4.2.1. Co-eruptive TSX Observations

Co-eruptive TSX amplitude images reveal a tongue-like reflectivity change at the western dome, which depicts the emplacement of an extensive lava flow (flow 1) (Figure 5a2 and Figure S5). Its lower parts are marked by radar shadow casting crevasses, as well as radially and flow parallel oriented shadow casting ridges that are bisected by a significantly larger and irregularly oriented, but also flow perpendicular, shadow-casting ridge (Figure 6a). Between 17 November and 20 December 2016, the lava effusion is accompanied by northward-directed rigid (inelastic) bulging of the northern composite dome flank, and the detected azimuth offset rates increase with elevation from approximately 0.05 to 0.1 m d−<sup>1</sup> (Figure 5a3 and Figure S5). During the end of December 2016, no motion of the northern carapace was determined (Figure S6), whereas between 31 December 2016 and 2 February 2017 the same motion direction and elevation-related distribution is observed, but at significantly higher rates of approximately 0.1–0.5 m d−<sup>1</sup> (Figure 5b3 and Figure S5). A substantial increase in seismic activity is observed during the second half of January 2017, which rapidly decreases by the end of January 2017 (Figure 2e).

**Figure 5.** Ground motion detected in TSX amplitude images during the 5 December 2016–28 February 2017 effusive eruption. Extent is indicated in Figure 1b. (**a1,2**), (**b1,2**), (**c1,2**), (**d1,2**) Geocoded TSX amplitude images with indicated changes. Left column are master images, central column are slave images. RB = rigid body. The colour coded outlines show extent of lava flow 1 compared with previous extent. (**a3–d3**) Azimuth offset maps and 50 m contour lines of Bezymianny. Red and blue colours reflect movement along (approximately southward) or against (approximately northward) the TSX flight direction (LOS). Employed cross-correlation patches are indicated. Arrows in close-up views indicate relative increase of azimuth displacements from the foot to the summit of the dome. Displacement increases towards the summit. Compare Figure S5 for swath profiles A–A'.

**Figure 6.** Topographic changes at Bezymianny between December 2016 and June 2017. (**a**,**b**) show amplitude images of Bezymianny after the 5 December 2016 and 9 March 2017 eruptions, both marked by large lava flows. (**c**) Amplitude change difference map and (**d**) aerial image after the 16 June 2017 eruption. White and black arrows (**c**,**d**) indicate deposition of pyroclastic deposits and an extrusive body, respectively. Both, the pyroclastic deposits and the extrusive body are indicated by reflectivity increases in (**c**) (green colouring), whereby the extrusive body is barely perceptible.

By beginning of February 2017, the surface fractures of flow 1 are widely distributed, and a new contrasting radar shadow appears at the summit that indicates uplift of a new rigid body (Figure 5b2). The subsequent scene from 13 February 2017 shows that the reflectivity pattern of flow 1 at the summit area significantly extended, and the shadow-producing rigid body moved westwards (Figure 5c2). Simultaneously, the northern flank bulges again northwards with rates between 0.1–0.3 m d−<sup>1</sup> that increase with elevation (Figure 5c3 and Figure S5). In addition, azimuth offset maps between December 2016 and February 2017 reveal differential motion of flow 1, where its northern and southern segments move approximately north- and southwards, respectively. The following amplitude pair does not reveal significant azimuth ground motion anymore (Figure S6).

The azimuth offset map of the last descending amplitude image pair (24 February–7 March 2017) again reveals northward directed motion of the northern composite dome flank, where the rates repeatedly increase with increasing elevation from 0.1 to 0.4 m d−<sup>1</sup> (Figure 5d3 and Figure S5). In addition, minor southward directed motion indicates the bulging of the southern flank. Lastly, the only ascending amplitude image pair (17–28 February 2017) of this episode shows southward directed motion of the southern composite dome flank (Figure S11), which implies that the southward and northward directed bulging detected in the last descending pair occurred at different times.

