Investigations of Nyamuragira and Nyiragongo Volcanoes (Democratic Republic of the Congo) Using Insar

Investigations of Nyamuragira and Nyiragongo volcanoes (Democratic Republic of the Congo) using InSAR

Sarah Colclough (1) (1) Cambridge University, Earth Sciences, Cambridge, CB2 3EQ, United Kingdom

ABSTRACT Nyamuragira and Nyiragongo are neighbouring volcanoes in the DRC (Democratic Republic of the Congo), and are amongst the most active and dangerous in the world. Eleven ERS SAR scenes were obtained, spanning the period between Jun 1997 - Jan 2003, and were combined to form interferograms and associated coherence images. During this period, Nyamuragira erupted four times (in Oct 1998, Jan 2000, Feb 2001 and Jul 2002), and Nyiragongo erupted in Jan 2002.

The coherence images enabled mapping of lava flows, and based on assumed flow thicknesses, minimum erupted volumes were estimated to be 22 × 106 m3, 71 × 106 m3 and 133 × 106 m3, for the 2002 Nyiragongo, and 1998 and 2001 Nyamuragira eruptions, respectively. SRTM data were used to remove the topographic signal from the interferograms, revealing previously undetected volcanic deformation signals over both long (years) and short (weeks) time-scales. Persistent subsidence in Nyamuragira’s NE flow field was attributed to flow cooling and substrate relaxation, and localised deformation within Nyamuragira’s summit caldera and NE flow field, were interpreted to result from shallow magma transport. Inflation on the NW flanks of Nyamuragira was interpreted as a response to magma accumulation prior to the 2002 eruption.

This study shows that InSAR can provide insights into the behaviour of these otherwise little studied volcanoes, and can usefully support hazard assessment. The study also emphasises the value of InSAR, even when applied to densely vegetated volcanoes in the humid tropics.

1. INTRODUCTION 1.1. Geographic setting Nyamuragira and Nyiragongo are situated within the Western branch of the East African Rift, and are the western-most, and only recently active volcanoes of the Virunga Mountains. Nyamuragira is a shield volcano and Nyiragongo is a strato-volcano (Fig. 1). Historical eruptions have mostly originated from the numerous flank fissures and cinder cones, and less frequently from the summit calderas. Activity is characterised by effusion of basic magmas, and lava lake activity has also occurred at both volcanoes. The lavas are of low viscosity, resulting in flows that extend for tens of kms from the summits. The city of Goma lies just 18 km south of Nyiragongo, and other smaller towns, villages and refugee camps also lie within close proximity, meaning that the hazards posed by these volcanoes threaten a large population [1].

Nyamuragira Nyiragongo N

Lake Kivu Goma

Fig. 1. August 1987 Landsat scene (RGB: bands 5,4,1), draped over SRTM. Vertical exaggeration x4. 1.2. Data selection and interferometric processing ERS track 228, frame 7155, provided the most complete coverage of the study area, and eleven raw SAR scenes were obtained (Table 1). Scenes were acquired during ascending night time passes, minimising atmospheric contamination. Perpendicular and temporal baselines and DCF variability of potential InSAR pairs were considered to minimise decorrelation, and time spans of potential pairs relative to eruptions were taken into account. Initial results revealed excellent coherence over barren lava surfaces, and limited coherence over vegetated areas and steep upper slopes of the volcanoes. For each interferogram, the overall coherence quality was a trade-off between the temporal and perpendicular baselines, and DCF variability (Table 1). Twenty-five InSAR pairs possessed sufficient coherence, and were processed using GAMMA software. The interferograms were multi-looked at a 2:10 ratio, and the GZW branch cut unwrapping algorithm with a coherence threshold of 0.3 was used. Dense vegetation resulted in isolated patches of coherence, which despite modifications to the processing technique [2] could not be bridged together during unwrapping, without generating artificial phase ramps. Hence, the isolated areas were unwrapped individually. The two-pass technique was employed to obtain the differential interferograms, whereby SRTM data were used to remove the topographic signal.

Table 1. Interferograms processed and their properties. (Mean coherence values were extracted from the same location in each image).

