| 摘要: | 在地震分布上，台灣西北部向來是地震活動較少的區域，但近年來的研究顯示本區域在地質構造上有其特殊之處，此外，本區域有許多重要的公共設施與工商重鎮，且在1999年九二一大地震時，位於新竹的強震站曾記錄到異常大的震度，再加上1935年的新竹-台中烈震在本區域南端發生，儘管在本區域地震發生的次數相對較少，但是否有可能發生大地震，仍然是值得關注的焦點，因此本研究針對本區域的地震活動與地下構造加以探討。 本研究自2001年1月起，陸續於桃園、新竹與苗栗一帶設置十個地震站，構成一個臨時微震網。每個地震站包含一部加速度地震儀，外接三分量速度型感應器，並配有GPS全球定位系統，以及資料儲存裝置。由於本微震網備有GPS的絕對時間系統，有利於地震的精確定位。為了收錄規模較小的地震，或是對於小區域地震能有量多質優的記錄，在地震站的選址上，設置密集的地震站是必要的，但是在儀器數量有限的情況下，局部區域過於密集的測站分布，容易使微震網外的地震失去測站的包覆性，影響定位品質，故本微震網之測站採均勻佈設於全研究區域內。 本研究利用所佈設的臨時微震網，結合中央氣象局地震網（CWBSN）的資料進行地震定位，並利用地震群重新定位的方法對於本區域的地震活動進行更深入的分析，嘗試與活斷層分布、地質剖面比對。另外也定出大批地震的震源機制，有助於對本區域地體構造應力特性之瞭解。最後利用地震定位所得到的P波平均走時殘差與加速度記錄頻譜所得到的共振主頻比較，估算沖積層厚度。 由本微震網結合CWBSN資料的定位結果，可看出加入距離震源較近的微震網測站資料後，地震在深度上有明顯的變化，本區域的震源深度大都在15公里以內。使用一維速度模型造成地震定位系統性的偏差，在利用DD和JHD做地震群重新定位後，地震位置有明顯地改善，且在水平方向上呈現往西北偏移的情況，深度上集中在5到10公里處。JHD的測站修正量則反映出本區域西北－東南方向速度構造的差異，與地質和地形的趨勢皆甚一致。而由重新地震定位後的地震分布與地質剖面比對，則顯示地震位置與地下構造有關。 在利用加速度資料以決定震源機制部分，先將單站的三分量加速度記錄積分成位移波形，再用波形逆推方法定出震源機制，共計得到88個芮氏規模為1.35到3.33之間的地震震源機制。由這些震源機制區分出不同斷層類型，結果顯示本研究區域除了有逆斷層和走向滑移斷層的震源機制外，也存在不少正斷層的震源機制。本微震網波形逆推所得到複雜的震源機制結果，顯示許多小地震的發生可能是由次生構造所引起。 本研究最後嘗試結合速度和加速度記錄資料，對各地沖積層厚度做一比較和估算。利用速度到時資料做地震定位所得到的測站P波平均走時殘差和利用加速度波形資料計算傅立葉譜得到的測站共振主頻，兩者得到本區域的等值圖並不一致，且利用這兩個參數估算出來的沖積層厚度也有很大差異。另外比較測站P波平均走時殘差和由JHD所得到的P波測站修正量，前者主要反映縱向的速度構造，後者則以側向速度構造為主。然而P波平均走時殘差與地表地質和地形有良好的對應，並且在與地質剖面比對時，顯示兩者也有其相關性，因此可利用P波平均走時殘差的等值圖反映地下構造的趨勢。 The seismicity is low in northwestern Taiwan for a long time. However, recent studies indicated the geological characteristics in this area are special. Besides that, many important facilities, industrial and economic cities are in this area. Significant intensity was recorded at seismic stations in Hsinchu area during the 1999 Chi-Chi earthquake. And the disastrous Hsinchu-Taichung earthquake in 1935 occurred in the southern part of this area. Thus, potential recurrence of large earthquakes in this area becomes important topics. In response, the seismicity and subsurface structures are analyzed in this study. A temporary seismic network including ten seismic stations was deployed in Taoyuan, Hsinchu and Miaoli counties in northwestern Taiwan since January 2001. Each station has one triaxial accelerograph, three external one-component velocity sensors, global positioning system, and data storage device. Seismic records with absolute timing are critical to obtain accurate earthquake locations. Dense station distribution is necessary to get earthquakes with lower magnitude from clear seismic signals recorded in local area. Based on considerations of available instruments and recording sites, the stations were deployed uniformly in the study area. In this study, the arrival time data of the temporary seismic network and CWBSN are combined to locate earthquake. Two dense earthquake clusters were relocated to compare several geological cross sections. In order to study the stress patterns, numerous focal mechanisms were determined by waveform inversion. Finally, the thicknesses of alluvium were estimated by using the P-wave travel-time residuals from earthquake location and the dominant frequencies identified from spectra of ground acceleration. By combining the temporary seismic network and CWBSN data, the results of earthquake location show significant convergent in focal depths due to adding of the near source arrival time data. Most of the hypocenters are located shallower than 15 km at depth. Relocation of two dense clusters using the JHD and DD methods removed systematic bias due to one-dimensional velocity model. The events were shifted toward northwest in the horizontal direction and became clustered at depth from 5 to 10 km. The station corrections of JHD reflected the difference in geology of the northwest and southeast parts in the study area. It is also consistent with distinct topographic features. By comparing the relocated events with several geological cross sections, we found that the seismicity and subsurface structures are related. To determine the focal mechanisms using waveform inversion, the acceleration records were used. The acceleration records of a single station with three-component sensors are doubly integrated to get displacement waveforms. Then the focal mechanisms are determined by waveform inversion. In total, 88 focal mechanisms were determined with local magnitudes from 1.35 to 3.33. The widespread presences of complex fault types of focal mechanisms imply that the microearthquakes might be triggered by subfaults. Finally, the thicknesses of alluvium were estimated by correlating the velocity and acceleration data. Results from the averaged P-wave travel-time residuals from earthquake location and from the dominant frequency from Fourier spectra are not consistent to each other. The thicknesses of alluvium obtained by these two methods are different. However, the averaged P-wave travel-time residuals can be correlated with geology and topography. There is good relation between travel-time residuals and geological cross-sections. The results imply that the averaged P-wave travel-time residuals can be used to prospect the subsurface structures. |
