## 4 Results
### Damage and Stress Analysis  
We used analytical stress and damage solution in July 2017 to assess diagnostic fracture stability and to analyze the calving process. Figure 5a illustrates transverse stress distribution that stretches across the flow line forming in a transverse extension zone with large crevasses. The longitudinal stress showed crevasses in an ice divergence zone where opening rift existed in Figure 5b. Splashing crevasses developed a shearing zone in Figure 5c with adjacent tributary glacier terminate. Local stress fields promoted crevasse opening and enhanced ice flow near the shear margin, leading to a relative rotation of the flow in the convergence zone. Additionally, the middle part in the crevasse, marked green line in Figure 2a, was bended convex upward shape with respect to local flow in red rectangle region of Figure 2a,b. In general, crevasses are typically parallel to the least tensile principal stress axes and normal to the most tensile principal strain rate axes in the horizontal plane [52]. We computed the axes of the principal stress by using the surface stress to compare with the orientation of the crevasses on glacier terminate position in Figure 5d. On the connected point with tributary glacier terminate, high shear stress showed unbalanced force from different direction ice movements producing vulnerable region in Figure 5c. However, we did not find the obvious velocity change from crevasses region during the period of 18 July to 24 July in Figure 3c until the day of 25 July despite of remarkable signals on stress from high longitudinal and transverse stress values in Figure 5. The stress characteristics serves as preconditioning factor describing the fracture instability. An evident sign was seen that the gradually increased effective stress series exerted within the front of glacier during July 2017 in Figure 6. The weakness of structural integrity was mainly distributed at zone of fracture which have consistently yielded an effective stress field. The magnitude of effective stress ranged from 300-800 kPa in the shear zone. As long as weakened ice shelf stepped into an unstable state of rheology in Figure 6, unstable regime controlled by the plastic rheology of crevasse may reduce ice tongue strength under the increased effective stress burden. The spatial-temporal changes of effective stress present the instantaneous effect to destroyed crevasse. One day prior to collapse, pressure on the tongue reached the ultimate threshold in Figure 6d; glacier strength was not sufficient to carry more effective stress, hence the calving event. An example of this preconditioning can be seen in the interaction of these damaged shear zones with the existing fractures perpendicular to flow in the center of the PG's ice tongue. To quantify how ice-shelf stress affects fracture stability, we constructed a fracture stability damage diagram referring to the Equations (2) and (3). The increasing damage scalar series of ice tongue before calving gradually weaken and impair the glacier integrity in Figure 7, which could not offer enough buttressing at the glacier's terminus. In days prior to collapse, there was obvious damage region in the crevasse which reached the threshold (0.5), even the completed damaged state. A highly damaged region means the crevasse has been penetrated, which could not forbear an occurrence of caving soon afterwards. When these rafts connect with the damaged shear zone, the ice front is no longer stabilized due to the structural weakening, resulting in large calving events. The high effective stress and damages in frontal region shows that an underlying and vulnerable condition for this type of glacier flow instability was reaching an ultimate disintegration. Figure 5: Ice stress condition distribution at the glacier’s terminus corresponding to the period of Figure 3c (average daily stress from 18 July to 24 July in 2017) before the calving event. (**a**) The transverse stress, (**b**) the longitudinal stress, (**c**) the shear stress and (**d**) the first principle stress.