A 6.8 Ms earthquake struck the Spitak region of Armenia, killing at least 25,000 people (aftermath pictured).

The surface wave magnitude scale is one of the magnitude scales used in seismology to describe the size of an earthquake. It is based on measurements of Rayleigh surface waves that travel along the uppermost layers of the Earth. This magnitude scale is related to the local magnitude scale proposed by Charles Francis Richter in 1935, with modifications from both Richter and Beno Gutenberg throughout the 1940s and 1950s. It is currently used in People's Republic of China as a national standard for categorising earthquakes.

The successful development of the local-magnitude scale encouraged Gutenberg and Richter to develop magnitude scales based on teleseismic observations of earthquakes. Two scales were developed, one based on surface waves, , and one on body waves, . Surface waves with a period near 20 s generally produce the largest amplitudes on a standard long-period seismograph, and so the amplitude of these waves is used to determine , using an equation similar to that used for .

The 1988 Armenian earthquake, also known as the Spitak earthquake, occurred on December 7 at 11:41 local time with a surface-wave magnitude of 6.8 and a maximum MSK intensity of X (Devastating). The shock occurred in the northern region of Armenia which is vulnerable to large and destructive earthquakes and is part of a larger active seismic belt that stretches from the Alps to the Himalayas. Activity in the area is associated with tectonic plate boundary interaction and the source of the event was slip on a thrust fault just to the north of Spitak. The complex incident ruptured multiple faults, with a strike-slip event occurring shortly after the initiation of the mainshock. Between 25,000 and 50,000 were killed and up to 130,000 were injured.