Figure 4. (Top) ALOS2
interferograms during the five periods, (a—e). (Bottom) Sentinel-1
stacked interferograms during the five periods, (f)—(j), derived so
that the temporal coverage could nearly match those from (a) to (e); all
the interferograms are overlaid on shaded relief maps. Warm and cold
colors indicate LOS changes away from and toward the satellite,
respectively.
We compared the ALOS2 and stacked Sentinel-1 interferograms for five
periods (Figure 4) and assessed their differences (Figure 5). Despite
differences in look directions both ALOS2 and Sentinel-1 indicated
extensions in the LOS during periods (a, f) from the middle of June to
the end of July and (b, g) from the end of July to the early October.
Also, their deformation areas and amplitude were mostly consistent,
suggesting that LOS changes were largely due to summer subsidence (see
section 5.1 below for details). In terms of the spatial distribution of
deformation signals, we noticed that the LOS changes over
higher-elevation areas such as ridge and peak were insignificant,
whereas the boundaries between the burned and unburned areas were clear.
The north-western area, however, showed few LOS changes (see section 5.2
for the relationship between LOS changes and burn severity). During the
period (c, h) from early October to early December both ALOS2 and
Sentinel-1 indicated shortening in the LOS by an approximate 5 cm
maximum, and the deformation areas and amplitude were quite similar.
This observation presumably indicated frost heave in the early freezing
period. In view of the previous three periods, both subsiding and
uplifting areas were nearly the same. The following period (d, i) from
early December to the middle of March also included the winter season
with much colder air temperatures, but we did not observe any
significant deformation signals, indicating that frost-heave virtually
stopped in early December. While the good interferometric coherence
during mid-winter was an unexpected result, we speculate that it could
have been due to drier, lower amounts of snowfall.
In the periods (e, j) from the middle of March to early June, both ALOS2
and Sentinel-1 suffered from decorrelation, and we could not identify
clear deformation signals. However, in light of Figure 6 below, each of
the Sentinel-1 interferograms had overall good coherence with the
exception of the data acquired in the middle of May. These observations
suggested that the decorrelation may be attributable to the rapid
changes on the ground surface during the initiation of the thawing
season when the air temperature rises above the freezing point and the
active layer begins to thaw.