The beginning of March 2017 is characterised by a significant increase in seismic activity that culminated on 9 March 2017, when the first explosive eruption occurred (Figure 2e). The descending amplitude image from 18 March 2017 shows another tongue-like reflectivity change at the north-western composite dome flank, which reveals the outpouring of the second lava flow (flow 2) (Figure 6b). Subsequent scenes do not unveil significant changes neither of flow 2 nor of the dome flanks (Figure S2). The surface reflectivity of flow 2 is more homogeneously distributed than that of flow 1.

By end of May 2017, seismicity picked-up and significantly increased until 16 June 2017, when the second explosive eruption occurred. The descending change difference map between the 18 March and 25 June 2017 amplitude images shows strongly lifted reflectivity within the western and northern 1956 crater moat (Figure 6c). Most of the surface of the two previously emplaced lava flows is now covered by new material, which corresponds to the pyroclastic deposits that were observed in aerial photographs from July 2017 (Figure 6d).

#### 4.2.2. Co-eruptive Webcam Observations

Only one clear image could be selected to determine morphologic changes during the co-eruptive episode because of strong fumarolic activity at Bezymianny and predominant poor weather conditions between 8 December 2016 and 9 March 2017 (cf. Figure 2e). The corresponding Mimatsu diagram between 7 December 2016 and 14 January 2017 reveals growth of the northern and southern summit with approximately 15 m and 11 m, respectively (Figure 4c). The timing of summit growth correlates with the onset of enhanced seismicity, and may also correspond to the beginning of the uplift of the rigid body that produce the significant radar shadow detected on 2 February 2017 (Figures 2e and 5b2).

The 12 March 2017 Mimatsu image demonstrates partial destruction of Bezymianny's summit after the explosive 9 March 2017 eruption, yet most of the previously developed morphology remained (Figure 4d). The latter agrees with radar shadows that are discernible in both the 7 and 18 March 2017 descending amplitude images (Figures 5d2 and 6a). However, the subsequent 24 April 2017 Mimatsu image shows growth of the summit by 24 m, while during May and June 2017, a repeated partial destruction of the summit is observed (Figure 4e).

Eventually, the 10 August 2017 Mimatsu image reveals that significant parts of the previously determined summit accumulation are destroyed, whereas the southern summit portion is characterised by extrusion of new material (Figures 4e and 6c,d). This last and most significant optically detected summit morphology change corresponds well with the compelling reflectivity change of the summit determined in the 25 June 2017 TSX amplitude image, whereby the southern summit growth is only weakly represented in this amplitude image.

#### *4.3. Three Stage activity*

Overall, the TSX amplitude data provided the most detailed (spatially and temporally) observations of precursory and co-eruptive ground motion during the 2016–2017 eruption sequence. Near constant precursory ground motion is observed between January and October 2016, which then rapidly increased two months (stage 1) prior the 5 December 2016–28 February 2017 eruption. The latter observation agrees well with Mimatsu-derived topographic growth of Bezymianny's eastern summit that was detected in December 2016. During the effusive December 2016–February 2017 eruption (stage 2), the radar data unveil recurrent flank motion at different rates that always increase with increasing elevation. Moreover, SAR data show differential lava flow motion as well as the uplift of a rigid body during January–February 2017 that subsequently moved westwards as the summit reflectivity significantly changed. The timing agrees well with considerably enhanced seismicity and optically derived growth of the eastern summit. The surface texture of the two lava flows 1 and 2 (stage 3, 9 March 2017) differ markedly, as flow 1 is characterised by shadow casting crevasses that are absent on flow 2. Eventually, the 16 June 2017 eruption (stage 3) produced pyroclastic deposits that cover most of the two lava flows and fill the northern 1956 crater moat.

#### **5. Discussion**

Our data set captured seven to nine months of precursory ground motion as a rigid body extruded at the summit prior to the first documented effusive December 2016–February 2017 eruption. We interpret the rigid body as extruded, solidified conduit material that we refer to as a plug. Subsequent determined differential lava flow motion was accompanied by a second plug extrusion that rafted westwards as new lava was emplaced near the summit. Besides exogenous growth, the SAR amplitude images also unveiled distinct, recurrent endogenous growth stages as Bezymianny's dome bulged northwards multiple times. Hereinafter, we first shed light on the limitation of employed techniques, and we will then discuss our observations of the different dome growth stages at Bezymianny.

