The Qayen earthquake, also known as the Ardekul or Qaen earthquake, struck northern Iran's Khorasan Province in the vicinity of Qaen on May 10, 1997 at 07:57 UTC (12:57 local time). The largest in the area since 1990, the earthquake registered 7.3 on the moment magnitude scale and was centered approximately 270 kilometers (170 mi) south of Mashhad on the village of Ardekul. The third earthquake that year to cause severe damage, it devastated the BirjandQayen region, killing 1,567 and injuring more than 2,300. The earthquakewhich left 50,000 homeless and damaged or destroyed over 15,000 homeswas described as the deadliest of 1997 by the United States Geological Survey. Some 155 aftershocks caused further destruction and drove away survivors. The earthquake was later discovered to have been caused by a rupture along a fault that runs underneath the IranAfghanistan border.

Damage was eventually estimated at $100 million, and many countries responded to the emergency with donations of blankets, tents, clothing, and food. Rescue teams were also dispatched to assist local volunteers in finding survivors trapped under the debris. The destruction around the earthquake's epicenter was, in places, almost total; this has been attributed to poor construction practices in rural areas, and imparted momentum to a growing movement for changes in building codes for earthquake-safe buildings. With 1 in 3,000 deaths in Iran attributable to earthquakes, a US geophysicist has suggested that a country-wide rebuilding program would be needed to address the ongoing public safety concerns.



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.