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The critical taper model has been widely used to evaluate the strength contrast between the wedge and the basal detachment of fold-and-thrust belts and accretionary wedges. However, determination of the strength parameters using the traditional critical taper model, which adopts the Mohr–Coulomb failure criterion, is difficult, if not impossible. In this study, we propose a modified critical taper model that incorporates the non-linear Hoek–Brown failure criterion. The parameters in the proposed critical Hoek–Brown wedge CHBW model can be directly evaluated via field investigations and laboratory tests. Meanwhile, the wedge strength is a function of the wedge thickness, which is oriented from stress non-linearity. The fold-and-thrust belt in western central Taiwan was used as an example to validate the proposed model. The determined wedge strength was 0.86 using a representative wedge thickness of 5.3 km; this was close to the inferred value of 0.6 from the critical taper. Interestingly, a concave topographic relief is predicted as a result of the wedge thickness dependency of the wedge strength, even if the wedge is composed of homogeneous materials and if the strength of the detachment is uniform. This study demonstrates that the influence of wedge strength on the critical taper angle can be quantified by the spatial distribution of strength variables and by the consideration of the wedge thickness dependency of wedge strength.

This study aims to quantify the tectonically induced thrusting and erosion. Here, we propose and evaluate a laboratory-based method for determining the maximum effective stress ever sustained by a sedimentary-rock formation. The porosities of nineteen samples collected from two deep boreholes of the Taiwan Chelungpu-Fault Drilling Project were measured under different confining stresses. The relation between measured porosity and confining pressure (compaction curve) is bilinear in log–log scale. The maximum effective stress of the tested rocks was determined from the intersection point of the two straight lines of the compaction curve. The maximum effective stresses of the rocks below the Sanyi Fault (reverse fault) are close to the in-situ vertical effective stress. Above the Sanyi Fault (hanging wall), where the rocks have been thrust, the maximum effective stresses of the rocks are significantly greater than the in-situ stress with an average value of 29.48 MPa. That is, the eroded thickness of the formation on the hanging wall of the Sanyi Fault is 2482 m, a value close to that determined from the geological profile (2773 m). It is inferred that the total displacements along the thrust Sanyi Fault is 11.5 km, a value close to that determined from the geological profile (14–16 km). These results indicate that the proposed laboratory-based method is potentially useful for determining the thrusting and erosion in a fold–thrust belt and the total displacements along a thrust fault.

The Chi-Chi earthquake (Mw = 7.6) of September 21, 1999 triggered many landslides in central Taiwan. Two of these landslides, Hungtsaiping (HTP) and Jiufengershan (JFES) were situated as close as 2 km from each other but had significant differences in their kinematics. JFES landslide was a catastrophic rockslide-avalanche and the HTP landslide was relatively slow-moving. The authors conducted a study to explore the reasons for such differences. Factors such as the characteristics of strong ground motion, sliding direction of landslide, and friction angle of the sliding surface were considered in the study. An analysis of 12 strong-motion records collected in the study area showed that the distribution of horizontal pseudostatic coefficients, earthquake energy ratio and permanent sliding-block displacements (Newmark displacement) were anisotropic with their predominant direction mostly in the E/W–ESE/WNW trending. This direction is perpendicular to the axis of the main geological structures of the studied area. The computed Newmark displacement in the sliding direction of the JFES landslide is larger (44%) than that of the HTP landslide with sliding surface inclination of 21° and friction angle of 28° We can conclude that the seismic anisotropy and the corresponding sliding direction are important contributing factors to the kinematics of studied landslides. The back-calculated friction angle of the sliding surface that corresponds to a critical Newmark displacement for the JFES landslide is about 3.5° higher than that of HTP landslide. The material (colluvium) on the sliding surface in HTP should be less velocity-dependent than that of the JFES landslide (rock) according to the back calculations. The importance of seismic anisotropy, sliding direction, and mechanical properties of sliding surface on the kinematics of deep-seated landslides is demonstrated.