利用超導重力儀訊號偵測地震造成的永久性重力變化

Abstract

本研究可分為兩個部分來探討。第一部分利用超導重力儀重力資料及中央氣象局所發布的地震彙報，欲計算出因地震發生時所產生的永恆重力變化。資料時間是自2006年4月至2010年3月。利用三種方法進行檢測：(I) 傅立葉函式進行海潮擬合；(II) 利用外插函式進行擬合；(III) 簡單法，利用地震前後帄均計算差異量。發現目前以Method I進行擬合其相關係數最高，且計算後的重力變化與Method III趨勢一致。為了研究2010年3月所發生的甲仙地震，SG48及SG49各別得到4.288μgal及0.633μgal的重力變化；採用Sun ( 1998 )模式所計算出的新竹站重力變化量為4.32ngal。而GPS計算高程變化，發現地表於震後上升0.575 cm( 相當於–1μgal )。 第二部分，則探討兩台儀器於同震的情況下SG48與SG49不同反應的原因，分別針對儀器擺設的距離與方位角、地震能量及儀器設計問題等進行詴驗，結果發現 (1) SG48及SG49的重力變化在同震時反應皆不同；(2不同地震種類或儀器回饋機制是造成重力變化的主因。

This research is divided into two parts. In the part, superconducting gravimeter (SG ) from the Hsinchu SG station and the earthquake data from the Central Weather Bureau of Taiwan are used to compute permanent gravity changes caused by quakes, covering the time span from April 2006 to March 2010. Three methods are used compute the gravity changes: (I) ocean tide fitting by Fourier function, (II) extrapolation fitting, and (III) difference of simple averages of SG values before and after an quake. Method I produces the highest correlations between the original and model SG value, and result in gravity changes consistent with those from Method III. In a case study for the Jiasian earthquake in March 2010, SG48 and 49 show gravity changes of 4.288 and 0.633 μgal, while the model value of Sun (1998) is 4.32 ngal and the GPS-derived height change is 0.575 cm (equivalent to -1 μgal ), highlighting the disagreement among these sensor results. In the second part, the responses of SG48 and SG49 to the same quakes are investigated with respect to azimuth to the earthquake source, distance and seismic energy. The result shows that (1) SG-derived gravity changes from SG48 and 49 are different in most events, and (2) the causes of gravity changes can be of tectonic origin or instrument origin.