RyanQuintana2 at 04:24, 12 April 2026

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{{Short description|Earthquake in India}}
{{Infobox earthquake
| name = 1984 Cachar earthquake
| location = {{coord|24.641|92.891|display=inline, title}}
| map2 = {{Location map|India Assam#India
|lat=24.641
|long=92.891
|mark=Bullseye1.png
|marksize=50
|position=top
|width=260
|caption=
|relief=yes
|label=}}
| timestamp = 1984-12-30 23:33:37
| isc-event = 537963
| anss-url = usp0002ajx
| local-date = December 31, 1984
| local-time = 05:03:37 ([[Time in India|IST]])
| magnitude = 5.6 {{M|b|link=yes}}<br>6.0 {{M|w|link=yes}}
| depth = 22.6 km
| type = [[Oblique-slip|Oblique–reverse]]
| affected = [[Assam|Assam, India]]
| intensity = {{MMI|8}}
| casualties = ~20 deaths<br>~100 injuries
}}

The '''1984 Cachar earthquake''' rattled much of Southern [[Assam]] on December 31, 1984, at 5:03 a.m. ([[UTC+05:30|UTC+5:30]]) with an epicenter 20 kilometers southwest of [[Lakhipur]]. The quake measured with a magnitude of 6.0 on the [[moment magnitude scale]] and an estimated intensity of VIII (''Severe'') on the [[Modified Mercalli intensity scale]].<ref name=":0">{{Cite web|title=M 6.0 - 20 km SW of Lakhipur, India|url=https://earthquake.usgs.gov/earthquakes/eventpage/usp0002ajx/executive|url-status=live|access-date=2021-11-16|website=earthquake.usgs.gov|archive-url=https://web.archive.org/web/20211117055500/https://earthquake.usgs.gov/earthquakes/eventpage/usp0002ajx/executive |archive-date=2021-11-17 }}</ref> About 20 people died and 100 others sustained mild to severe injuries.<ref name=":1">{{Cite web|title=M 6.0 - 20 km SW of Lakhipur, India—Impact|url=https://earthquake.usgs.gov/earthquakes/eventpage/usp0002ajx/impact|url-status=live|access-date=2021-11-16|website=earthquake.usgs.gov|archive-url=https://web.archive.org/web/20220418140521/https://earthquake.usgs.gov/earthquakes/eventpage/usp0002ajx/impact |archive-date=2022-04-18 }}</ref>

== Geological setting ==
[[Cachar district|Cachar]] lies on the [[Shillong Plateau]], in the northeast region of India. It is one of the most seismically active regions in the world, due to its being located between the boundary of the [[Indian Plate]] and the [[Eurasian Plate]], thus creating the Himalayan arc. The continental Indian Plate collides with the [[Burma Plate]] (part of the Eurasian Plate) at an oblique angle.<ref>{{Cite journal|last=Kayal|first=J.|date=1991-07-01|title=Earthquake prediction in northeast India—A review|url=https://www.researchgate.net/publication/227260798|journal=Pure and Applied Geophysics|volume=136|issue=2–3|pages=297–313|doi=10.1007/BF00876379|bibcode=1991PApGe.136..297K|s2cid=128679941}}</ref> The [[continental collision]] zone is seismically active with the potential to generate large earthquakes.

The events following this earthquake (1984) are said to be the beginning of a re-activation of seismic activity in the area,. The area has been predicted to be home to another large earthquake with a magnitude of ≥6 in the near future.<ref>{{Cite journal|last=Gupta|first=Harsh K.|date=1985-02-01|title=Cachar Earthquake of December 31, 1984 is it a Signal for the Beginning of Seismic Activity?|url=http://ischolar.info/index.php/JGSI/article/view/69934|journal=Journal of Geological Society of India|language=en|volume=26|issue=2|pages=145–147|issn=0974-6889}}</ref> Cachar has also been hit by a [[1869 Cachar earthquake|much larger earthquake]] in the past with a magnitude of 7.4 in the same area in 1869.