#### *5.1. Limitations*

Seismic activity beneath Bezymianny is monitored by a widespread array of seismometers that covers activity of all volcanoes within the Klyuchevskoy Group of Volcanoes (see locations for stations in Shapiro et al. [19]). The data used here is from a local catalogue and reflects seismic events detected beneath the volcano within a radius of 6 km. However, accurate allocation of events is impeded when other nearby volcanoes are active. In fact, Klyuchevskoy was very active in 2016 (cf. Figure S7), which caused the detection of only few events that are directly associated with Bezymianny. This attracts the attention to other methods to determine activity at Bezymianny, such as SAR and optical observations. However, these techniques are also not immune to shortcomings that have to be considered for interpretation.

Visual observations of volcanic unrest and eruptions are important at volcano observatories to examine topographic changes, levels of gas emissions, and other processes. Time-lapse cameras are increasingly used for documentation and observation as they require low budget and maintenance [13,51]. However, the number of cameras, their location, installation, and weather conditions have strong effects on the resolution to retrieve quantitative information. Multiple terrestrial cameras enable to break down the three dimensional deformation over time, as was demonstrated for dome growth at, for instance, Mount St. Helens during the 2004–2008 eruption [17]. Single cameras, in turn, may have the disadvantage of reducing the three-dimensional displacement into its two-dimensional FOV. Therefore, determined and quantified topographic changes at Bezymianny may over- or underestimate the total amount of deformation, as the absolute displacement may encompass deformation further away or closer towards the camera's FOV. Moreover, the low spatial resolution is confined to the large distance (7 km) of the camera, which is focused on Bezymianny and its neighbouring volcanoes. This significantly lowers the image contrast and it causes blurry edges at Bezymianny's dome (Figure S8), which, together with fumarolic activity and background clouds, may have strong impact on the outline mapping quality. While most of the error contributions cannot be further quantified, the mapping error may be equal to the calculated pixel-size-range (i.e., 4.2–5.3 m/pixel), as the choice of the pixels along the cone outline depends on subjective and biased decisions during the mapping. In addition, the employed metric pixel conversion strongly depends upon topography, distance, and image distortion, which was not corrected for in the images. Thus, the mapping error may be even higher. Yet, the webcam imagery provided valuable qualitative information that supports and complements deductions from seismic and TSX observations, such as deformation in foreshortening areas of the radar data. Thus, time-lapse camera observations constitute an indispensable tool to monitor Bezymianny.

Tracking changes at dome building volcanoes is vital for hazard assessment because of the close link to their explosive potential, dome collapse, and associated pyroclastic density currents generation [5,14]. Yet, lava domes are often tied to the volcanic summit, which is often obscured by frequent cloud cover. SAR systems, in turn, penetrate this cover, and hence may significantly aid in identifying dome growth processes. Here, we analysed the amplitude information of TSX data and employed a pixel offset tracking technique to estimate deformation at Bezymianny during the 2016–2017 eruption series. However, specific steps within the processing chain may have substantial

