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Sagaing Fault

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Sagaing Fault
A pressure ridge formed along the fault, obscured by vegetation.
Coordinates21°55'06.8"N 95°59'01.6"E
CountryMyanmar
CitiesYangon, Nay Pyi Taw, Meiktila, Mandalay, Myitkyina
Characteristics
SegmentsSouthern section: Bago, Pyu, Nay Pyi Taw, Meiktila, Sagaing. Northern section: Tawma, Ban Mauk, Indaw, Mawlu, Shaduzup, Kamaing, Mogang
Length1,400 km
Displacement18-20 mm/yr (0.71-0.79 in/yr)
Tectonics
StatusActive
TypeTransform fault
MovementDextral
Age15-22 MYA

The Sagaing Fault is a major fault in Myanmar, a mainly continental right-lateral transform fault between the Indian Plate and Sunda Plate. It links the divergent boundary in the Andaman Sea with the zone of active continental collision along the Himalayan front. It passes through populated cities of Mandalay, Yamethin, Pyinmana, the capital Naypyidaw, Toungoo and Pegu before dropping off into the Gulf of Martaban, running for a total length of over 1200 kilometers.[1][2]

Discovery and early studies

A partial visualization of an active fault trace that aligns with the present-day Sagaing Fault trace was recorded by Fritz Noetling, a geologist, in the book The Miocene of Burma published in 1900. In 1913, Thomas Henry Digges La Touche of the Geological Survey of India[3] acknowledged the existence of a plate boundary feature along the Shan Plateau's western margin in Mandalay, including the one dividing metamorphic stratas of the Sagaing Hills from the Central Tertiary Basin. While analysing historical earthquakes in Myanmar, Harbans Lal Chhibber (1934) discovered a linear trace when joining their epicenters which he inferred to be along the same fault.[4] In 1970, Aung Khin and others confirmed the fault's existence using geophysical techniques and a Bouguer gravity investigation from Taungoo to Thabeikkyin. That same year, Win Swe nomenclated it the "Sagaing Fault". He also made the first description of the fault in Rift features at the Sagaing Tagaung Ridge at the Fifth Burma Research Congress. He determined the fault running nearly the entire length of Myanmar from south to north, had a strike-slip mechanism and linked it to historical earthquakes. A 1991 study of the fault determined a total displacement of 203 km (126 mi) and slip rate of 22 mm (0.87 in) per year by analyzing two metamorphic units and assuming the fault's origins in the end Oligocene or early Miocene.[5]

Geology

Sagaing Fault represents a boundary of the Burma Microplate (bottom)

The Sagaing Fault begins offshore in the Andaman Sea at a divergent boundary before passing through the central Myanmar basin. The fault has a relatively low topographical relief for most of its length compared to the Shan Scarp Fault to the west.[6] Its total length from the Eastern Himalayan Syntaxis to the Andaman Sea is 1,400 km (870 mi).[7] While the fault for most of its length runs uninterrupted by large stepovers, its northern 400 km (250 mi) branches outwards in a "horsetail" fashion into individual segments 100 km (62 mi) in width.

The total slip rate across the Indian–Sunda Plate boundary is about 35 mm/yr, of which 18 mm/yr is accommodated by the Sagaing Fault, according to GPS data.[8] The measured maximum displacement along the fault is about 100 km, although several authors have proposed between 360 km to 400 km.[9][10]

Southern section

The southern section comprises, from south to north, the Bago, Pyu, Naypyidaw, Meiktila and Sagaing segments. The 170 km (110 mi) Bago segment runs from Myanmar's coast, southeast of Yangon, to 18°N, where the fault experiences a sharp bend. The southern termination and offshore section remains uncertain, however, several tens of kilometers offshore, it connects to normal faults striking east–west. Approximately 100 km (62 mi) of the Bago segment ruptured during the May 1930 earthquake with offsets of at least 3 m (9.8 ft).[9]

The Pyu segment begins where the Bago segment terminated and continues 130 km (81 mi) to 19.1°N. It runs along the base of the Bago-Yoma range. Along the central part of the segment lies a terrace east of the fault trace which represents the hanging wall of a reverse fault expressed on the feature's eastern side. This feature is due to a small transpressional component where the segment experiences crustal compression. A majority of the segment ruptured during the December 1930 earthquake which originated at its southern limit.[9]

For its entire 70 km (43 mi), the Naypyidaw segment consists of two parallel strands that branches out at 19.1°N. Both strands cuts through channels and alluvial fans. The western strand runs east of Naypyidaw. A damaging but moderate earthquake in close proximity to the segment in August 1929 damaged railroads and bridges 40 km (25 mi) south of Naypyidaw.[9] The International Seismological Centre assigned the earthquake Mw  6.5.[11]

