Cooperative Institute for Research in Environmental Sciences

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Earthquakes in the Himalaya

Roger Bilham
Professor, Geological Sciences
Associate Director, Solid Earth Sciences

Everest - the nasty bits: Living (and Dying) with Earthquakes in the Himalaya

Deep beneath Mount Everest, and beneath the entire Himalayan range, a series of earthquakes are slowly preparing their nasty blend of mountain uplift and societal ruin. Most Himalayan earthquakes are harmless, several each day that are detected only by sensitive seismometers. But once every few decades a village somewhere along the range will experience damaging shaking. Every few centuries a chunk of the Himalaya a quarter the size of Colorado will shudder southwards and upwards over the descending Indian plate, destroying nearby cities, and wreaking havoc deep in the Ganges Plain, and in southern Tibet. Two have occurred in the past century - in Assam and E. Nepal. New research shows that as many as seven others could occur at any time.

Earthquakes are the smoking gun of an active mountain range, a range that persists despite erosion processes that would otherwise wear it down to gentle hills in a few million years. The Rocky Mountains have thrice been eroded to flatlands in the past 3 billion years; the Himalaya are presently holding their own. Studies of historical archives in Tibet and India reveal that three giant earthquakes ripped through the western Himalaya in the early sixteenth century, but subsequent earthquakes have either been relatively modest, or have occurred elsewhere.

The scientific challenge is to find out more about these giant earthquakes. From GPS geodesy we know how much closer India approaches Tibet each year (2 cm/yr). From geological excavations of the faulted frontal range of the Himalaya we know how much slip occurs in a few giant earthquakes (8-10 m). From seismological studies we know how much slip occurred in recent great earthquakes (2-6 m). Simple arithmetic yields the astounding possibility that almost 3/4 of the Himalaya could have a magnitude 8 earthquake today, and that in parts of the west-central Himalaya we could have an earthquake as big as M=8.2.

Recent studies show that these giant earthquakes are but one kind of earthquake that will shake the Himalaya. The Indian plate, despite a strength close to that of a 40-mile-thick concrete bridge girder is bent more than 5 miles downward by the crushing weight of the Himalaya. Earthquakes at depths of 15-40 miles tell us of the gigantic wrenching and compressional forces that are now rupturing through the ancient Indian plate deep beneath the Ganges and Himalaya . In 1998 one such earthquake occurred in southern Nepal causing severe damage in northern India and southern Nepal, burying 2000 people alive.

The biggest problem with future Himalayan earthquakes is not the determination of their timing, it is the problem that they will inevitably occur. The longer we wait, the larger they will be. Since population densities have increased tenfold or more since the last of the great Medieval earthquakes, and since building styles have in many cases become more vulnerable to seismic shaking, the next great Himalayan event will be much more disastrous than those in recorded history.