Historical eventOctober 201991

Moment magnitude scale

A 6.8 Mw earthquake strikes the Uttarkashi region of India, killing more than 1,000 people.

Everything on October 20Original source

In context

When
Sunday, October 20 199135 years ago
Category
Historical eventA dated happening — a battle, treaty, launch, discovery, disaster or decision — recorded on this calendar day.
Era
20th century · 1990s
Recorded from
Wikipedia “On this day” feed

At a glance

Dates: From 1991.

What happened

The moment magnitude scale is a measure of an earthquake's magnitude based on its seismic moment. Mw was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. Before Hanks and Kanamori (1979), Kanamori (1977) developed Mw scale for large earthquakes above 7.5. Thus Mw and M are not the same mathematically even though they are considered the same. Similar to the local magnitude/Richter scale (ML) defined by Charles Francis Richter in 1935, it uses a logarithmic scale;

October 20 through the years

Other entries on the same calendar day, in order.

  1. 1991Moment magnitude scale
  2. 1995Space Shuttle Columbia
  3. 2002Top Gear (2002 TV series)
  4. 2003Sloan Great Wall
  5. 2005UNESCO
  6. 2011Libyan crisis
  7. 2011Libyan civil war (2011)
  8. 2017Syrian civil war
  9. 2022Liz Truss
  10. 20222022 Chadian protests
  11. 2024Prabowo Subianto

Also in 1991

  • September 9TajikistanTajikistan declares independence from the Soviet Union.
  • September 11Continental Express Flight 2574Continental Express Flight 2574 crashes in Colorado County, Texas, near Eagle Lake, killing 11 passengers and three crew.
  • March 14EscondidaEscondida, one of the most productive copper mines in the world, was officially inaugurated.
  • September 12Space Shuttle DiscoveryNASA launches Space Shuttle Discovery on STS-48 to deploy the Upper Atmosphere Research Satellite.
  • September 17EstoniaEstonia, North Korea, South Korea, Latvia, Lithuania, the Marshall Islands and Micronesia join the United Nations.
  • September 17LinuxThe first version of the Linux kernel (0.01) is released to the Internet.
  • September 5Indigenous and Tribal Peoples Convention, 1989The current international treaty defending indigenous peoples, Indigenous and Tribal Peoples Convention, 1989, comes into force.
  • November 27Ronald Reagan Space and Missile Test RangeOperations at the Reagan Test Site were disrupted due to the nearby formation of Tropical Storm Zelda.

Everything we record for 1991 →

Sources

Cite this page

“Moment magnitude scale”, What's On This Date, https://whatsonthisdate.com/app/event/moment-magnitude-scale-1991-october-20, accessed October 27, 2026.

Cite the underlying source as well — the source links above go to the record this entry is adapted from.

Questions about this entry

When did Moment magnitude scale happen?

October 20 1991 — 35 years ago, on a Sunday.

Where does this text come from?

Wikipedia “On this day” feed. Summaries are short adaptations that link back to the source, not reprints.

How do I cite this page?

Use the ready-made citation near the sources at the bottom of the page, and follow the source link to cite the underlying record as well.

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