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Search for New Sources of Ambient Tremor Using Local Seismic Network
|Abstract:|| 非火山自發型長微震(non-volcanic tremors)是慢地震家族的一員，常見於大型隱沒帶或板塊邊界型斷層，在孕震帶更深處，以數分鐘至數小時的持續時間釋放能量，對應的地震矩能量可高達規模六的一般地震，是了解板塊邊界的孕震潛能不可或缺的課題。臺灣的自發型長微震，過去被發現頻繁地發生在中央山脈南段下方，其特徵為地震波持續時間數分鐘至半小時，振幅與背景噪訊相當且無明顯體波波相，需依賴多測站微小波包之相近到時方能定義。長微震震源區是否僅限於中央山脈南段？測站密度能否影響震源位置？為釐清此點，本研究利用架設的臨時中央山脈中段地震網（位於長微震震源區北方，彌補原有地震網的空區），以進行長微震的自動化偵測和目視確認，進一步分析臺灣長微震的空間特徵。
Non-volcanic tremor is one type of of slow earthquakes phenomena. It has been detected mainly in plate boundaries including subduction zones and transform faults and is usually found to occur below the seismogenic zone. The seismic characteristics include small amplitude, long duration of several minutes to hours, no obvious arrival of body waves, and the main frequency band of 2-8 Hz. By geodetically detected slow slip events, the equivalent seismic moment could be up to Mw 6. In Taiwan, ambient tremor events were previously detected in south Central Range with duration ranging from several minutes to thirty minutes. The location of ambient tremor, although showing location uncertainty below 10 km, is determined using the stations located to south of 23.5°. Whether different source of tremor can be found using widely distributed stations remains unknown. In this study, we aim at adding temporary mountain array to the north of 23.5° in middle Central Range, to address: Are there any tremor source located outside the previously recognized source area? Does coverage of station distribution influence the tremor location? And what is the spatial and temporal characteristics of ambient tremor detected using the mountain array? During the study period from January 2013 to January 2014, we found tremor mainly concentrated in the two areas: (1) South Central Range, here tremor events are located in a depth range of 15 to 50 kilometers with north-south striking feature, consistent with the previous discovery (2) middle Central Range ( northern than 23.5°N), here tremor events are vertically aligned and located deeper than 15 kilometers, roughly underneath the eastern flank of the Central Range. The new tremor source is located near the edge of seismic station coverage, leading to large location uncertainties of greater than 10 km. From the mapview, however, we found that the newly discovered tremor activity is roughly overlapped with the location of repeating events along the Central Range fault. By integrating with previous tremor catalogs, the temporal evolution of tremor activities in Taiwan illustrates a quiescence during the period of 2013-2015, while the rest of the study period from 2007 to 2016 reveals the concentration in southern Central Range. The cause of the tremor quiescence may provide the clue for the generation mechanism of tremor, however, requires the analysis on the long-term tidal, GPS, precipitation data in the future.
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