influence on the distribution of pixel offsets. Speckle, for instance, causes random noise in the amplitude information that may result in the occurrence of randomly distributed offsets. On the other hand, speckle on surfaces also increases the tracking quality of these features. Multilooking (down-sampling), in turn, may significantly reduce amplitude noise. Yet, it also decreases the spatial resolution and may lead to the omission of small scale pixel offsets, such as the episodically detected distension of the dome or smaller differential offsets during the precursory episode. Additionally, strong scatterers within the cross-correlation window may obtain a high displacement weighting that dominates the whole patch. This causes the appearance of patch-like offsets, where the strong reflector related offset propagates throughout several overlapping and adjacent windows [52]. Patch like offsets occur, for instance, outside the 1956 crater rim (Figure 3e–h), or in the azimuth offset map of the northern dome between images 31 December 2016–2 February 2017 as offsets decrease stepwise downslope (Figure 5b3 and Figure S5). The latter may be caused by bright scatterers at the edges of older lava flows located along the northern dome flank. In addition, offset tracking between May and September 2016 showed that larger cross-correlation windows (64 and 96 pixels) detected near continuous range offset rates with minor errors, whereas the smallest window (32 pixels) revealed varying offsets with much larger errors (Figure 3i and Figure S4). Also, calculated SNRs do not sufficiently aid in the identification for erroneous offsets as both low and high SNRs were calculated for offsets close to zero in the stable areas. The latter is most prominent during the summer months where SNRs in stable areas are temporarily significantly higher, while SNRs in the summit region do not reveal a considerable change (Figure S4). However, other potential offset tracking error sources may result from changing amplitudes that are related to slope processes (e.g., gravity driven toppling rocks as moving bright scatterers), downslope block addition onto lava flows after its emplacement increasing the surface roughness, changing amplitude values due to intermittent snow cover [46], or layover effects as observed at Cleveland volcano [16]. Layover effects may be observable at the 1956 collapse scar rim, but they could not be observed at the summit. Other limitations in pixel offset tracking occur when pixels disappear due to strong motion as observed at the front of flow 1, or when surfaces are shifted into foreshortening areas as observed at the summit crater rim during the precursory plug extrusion episode. Despite the error sources, the method enabled the detailed quantification and analysis of exogenous and endogenous growth stages at Bezymianny.

#### *5.2. Implications and Interpretations of Eruptive Events*

The observations from the TSX data allowed us to identify different stages of ground motion activity and to distinguish processes from plug extrusion over endogenous dome deformation to lava flow emplacement. This enables us to derive a conceptual model of volcanic growth at Bezymianny.

#### 5.2.1. Precursory Deformation

After approximately four years of quiescence, our amplitude data reveal persistent range motion at Bezymianny's summit seven to nine months prior to the first documented effusive December 2016–March 2017 eruption. We associate this motion with the extrusion of cold crystalline upper conduit material (plug) along a pre-existing, reactivated fracture network (Figure 7a, Table 1). Initially, the plug extrusion was characterised by intermittent offsets and few detected seismic events, which may constrain the extrusion onset to January–April 2016. The subsequently derived range offset rates remained near constant until August 2016, whereas the seismicity of Bezymianny could not be differentiated from that of the active Klyuchevskoy volcano. Moreover, the plug extrusion was accompanied by observed intermittent degassing (cf. Figures S1 and S7), which, in contrast to the observed continuous range offset rates, might indicate a discontinuous precursory plug-extrusion behaviour. Yet, alternating weather conditions, such as daily changing wind directions and atmospheric pressures, may have had major impact on the irregular degassing pattern [53]. Previous seismic and petrographic studies at Bezymianny, in turn, showed that the rising magma is being stored at different

depths prior to eruptions, which may also reflect alternating emission patterns [18,20,22,54] during the 2016 precursory stage.

**Figure 7.** Schematic sketch of endogenous and exogenous growth episodes at Bezymianny. (**a**) Plug extrusion accompanied with degassing through a pre-existing, reactivated fracture network. Circles indicate gas pressurisation, ellipses show shearing at the conduit walls. (**b**) December 2016 lava flow emplacement and mixing of flow with precursory plug material causing significant compressional folding. The remaining magma batch in the upper conduit starts to solidify. (**c**) The new plug clogged the vent and (**d**) deflected the rising magma into distinct parts of the carapace. These mechanisms may account for all detected distension episodes. (**e**) Gas pressurisation exceeds yield strength of flow 1, and the previously formed new plug extruded. (**f**) Lava replenishment pushed the new plug and flow 1 westwards, which caused enhanced fracturing of the lava flow.

However, between September–October 2016, we observed a gradual change from slower to faster plug extrusion rates, which may have been related to the gradual ascent of magmatic fluids into shallower reservoirs. The simultaneously observed Mimatsu-derived summit growth, as well as new radar shadows at the rim, may have formed due to the presumably related increased gas pressurisation, which eventually pushed volcanic material over the rim, and stress-parallel oriented shadow-casting tensile fractures were formed at the rim. During November 2016, the observed seismicity and extrusion rates in range direction increased simultaneously, and the plug-related radar shadow was bisected by brighter surface reflectivity. This may indicate plug disintegration due to exhumation and concurrent loss of the previously existing circumferential pressure that was induced by the surrounding dense composite dome crust. As the largest Mimatsu displacement was determined between October–December 2016, and because the observed seismicity substantially increased during

end of November 2016 and the beginning of December 2016, we assume that the determined cumulative range motion (~40 m) constitutes the minimum of the total amount of plug extrusion.

