Earthquakes and slip rate of the southern Sagaing fault: insights from an offset ancient fort wall, lower Burma (Myanmar)

SUMMARY Field investigations of an ancient fortress wall in southern Myanmar reveal an offset of ∼6 m across the Sagaing fault, the major right‐lateral fault between the Sunda and Burma plates. The fault slip rate implied by offset of this 16th‐century fortress is between 11 and 18 cm yr –1 . A palaeoseismological excavation within the fortress reveals at least two major fault ruptures since its construction. The slip rate we obtained is comparable to geodetic and geological estimates farther north, but is only 50 per cent of the spreading rate (38 mm yr –1 ) at the Andaman Sea spreading centre. This disparity suggests that other structures may be accommodating deformation within the Burma Plate. We propose two fault‐slip scenarios to explain the earthquake‐rupture history of the southern Sagaing fault. Using both small offset features along the fault trace and historical records, we speculate that the southern Sagaing fault exhibits a uniform‐fault‐slip behaviour and that one section of the fault could generate a  M 7+ earthquake within the next few decades.