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Interpretation of earthquakes In South-Western China and the Tibetan Plateau suggests that Longmenshan isn't an isolated feature, but behaves as a temporary release valve for compressive energy that affects the entire plate - other areas may follow.
Longmenshan. Who’s next in Line?
A long time ago it was known that the Longmenshan Mountains were a zone of earthquakes. But, as the towns and cities of the area grew, the earth remained quiet. A few earthquakes, scattered over the landscape, no particular reason to worry. In 2007, two small but deep quakes of 150 km plus focal depth occurred beneath the trustbelt, followed by several near-surface quakes. Then, out of a sudden, in April 2008, the dragon woke up, causing a devastating quake, breaking dams, shaking mountains, and killing more than 100000 people. Science fiction? Unfortunately not.
Ever since, the earth hasn’t calmed down in Longmenshan. More than two hundred quakes have been reported, all in the same area, all along the some overthrust zone. Faults are moving and mountains are rising. This breathtaking happening shows, above all, one important fact: Asia, namely the Tibetan Plateau, is (according to Simons et al 2007) put under considerable horizontal stress by the north-eastwards moving Indian Plate, and the slowly eastwards moving South-Chinese Plate. The Tibetan Plateau is “mushrooming” and overthrusts both the southern and eastern borderlands.
Some of this, often deadly, energy is released in Longmenshan, but it would be naive to assume that one thrustbelt alone would take care of all the loaded elastic energy of the area. My best guess is, given a continuous stress fields of the same as current orientation that systems parallel to Longmenshan will move. There is a chance that some old faults in the Sichuan/Gansu are being remobilized, though this is unlikely to happen, unless the Longmenshan fault system is being fully locked. Since Longmenshan is moving, and adjacent areas don’t, a build-up release of stress, and energy release in the fault zone’s northern and southern “hinges” will happen.
Since a couple of months, I studied earthquake patterns in Asia, and my conclusion is as follows: Longmenshan is not alone. Having merged in one map some 27 USGS maps, showing quakes of the last 28 years, I realized a young system of SW-NE striking overthrusts. These, parallel to Longmenshan, are crossing the Tibetan Plateau. Considerations such as topography seem to indicate that these fault systems are very young – perhaps a consequence, if I may speculate, of recent changes of the vectors of plate motion. Earthquake activity is mostly located around 10-30 kms depth, and concentrated in pockets and clusters, possibly indicating an initial break-up phase.
Sooner or later (assuming current stress continues) such clusters could or will link up and produce another “big one.” Although this process may take months, years, hundreds or thousands of years, there is some certainty where it will happen.
Most areas of future earthquake activity are in thinly populated steppes and deserts. About five plus systems, however, transect the Southern Plateau rim into the crescent of the Himalaya Mountains bordering India and Nepal. This area being more densely populated, the following districts carry particular risks:
- South and Southwest of Chamdo, China;
- The (eastern) Bhutan/India border area;
- The greater Lhasa region, Tibet;
- The Lhatse, Namling (Tibet area);
My hopes are that trough increased awareness – respect of building codes, contingency plans, fault monitoring - human disaster can be avoided. Earthquakes do not respect government policy, opinions or anyone’s time tables. They simply do happen, and the only thing we can do is to understand them and restructure our lives accordingly.
© 2008 by Franz L Kessler