**Table 1.** Chronology of volcanic processes at Bezymianny between January 2016 and August 2018. The ra- and az-rates correspond to range and azimuth offset rates, respectively. FOV = field of view.


In general, plug extrusions at Bezymianny were commonly observed days to weeks prior to explosive eruptions [30,31]. Similarly, satellite thermal observations showed enhanced anomalies that are associated with exogenous growth 15 to 20 days prior to >20 eruptions between 1993 and 2008 [39]. Our data set, in turn, documented seven to nine months of precursory ground motion related to plug extrusion prior to the documented effusive 5 December 2016–28 February 2017 eruption. The absence of explosive activity of Bezymianny during the effusive eruption may be caused by insufficient gas pressurisation during magma ascent likely as a consequence of persistent degassing. Similar observations were made prior to a non-explosive eruption at MSH during the dome building phase in 1981 [55]. Lastly, pixel offsets that are derived from the precursory plug extrusion episode may be used in future to refine models and investigate the corresponding source of deformation with, for example, a discrete element method, as described in [56,57].

#### 5.2.2. Effusive 5 December 2016–7 March 2017 Eruption

Our data set has shown that the so far first registered effusive eruption initiated with emplacement of flow 1, its surface characterised by few crevasses and a dominant, radar-shadow producing, flow perpendicular ridge. This ridge does not exhibit the typically observed radial orientation of surface folds of many silicic lava flows, which form as the flows stretch and rotate in the flow direction [58]. Therefore, the ridge on flow 1 could be interpreted as a result of a pronounced step in the paleotopography, but our terrain model from 2014 does not reveal any significant elevation changes on the western composite dome flank (see inset in Figure 2b). Instead, it may reflect the compressional folding [59] of a mixture of the lower viscous flow 1 with the highly viscous precursory plug material (Figure 7b and Table 1). The partial preservation of the plug would emphasize the weak explosiveness of the effusive 5 December 2016–28 February 2017 eruption.

Between the end of December 2016 and the beginning of February 2017, we observed significant unidirectional bulging of the northern dome flank, which were not reversed again and therefore can be considered inelastic. This was either accompanied or preceded by the emergence of a new radar shadow at the summit as well as enhanced seismicity during the second half of January 2017. The expansion may be explained by near vertical magmatic intrusions into the carapace without an existing plug. Yet, deformation experiments of conical shaped volcanoes have shown that, under these conditions, the summit always concurrently subsides [60].

Since summit subsidence was not observed at Bezymianny, we assume that a second plug formed between December 2016 and beginning of January 2017 that caused the unidirectional bulging (Figure 7b,c). This plug may have formed in response to low effusion rates, shallow degassing (see degassing in Figure 4c), and microlite crystallisation [7]. A process also inferred for spine formations during dome growth episodes at Unzen and Soufrière Hills volcanos [6,7]. The second plug at Bezymianny possibly clogged the upper conduit, thereby causing unidirectional bulging of magmatic fluids in the upper conduit, or the deflection of magmatic fluids into structurally weaker parts of the composite dome (Figure 7d). Layer boundaries, interlayered unconsolidated pyroclastic deposits, or pre-existing fractures related to explosive events that formed older summit craters might depict the latter. Eventually, the new summit radar shadow indicates that the second plug was extruded by the beginning of February 2017. By mid February 2017, we observed a general amplitude change at the summit, which we associate with a second pulse of lava emplaced at the summit. Simultaneously observed enhanced crevasse density on flow 1 and the location change of the plug related summit shadow indicate that the new lava flow pushed both flow 1 and the previously extruded plug westwards (Figure 7f).

