Offset ancient city wall yields plausible slip rate for the Sagaing fault, Burma (Myanmar)

 

論文發表人：王昱（加州理工學院 地質與行星科學系 博士班）

http://www.agu.org/meetings/fm08/

 

The Sagaing fault offers great potential for paleoseismology study, because it traverses a region with a long history and high rates of sedimentation.  Buddhist documents from ancient Pegu (Bago), in southern Myanmar, record 34 strong earthquakes in the past 2.3 millennia. The latest of these is the 1930 Pegu earthquake (M 7.3), which had high intensities along a 90 km stretch from Pegu to the southern coastline of the country. 

We have found evidences for surface rupture in 1930 in the stories of village elders and in offset paddy fields.  These reports and offsets suggest that coseismic displacement decreased from several meters in Pegu to liquefaction without faulting near Payagyi Township, 15 km farther north. West of Payagyi, the fault trace cuts through and offsets an ancient city wall. The age of the ancient city is uncertain, but descriptions from Burmese history indicate it was built in the late 16^th century, probably about 440 yrs ago.

Determination of the offset of the 440-year-old city wall is possible, but not simple, in part because vertical displacements across the fault have resulted in differential sedimentation on the flanks of the wall.  After accounting geomorphologically for the differential sedimentation, the offset of both the outer and inner edges of the city wall appear to have sustained a right-lateral offset of about 6 meters.

This yields an approximate slip rate of 14 mm/yr, which is slightly lower than the slip rate determined by others from GPS geodesy. The number of earthquakes involved in creating the 6-meter offset is currently unknown, but paleoseismic excavations within the ancient city may well yield evidence of discrete offsets that we will be able to ascribe to specific large earthquakes in the historical record.  Candidates include historical earthquakes 1582, 1644, 1768, 1830, 1888, 1913 and 1917 C.E.

 

實皆斷層(Sagaing fault)為南北向貫穿緬甸全境的主要走滑斷層。由於該斷層通過具有長期歷史紀錄及高沉積速率的地區，為一具高潛力的古地震研究區域。

於過去的二千多年中，位於勃古(Bago)的佛教典籍紀錄了南緬甸地區多達34次的強烈地震，最後一次規模7.3的強震發生於1930年，並造成該地區的嚴重死傷。然而，該事件僅留下部分片段的地表破裂紀錄，對於古地震事件的研究並無太多的助益。

本研究透過現地調查及口述歷史的方法，揭露了1930年地震的同震變形可能情境。調查顯示實皆斷層(Sagaing fault)1930年的地表破裂在通過勃古城後即向北遞減，由數公尺遞減至難以辨認的程度。 距離勃古城以北15公里處，調查亦發現該斷層穿過一古城遺跡，並錯移該遺跡之北面城牆。雖然至目前為止，對於該遺跡之年代並無科學上的定論，但透過緬甸歷史之描述，我們相信該遺跡應建於16世紀末期，據今約有440年之譜。透過電腦模擬方式，本研究建議該古城牆所紀錄的錯移量約在6-8公尺之譜。

此結果顯示實皆斷層(Sagaing fault)的大約滑移速率約在每年1.4 -1.8 公分之間，與近年的GPS觀測結果相近。目前我們並不清楚這6-8公尺的位移與哪些歷史地震有關，但應不脫公元1582，1644，1768，1830，1888，1913 及1917 的地震事件。對於該遺跡的更進一步調查或可揭露實皆斷層(Sagaing fault)於過去數百年間的詳細活動行為，並將對東南亞該地區的潛在地震災害有更進一步的認識。