由歷史航照重建探討花東縱谷北段活動構造與1972年瑞穗地震之關係

The relationship between active tectonics in the northern Longitudinal Valley and 1972 Rueisuei earthquake assessed by historical aerial photos reconstruction

李應頎(Ying-Chi Lee)

指導教授 ： 王昱

國立臺灣大學/理學院/地質科學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202200312

摘要

台灣東部於1972年4月22日發生地震矩規模(Mw) 6.7之瑞穗地震。過去研究發現1972年瑞穗地震主震之震源機制解為逆衝斷層帶走滑分量，且主震與餘震序列位於1951年10月花蓮縱谷地震群以南、11月縱谷地震序列以北，顯示該地震發震斷層與花東縱谷北段的活動構造有關。為了解1972年瑞穗地震與花東縱谷北段活動構造之關係，與花東縱谷北段海岸山脈側的活動構造特性，本研究使用在1972年瑞穗地震前、後之歷史航照影像，以運動回復結構(SfM)及多視立體(MVS)之方法建置該地區的歷史數值地表模型(DSM)與正射影像，透過構造地形判釋與次像素關聯分析，以及歷史文獻收集及實地訪談方式，試圖了解該區域之活動斷層特性與1972年地震可能伴隨的同震地表破裂。
本研究透過1972年地震災害報導，以及本研究現地訪談所得的主震地震震度分布圖，發現該地震震度分布與花東縱谷中段與北段之活動構造位置相符。本研究亦透過歷史數值地表模型分析發現，在花東縱谷北段-光復鄉至壽豐鄉一帶總長約24公里的海岸山脈西翼，存在連續分布之斷層跡與相關活動構造地形。然而沿著斷層跡之口述歷史與地震前後的次像素關聯位移分布皆未發現明顯可見的地震地表破裂，暗示1972年瑞穗地震可能並未伴隨明顯的地表斷層破裂。該段活斷層跡沿線可見斷錯沖積扇、閉塞丘、斷層谷、斷錯河等多樣的左移活斷層地貌，顯示海岸山脈西側近地表處的活動構造強烈受到走向滑移錯動控制，另有發現開花構造與右階斷層之現象。本研究推測這些活動構造是由斜碰撞構造環境下形成，且屬於較年輕的斷層系統。綜上所述，本研究認為1972年瑞穗地震在花東縱谷北段沒有伴隨明顯可見的同震地表破裂，地表所見之左移活動構造應為早於1951年的同震斷層錯動所造成的結果。

關鍵字

1972年瑞穗地震 ； 歷史航照 ； 縱谷斷層 ； 活動構造 ； 運動回復結構 ； 多視立體技術 ； 數值地表模型

並列摘要

On 24 April 1972, a destructive earthquake (Mw 6.7) struck Rueisuei, eastern Taiwan. Published mainshock focal mechanisms suggest the Rueisuei earthquake occurred on an oblique reverse fault, and the earthquake sequence’s relocation results suggest the sequence lies between the October 1951 earthquake sequence at Hualien, and the November 1951 earthquake sequence at Taitung. Altogether, these seismological analyses suggest the 1972 Rueisuei earthquake was likely caused by the fault rupture along the eastern margin of the northern Longitudinal Valley. In order to understand the relationship between active structures along the northern Longitudinal Valley and the 1972 Rueisuei earthquake sequence, we use historical aerial photos taken before and after the 1972 earthquake to construct the historical Digital Surface Models (DSM) and orthoimages using SfM-MVS methods. With these historical DSM data, we are able to map active fault features along the western flank of the Coastal Range, and use sub-pixel correlation to look for the plausible coseismic surface fault rupture. We also collect historical earthquake documents and eyewitness accounts from village elders to understand the ground motion distribution and possible coseismic ruptures along this section of LVF.
Our result suggests the 1972 Rueisuei earthquake’s intensity distribution agrees with active tectonics in the middle to northern Longitudinal Valley, where we found a section of ~24-km-long active fault traces at the western flank of the Coastal Range (CR), from Kuangfu to Shoufeng Township. Although no surface rupture records associated to the 1972 earthquake can be found along this section of the fault, various left-lateral active fault features, such as offset alluvial fans and rivers, shutter ridges and fault valleys can be observed along this section of the fault, suggesting the active structures at the northwestern flank of the Coastal Range are dominantly controlled by left-lateral strike-slip. We also observed flower structures resulted from the oblique collision condition and several right-stepping fault step-over, indicating the active fault trace is relatively immature.
In conclusion, we suggest 1972 Rueisuei earthquake was triggered by faulting with no apparent coseismic surface rupture in northern Longitudinal Valley. Left-lateral active fault geomorphic features observed on the surface may result from active fault offset early than the 1951 earthquakes.

並列關鍵字

1972 Rueisuei earthquake ； Aerial photos ； Longitudinal Valley Fault ； active tectonics ； DSM ； SfM ； MVS

參考文獻

References

Agisoft. (2021). Agisoft Metashape User Manual Professional Edition, Version 1.7.

Ayoub, F., Leprince, S., Avouac, J.-P. (2009). Co-registration and correlation of aerial photographs for ground deformation measurements. ISPRS Journal of Photogrammetry and Remote Sensing, 64, 551-560.

Ayoub, F., Leprince, S., Keene, L. (2009). User's Guide to COSI-CORR Co-registration of Optically Sensed Images and Correlation.

Barrier, E., Angelier, J. (1986). Active collision in eastern Taiwan: the Coastal Range. Tectonophysics, 125(1-3), 39-72.