The welding heat affected zone of the as-welded welding joint was the weakest zone.
The fatigue test showed welded zone is the weakest zone due to the existence of micropores defects in the surface of the joint, even if there is softening zone in HAZ which is shown in hardness curve.
Although it is unclear when the weak zone began to form, this could be a representative magma pathway for past or future activity.
The shear modulus and Poisson's ratio of the weak zone are assumed to be 56 GPa and 0.25, respectively.
This idea is in agreement with the weak zone model as a cause of the NKTZ proposed by Iio et al. (2002, 2004), based on the observations of rich water content (Sano and Wakita 1985) and low velocity (Nakajima and Hasegawa 2007) in the lower crust beneath the NKTZ.
Therefore, when tunnel excavation approached the altered weak zone, as the weak zone was not identified and no measures were taken to avoid the accidents, inrush hazards were triggered due to excavation disturbance, stress redistribution, blasting vibrations and groundwater drainage.
As long as the bottom depth of the weak zone is greater than the thickness of the continental plate (40 km), surface deformation is not sensitive to the lower boundary of the weak zone (Additional file 1: Figures S10 and S11).
Hu et al. (2016a) suggested that the viscosity of this weak layer, at least 1 × 1018 Pa s, is sufficient to produce landward motion, although they assumed the thickness of the weak zone to be 80 km.
The weak zone is always appear around the sink nodes (shown in Figure 7).
The plan-view width of the weak zone is 50 km.
