Using Regional Waveform Source Inversion to Investigate the Earthquake Properties in Eastern Black Sea and the Javakheti Highland in the Lesser Caucasus

阿拉伯板塊和歐亞板塊之間的碰撞在土耳其產生大型橫移斷層、將東安納托尼亞抬升為高原、並在高加索形成山脈。然而高加索旁邊的黑海區域，是個相對穩定的構造，也鮮少中大規模地震的發生。過去的研究裡高加索地區的震源機制多是利用遠震的波形資料針對中大規模的地震求解，小地震的研究較為缺乏，約束也較差。本研究使用新的測站陣列，設定東經40–47度、北緯40–44度為研究範圍，以區域波形資料分析2012年1月1日至2014年5月30日間、規模3.5以上的地震之震源機制和深度，新增地震共20筆，並更新4筆前人結果，主要成果在東部黑海以及小高加索扎瓦赫季高地兩區域。 其中2012年12月23日在東黑海區域的Shatskty Ridge發生一起規模5.8之地震序列，為地震網架設期間所難得紀錄到之地震事件，位處複雜的拉張大陸地殼上。本研究之震源機制結果顯示這一序列共六個主餘震全部為走向滑移的型態，破裂深度為20-30公里，五個機制解的位置呈西北西–東南東走向並與其中一斷層面方位相符，判定主震為左移斷層，另一群餘震向東北方較淺處分布。根據鄰近區域的震測剖面資料，我們推斷本地震的發生可能發生在過去弧後張裂所生成之舊有正斷層系統上，受現今擠壓應力重新發育成走向滑移斷層，而主震發生後，又引發了東北方另一條走向滑移斷層的錯動，次斷層走向與主斷裂垂相直，可對應至洋盆張裂期形成的轉型斷層。 另外，在群震頻繁的小高加索扎瓦赫季火山高地上已累積共20個震源機制，大多為走向滑移斷層，僅在北部出現3個小的逆衝事件，我們發現在扎瓦赫季山脊東部的走向滑移地震帶有少許東西拉張分量，和地表上的正斷層及GPS的東西相對運動相符。The continental collision between Arabia and Eurasia created the strike-slip fault conjugates in Anatolia as well as the folds and thrust belts in the Caucasus Mountains. The Black Sea, on the contrary, is tectonically stable and absent of notable earthquakes. In previous studies, focal mechanisms in the Caucasus regions are determined primarily for large earthquakes using global waveform data. Small earthquakes are primarily studied with polarity data, thus the reliability of focal mechanisms highly depends on station coverage. In this study we use regional waveforms of a new seismic array to constrain the focal mechanisms and depths of the earthquakes with M>3.5 in the Greater Caucasus, Trans-Lesser Caucasus (mainly in Javakheti Highland) and the eastern Black Sea. There is a total of 20 new events for the studied area (Latitude 40–44⁰N and Longitude 40–47⁰E) in time period 2012.01.01–2014.05.30. We also update 4 focal mechanisms from previous study. On December 12 of 2012, an M 5.8 earthquake sequence occurred in the eastern Black Sea region beneath an extended continental margin called Shatsky Ridge. The fault plane solutions of the main shock and 5 aftershocks are all in strike-slip faulting. Depth of rupture is as deep as ~30 km. Location of five focal mechanisms is aligned at WNW-ESE direction, which coincides with one of the fault plane, indicating left-lateral slip. A group of small aftershocks scattered toward northeast direction at shallower depth. Based on the seismic reflection profiles nearby, we conclude that the main rupture fall along the existing normal fault created associated with back-arc spreading of Eastern Black Sea basin in the past, but now reactivated as strike-slip due to compressive stress. A second rupture is triggered along a conjugate fault perpendicular to the main fault, which can be correlated with a transform fault developed during the ocean spreading. Javakheti is a volcanic highland where swarms of small earthquakes are continuously occurring in its eastern ridge. Collectively, the total of 20 earthquakes here is dominated by strike-slip faulting. Only three small thrust events are found in north of the swarm area. Earthquakes in eastern foothill of Javakheti ridge show small amount of normal-faulting component, which indicates east-west extension, consistent with the fault outcrops nearby and the east-west relative motion form GPS data