Perpendicular Temporal Absolute DCF Mean Interferogram pair Baseline (m) Baseline (days) Difference (Hz) Coherence 04Jun’97-22Oct’97 23 140 0.0 0.56 04Jun’97-10Nov’98 102 524 153.6 0.55 04Jun’97-11Nov’98 233 525 0.0 0.53 04Jun’97-06Sep’00 98 1190 355.3 0.53 22Oct’97-10Nov’98 79 384 153.6 0.50 22Oct’97-11Nov’98 256 385 0.0 0.45 22Oct’97-29Jan’03 154 1925 1690.4 0.41 10Nov’98-11Nov’98 335 1 153.6 0.68 10Nov’98-06Sep’00 200 666 201.7 0.54 11Nov’98-19Apr’00 347 525 88.3 0.43 11Nov’98-06Sep’00 135 665 355.3 0.53 11Nov’98-20Dec’00 409 770 175.9 0.46 19Apr’00-06Sep’00 482 140 267.0 0.28 19Apr’00-20Dec’00 62 245 87.6 0.54 19Apr’00-29May’02 52 770 902.4 0.29 06Sep’00-20Dec’00 544 105 179.4 0.28 06Sep’00-13Feb’02 108 525 821.7 0.29 06Sep’00-03Jul’02 46 665 298.9 0.44 20Dec’00-13Feb’02 436 420 1001.1 0.32 20Dec’00-29May’02 10 525 814.8 0.30 13Feb’02-03Jul’02 62 140 522.8 0.47 13Feb’02-29Jan’03 75 350 513.3 0.24 29May’02-03Jul’02 488 35 336.5 0.43 29May’02-29Jan’03 501 245 699.7 0.18 03Jul’02-29Jan’03 13 210 1036.1 0.38

2. METHODOLOGY 2.1. Lava flow mapping Lava flows erupted onto previously vegetated land were revealed as newly coherent areas. However, for newly erupted flows emplaced upon older flows, the contrast in coherence was less marked, and it was not always possible to distinguish their extent/edges. Coherence images were resampled to a UTM projection, and flow areas calculated by addition of the pixel areas comprising the new flows. Mean thicknesses of 1.0, 1.5 and 3.0 m were assumed for Nyiragongo pāhoehoe and ‘a‘a flows [3a], and Nyamuragira flows [4 and 5], respectively.

2.2. Deformation mapping It was assumed that the only contributions to the differential phase signals were true ground displacements, so that each displacement fringe represents 28.3 mm of relative range change in the satellite LOS direction. The 23º ERS nominal look angle means that interferograms are more sensitive to vertical than horizontal deformation, and it was assumed that all movement was vertical. Deformation signals were analysed quantitatively. 3. RESULTS 3.1. Lava flow mapping Three cases of new lava flow emplacements were revealed. Flows to the S of Nyiragongo first appear in InSAR pairs with the earliest scene acquired on or after Feb 2002. The only extra-caldera Nyiragongo eruption between 1997-2003 was in Jan 2002, and emplaced flows on the S flanks. Flows NW of Nyamuragira show coherence for pairs with the earliest scene acquired on or after Nov 1998, and flows to the N and SE of Nyamuragira first appear in pairs with the earliest scene acquired on or after Feb 2002. By similar reasoning to the above, these flows were attributed to the Oct 1998 and Feb 2001 Nyamuragira eruptions, respectively. Associated flow areas that were not evident in the earliest InSAR pair following an eruption, but became coherent in later pairs, were mapped as ‘a‘a flows, where the delay in coherence was attributed to postemplacement rotations and compactions [e.g. 6]. Fig. 2 illustrates the mapped areas, and Table 2 lists the areas and volumes. Flows to the SE of Nyamuragira reveal coherence in pairs for which the earliest and latest scenes were acquired after the 2000 and before the 2001 Nyamuragira eruptions, respectively. These flows can be attributed to the 2000 Nyamuragira eruption, but poor coherence quality precluded mapping.

2a 2b 2002 Nyiragongo pāhoehoe flows

2002 Nyiragongo ‘a‘a flows

2001 SE Nyamuragira flow

2001 N Nyamuragira flow

1998 NW Nyamuragira flow

2c Coherence image RGB combinations: Fig. 2a. R:13Feb’02-3Jul’02, G:29May

’02-3Jul’02, B:29May’02-3Jul’02

Fig. 2b. R:19Apr’00-20Dec’00, G:13Feb

’02-3Jul’02, B:13Feb’02-3Jul’02

Fig. 2c. R:4Jun’97-22Oct’97, G:13Feb

’02-3Jul’02, B:13Feb’02-3Jul’02

Fig. 2. Nyiragongo and Nyamuragira mapped lava flow areas superimposed on coherence images. Fig. 2a: 2002 Nyiragongo lava flows. Fig. 2b: 2001 Nyamuragira N and SE flows. Fig. 2c: 1998 Nyamuragira NW flows.