== Earthquake ==
[[File:1984 Cachar earthquake.jpg|thumb|A USGS Shakemap of the 1984 Cachar earthquake]]
The earthquake happened in the early morning of December 31, 1984 local date or the last day of the year. There are two recorded magnitudes for this event, one being 6.0 on the moment magnitude scale and the other is a 5.6 on the [[body-wave magnitude]] scale.<ref name=":0" /><ref name=":2" /> It was assigned a maximum intensity of VIII (Severe) on the Mercalli intensity scale. According to [[isoseismal map]]s. intensity VIII was felt in a narrow area near the epicenter. Isoseismal maps with intensity contours in the area also suggest the depth of focus is shallower than initially finalized; at 10 kilometers or less.

The tremor triggered numerous surface fissures on flat grounds spewing sand and water out of it.<ref name=":3">{{Cite book|last=Kayal|first=J. R.|url=https://books.google.com/books?id=POLg0eGi3EAC&dq=december+31+1984+cachar+earthquake&pg=PA286|title=Microearthquake Seismology and Seismotectonics of South Asia|date=2008-08-22|publisher=Springer Science & Business Media|isbn=978-1-4020-8180-4|language=en}}</ref> Multiple ground failures and liquefaction were observed, which was considered unusual due to it being too moderate in size to generate an effect similar to this normally seen in larger earthquakes. However, it was later realized that the region is filled with loose soil and a main component of liquefaction which is the flat land in the region being filled with [[sedimentary rock]].<ref>{{Cite journal|last=Agrawal|first=Pramod N.|date=14 Mar 1991|title=Liquefaction During Two Moderate-Recent Earthquakes in India|url=https://core.ac.uk/download/229083491.pdf|journal=Missouri University of Science and Technology|pages=7}}</ref> The focal mechanism of the earthquake is a combination of a thrust and a strike-slip.<ref name=":2">{{Cite journal|last=Singh|first=D. D.|date=1991-04-01|title=Source Mechanism of the Cachar Earthquake of December 30, 1984, NE India from the Synthesis of Body Waves|url=http://ischolar.info/index.php/JGSI/article/view/71082|journal=Journal of Geological Society of India|language=en|volume=37|issue=4|pages=365–373|issn=0974-6889}}</ref>

== Damage and casualties ==
Damage was concentrated in a [[meizoseismal area]] of 250 km<sup>2</sup>. Places with weak infrastructural management, most notably homes made out of soft mud, clay and bricks were a contributing factor to the deaths reported.<ref name=":3" /> Two children died after their home collapsed. Many injured were sent to a medical college in nearby [[Silchar]]. An embankment in the Sonai river collapsed, flooding multiple villages and towns, and causing many to lose their homes. Multiple bridges also suffered considerable damage.<ref name=":3" /> In total, 20 people died about a hundred others sustaining injuries and over 10,000 were reported homeless.<ref name=":1" /> The earthquake is considered one of the deadliest in northeastern India since the [[1950 Assam–Tibet earthquake]].<ref>{{Cite web|title=A strong earthquake shook eastern India Monday, collapsing mud-and-brick...|url=https://www.upi.com/Archives/1984/12/31/A-strong-earthquake-shook-eastern-India-Monday-collapsing-mud-and-brick/9636473317200/|access-date=2021-11-17|website=UPI|language=en}}</ref>

== See also ==
* [[List of earthquakes in India]]
* [[Lists of 20th-century earthquakes]]

== References ==
{{reflist}}

{{Earthquakes in 1984}}
{{Earthquakes in India}}

[[Category:Earthquakes in India]]
[[Category:1984 earthquakes]]
[[Category:1984 disasters in India]]
[[Category:December 1984 in India]]
[[Category:Cachar district]]

1984 Cachar earthquake - Revision history

RyanQuintana2 at 04:24, 12 April 2026