The Surface Deformation and Earthquake History Associated with the 1975 M 6.8 Bagan Earthquake in Myanmar

Abstract

The 1975 M 6.8 Bagan earthquake occurred on 8^th July 1975, at 12:04:38 (UTC). The epicenter was at 21.50N 94.70E with the depth of 112 km (ISC) and focal mechanism is thrusting (USGS).  The intensity of earthquake was severe which involved destruction of many pagodas with a loss of at least one live and one injury. Most of the pagodas were damaged with their tops falling down to the ground.  Bagan, the land of temples and stupas was shaken by a series of earthquakes since ancient time and the earliest records are about 25 November,1372; 14 July 1485 A.D. and another event in 1550 A.D.by which event Shwe-gu-gyi temple was damaged. Although the 1975 earthquake was one of the significant earthquakes that have occurred along inland zone in the western Myanmar, this event has not been analyzed within the context of present-day understanding of earthquake seismology. The mode of deformation and seismic history of these earthquakes remain unresolved. Due to another large earthquake, Chauk earthquake in 2016 with M 6.8, approximate numbers of pagodas of (400) were damaged as the previous 1975 Bagan earthquake. Pagodas left intact and withstand as before.  Geologists and earthquake engineers went to Bagan city and neighboring towns for damage assessment a few days after the 2016 Chauk earthquake event. People explained that the main shock was very powerful and the houses and religious building were lifted about 3 times at initial shaking and then lateral shaking continued for a minute. It is due to the ground motion that was strong enough to fling up the buildings as the fault rupture beneath it.

On the base of field investigation that was carried out to map the surface rupture associated with this earthquake event, the 1975 Bagan earthquake and 2016 Chauk earthquake are intermediate-depth subduction earthquakes and such inland intermediate-depth earthquakes are hazardous earthquakes for the area along the Rakhine Western Ranges (Indo-Andaman belt) under which the India plate is subducting beneath the Burma plate. These earthquakes are the most significant events that occurred in intra-plate subduction zone setting.