The article “Himalayan Seismic Hazard” discusses how India’s basement rock flexes and slides beneath the Himalayas during large earthquakes. This is similar to what happens in deep ocean trenches when plates converge rapidly, causing the ocean floor to bend downward, and thus create earthquakes. As India and Tibet converge at a rate of 20+3 mm per year, the stress is not spread out over the entire plate. 80 percent of strain is absorbed by a 50 km stretch along the southern edge of the Tibetan Plateau. Small earthquakes and uplift are most common in this region. The rest of the Himalayan mountain range absorbs the remaining 20 percent of the strain. The accumulation of stress, from the converging plates, is measured in potential slip, and on average the potential slip of the region is about two meters per year, which is released during periodic earthquakes (Bilham, et al., 2001). The article “Great Himalayan Earthquakes and the Tibetan Plateau,” argues that the the region accumulating the stress from plate convergence, and then releasing it, is much broader than the previous paper suggested, but only one fifth of that region actively engages with the earthquake cycle (Feldl, Bilham, 2006).
During great earthquakes, the displacement of formerly adjacent points on opposite side of a fault average to be about 4 meters (Bilham, et al., 2001). However, the average slip may not be a reflection of the potential slip, or stress, that the plate is under, and depending on the the length of the rupture during an earthquake, all of the stress may not necessarily be released. Rupture lengths shorter than 150 km leave significant strain in southern Tibet, thus shortening the renewal time for another earthquake in that region. For ruptures longer than 150 km, generally all the strain is released (Feldl, Bilham, 2006).
Because of the lack of any great earthquakes in the region over the past three centuries, it is now estimated that the slip potential for the for the Himalayan region is greater than 6 m. And in some isolated areas it is possible that there has not been any major earthquakes for the past five to seven centuries, which could cause a slip of more than 10 m (Bilham, et al., 2001).
The lack of any great earthquakes in the Himalayan region over the past few decades, and in some areas, centuries, the buildup of stress on the plate, along with exponential population growth, could spell disaster for some of the major cities. 1950 was the last great Himalayan earthquake, and since then the population has doubled, and since 1905 the population has increased by a factor of 10. The 1905 earthquake killed close to 20,000 people in the region, today about 50 million people are at risk in the same region (Bilham, et al., 2001).
The lack of infrastructure strong enough to withstand a serious earthquake, and loose building codes, make the situation even more dangerous. Were a great earthquake to strike the himalayan region today, it would likely lead to the death of 200,000 people, and hundreds of thousands more injured (Bilham, et al., 2001). The countries of India, Nepal, Bangladesh, Bhutan, and Pakistan need to drastically strengthen the infrastructure, and the strength of buildings, so as to minimize death and injury. And although one might think that a region would be safe after a major earthquake, due to rates of renewal time for plate convergence, Feldl and Bilham’s model shows that great earthquakes can be followed by major earthquakes much more rapidly than one might assume for typical rates of plate convergence. An example of this is the 7.8 magnitude earthquake that struck Nepal in 1833, and then in 1934, an 8.2 magnitude earthquake struck Nepal and Bihar. Because of this phenomena, areas struck by earthquakes cannot assume that the next one is centuries away, and must rebuild infrastructure to withstand the force of more earthquakes.
Bilham, R., Vinod, K., & Molnar, P. (2001). 22 himalayan seismic hazard. Science, 293, 1442-1444.
Feldl, N., & Bilham, R. D. (2006). Great himalayan earthquakes and the tibetan plateau. Nature, 444, 165-170. Retrieved from http://cires.colorado.edu/~bilhamHimalayanEarthquakes/ natureHimalaya06.pdf
No comments:
Post a Comment