The Meiktila segment runs 220 km (140 mi) through a wide valley from Naypyidaw to Mandalay. This segment does not produces any dip-slip component evident from the absence of elevation differences across the fault. At 20°N, the fault offsets a stream by 2.4 km (1.5 mi); the largest observable offset along the segment. Although no major historical earthquake has been clearly associated with the segment, a possible contender is the the 1839 Ava earthquake which destroyed the city of Inwa and caused severe damage to areas east and south of the Irrawaddy River.[9]

The geomorphologic features of the Sagaing segment led to its discovery in 1970. This segment runs relatively linear; from 21.9°N to 22.6°N, it runs west of the Irrawaddy River where it offsets alluvial fans of the post-Pliocene. It cuts through the Singu Plateau and splits into two segment running east and west of the river. Its western strand to the north would be the Tawma segment. In 1946, a magnitude 7.7 earthquake was likely associated with the segment's northern two-thirds. This segment also ruptured during a smaller earthquake in 2012. The remaining portion to the south likely ruptured during the destructive Mw  7.1 earthquake of 1956.[9]

Northern section

The Sagaing Fault begins to fan outwards in a "horsetail" fashion into four separate fault zones from 23.5°N. The Tawma segment runs along an east-facing scarp and streams display destral offsets. The western strand of the Sagaing segment extends north and is represented by the Tawma segment. The trace is no longer visible at 24°N where a left stepover leads to the Ban Mauk segment 10 km (6.2 mi) west. Between the stepover is a series of northeast–southwest striking faults that cuts through a transpressional ridge. Many are shorter than 20 km (12 mi) long and some show normal-component in their slip; however their weak expression suggest small slip rates compared to the Sagaing Fault's main branches.[9]

The Ban Mauk segment runs from 23.8°N and continues for 150 km (93 mi). Due to the rarity of clear geomorphic offsets associated with this segment, its slip rate is inferred to be low. The segment marks the boundary between the Neogene volcanics of the west and Miocene sedimentary strata of the east. At 25°N, the fault trace disappears, buried by clastic deposits of the Taungthonton Volcano to the east. At 23.7°N, the In Daw segment branches away from the Tawma segment at the Irrawaddy River's northern bank; continuing its north–northeasternly trace. A 3 km (1.9 mi) wide pull-apart basin at 24.25°N separates it from the Mawlu segment where it continues north–northwest for 90 km (56 mi). The combined length of both segments is estimated at 170 km (110 mi). Geodetic analysis of these segments revealed an annual strain accumulation of about 2 cm (0.79 in). A magnitude 7.3 foreshock occurred along the In Daw segment three minutes before the magnitude 7.7 earthquake in 1946.[9]

Where the Mawlu segment terminates, the fault branches into three segments; the Shaduzup, Kamaing, and Mogang segments. The Shaduzup segment, the westernmost of the three, is not believe to exceed 120 km (75 mi). The lack of clear geomorphic features suggest less activity occur on this segment. The Kamaing segment continues further north into the Naga Hills, beyond 26.7°N. Right-lateral displacements are also well-expressed in drainage channels on the eastern flank of the Naga Hills at its northern termination. The fault continues north and joins a thrust fault system associated with the Assam Valley. The most active of the three, but less seismically active that the Kamaing segment, the Mogang segment runs in a broad arc shape from 24.8°N. It ends at 26.8°N where it transforms into a northwest–southeast striking thrust fault. Either of these segments produced the magnitude 7.6 earthquake of January 1931; due to the lack of seismic data and isoseismal map its origin segment remains tentative. However, shaking was more intense along the Kamaing segment.[9]

Seismicity

See also: List of earthquakes in Myanmar
A large fracture on the Mingun Pahtodawgyi caused by the 1839 Ava earthquake.

Damaging earthquakes have been associated with the fault for centuries. The great 1839 Ava earthquake killed hundreds and damaged many cities, including the then imperial capital Inwa.[12] That earthquake is thought to have a moment magnitude of 8.0 or greater. The large magnitude would imply a rupture length of at least 300 km along the fault.

In the early 20th century, from 1929–1931, more than half the length of the Sagaing Fault was involved in significant earthquakes. The first earthquake that would be followed-up by subsequent larger events struck southwest of Nay Pyi Daw, and east of Thayetmyo in the Pegu Range in August 1929.[13] There is insufficient reports of damage caused by this earthquake although it was said that buildings were swaying and objects were displaced at Yamethin, roughly 133.6 km north of the quake epicenter.[14] The magnitude of this earthquake was no greater than 7.0.

