When a Fault System Re-Awakens
Observations on China’s Longmenshan Thrust Belt
When a Fault System Re-Awakens, ICGR - Institute for Creative Geology Research
Dr. Franz L. Kessler, ICGR - Institute for Creative Geology Research
On May 12, 2008, China’s most powerful earthquate in 58 years occurred along the Longmenshan Thrust Belt in Sichuan Province. The 7.9-magnitude earthquake caused widespread destruction and killed more than 69,000 people. The Longmenshan Thrust Belt constitutes a series of Mesozoic overthrusts that offset the Tibetan Plateau on the northwest from
the Gansu Plain to the southeast
(Figures 1 and 2).
The possibility of a severe earthquake along this thrust belt had been anticipated in academic circles, but there was little evidence that it would happen so soon and with such devastating consequences. Li Yong, a geological expert at the Chengdu University of Technology in Sichuan,
quoted in an International Herald Tribune article of May 30th, 2008, stated: “The line of the middle fault is as clear as a string. It suggests continuous and strong movement. Such a long and clear lineament should trigger a big quake.
Other scientists have had similar ideas.” In July 2007, Li co-wrote a paper that raised the likelihood of a 7.0-magnitude earthquake in this same area, along the
Longmenshan Thrust Belt. He spoke again of the dangers associated with this fault zone at a conference in China only a month before the disaster struck. Seismic activity in the area was sparse from 2004 to late 2007 , with no strong seismic evidence to indicate an imminent fault reactivation in the thrust belt area. Hence, the Longmenshan Thrust Belt provides an excellent example for a sudden rejuvenation of an older fault system.
The following conclusions are offered:
• The ‘awakening’ of the thrust belt occurred in an extremely short time span. This could be significant for other dormant fault belts.
Figure 2:
Relief maps of the Longmenshan Thrust Belt area. The star indicates location of the
city of Chengdu. The SW-NE Longmenshan Thrust Belt forms an obvious landscape element
that separates lowlands south and south-east of Guangyuan from the Tibetan Plateau. Seismic
activity, as recorded since 1973 by the USGS, indicates scattered events of shallow and intermediate depth, but without emphasizing the fault belt area.
Figure 1:
Geological cross-section through the Longmenshan Thrust Belt to the northwest, and stable Paleozoic
and Mesozoic foreland. Although the thrust belt’s origin is clearly Mesozoic, there remain questions about Tertiary
and Quarternary tectonic activity. From the point of human settlement history, the thrust belt re-awoke to life in
2008. Courtesy of Tai-Ran Jiang Miri, Malaysia
• Data clearly indicate that seismic precursor activity was extremely brief; there was neither strong seismic evidence nor sufficient time for disaster prevention.
• Prediction of earthquake risk needs to be carried out plate-wide, given that risk cannot be calculated nor earthquakes forecasted based on analysis of isolated areas only. ■
Figure 3:
Location of earthquake foci in map view. The position of the Longmenshan Thrust Belt is indicated by the dashed line. In early 2008, four months before the big quake, seismic activity suddenly awakened. Quake foci are taken from the USGS database. Color scale indicates depth of earthquake foci inkilometers.