## 4 Results  
### The Sub-Week Timescale Ice Flow  
Sentinel 2A/2B images characterized the frontal collapse process near the glacier terminus in PG, as shown in Figure 2. We acquired the high-resolution 20-m ice velocity with a sub-week or even daily overpass timescale in July in 2017. There is a sign of widening of the fracture taking place on the day (25 July 2017), prior to the calving event date (26 July 2017). The contours of the PG terminus position are shown in Figure 3, and we calculated the mean velocity to analyze the trend of the flow rates. Average surface velocities on the calving region show a relatively steady state of 3 m/d, with a huge peak of 31 m/d observed one day before the calving, which corresponds to evident accelerated signal shown in Figure 3d. In the glacier velocity field results, the average displacement of bedrock in an off-glacier region was used as an accuracy measure to assess uncertainty of the ice speeds [21]. The uncertainty of ice velocity for six-days internal was approximately 4.2 m (0.69 m/d), and the measurement error of ice velocity between 24 July and 25 was 2.52 m for the one-day internal, as shown in Figure 4. Additionally, the speed of the upstream region at one-day internal showed less than other dates in Figure 3d due to the measurement error limited by the capacity of feature tracking algorithm extracting small displacement. In general, the accuracy of velocity is reliable to analyze the changes of time series at the terminus calving region within considerable accuracy range.  
Figure 2: The sequence of Sentinel-2 NIR-band images showing the Petermann Glacier terminus collapse event in 2017. **(a)** The fracture of the terminus region occurring; **(b)** the fracture of the terminus region widening; **(c)** the frontal collapse event that occurred on 26 July 2017; **(d)** iceberg drift after collapse. The red polygon region shows the shear zone and the green line denotes the bending deformed ice.  
Figure 4: The time series of Petermann Glacier’s velocity, the blue polygon in Figure 3, during July 2017, prior to the calving event. Additionally, the red error bar represents the velocity accuracy evaluation.  
Figure 3: Surface daily velocity field of the Petermann glacier in four periods of July 2017, prior to the frontal collapse. (**a**) The daily velocity field from 08 July to 13 July in 2017; (**b**) the daily velocity field from 13 July to 18 July in 2017; (**c**) the daily velocity field from 18 July to 24 July in 2017; and (**d**) the daily velocity field from 24 July to 25 July in 2017.  
Widening of the fracture of the terminus region in the outlet glacier was accompanied by an abrupt increase of the scalar flow rates. Although we cannot directly capture the subpixel change due to the restriction of the resolution (10 m) of the remote sensing image shown in Figure 2b, we could monitor the significant increase--by about 10 times--of the speeds in Figure 3d. Pre-collapse velocity served as a kinematic precursor to enlargement of the crack before glacier calving occurrence. In addition, the tributary glacier feeds and drains from the perpendicular direction and pushes the joint region of the main glacier's terminus, leading to a deformed shear zone which weakens the outlet glacier surface structure with continuing evolution of the rift, and afterwards the calving will occur sooner or later. The analysis for the calving cause from inflow of a tributary is discussed in Section 5.1.