Geophysical Research Letters
Supporting Information for
Dome growth at Mount Cleveland, Aleutian Arc,
quantified by time-series TerraSAR-X imagery
Teng Wang1, Michael P. Poland2, Zhong Lu1
1, Roy M. Huffington Department of Earth Sciences, Southern Methodist University, Dallas, Texas 75275, USA
2, Cascades Volcano Observatory, U.S. Geological Survey, Vancouver, Washington, 98683-9589, USA
Contents of this file
Text S1 and S2
Figures S1 to S6
Table S1
Text S1. InSAR observations
InSAR processing
We coregistered all 10 Stripmap TerraSAR-X images using orbital information provided by the
German Aerospace Center and the Digital Elevation Model (DEM) from the USGS National
Elevation Database (NED) [Wang et al., 2014], and then formed 55 differential interferograms.
One additional interferogram was generated from the two coregistered spotlight images. The
overall coherence of these interferograms is represented by color-coded lines in Figure S1.
Unfortunately, all interferograms that include images acquired during early 2012 have low
coherence values due to the heavy snow coverage in winter. The sequence of stripmap
interferograms referenced to the first acquisition on Aug. 07, 2011, is depicted in Figure S2. All
interferograms were multilooked (10-by-10 pixels) and smoothed with a two-dimensional
complex convolution filter [Perissin and Wang, 2012].
InSAR observations of Mount Cleveland and surrounding islands
1
Most of the interferograms show clear fringes—including occasionally quite complicated
patterns—on the three islands in the SAR frame. We exclude the possibility of large topographic
errors as a source for the fringes because the interferogram with the largest perpendicular
baseline among the interferograms shown in Figure S2 (spanning August 7 to September 20, 186
m baseline; Figure S1) shows a relatively smooth fringe pattern. Dense fringes are apparent on
the east flank of Mount Cleveland and also on the east side of the other two islands in the
interferogram spanning August 7 and 18 (Figure S2, upper left). Similar patterns can be observed
on all three islands in the interferogram spanning August 7 and October 12 (Figure S2, middle
right), although the densest fringes in this image are on northeast flanks of the islands. In the
interferogram spanning August 7 and October 23, fringes appear correlated with topography as
well as present on the east sides of each island. Within individual interferograms in Figure S2,
however, fringes exhibit broadly similar patterns on all three islands. This similarity suggests that
the origin of the fringe patterns is probably related to stratified atmospheric conditions.
We unwrapped all interferograms and converted the unwrapped phase to radar-to-target
distance change in the line-of-sight (LOS) direction. For X-band interferograms, each fringe
represents about 1.5 cm of LOS change. To investigate the potential relation between the
observed fringes and topography, we drew east-west profiles across each island (Figure S2
(upper-left panel), and Figure S3). In these profiles, it is clear that the west flanks of the islands
generally show smooth changes in LOS distance, while the east flanks show higher gradients in
many interferograms. It seems that some interferograms exhibit a high correlation between LOS
distance change and topography, but the correlation is not a simple linear relation. For example,
profiles from the August 7–18 interferogram (dark blue lines in Figure S3) exhibit very similar
patterns for all the three islands, which are correlated with topography but with a shift of about
1.5–2 km toward the east. Although the mechanism for the topography-correlated and shifted
LOS change is not clear, we conclude that such a feature is probably related to atmospheric delay
rather than surface deformation.
If there is any deformation signal related to 2011 eruptive activity, we would expect to see some
unique spatial patterns in the fringes around Mount Cleveland; such patterns are absent. For
interferograms that range from August 7 to September 9, October 1, and October 23, a patch of
phase change is obvious on the southern flank of Mount Cleveland (Figure S2); however, we could
not identify the same feature in the interferogram spanning August 7–November 3, which is just
11 days after the latest image containing the south flank feature. Moreover, a similar pattern
appears on the southern flanks of Herbert and Carlisle islands in the August 7–October 1
interferogram. We are therefore not able to distinguish with confidence any true deformation
signal from this set of TerraSAR-X interferograms. This observation is consistent with previous
studies of Cleveland volcano, which concluded that magma may ascend through an open conduit
without producing large-scale surface deformation [Lu and Dzurisin, 2014].