As inflations were recurrently observed at the northern composite dome flank, a clogging plug may also explain the other observed distinct lateral flank movements. Lava flow emplacements followed the bulging events during February and March 2017 (including flow 2), which reveals that exogenous growth succeeded endogenous growth. Therefore, it may be possible that flank bulging between November and December 2016 also preceded flow 1. This would agree with precursory endogenous growth of Bezymianny that was derived from remotely detected enhanced thermal activity prior to most eruptions during 1993–2008 [39]. Since each bulging event of the recent eruption series increased in magnitude towards the summit (Figure 5 and Figure S5), the nucleus of unidirectional intrusions was likely located in the uppermost few hundred meters (~100–400 m) of the volcanic conduit, thus above the base of Bezymianny's composite dome (cf. Figure 1b(inset) and Figure 7). This agrees with the deformation observations at Colima in Mexico, where an inferred shallowly located (~200–300 m) clogging plug may have also governed the pathway of rising magmatic fluids prior to the volcano's 2013 eruption series [9]. Evidence of shallow conduit pressurisation that is derived from ground deformation and seismicity was also identified at other dome building volcanoes, such as Soufrière Hills on Montserrat [61], Unzen in Japan [6], or Lascar in Chile [62].

Lastly, we cannot differentiate this motion from potential endogenous growth motion, since the size of flow 1 continuously changed between adjacent amplitude scenes. Thus, we did not consider employing an elastic modelling approach to identify the source of deformation at the northern flank. In fact, the detected southward motion of the southern flank in the descending and ascending data (Figure 5b3 and Figure S11) indicates that magma may have also been deflected into the southern carapace, but this was observed only once and it occurred very localised near the summit.

### 5.2.3. Exogenous and Endogenous Dome Growth—Comparison with other Volcanoes

Volcanic activity at Bezymianny identified by TSX radar and optical data indicate precursory plug extrusion, as well as explosive eruptions, both being followed by lava flow emplacements and renewed summit excavation. All of these activities originated from the central summit. At the dome building Colima volcano, Mexico, activity between 1998 and 2010 also originated from a central summit, yet in this case the recurrent growth of blocky domes, from which short lava flows emanated, dominated the summit [63]. Soon after their formation, the summit domes were destroyed by recurrent Vulcanian eruptions that excavated new summit craters similar to the observed reshaped summit craters of Bezymianny. Yet, the repeated emergence of blocky domes at Colima contrasts with the recurrently observed plug extrusions at Bezymianny that depict the stiffened upper conduit. However, minor summit deformation at Colima's summit (2013) also suggested the existence of a shallow plug, which caused the deflection of the rising magmatic fluids, but was not extruded after all [9]. Yet, deformation at Colima only occurs days or hours prior to new eruptions [9]. Moreover, our data has shown that Bezymianny produced multiple eruptions within one year that significantly increased in explosiveness, whereas Colima produced eruptions with rather similar explosive character [63].

In addition, we showed that the recurrent bulging events of Bezymianny's northern dome flank occurred at strikingly different azimuth rates near the summit (0.1–0.6 m d<sup>−</sup>1). These were likely related to intermittent conduit plug formation, which clogged the vent as the magmatic fluids were deflected into the northern carapace. The variance of observed endogenous growth rates at Bezymianny's mature composite dome strongly contrast with observations of linear endogenous growth rates during the formation of the relatively young domes of Mount St. Helens (1980–1986) and Unzen (1990–1995), which emphasizes the very distinct character of dome growth behaviour at different volcanoes.

Overall, we have shown that Bezymianny evolved during 2016–2017 from precursory plug extrusion over mostly effusive and unidirectional endogenous growth to successively stronger explosions, which produced large amounts of pyroclastic deposits that cover most of the two major lava flows. This may point out that Bezymianny's dome evolution is on the verge to a stratocone volcano, as was described prior to the 1956 eruption. In fact, all of the observed eruptive activity during the 2016–2017 eruption sequence at Bezymianny was confined to the central summit crater, which is a common feature for many stratocone volcanoes [64].

