Geology of Himalayas
The northeastern end of the Gangetic Plain has experienced four "great" earthquakes with a Richter magnitude exceeding 8.0 in the past 100 years, beginning with the Assam earthquake in 1897. Over 30,000 people perished in these quakes. The origin of the earthquakes is the same tectonic, plate-moving action that welded the Indian subcontinent to Eurasia and formed the Himalayan Mountain Range as a kind of massive suture. Along the line where the outer Himalayas border the Gangetic Plain, seismic gaps store the strain from tectonic movement. One of these, called the Central Seismic Gap, has not released its strain in the form of an earthquake in an estimated 745 years (since a great quake appeared to have killed the king of Nepal in 1255). The Central Seismic Gap is about 500 mi (800 km) long and lies between the regions struck by great earthquakes in 1905 and 1934. Northern India and Nepal will experience significant devastation if a great earthquake occurs in this region with a population of 100 million.
The faults along which earthquakes occur were generated by the pressure of the Indian subcontinent. After the period when it was subducted under the Eurasian plate, the plate's direction shifted, and it pushed toward Tibet, compressing the edge of Tibet. When the resisting rock had folded as much as possible, it began to tear and the faults were born. Geologists know plate movement is continuing from the earthquake activity but also from the continuing formation of hills along the southern limits of the Himalayas. The longest fault, called the Main Detachment Fault, is as long as the Himalayan Range from west to east. If the fault does rupture, it is most likely to occur where the greatest strain has accumulated at the Central Seismic Gap. Although the prospect of such an earthquake is frightening, study of seismic activity has helped geologists to better understand the complex processes that have formed Earth's most upstanding mountain range.
In analyzing geologic processes and earthquake hazards, geologists have used technology to measure movements in areas that are remote, frigid, and impassable. An array of 24 satellites bounce radio signals over the earth's surface in the Global Positioning System (GPS). Hikers can tune into the GPS signals and determine their own exact locations, and geologists can similarly find the locations of mountaintops and continents. Comparisons of data over time show relative movements. By routinely using GPS data to survey a line of reference points, scientists are understanding geophysics, geomechanics, and the convergence of continents. They have found that India is shifting to the northeast toward Asia at a rate of about 2.5 in (6 cm) per year. Studies of paleontology (fossils) in the Himalayas have also added pieces to the puzzle of the explanation of the range's geologic and seismic history. Comparison of fossilized trilobites (ancient crustaceans) found in different locations in the Himalayas and the places where they were known to have lived helps superimpose the geologic timetable on the components of the comparatively young Himalayas.
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Gillian S. Holmes