On the night of May 5, a large shock registering Mw  7.5 struck north of the city of Pegu resulting in widespread deaths and destruction.[15] There were no foreshocks that preceded. The earthquake had a maximum intensity of IX to X on the Rossi-Forel scale.[9] The city of Pegu, Rangoon and several other towns were destroyed. In Pegu, fires erupted, and severe liquefaction caused further damage. Approximately 500 people died in Pegu while 58 were killed in Rangoon, more deaths were recorded in other villages.[14] It triggered a small tsunami in that inundated villages along the coast.[12] This is the deadliest earthquake in the 1929–1931 sequence.

The night of December 3 was disturbed by two moderate foreshocks, the first lasting 5 seconds generated some panic, it was felt in Pyinmana and Rangoon and another that was described as being stronger than the first but was not felt as widespread as the first.[16][17][18]

The early morning of December 4 in Pyu, Taungoo District was suddenly disrupted by violent shaking from an Mw  7.3 earthquake.[19] The December 4 event occurred further north of the epicenter of the May earthquake, about 6.4 to 9.7 km west southwest of Pyu. Damage was severe, a railroad was shifted off and twisted while many buildings in the city collapsed. About 30 people were killed. The maximum intensity was assigned X on the Rossi–Forel scale.[16] This event is not an aftershock of the May quake because it ruptured a different segment of the Sagaing Fault.[9]

Although the May and December 1930 earthquakes occurred during this active period, they were separate events, unrelated to the activity in the northern part. The December 1930 earthquake, however, was triggered due to stress transfer from the previous event in May.[20]

A lesser-known earthquake on July 18, 1930, in the Ayeyarwady Region killed about 50 people.[21] According to the National Centers for Environmental Information database, there is no magnitude assigned to this event.[12] The book Southeast Asia Association of Seismology is the only published work mentioning this event.[22]

The largest earthquake in the sequence, an Mw  7.6 quake struck next to Indawgyi Lake resulting in large landslides and ground failures.[14][23] There were no casualties in this earthquake.

Another violent earthquake of unknown magnitude occurred 146.5 km north of Pyu. It was felt in Mandalay and Thanatpin.[14] Six brick buildings sustained damage. On August 19, another tremor caused cracks to appear in buildings in Mandalay and lightly affected Kalaw.[16] The city was rocked another time, resulted in the collapse of the Shwe Sandaw Pagoda in Taungoo.[14]

Several short tremors were felt that woke people up in Taungoo, Pyuntaza and Nangyun.[16]

Two powerful earthquakes north of Mandalay measuring 8.0 and 7.7 on the moment magnitude scale occurred on September 12, 1946.[11] The doublet earthquake sequence would remain as one of the largest in the country. Not much about this event is known due to sparse records.

Looking back at the historical records of earthquakes, the years 1906 and 1908 saw two major events in the northernmost end of the Sagaing Fault.[20] The 1906 Putao earthquake on August 31 had an estimated moment magnitude of 7.0, and the 1908 earthquake measured Mw  7.5.[24][25]

The 1908 earthquake resulted in the accumulation of stress towards the south, where the future 1931 quake would take place. Similarly, the 1946 earthquake rupture segments were directly south of the 1931 rupture. The first mainshock in the 1946 doublet sequence then triggered the second mainshock due to the sudden increase in stress levels on the fault.

Ten years later, an Mw  7.1 earthquake near Mandalay killed at least 40 people.[12] That earthquake broke a segment south of the 1946 rupture. In 1991, a small seismic gap between the two 1946 ruptures generated an Mw  7.0 earthquake, partially re-rupturing a small section of the 1946 quakes, killing two.[26]

The 2012 Shwebo earthquake was the most recent major event on the Sagaing Fault. It had a moment magnitude of 6.8 and ruptured the fault north of Mandalay. A detailed and thorough evaluation of the event suggest the rupture was estimated at 45-km-long. The centroid moment tensor solution suggested the earthquake ruptured a north-south trending and sub-vertical fault that steeply dipped to the east.[6]