Table 2. Mapped lava flow areas and calculated erupted volumes (to 1 and 0 d.p., respectively).

Surface area (m2) Erupted volume (m3) 2002 pāhoehoe flows 8.6 × 106 9 × 106 NYIRAGONGO 2002 ‘a‘a flows 9.1 × 106 14 × 106 2002 Total 17.7 × 106 22 × 106

1998 NW flow 23.7 × 106 71 × 106 2001 N flow 15.6 × 106 47 × 106 NYAMURAGIRA 2001 SE flow 28.7 × 106 86 × 106 2001 Total 44.2 × 106 133 × 106

3.2. Deformation mapping Significant areas of deformation included: a large area of persistent subsidence within the NE Nyamuragira flow field, inflation and deflation signals within Nyamuragira’s caldera and inflation to the NW of Nyamuragira’s caldera (Fig. 3). Surface deformation rates were calculated for transects taken along the NE flow field (Fig. 3a) and the summit caldera (Fig. 3b). Two examples of the resultant deformation rate profiles are depicted in Fig. 4.

3a 3b 2.83 cm Fig. 3a. NE Nyamuragira flow field A1 transects, superimposed on the 22Oct’97-10Nov’98 differential inter- A2 ferogram. UTM projection. 0 B2 Range Change B1 Fig. 3b. Nyamuragira’s caldera transects, superimposed on the 06Sep’00- 3c 20Dec’00 differential interferogram. SAR geometry. Circle A

Fig. 3c. Inflation NW of Nyamura- gira’s summit for the 03Jul’02- 29Jan’03 differential interfereogram. Circle B Circles A and B represent potential continuations of the inflation fringes, with the crosses representing the deformation source centres at the 0 4 km 0 2 km surface. UTM projection.

Transects through the NE Nyamuragira flow field (Fig. 3a, 4 and [2]) revealed the following trends: (1) For longer time-period interferograms, a generally decreasing subsidence of ~1-4 cm/yr of the flow interiors relative to the margins. (2) A consistently larger variation in deformation rates for shorter time-period interferograms. (3) A generally opposite sense of motion for the 6Sep’00-20Dec’00 pair, compared to the other interferograms.

Transects through Nyamuragira’s caldera (Fig. 3b, 4 and [2]) revealed the following trends: (1) For longer time-period interferograms, transect B1-B2 revealed a caldera wide subsidence of 1-6 cm/yr (Fig. 3b). Elevated deformation rates correlate with the 1938 crater rim and the 1989 SE caldera fissure [3b]. Transect A1-A2 revealed higher deformation rates, although no consistent trends were observed [2]. Elevated deformation rates correlate with the pit crater and caldera rim [3b]. (2) A consistently larger variation in deformation rates for shorter time-period interferograms. (3) A generally opposite sense of motion for the 6Sep’00-20Dec’00 pair, compared to the other interferograms.

1-1.5 fringes (2.83–4.24 cm) of relative inflation over a distance of ~1 km to the NW of Nyamuragira’s summit was revealed by the 3Jul’02-29Jan’03 pair (Fig. 3c), and about half this amount for the 13Feb’02-29Jan’03 pair. It was assumed that the arc of inflation fringes constitute part of a larger concentric pattern around the caldera, with the absence of any inflation SE of Nyamuragira providing a maximum extent for the conjectured inflation fringes. Two potential deformation source centres are depicted in Fig. 3c. Simple Mogi modelling [7] indicated that the inflation could not be accounted for by a spherical deformation source beneath either location [2].

Fig. 4. Topographic profiles and surface deformation rates for the NE Nyamuragira flow field along transect H-H’ (left), and for Nyamuragira’s caldera along transect B1-B2 (right). The shorter time-period interferograms revealing higher deformation rates are plotted below the longer time-period interferograms.

