由地表破裂與震度紀錄重建1839緬甸阿瓦地震之規模

Re-estimating the magnitude of 1839 Ava earthquake through geomorphic mapping and macroseismic records, central Myanmar

吳昇翰(Sheng-Han Wu)

指導教授 ： 王昱

國立臺灣大學/理學院/地質科學系/碩士(2022年)

https://doi.org/10.6342/NTU202203906

摘要

西元1839年3月23日，緬甸中部遭遇極嚴重的的地震，甚至遠達曼谷也能感受到晃動。歷史紀錄顯示1839年地震震央應位於阿瓦(Ava)附近，且幾可確定此次事件的孕震構造是緬甸與巽他地塊間的轉型斷層：實皆斷層(Sagaing Fault)。實皆斷層以南北向貫穿緬甸的右移斷層系統，總長度超過1000公里，持續以約20mm/yr的平均速率滑移。
實皆斷層沿線在過去150年間曾發生多次伴隨地表破裂的災害性地震，唯獨密鐵拉(Meiktila)至實皆(Sagaing)間的區段除可能造成1839年地震外，在過去近兩世紀中未有明確的地表同震破裂與地震記錄。以地震再現週期與斷層活動速率而言，該區段在過去超過100年以上未發生地震，可能表示斷層在該區段處於長達數百年的間震期閉鎖之狀態，並持續累積應力。為了解該地區的斷層活動性質，本研究試圖確定1839地震的地表破裂範圍，以及透過古地震研究的方法，嘗試重建密鐵拉區域的斷層活動歷史。
本研究透過高解析度數值地形模型，標記達西(Thazi)至內比多(Nay Pyi Taw)附近因斷層活動所造成的地表變形，並描繪斷層跡以及斷層跡上的錯移量紀錄以判斷1839阿瓦地震的平均同震位移量。其結果顯示此段沿線最後一次事件的平均右移量約為3.8 m。透過經驗公式計算顯示3.8 m平均同震位移量所對應之地震規模應介於MW 7.6 – MW 8.0間。除此之外，本研究試圖透過歷史文獻紀錄以及古地震露頭觀察重建1839地震的震度分布圖，並將之與1912 Maymyo earthquake (MW 7.7)等20—21世紀的大型平移斷層地震事件相比較。本研究於達西以南斷層附近發現一晚近地震事件所造成之噴砂露頭，顯示水平層狀湖相沉積物覆蓋於受擾動且被砂質侵入體貫穿的混亂湖相泥質沉積物之上，可能與1839年地震所引發的強地動相關。根據ESI 2007環境震度分級，該露頭代表IX級的震度。基於以上比較的結果，本研究認為1839阿瓦地震的規模顯著大於MW 7.7，震度分布與MW 7.9的地震相當。
參考前人對阿瓦地區的錯移量測量結果，本研究認為1839年地震可能在破裂區北段(阿瓦地區)造成5.5 m的平均地表錯移，而在破裂區南段(達西至內比多間)造成約3.8 m的平均同震錯移。使用前人GNSS研究得到的實階斷層斷層面寬度，本研究進一步計算1839年地震的地震矩規模應介於MW 7.8—MW 7.9之間，且其斷層同震破裂範圍應該至少達到285 km。本研究推論除上述之285 km外，北邊尚有40公里可能的破裂，最終同震破裂可能達325 km。本研究以錯移量經驗公式、震度比較、地震矩三種方法推估的地震規模很相近，都在MW 7.9上下。雖然1839阿瓦地震跨過兩個斷層段，且兩段上的1839地震同震錯移量並不相同，但若假設該同震錯移量在歷史上反覆發生，且斷層於間震期皆處於完全閉鎖的狀態，則兩個斷層段分別可能具有120年(阿瓦以北)與180年(阿瓦以南)的地震周期。這兩個假想的週期在歷史紀錄中幾乎都有可對應的地震紀錄。

關鍵字

活動斷層 ； 地震地質 ； 古地震 ； 實皆斷層 ； 緬甸

並列摘要

The severe Ava earthquake of 1839 occurred on the central part of the dextral Sagaing fault, and destroyed several Burmese cities. Since then, no major earthquake occurred along this segment of the fault. Also, Sagaing fault is a major structure in Myanmar, which is active and moving at a pace of 20 mm/yr. Therefore, Myanmar is now under great seismic threat. While the 1839 Ava earthquake is well documented in written records, its earthquake magnitude is not constrained. Thus, this study decided to incorporate both active fault mapping and historical earthquake accounts to estimate the plausible magnitude of the 1839 earthquake, and to infer the possible scenario for the upcoming earthquake on the central segment of the Sagaing fault.
To achieve this goal, this study measured cumulative dextral offsets on single fault surface trace along the Sagaing fault’s plausible 1839 ruptured extent. On Meiktila segment, the southernmost part of the 1839 earthquake’s plausible ruptured extent, our observations on airborne LiDAR DEM suggest the mean smallest-dextral-offset reached ~3.8 m. It corresponds to the mean surface fault offset of MW 7.6-MW 8.0 earthquakes through various empirical relationships. The macroseismic records of 1839 earthquake are also more intense than the records of nearby 1912 Maymyo earthquake (MW 7.7), supporting our interpretation that the magnitude of 1839 earthquake is higher than MW 7.7. While we compare intensity map of 1839 earthquake with 2002 Denali earthquake (MW 7.9), we found they are roughly compatible, considering great difference in their site effects. Therefore, the result magnitude should be larger than MW 7.7 and equal to or smaller than MW 7.9. Previous researchers measured 7 m dextral slip on Sagaing segment, northern part of the 1839 earthquake’s plausible rupture. We re-interpret 5.5 m of it was contributed by 1839 earthquake. Combine it with our ~3.8 m dextral slip in the southern part of the fault rupture and fault plane geometry from previous GNSS result, we calculated the moment magnitude of the 1839 Ava earthquake to be MW 7.9. This result almost reach the upper limit estimated in our other methods. Therefore, the rupture length of the 1839 Ava earthquake might be equal to or slightly longer than 285 km, sum of previous research and our study area. We further suggested the most possible rupture length of the 1839 Ava earthquake might reach 325 km from Naypyitaw to the southern margin of Singu Basalt. The rupture extent of 1839 Ava earthquake broke across two segments, which have different activity history in the 20th -21st century. If the observed offset of the most-resent-earthquake on these segments kept occurring in the future, they are likely to rupture at a cycle of 120 years in the north and 180 years in the south, respectively. Moreover, corresponding earthquake events in historical records can be found.

並列關鍵字

Active Faults ； Earthquake Geology ； Paleoseismology ； Sagaing Fault ； Myanmar

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