See also

References

  1. ^ Win, Nilar (August 25–31, 2008). "Sagaing Fault surveyed to prepare for future quakes". The Myanmar Times. Archived from the original on 2012-06-07. Retrieved 2011-04-04.
  2. ^ Wang, Yu (2011). "Earthquakes and slip rate of the southern Sagaing fault: insights from an offset ancient fort wall, lower Burma (Myanmar)" (PDF). Geophysical Journal International. 185: 49–64. Bibcode:2011GeoJI.185...49W. doi:10.1111/j.1365-246X.2010.04918.x.
  3. ^ Tipper, G. H. (7 May 1938). "Mr. T. H. D. La Touche". Nature. 141: 819–820. doi:10.1038/141819a0.
  4. ^ Chhibber, H. L. (1934). The Geology Of Burma. Macmillan & Company.
  5. ^ Tun, Soe Thura (2005). Sagaing Fault: A desk study report on seismotectonic implications in Myanmar. Myanmar Earthquake Committee. doi:10.13140/RG.2.1.4048.6249.
  6. ^ a b Soe Thura Tun; Ian M. Watkinson (2017). "Myanmar: Geology, Resources and Tectonics". Geological Society, London, Memoirs. 48: 413–441. doi:10.1144/M48.19. S2CID 134291037.
  7. ^ Chit Thet Mon, Xuan Gong, Yun Wen, Mingming Jiang, Qi-Fu Chen, Miao Zhang, Guangbing Hou, Myo Thant, Kyaing Sein, Yumei He (2020). "Insight Into Major Active Faults in Central Myanmar and the Related Geodynamic Sources". Geophysical Research Letters. 47 (8). doi:10.1029/2019GL086236.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Tsutsumi, H.; Sato, T. (2009). "Tectonic geomorphology of the southernmost Sagaing fault and surface rupture associated with the May 1930 Pegu (Bago) earthquake, Myanmar". Bulletin of the Seismological Society of America. 99 (4): 2155–2168. Bibcode:2009BuSSA..99.2155T. doi:10.1785/0120080113.
  9. ^ a b c d e f g h i j k Yu, Wang (2014). "Active tectonics and earthquake potential of the Myanmar region" (PDF). Journal of Geophysical Research: Solid Earth. 119 (4): 3767. Bibcode:2014JGRB..119.3767W. doi:10.1002/2013JB010762 – via Caltech.
  10. ^ Yu Wang, Kerry Sieh, Thura Aung, Soe Min, Saw Ngwe Khaing (2011). "Earthquakes and slip rate of the southern Sagaing fault: insights from an offset ancient fort wall, lower Burma (Myanmar)". Geophysical Journal International. 185 (1): 49. Bibcode:2011GeoJI.185...49W. doi:10.1111/j.1365-246X.2010.04918.x.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ a b ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009), Version 9.1, International Seismological Centre, 27 June 2022 Cite error: The named reference "ISC-GEM" was defined multiple times with different content (see the help page).
  12. ^ a b c d National Geophysical Data Center / World Data Service (NGDC/WDS) (1972), Significant Earthquake Database (Data Set), National Geophysical Data Center, NOAA, doi:10.7289/V5TD9V7K
  13. ^ "M 6.6 - Myanmar". USGS.
  14. ^ a b c d e Hla Hla Aung (2012). "Reinterpretation of historical earthquakes during 1929 to 1931, Myanmar". Advances in Geosciences. 31: 43. Bibcode:2012aogs...31...43A. doi:10.1142/9789814405775_0005. ISBN 978-981-4405-76-8 – via Researchgate.
  15. ^ "M 7.5 - Myanmar". USGS.
  16. ^ a b c d J. COGGIN BROWN AND P. LEICESTER. "THE PYU EARTHQUAKES of 3RD AND 4TH DECEMBER 1930 AND SUBSEQUENT BURMA EARTHQUAKES UP TO JANUARY 1932". Memoirs of the Geological Survey of India. LXII.
  17. ^ "Event 907776 Myanmar". International Seismological Centre.
  18. ^ "Event 907775 Near south coast of Myanmar". International Seismological Centre.
  19. ^ "M 7.4 - Myanmar". USGS.
  20. ^ a b Xiong, X., B. Shan, Y. M. Zhou, S. J. Wei, Y. D. Li, R. J. Wang, and Y. Zheng (2017). "Coulomb stress transfer and accumulation on the Sagaing Fault, Myanmar, over the past 110 years and its implications for seismic hazard". Geophysical Research Letters. 44 (10): 4781–4789. Bibcode:2017GeoRL..44.4781X. doi:10.1002/2017GL072770. hdl:10220/46403. S2CID 55509712.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  21. ^ "Significant Earthquake: MYANMAR (BURMA): THARRAWADDY". NOAA.
  22. ^ Nutalaya; P.S. Sodsri; E.P. Arnold (1985). SEASEE Series on Seismology. Vol. II–Thailand. U.S. Geological Survey.
  23. ^ "M 7.6 - Myanmar". USGS.
  24. ^ "Event 914281 Myanmar-India border region". ISC: On-Line Bulletin. International Seismological Center. Retrieved 3 March 2021.
  25. ^ "Event 16958007 Myanmar". ISC: On-Line Bulletin. International Seismological Centre. Retrieved 3 March 2021.
  26. ^ "M 7.0 - Myanmar". USGS-ANSS. USGS. Retrieved 3 March 2021.
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