak ground acceleration (PGA) in Yilan area. In the first step, the frequency function at the PGA time is calculated by using the complex demodulation to the seismic record of the strong earthquake stations from 1996 to 2009 of the CWBSN. Then, the source frequency function is estimated by using the ray tracing method, the attenuation structure and the average method. The site effect of each station is derived from the source function and path effect correction. Finally, the source frequency function, path effect and site effect are compiled to estimate the peak ground acceleration of each station in Yilan area. The conclusions in this study are shown in the follow: (1) The instantaneous frequency function at the PGA time shows well correlation to the frequency function of the whole seismic record. (2) The amplification frequency range of Yilan area is mainly within 1~5Hz, and the frequency near the mountainous area has a tendency to become larger. The site effect in Yilan area indicates that the first magnification frequency mostly falls between 0.9 and 2.5 times. Moreover, the magnifying ratio near the strong earthquake station in the northwest of Lanyang Plain is low (1.2~1.8 times), and the magnification in the eastern plain is higher (2.4~3.0 times). (3) The estimated of the peak ground acceleration in Yilan area, using the source function, path effect and site effect, is compared to the observed data. It shows a little inconsistence in most stations except for the Nanao station.