The 1991 Uttarkashi earthquake (also known as the Garhwal earthquake) occurred at 02:53:16 Indian Standard Time (UTC+05:30) on 20 October with a moment magnitude of 6.8 and a maximum Mercalli intensity of IX (Violent). This thrust event was instrumentally recorded and occurred along the Main Central Thrust in the Uttarkashi and Gharwal regions of the Indian state of Uttarakhand (then still part of Uttar Pradesh). High intensity shaking resulted in the deaths of at least 768 people and the destruction of thousands of homes.

The moment magnitude scale (MMS; denoted explicitly with Mw  or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 paper by Thomas C. Hanks and Hiroo Kanamori. Similar to the local magnitude scale (ML ) defined by Charles Francis Richter in 1935, it uses a logarithmic scale; small earthquakes have approximately the same magnitudes on both scales.

Moment magnitude (Mw ) is considered the authoritative magnitude scale for ranking earthquakes by size. It is more directly related to the energy of an earthquake than other scales, and does not saturate—that is, it does not underestimate magnitudes as other scales do in certain conditions. It has become the standard scale used by seismological authorities like the U.S. Geological Survey for reporting large earthquakes (typically M > 4), replacing the local magnitude (ML ) and surface wave magnitude (Ms ) scales. Subtypes of the moment magnitude scale (Mww , etc.) reflect different ways of estimating the seismic moment.