Phyu Earthquake (1930) Part 2: rupture

In 1930 the same year of the Bago Earthquake with Magnitude 7.3 happened, another strong earthquake had occurred 150 km north of Bago Epicentre on 4th of December (1:22 local time). Coggin Brown,
J and Leicester (1933) highlighted that the epicentre was near Phyu City based on the most felt locality, a village named Kintangyi SW of Phyu. Extensive survey and records on the effect of this earthquake suggested the zone of maximum intensity IX in R-F scale about Phyu, Penwegone, Kanyutkwin in the south and Zeyawady, Oktwin and villages SW of Taunggoo and VIII was felt a large areas in the northern Pegu District and Taunggoo District partly in the eastern flank of Bago Yoma. Brown et al (1933) listed cities that felt different intensities in their report.
     Present author attempts to locate a possible source of this earthquake and associated rupture based on 1933 records. In viewing that, a new map showing earthquake intesity was developed and then correlate to the Sagaing Fault, the nearest active tectonic source. The Sagaing Fault (Win Swe, 1980) is a major strike-slip fault crossing the central  part of Myanmar from south to north with 1000 km strike length and zone of maximum felt area was within 10 km from this transform boundary. In the map, lower limit was set to MMI VI (Fig. 1).

Figure 1. Distribution of earthquake intensity by the Phyu Earthquake (1930)
      The Sagaing Fault is the well known active tectonics structure in the region which lies just 5 kilometers away west of Phyu. The geomorphological expressions like trancated stream courses, abrupt changes in elevation, continuous erosion at valley bottoms, etc., are very prominant for an active transform fault for this particular section. Based on the felt areas (Brown, et al 1933) and those active features along the Sagaing Fault, the possible rupture lenght for Phyu Earthquake is definitely not less than 80 kilometers, measured from Myochaung in the south to west of Oktwin in the north, (Fig. 2)

Figure 2. Earthquake rupture for Phyu Earthquake (1930) (North is towards the left side of picture)
Fig. 2 is divided into 3 parts (3 Figures for your clear view). In these figures, north is towards the right side of pictures.

 Figure 2.a

 FIgure 2.b

Figure 2.c
     Brown et al (1933) indicated that the epicentre of Phyu Earthquake was near Kintangyi Village, SW of Phyu. This village situated on the saturated alluvial plain at the foothill of the Bago Yoma appeared to be totally damaged as it was very close to the Sagaing Fault (1.5 km only). Based on this, the possible epicentre is plotted exactly on the Sagaing Fault and the location become at the bending point of the Kun Chaung (96.398, 18.427), the yellow star in Fig. 2. I have visited this location in 2002. The rocks in this fault zone were exposed for a major civil engineering project. These rocks; sandstone and shale of the Upper Pegu group, have been slightly metamorphosed due to shearing along the transform fault (Fig. 3)

Figure 3. Excavated section  of the Sagaing Fault near Phyu Chaung bending, NW of Phyu.
(Photo: 2002)
     This is a preliminary determination of epicentre and fault rupture for Phyu Earthquake (1930). Detail survey and research works are required to varify this premise.
     I notice that three large dams were or having been constructed in the eastern flank of Bago Yoma, which are close to this Phyu Rupture. A series of towns and villages are also lying in the alluvial plain in east of the Sagaing Fault and population in those cities are being growth many times of 1930s. A new Hiway is parallel to the sagaing Fault. There was no significant earthquake since 1930 Phyu event (within 81 years span) and probability of a strong earthquake to occur is also high. The higher the vulnearbility and the probability of a big earthquake, the higher the seismic risk in the region.