Figure 7. LOS-changes of ALOS2 interferograms overlaid on shaded relief
map. Details of each image are described in Table 1; imaging was
performed by ascending, right-looking orbit. Warm and cold colors
indicate LOS changes away from and toward the satellite, respectively.
Black dashed line indicates the boundary between the burned and unburned
area confirmed with Landsat optical images.
Figures 7a—7h show ALOS2 interferograms, each of which covers nearly
one-year after October 2015 with some overlaps in its temporal
coverages. Figure 7a, derived at the earliest period after the fire,
indicates the maximum one-year subsidence to be as much as 10 cm or
more.
If the amplitude and timing of seasonal subsidence/uplift cycle are
invariable over time, a one-year interferogram will tell us only the
irreversible displacements regardless of the acquisition times of
master/slave images, which corresponds to the “pure ice” model in Liu
et al (2015). Figure 7 sequentially shows the periods from October 2015
to June 2019 and indicates that the yearly subsidence rate slowed down.
However, the variations of the one-year LOS changes in Figures 7 suggest
that the actual deformation processes were more complex.
Figure 8a shows the cumulative LOS changes from October 2015 to June
2019 derived from SBAS-type time-series analysis, and that the maximum
LOS extension reached as much as 25 cm; the 2\(\sigma\) errors for
Figure 8a were ±1.5cm. Considering that the LOS changes during the first
year after the 2014 fire were not included, the total LOS changes were
presumably much greater than 25 cm, which meant that the subsidence was
greater than 30 cm on account of the 36° incidence angle. As mentioned
earlier, however, the higher-elevation areas such as the ridge did not
undergo significant deformation, which probably would have been the case
even during the first year after the fire. In addition to the high
elevation areas, we realized clear contrasts in the LOS changes between
the east- and the west-facing slopes near the northwestern area and the
central north-south trending ridge; this spatial heterogeneity could
also be recognized in Sentinel-1 (Figure 4). Their possible mechanisms
comparing the burn severity (Figure 8b) and local landform (Figure 8c)
are discussed in section 5.2.
We show the estimated time-series data at four representative sites
(Figures 9a-9d), whose locations are indicated in Figure 8a. The sites
(a) and (b) underwent nearly the same cumulative LOS changes by roughly
20 cm but were located at different slopes that are 4.3 km apart. On the
other hand, the cumulative LOS changes at the site (d) were relatively
small (approximately 10 cm). The site (c) located in the ridge did not
show either significant seasonal or long-term deformation.
Time series data in Figures 9a and 9b clearly indicate that the largest
subsidence took place from 2015 and 2016. We believe, however, that the
most significant subsidence probably occurred only during the thaw
season in 2016, as we have observed earlier, that no deformation
occurred from December to March. Thus, the actual subsidence rate from
October 2015 to July 2016 should have been more complicated than that
expected from the linear trend in Figures 9a and 9b. The error bars in
Figures 9a-9d indicated an estimated standard deviation with 2σ and
attained ±1.5cm in the last epoch.