The 2011 Tarlay earthquake occurred on 24 March in Shan State, Myanmar. The earthquake measured 6.8 and had an epicenter northwest of the border between Myanmar, Thailand and Laos. It occurred in a region accommodating tectonic deformation brought by the collision between the Indo-Australian and Eurasian plates. Strike-slip faulting along the Nan Ma Fault was identified as the cause. There were between 75 and 151 fatalities; including one death in Thailand. An additional 212 people were injured. Hundreds of buildings and some transport infrastructure were damaged in Myanmar and Thailand. In the aftermath of the disaster, the Burmese government provided aid and relief supplies to the affected region. Neighbouring countries China, India and Thailand provided monetary assistance. Several international humanitarian organizations also supported in the relief and recovery.
Tectonic setting
Myanmar lies at the junction of the Alpine–Himalayan Orogenic Belt and Indonesian Island Arc System. In northern Myanmar, continental collision is ongoing between the between the northward moving Indian Plate and the Eurasian Plate. Collision also occurs along the Indo-Burman Ranges (Patkai, Naga and Chin Hills) and Arakan Mountains of western Myanmar. In this zone of highly oblique collision, most of the motion is accommodated by the north-south trending Sagaing Fault, a major right-lateral strike-slip fault that runs through the western and central part of Myanmar. Additional deformation is distributed and accommodated within eastern Myanmar and Thailand extending into Laos. Deformation within the Shan Plateau is partly accommodated by a set of southwest-northeast trending left-lateral strike-slip faults. Major faults in the Shan Plateau include the Mae Chan, Nam Ma, Menxing, Menglian, Nantinghe, Wanding, and Longling faults. The faults closest to the epicenter are the Mae Chan and Nam Ma faults, about east of the Sagaing Fault.
Earthquake
The 6.8 earthquake occurred with a focal depth of and epicenter north-northwest of Tachilek, Shan State; north of the Golden Triangle region. It was one of the largest earthquake recorded in the area. A focal mechanism solution obtained for the earthquake indicated left-lateral slip along a vertical fault plane. Large earthquakes also struck the area in 1988 ( 7.0) and 1995 ( 6.8).
Geology
The northeast-southwest trending Nan Ma Fault runs through Myanmar, northern Laos and China for a length of . Geomorphic studies along the central section of the fault where the Mekong River crosses it suggest it was previously a right-lateral fault. It produced up to of right-lateral displacement between 5 and 20 million years ago. The fault reactivated as a left-lateral fault and its average slip rate is estimated at per year. The fault branches from a single structure into multiple subparallel splay segments at its westernmost end. The earthquake ruptured one of these segments at the westernmost Nan Ma Fault.
Surface rupture
The mainshock produced a long surface rupture along the westernmost segment of the Nam Ma Fault. The surface offset peaked at almost . Based on inferring InSAR data, the coseismic slip along the fault rupture was concentrated within the shallow of the crust. The maximum slip was estimated at at depths of . Clear surface faulting was observed on farmland; at the paddy fields southeast of the epicenter, of surface rupture was mapped. The measured offset ranged between and with an average of . In Tarlay, offsets were in the order of several tens of centimetres.
Strong ground motion
The earthquake was associated with 24 seconds of significant ground motion. Resonance may have occurred in the frequency range of 1.82–2.1, affecting many low to medium-rise buildings. The horizontal ground motion was the primary cause of building destroyed during the earthquake. The highest peak ground acceleration, 0.20 g, was instrumentally measured south of the epicenter in Mae Sai, Thailand. This measurement was the highest ever recorded in Thailand from an earthquake.