4. DISCUSSION 4.1. Lava flow mapping The 22 × 106 m3 volume of the mapped 2002 Nyiragongo lava flows is of the same order as the 20 × 106 m3 GVN estimate [3c] and also the 30 × 106 m3 bulk erupted volume (lava and ash) estimate [3d] (which was based on the new level of Nyiragongo’s summit crater floor after its post-eruptive collapse). These estimates are very similar to the 20-22 × 106 m3 volume estimated for the 1977 Nyiragongo eruption [8 & 9]. Both the 1977 and the 2002 eruptions were short- lived (hours to days), and involved the draining of the summit lava lake. The volume estimates suggest a maximum capacity of ~20-25 × 106 m3 for the lava lake, before the crater’s yield strength is approached.

1901-91 Nyamuragira erupted volumes vary between 3-200 × 106 m3 (with an average of 80 × 106 m3 ) [4], which are of the same order as the 71 and 133 × 106 m3 volumes found here for the 1998 and 2001 eruptions. The mapped areas provide minimum estimates for the erupted volumes, since lava flows emplaced over pre-existing flows could not be distinguished. Reference [5] identified a 2.5-fold increase in the magma supply rate since 1980, achieved mainly by a reduction in the repose period between eruptions (a trend that continues within the time-period of this study).

4.2. Deformation mapping The NE flow field constitutes many superimposed flow units; most recently flows from the long-lived 1991-92 eruption. Hence the persistent and gradually decreasing subsidence was interpreted as due to ongoing cooling contraction and substrate relaxation due to loading. The presence of one or more shallow magma bodies beneath the NE flow field was invoked to explain localised deformation superimposed on the overall subsidence profiles. Deflation due to magma withdrawal or cooling can account for some of the observed subsidence, whilst inflation episodes prior to the 2000, 2001 and 2002 eruptions may result from the injection of fresh magma. A shallow magma system was also invoked to explain much of the deformation within Nyamuragira’s caldera.

The high deformation rates recorded by the shorter time-period interferograms for both the NE flow field and the caldera, can be related to particular pre-, syn-, or post-eruptive episodes. For example, inflation recorded by the 6Sep’00-20Dec’00 pair can be interpreted as a response to magma accumulation in the months prior to the 2001 eruption. That the 6Sep’00-20Dec’00 inflation profiles mirror the 19Apr’00-6Sep’00 subsidence profiles, suggests that during this time, pressure increases and decreases within the underlying magma body were preferentially taken up by the same zones of weakness in the overlying shallow crust, although atmospheric contamination could not be ruled out.

Several deformation episodes result in the overall inflation of Nyamuragira’s NW upper flanks between 3Jul’02- 29Jan’03, since eruptions occurred in Jul-Sep 2002 and in May 2004. An increased inflation rate in the few weeks prior to the 2002 eruption can be anticipated (e.g. [10]), whilst seismicity [3e] suggests that inflation may also result from a build up of magma following the 2002 eruption. In addition, deflation during the 2002 eruption due to lava extrusion is probable. The inflation cannot be accounted for by the standard Mogi model at either of the proposed locations, but the observed inflation certainly suggests the existence of a pressure source, implying that one or more of the model’s assumptions were violated, and/or that the true extent of the inflation was not observed, and/or a different source location(s). In particular, it is likely that the source shape was more complex, which is supported by reports that the caldera was cut by three fractures during the 2002 eruption [3f].

5. CONCLUSIONS 1998 and 2001 Nyamuragira and 2002 Nyiragongo lava flows that were revealed as new areas of coherence have been mapped in finer detail than previously available, and the resultant (minimum) erupted volume estimates of 71, 133 and 22 × 106 m3, respectively, are of the same order as historical eruptions. Persistent and gradually decreasing subsidence in the NE flow field was explained in terms of cooling of the 1991-92 lavas, and substrate relaxation. Localised inflation and deflation signals in the NE flow field and summit caldera were considered to result largely from shallow magma transport, and the inflation observed NW of Nyamuragira summit caldera was interpreted to be due to pre-2002 eruptive inflation.

Political and civil unrest in the region has limited geophysical and ground surveys in recent years, and this study demonstrates the potential for InSAR to provide valuable insights into the behaviour of these volcanoes.

ACKNOWLEDGEMENTS: This study was funded through the U.K. Natural Environment Research Council

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