Past and future Richter M>8 earthquakes in the Himalaya
The Himalaya were formed by a collision between the Indian plate and southern Tibet that started about 50 million years ago. The collision was fast at first when the full plate collision velocity was manifest at the north edge of India, but it has now slowed to 2 cm/year across the Himalaya with distributed convergence extending northward toward Mongolia. A slow anticlockwise rotation of India should result in the collision velocity in Pakistan being slower than in Assam, however, this has yet to be verified by measurement. The upper map is drawn with its right edge parallel to the current convergence direction. Steady movement of the Indian Plate towards Tibet over periods of centuries is accommodated by sudden slip events (great earthquakes) on a gently north-sloping surface beneath the Himalaya.
Danger zone. This view of the Indo-Asian collision zone shows the estimated slip potential along the Himalaya and urban populations south of the Himalaya (U.N. sources). Shaded areas with dates next to them surround epicenters and zones of rupture of major earthquakes in the Himalaya and the Kachchh region, where the 2001 Bhuj earthquake occurred. Red segments along the bars show the slip potential on a scale of 1 to 10 meters, that is, the potential slip that has accumulated since the last recorded great earthquake, or since 1800. The pink portions show possible additional slip permitted by ignorance of the preceding historic record. Great earthquakes may have occurred in the Kashmir region in 1555 (21) and in Nepal in 1255 and 1505 (8). The bars are not intended to indicate the locus of specific future great earthquakes, but are simply spaced at equal 220-km intervals, the approximate rupture length of the 1934 and 1950 earthquakes. Black circles show population centers in the region; in the Ganges Plain, the region extending ~300 km south and southeast of the Himalaya, the urban population alone exceeds 40 million. (inset) This simplified cross section through the Himalaya indicates the transition between the locked, shallow portions of the fault that rupture in great earthquakes, and the deeper zone where India slides beneath Southern Tibet without earthquakes. Between them, vertical movement, horizontal contraction, and microearthquake seismicity are currently concentrated (3-5). The radial attitude of the slip-potential bars indicate the probable direction of slip in Himalayan earthquakes.
Maturity of Himalayan seismic gaps:
The history of Himalayan earthquakes since 1800 is reasonably well known. The largest of these gaps in 1998 is the Central Gap in western Nepal. Earthquakes in 1803 and 1833 are believed to have been too small to permit the entire plate boundary to slip. Certainly the 1833 event cannot have filled much of the central gap because people in western Nepal hardly felt the earthquake. Information on earlier earthquakes is fragmentary. A great earthquake presumably occurred in 1505 and1255 either in the region of the 1934 earthquake or in the region to the west of Kathmandu because the reigning Nepalese King lost his life, and his son's reign was plagued by 3 years of what appear to be violent aftershocks. Geodetic measurements indicate that if an earthquake happened today in the Central Gap (western Nepal) it must slip by more than 4 m, and probably by more than 6 m, and possibly by as much as 10 m. These ruptures amount to roughly M=7.8, M=8.0 or M=8.3 earthquakes if the slip area measures 100 km (N/S) by 300 km (E/W).
Earthquakes prior to 1800 are known from Persian, Sanskrit, Arabic, Tibetan and Turkish writings, and from several Indian languages, and from the writings of travellers from Europe (Iyengar and Sharma, 1998). These earthquakes are difficult to quantify since reports are often from isolated observations. Where corroboration exists in the form of independent accounts from several areas it is possible to estimate the size of the rupture zone. Another estimate of magnitude can be obtained from the duration of the mainshock in minutes, and from the longevity of the period of aftershocks. For example, earthquakes in Kashmir in 1555 and in Nepal in 1255 may have been great earthquakes based on these criteria. However, it is possible that reporting is biased by the presence of literate population centers in Srinigar and Kathmandu.