| 摘要: | 台灣位於歐亞板塊與菲律賓海板塊交界處，台灣東部位於兩板塊交會的縫合帶，地體構造因板塊運動以及隱沒帶作用而相當複雜，相關構造至今仍具有高度研究價值。北花蓮區域地震活動頻繁區域，地震規模自M1至M8，震源深度由數公里至逾百公里，且蘇花路廊的鐵公路運輸為花蓮溝通北台灣重要交通動線。然而該區域中央氣象署地震觀測網（Central Weather Administration Seismographic Network，簡稱CWASN）佈設密度相對低，導致許多微地震事件可能無法被偵測，進而低估地震活動度。為提升微地震觀測能力，本研究自2020年起於北花蓮地區和平村仁水隧道之安全疏散隧道設置一地震監測系統，包含一部深度為一百公尺之井下寬頻地震儀和兩部速度型與兩部加速度型地震儀。
本研究首先針對不同儀器去除儀器響應得到真實地動值，並透過功率譜密度之機率密度函數（Probabilistic Power Spectral Density）分析儀器背景雜訊，結果顯示背景噪訊介於-80dB至-120dB之間，井下寬頻地震儀長週期噪訊較其他儀器低。另外藉由2021年中國青海（Qinghai, China）地震的長週期波形校正井下地震儀方位角，校正方位角為逆時針旋轉339.11°。於2022年10月至2023年12月期間，本研究從井下地震儀連續波形中人工辨識出10826筆地震事件，其中有2956個地震事件未被列在氣象署地震目錄中，佔總事件數的25%。本研究進一步利用台灣三維速度構造模型以及鄰近三個CWASN井下地震儀之P波到時，成功定位出496個地震事件，震央位置多集中於測站東北方。此外，本研究使用頻譜擬合法（Spectral Fitting Method）估算這些地震事件之震源參數，結果顯示地震矩規模在Mw0.0至Mw3.0的規模範圍內，應力降與地震矩之間並無明顯相關性，支持小規模地震具有自相關性（Self-similarity）特徵的觀點。
;The northern Hualien area, located at the collision boundary between the Eurasian and Philippine Sea plates, has a complex tectonic setting that remains a subject of debate. This region experiences high seismicity, with earthquake magnitudes ranging from M1 to M8 and focal depths from near the surface to about 150 km. Moreover, an important segment of Su-Hua Highway Corridor, including both highway and railway, passes through the study area, serving as a vital link between eastern and northern Taiwan. However, due to the lack of seismic stations in the local seismic network operated by Central Weather Administration （CWA）, the seismic monitoring capability in this area may be insufficient. To address this issue, we deployed a seismic array near Heping Village since 2020 to enhance the earthquake detection capability in the study area. This array consists of a borehole seismometer installed at a depth of 100 m, accompanied by a surface strong-motion accelerograph and a velocity seismometer.
In the present study, we correct the instrument response and examine the probabilistic power spectral density for the noise evaluation in each station. The background noise of the instruments ranges from -80 dB to -120 dB. The borehole broadband station exhibits a lower noise level in long-period signals compared to the other instruments. In addition, we calibrate the orientation of the borehole sensor by using long-period waveforms from the 2021 Qinghai earthquake in China. The corrected angle is 339.11° counterclockwise. To further investigate the microearthquake characteristics, we manually identify 10,826 events from continuous records of the borehole seismometer between October 2022 and December 2023 and obtain 2,956 events which are not listed in the CWA catalog, which account for approximately 25% of the total events. Using the Taiwan 3D velocity model and the P-wave arrival times from three nearby borehole seismometers, we locate 496 events, clustering in the north of the seismic array. We further estimate the source parameters of these events using a spectral fitting approach and find that earthquakes with magnitudes ranging from 0 to 3 show no significant relationship between stress drop and seismic moment. Our results suggest that small earthquakes in the study region exhibit a self-similar behavior. |