#### **6. Conclusions**

Here, we studied endogenous and exogenous dome growth before and during the 2016–2017 eruption sequence at Bezymianny. Multitemporal TSX amplitude imagery uncovered seven to nine months lasting precursory plug extrusion prior to the known onset of the eruption series. Deformation analysis of the ensuing effusive December 2016–March 2017 eruption revealed repeated exogenous lava flow emplacements that were accompanied and/or preceded by intermittent unidirectional bulging of the northern carapace. These events are likely related to the intermittent rapid formation of upper conduit plugs that deflected the rising magmatic fluids into the uppermost regions of the composite dome. The corresponding endogenous growth rates of Bezymianny's relatively mature dome significantly varied. Thus, dome growth at Bezymianny may have reached an advanced stage in its evolution close to the formation of a stratocone. Although endogenous growth could not be resolved by the webcam imagery, the images unveiled exogenous growth near the summit undetected by radar data. Yet, the images' poor resolution only contributed qualitatively to inferences that are drawn from seismic and SAR observations.

In this study, we have demonstrated the strengths of high-resolution SAR amplitude images as an effective observation tool to derive information regarding the detailed course of precursor activity as well as for differentiation of distinct lava flow surface and dome growth processes. However, as these processes rapidly changed, it becomes apparent that more frequent SAR acquisitions in different acquisition geometries would make it even more useful for real-time observations. This becomes obvious for the acquisition gaps prior the June 2017 eruption, which concealed possible precursor ground motion. In contrast, continuous seismic observations revealed a clear picture of magmatic activity and/or associated rockfalls prior to the eruption. Therefore, integration and analysis of different geophysical data sets is a vital base for the monitoring of remote volcanoes, such as Bezymianny, who poses a permanent threat to intercontinental aviation.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2072-4292/11/11/1278/s1, Table S1: Descending spotlight TSX amplitude image pairs of Bezymianny. Table S2: Ascending spotlight TSX amplitude image of Bezymianny. Figure S1: Selected clear view images that cover the 2016–2017 eruptive sequence of Bezymianny. Figure S2: Selected co-registered descending TSX amplitude images. Figure S3: Co-registration processing chain of the TSX amplitude data time-series. Figure S4: Summit range pixel offsets compared with stable areas (i.e., no deformation). Figure S5: Average azimuth pixel offset rates during December 2016–March 2017. Figure S6: Azimuth offset maps for descending acquisition intervals without northward dome bulging. Figure S7: Webcam images showing episodic degassing (cf. Figure S1) at Bezymianny during May–November 2016. Figure S8: Illustration demonstrating constraints on the Mimatsu mapping quality because of blurry edges of Bezymianny's dome. Figure S9: Ascending non-geocoded spotlight-mode TSX amplitude image acquired on 17 February 2017. Figure S10: Co-registered ascending TSX amplitude images. Figure S11: Azimuth offset map between the ascending TSX acquisitions from 17–28 February 2017.

**Author Contributions:** Conceptualization, R.M. and T.R.W.; methodology, R.M., T.R.W, S.B., and M.B.; software, R.M.; field work, R.M., T.R.W., M.B. and A.B.; formal analysis, R.M.; validation, R.M. and T.R.W.; formal analysis, R.M.; data curation, R.M.; writing–original draft preparation, R.M.; writing–review and editing, T.R.W., A.B., and M.B.; visualization, R.M.; supervision, T.R.W.; project administration, R.M.; funding acquisition, T.R.W. and S.L.S.

**Funding:** This is a contribution to VOLCAPSE, a research project funded by the European Research Council under the European Union's H2020 Programme/ERC consolidator grant ERC-CoG Q7 646858. We thank the DLR for support; the acquisition of the spot-mode TerraSAR-X data was realized through proposal GEO1505. DV-B is grateful to CONACYT-PDCAPN project 2579. This study was also supported by Scientific Research Work of Russian Academy of Science: "Complex geophysical studies of the volcanoes of Kamchatka and of the northern Kuril Islands in order to detect the signs of the future eruption, as well as forecast its dynamics with an assessment of the ash hazard to aviation", # AAAA-A19-119031590060-3.

**Acknowledgments:** We would like to thank Jacqueline Salzer and Henriette Sudhaus for the numerous discussions on the SAR processing and Bodo Bookhagen for his thoughts on the error analysis of the SAR data. Finally we thank Ilyas Abkadyrov for supporting us during field work.

**Conflicts of Interest:** The authors declare no conflict of interest.
