The Bagan Earthquake (1975) by Tint Lwin Swe

A large earthquake with Magnitude 6.5 hit Bagan Area on 8 July 1975 6:36 pm local time. Two peoples died on collapse of rural houses and other brick masonry structures and 15 injured. Many historical structures in Bagan including stupas and temples were
destroyed. The strongest intensity was felt in Nyaung-U, Pakokku, Yesagyo and Myaing townships on the confluence of Ayeyawady River. Further south, damages were also reported in Chauk and Natmauk townships.

Fig.1

This earthquake was located 100 km southwest of Mandalay City in the vicinity of Bagan, the ancient capital in 11th-13th Century AD. The epicentre location is 94.7E and 21.485N (Fig.1). The depth of earthquake according to USGS Earthquake Information Centre was 157 km below ground surface. However, ISC suggested the depth ranging from 84 to 157 km.The intensity was felt  in most of the Central Myanmar, partly in Rakhine and Chill hills in the west, some upper and lower Myanmar areas. MMI IV to V was also reported from Natmauk township 150 km from epicentre. The most damaged towns are Nyaung-U (Bagan), Pakokku, Yesagyo and Myaing. The intensity reached up to MMI VII – VIII. Damage records from ShweGaing Tha (1975) and Min Htwe Naung (1978) are summarized in Table 1. As the earthquake located in the sparsely populated areas, damage records were limited to some towns in the central Myanmar.

Table. 1

* This study

The felt intensity are interpreted based on those damage records and shown in the last column of Table 1. The correlated intensity levels are plotted as shown in Fig 2. This map is somewhat different from the earlier published (Min Htwe naung, 1978) due to the fact that intensity affected to stupas are interpreted according to pattern of failure (Saw Htwe Zaw, 2006) and that also consider damage report for Myaing township.

Fig. 2

Possible earthquake source is still controversy because of its depth information. Subduction and collision of India Plate and Burma Platelet is ongoing and this earthquake was happened at the interface of those two plates. Generally, intermediate depth earthquakes are likely in this region. The epicentre was located at the northern tip of Tantkyitaung Thrust and most damage areas and intensity level shown in Fig 2 are parallel to this range. Therefore, it is suggested that this earthquake is related to formation and growth of Tantkyitaung Thrust at the west bank of Ayeyawady River.

Ref.:
Shwe Gaing Tha (1976) Historical Earthquakes and the Bagan Earthquake, Sanay Literature Group, Yangon, Myanmar (in Burmese).
Min Htwe Naung (1978) Myanma Earthquakes, Sapay Beikman, Yangon, Myanmar (in Burmese).
Pivnik, D. K. (1998) Polyface Deformation in a Fore-arc/ Back-arc Basin, Salin Sub-basin, Myanmar (Burma), AAPG Bulletin, V-82, No 10 (October 1998), p1837-1856.
Saw Htwe Zaw (2006) Back Analysis of Earthquake Intensities from Old Records of Damages of Shwedagon Pagoda, presentation, Annual Conference of Myanmar Engineering Society, 2006.
http://earthquake.usgs.gov/learn/today/index.php?month=7&day=8&submit=View+Date