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Tectonic Geomorphology of the Southernmost Sagaing Fault and Surface Rupture Associated with the May 1930 Pegu (Bago) Earthquake, Myanmar

Department of Geophysics, Kyoto University, Kyoto 606-8502, Japan tsutsumh{at}kugi.kyoto-u.ac.jp

Takayuki Sato^*

Department of Geography, Hiroshima University, Higashi-Hiroshima 739-8522, Japan satotaka{at}star.odn.ne.jp

^* Now at: Meiji Gakuin High School, Kita-Kyusyu 804-8558, Japan.

Online Material: Field photographs of the southernmost Sagaing fault.

The Sagaing fault is a continental transform fault between the India and Sunda plates that connects spreading centers in the Andaman Sea and the continental convergence zone along the Himalayan front. Several M>7 earthquakes occurred along the fault in the last century, and Global Positioning System campaigns revealed a right-lateral slip rate of 18 mm/yr, about half of the total India–Sunda displacement rate of 35 mm/yr. However, there are few fundamental geologic data on the Sagaing fault, such as detailed fault trace locations, information on late Quaternary slip rate, or spatial extent of surface ruptures during historical earthquakes. We conducted geologic field investigations along the southernmost 120 km long stretch of the fault zone that ruptured during the M 7.3 1930 Pegu (Bago) earthquake. We found well-defined tectonic geomorphic features across a deltaic lowland, including fault scarps, tectonic depressions, stream offsets, and pressure ridges. The sense of displacement is predominantly right-lateral strike-slip with vertical motion less than 1/5 of horizontal motion. Based on right-lateral offsets of stream channels, terrace risers, and property boundaries, we estimate coseismic displacement during the 1930 earthquake as 3.0 m. This in turn gives us a preliminary recurrence interval of surface-rupturing earthquakes on the Sagaing fault east of Yangon as 160 yr. Coulomb stress change induced by the May 1930 Pegu (Bago) earthquake may have triggered the December 1930 Pyu earthquake immediately to the north. This study demonstrates that detailed geomorphic investigations are key for a better assessment of seismic hazard for the Sagaing fault.