Abstract:

Investigation and analysis on 2008.5.12 Wenchuan earthquake damages shows that some  major security hidden trouble problems for projects will leave out because of misjudging large earthquake by means of taking investigation based on active faults and historical earthquakes as the key evaluation method for engineering regional tectonic stability. Taking Longmenshan active nappe as an example, based on the research results on the crustal rock mass structure, new tectonic activities, historical earthquakes, using rock mass structure cybernetics, Byerlee’ law, mechanics characteristics of the Longmenshan crustal rock mass, the relationship between the shearing-slip strength of the controlling-seismic structural layer and earthquake magnitude, the relationship between earthquake magnitude and the seismic fortification intensity are quantitatively studied, the key problems to assess the regional tectonic stability are discussed. The main result is that the detachment layer within nappe boundary zone plays a main control role which is called a control shock layer. There is significant correlation among the earthquake magnitude, buried depth of the detachment layer and shear strength, namely, 8 earthquake focal depth close to 20 km, 7 earthquake focal depth close to 14 km, 6 earthquake focal depth near 10 km. Based on depth of ancient earthquakes ratified, possible scope of 6 above earthquakes and corresponding seismic fortification intensity are delineated in Longmenshan nappe-Minshan block and adjacent area. The result makes up drawbacks of the previous evaluation method based on the relation among active fault, seismogenic fault, historical maximum magnitude and corresponding seismic intensity, in which such problems as oversight of historical earthquakes, limit of scope and depth of fault zone and ignore the essence of rock mass mechanics in regional tectonic stability for proposed construction project lead to miscarriage of justice and leakage evaluation.

Key words: nappe, boundary zone, control shock layer, shear-slip, regional techonics, stability, active tectonic block