Ground motion behavior and rupture directivity of the Mw 6.8 Thabeikkyin earthquake of 2012 in upper Myanmar

In 2012, the Mw 6.8 Thabeikkyin earthquake devastated residential and public structures in Myanmar. The earthquake killed 26 people, with left 12 missing and 231 injured in the epicentral region. In addition, 201 houses, 25 schools, 13 hospitals, 35 monasteries, and 45 pagodas totally collapsed or were partially damaged in upper Myanmar. The USGS instrumental epicenter was near the Kyauktan fault, a western splay of the major Sagaing fault, with a shallow focal depth of ~9.8 km. Our post-earthquake field survey documented a ~45 km long surface rupture with a ~102 cm maximum dextral offset on the main fault, between Singu town in the south, passing through Thabeikkyin town, and terminating in Sabenago village in the north. The present study clarifies the macroseismic felt intensity of the Thabeikkyin earthquake in the EMS-98 scale, emphasizing the damage pattern and macroseismic effects in Myanmar and surrounding regions, based on press reports, digital news, and post-earthquake field survey data, supplemented with the finite fault solution based on nearest cGPS stations. In the epicentral region, the macroseismic intensity reached EMS 7-8, corresponding to grade 3 to 4 damage along the surface rupture. Light to moderate damage, corresponding to EMS 5-6 was distributed within a ~200 km wide region in Myanmar. Ground shaking was also reported several hundred kilometers away in northern and southern Myanmar while weak shaking was widely felt in the Assam region of India, in Kunming in western China, and in Chiang Mai and Bangkok in Thailand. High macroseismic intensities extend more widely in the NE-SW direction than in the NW-SE direction. Our study suggests that the ground motion attenuation relations for this region are comparable to those of the California region or Western North America. Moreover, the distribution of peak felt intensities reflects the dilation fields of the USGS mainshock focal mechanism and southward rupture directivity that caused light damage corresponding to EMS 5 in the Mandalay area, >100 km from the epicenter. In addition, preliminary assessment of macroseismic data for the Mw 5.5 earthquake of 29 July 2021, located ~14 km south of the 2012 Thabeikkyin event, suggests a similar intensity distribution and similar rupture directivity effects. Key Words: macroseismic mapping, Sagaing fault, EMS-98 scale, rupture directivity, ground motion attenuation