| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 105 === The Sunda megathrust is one of the major plate boundaries in South Asia, along which the Indian-Australian plate subducts northeastward beneath the Burma micro-plate offshore western Myanmar. In the past several centuries, the 1762 Arakan earthquake was the only giant event along this plate boundary, and caused significant co-seismic uplift along a large stretch of the western Myanmar coast. In previous studies, the magnitude of the event and the potential seismogenic structures were analyzed based on uplifted coastal features on the Ramree and Cheduba Islands, two of the largest coastal islands of western Myanmar.
However, in order to estimate average earthquake recurrence interval, inter-seismic deformation rate, co-seismic uplift, and long-term uplift rate are all necessary. We utilized coral microatolls as natural tide gauge to analyze relative sea-level history and to obtain information of land-level change during the inter-seismic period, and utilized the elevations of marine terrace shoreline angles and the age of marine terrace deposits to determine long-term uplift rate.
For some coral genus such as Porites, the highest level of survival (HLS) is constrained within a few centimeters of the lowest tide level. Therefore, once the relationship between HLS and the sea level is established, the morphology of the microatolls can provide us the relative sea-level history based on the patterns of their annual growth bands under x-radiographs. U-Th dating technique can constrain the age of the coral, and we can also identify sea level anomalies caused by climatic events through oxygen isotope analysis.
We collected two slabs of coral microatolls from the intertidal zone near Leik-Ka-Maw, a small village in northwestern Ramree Island, approximately 80 km away from the trench. The microatolls were uplifted and killed during a local earthquake event in 1848, and preserved HLS records of about 50 years prior to the 1848 event. We also surveyed four marine terraces with elevations between 1.0 and 11.4 meters.
Our results show that the coral recorded a HLS rise at a rate about 5.3 to 5.8 mm/yr, which represents land-level subsidence during the inter-seismic period. Several temporary HLS Diedown events are also present, likely produced by Indian Ocean Dipole events based on the microatoll morphology and results of oxygen isotopic analysis. The marine terrace level between 10.8 and 11.4 meters high formed during 7.5 ka and 8.2 ka, constraining a long-term uplift rate between 1.0 and 1.7 mm/yr. The recurrence interval of upper-plate splay fault earthquakes calculated using these results is between 300 to 360 years, and the average recurrence interval of both types of earthquakes (megathrust and splay fault) is between 220 to 280 years, both are shorter than the number obtained in previous studies. Therefore, it is important to further assess and prepare for earthquake hazards in this area.
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