InSAR observation of the summit crater area
Although we were not able to detect evidence of volcanic deformation in the flanks of Mount
Cleveland, the crater rim may deform during the extrusion of the dome. We therefore focused
on the summit area to analyze whether or not any localized displacements occurred during
eruptive activity in 2011–12. For most of the Aleutian volcanoes that have been studied thus far
[Lu and Dzurisin, 2014], deriving information from summit areas using C- and L-band SAR data is
difficult. This is due in part to snow coverage for most of the year in this region, and also to the
lower resolution of the SAR images used in the past (around 10–20 m). High-resolution SAR data
2
such as that from TerraSAR-X allow for better coherence in summit regions because when there
exist fewer scatterers within each pixel, these scatterers are more likely to be statistically
homogenous, and the signals reflect back from them are thus more likely to be coherent [e.g.,
Salzer et al., 2014 and Richter et al., 2013].
We oversampled all TerraSAR-X SAR images covering the summit area of Mount Cleveland by
factors of 4 in both azimuth and range directions and re-calculated all interferograms with
multilooking factors of 4 by 4, meaning that the interferograms are processed at their full
resolution. This procedure helped to improve coherence and also reduced the effect of signal
aliasing [Sousa et al., 2011]. Coherence maps show that high coherence is preserved in
interferograms comprised of SAR images acquired in August 2011, and that coherence of the
crater rim exceeds that of the upper flanks and crater interior (Figure S4a-b). For the
interferogram spanning August 29 and September 9, the coherence is much lower, probably due
to the snow coverage and the rapid growth of the lava dome (Figure S4c).
We processed interferograms of the crater areas (Figure S4d-f) without applying any filtering or
smoothing. Patches with different phase values can be observed from the two interferograms
spanning August 2011, demonstrating that the crater rim deformed during extrusion of the
dome. Although the dome itself did not change significantly between August 7 and 29, the crater
rim exhibited complicated phase-change patterns (Figure S4d-e). The interferogram spanning
August 29 and September 9 seems to exhibit a relative flat phase pattern, despite significant
motion of the dome from the far-field to the near field in the SAR images (Figure S4f and Figure
2 in the main text). Nevertheless, we could not conclude that the crater rim was stable during this
period, because reliability of the phase information can not be guaranteed given the fact that the
coherence is much lower than for the two interferograms acquired in August. The phase pattern
appearing in Figure S4f is likely random phase noise on bright scatterers, as it is very similar to
the phase patterns on the surface of the fast moving dome, on which the radar signal should be
completely decorrelated.
For the two interferogram with coherent phase observations, because phase information is
wrapped between ±π, we were not able to retrieve the sense of displacement from the wrapped
phase. Phase unwrapping was not practical because the localized deformation between adjacent
moving targets on the crater rim may be easily larger than half of the wavelength (1.5 cm) during
dome emplacement.
Text S2. Crater rim instability derived from pixel-offset tracking
As InSAR cannot retrieve the direction of the motion on the crater rim due to phase ambiguity,
we performed pixel-offset tracking on the oversampled SAR amplitude images. We are able to
estimate pixel offsets in both azimuth and range directions from the peak location of the crosscorrelation surface [Wang and Jónsson, 2015]. We applied 64-by-64-pixel cross-correlation
windows on the oversampled amplitude images and focused these cross-correlation windows on
bright pixels detected around the summit crater rim. Finally, we obtained a pixel-offset time
series on the detected reflectors, based on cross-correlating temporally consecutive images.
We plotted the cross-correlation peak values with azimuth and range offsets derived from
images acquired from August 7 to September 9, 2011, in Figure S5. As indicated by the small
3
black crosses, the dome moved from far-range to the near-range at some time point between
August 29 and September 9, consistent with the persistent thermal anomaly observed by AVO
starting on September 3 [McGimsey et al., 2014]. The offsets show high cross-correlation values
on most of the detected scatters in the far-range part of the crater (Figure S5, a, d and g). We
ignore the offsets in the near-range part of the crater rim as they are in the foreshortened areas
of the eastern flank. The crater rim in the far-range, i.e. the west section of the crater rim, exhibits
different kinds of motion before and during the growth of the dome.
From the pixel-offset results, the first impression is that the magnitude of crater rim motion
increased from August 7 to September 9, while over the same period the direction of the motion
reversed. Based on the differing directions of motions, we can divide the far-range part of the
crater rim into three sections (Figure S5f). Displacements between August 7 and 18 (Figure S5, bc) are concentrated in the section II, although those offsets exhibit low cross-correlation peaks
(Figure S5a), implying that they might not reliable due to the surface changes during dome
growth. Displacements became more apparent in offsets between images acquired on August
18 and 29 and August 29 and September 9 in Sections I and III (Figure S5e, f, h, i). If we assume
that motion along the crater rim is mainly horizontal, section I and section III of the crater rim
moved toward the northeast, while section II, moved toward the southwest during the slow
growth of the dome that occurred between August 18 and 29. During the rapid growth period
bounded by August 29 and September 9, sections I and III moved in the opposite direction
compared to the motion that characterized the slow growth stage. The whole crater rim moved
towards the south during rapid growth of the dome (Figure S5h). Different from the
interferograms shown in Figure S4, these offsets denote very localized surface motion on the
scale of decimeters. Such signal cannot be retrieved from InSAR as the deformation gradient is
larger than the maximum detectable gradient using phase information [Baran et al., 2005].
Therefore, pixel offsets can be useful in mapping gross localized deformation in the vicinity of
the crater.
Figure S1. Interferometric combinations for all images in the TerraSAR-X dataset. Each square
represents a SAR image, and lines connecting the images represent interferograms (colors
indicate the mean coherence of each interferogram, and Bn is the perpendicular baseline).
4
Figure S2. Temporally consecutive stripmap-mode interferograms that have the image acquired
on August 7, 2011, as the master.
5
Figure S3. Radar line-of-sight distance changes of the first 8 interferograms shown in Figure S2
along three profiles (see Figure S2 for locations of profiles) across Carlise Island, Mount
Cleveland, and Herbert Island.
Figure S4. Coherence maps and interferograms of the summit crater of Mount Cleveland. (a)-(c)
Coherence maps for three interferograms acquired in August and September 2011. High
coherence (warm colors) is preserved around the crater rim. White box in part (a) shows area
covered by enlargements in (d)-(f). Note that these coherence maps and interferograms are
displayed in radar coordinates; therefore, north is not exactly towards the top of the image, and
east and west are reversed.
6
Figure S5. Pixel offsets calculated for strong reflectors on the crater rim. From left to right, each
column shows cross-correlation, azimuth offsets, and range offsets from three image pairs
labeled at the top of each plot. Ground-pixel spacing is 0.5m and 0.4 m in azimuth and range,
respectively. For the azimuth offsets, negative values (blue) indicates motion toward the north,
while positive values (red) means motion toward south. For the range offsets, negative values
(blue) means motion toward the satellite in the line-of-sight direction (uplift and/or eastward),
while positive values (red) mean motion away from the satellite (subsidence and/or westward).
Detected bright scatterers from the dome (small black dots) are shown to provide context. Note
that these images are displayed in radar coordinates; therefore, north is not exactly towards the
top of the image, and east and west are reversed. The extent of each plot is about 250 m by 250
m.
7
Growing dome
h
Crater
r
conduit
Figure S6. Schematic dome geometry and volume calculation. We assume the dome is a halfellipsoid with radius r at the intersection between the dome and the crater floor, and with the
axis h in the vertical direction. We can calculate the dome volume V based on the derived areas
and heights using equation: V = 2/3r2h, where we can use the area we measured from the
boundary polygon to replace r2. We assume that h=r/2 for the initial dome geometry (see Table
S1 for the details in calculations).
Date
(yyyymmdd)
20110807
20110818
20110829
20110909
20110920
20111001
20111012
20111023
20111103
20120108
Area
(104m2)
2.0417
2.0341
1.9881
2.0277
2.0807
2.0741
2.0219
2.4323
2.5426
2.5225
LOS (m)
2.4844
3.7753
35.4461
51.3831
49.353
53.0277
54.3596
53.1479
44.8406
Height
(m)
3.1147
4.7332
44.4394
64.42
61.8747
66.4818
68.1516
66.6325
56.2174
Volume
(105m3)
0.0402
0.0483
5.97
9.041
-
Effusion Rate
(m3/s)
0.0423
0.0086
0.5773
0.3232
-
Table S1. Quantifications of the dome area, volume, height and effusion rate derived from
volume change. The initial dome height was assumed to be half of the radius of the dome
determined from the SAR image acquired on August 7, 2011, and the volume is calculated by
assuming the dome is a half- ellipsoid. Gray shading indicates calculations after the image
acquired on October 11, 2011, when overlapping of the dome and the crater rim in SAR images
introduces uncertainty in dome area.
8
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