Surface height adjustments in pyroclastic-flow deposits observed at Unzen volcano by JERS-1 SAR interferometry

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Journal of Volcanology and Geothermal Research 125 (2003) 247^270 www.elsevier.com/locate/jvolgeores

Surface height adjustments in pyroclastic-£ow deposits observed at Unzen volcano by JERS-1 SAR interferometry J.P. Matthews a , H. Kamata b , S. Okuyama a , Y. Yusa c;
, H. Shimizu d a Department of Geophysics, Faculty of Science, Kyoto University, Kyoto 606-8502, Japan School of Earth Sciences, Faculty of Integrated Human Studies, Kyoto University, Kyoto 606-8501, Japan c Institute for Geothermal Science, Kyoto University, Noguchibaru, Beppu 874-0903, Japan Shimabara Observatory, Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shinyama, Shimabara 855-0843, Japan b

d

Received 27 February 2002; accepted 20 March 2003

Abstract Pyroclastic flows from the 1990^1995 eruption of Unzen, a dacitic volcano in the southwest of Japan, descended the mountain along a variety of routes causing widespread damage and loss of life. Interferograms constructed from JERS-1 L-band Synthetic Aperture Radar (SAR) images show a number of features related to these pyroclastic flows and their secondary effects. The most useful interferogram in this respect is based on images acquired on 22 July 1993 and 1 December 1993 and shows the descent paths for pyroclastic flows occurring in four valley systems within this time window as zones of decorrelation caused by the repeated resurfacing. The 22 July 1993 SAR image was, through considerable good fortune, acquired only 2.6 days after a major pyroclastic flow had descended into the Mizunashi valley so that, in the absence of rainfall, the fresh 2-m-thick deposits were dry when first imaged. The largest differential surface height changes observed in the interferogram represent height decreases in the vertical of V12 cm and, significantly, lie within a small region of the Mizunashi valley which was resurfaced by the pyroclastic flow of 19 July 1993 but not subsequently. Within this small region, radar coherence is higher (maximum correlation value of V0.75) in a center-valley site where ash but relatively few large boulders are present. In a qualitative sense, the new ash surfaces exhibit higher levels of radar coherence than the older (pre-19 July) deposits. In other Unzen valleys visited by pyroclastic flows, smaller differential surface height decreases (V4 cm) are observed where the surface deposits were emplaced by events taking place between 1^3 months before the acquisition date of the 22 July 1993 image. The ‘extra’ V8 cm of surface height decrease observed in the case of the freshly laid Mizunashi deposits must result from a deflationary mechanism (or mechanisms) operating in a spatially uniform manner in order for radar coherence to be maintained. A number of possible causative mechanisms including hydroconsolidation initiated by the seepage of rainwater into a newly deposited ash sheet, deflation on gas release, basal erosion, settlement and substrate relaxation are discussed here. Given the relatively low emplacement temperatures of the Unzen ash (300^ 500‡C), it is unlikely that viscous deformation within the body of the fresh ash deposits is responsible for the subsidence. Comparison of the 22 July 1993^1 December 1993 interferogram with a LANDSAT 4 Thematic Mapper image of Unzen, acquired on 10 January 1994, highlights a ribbon-like zone of radar decorrelation running along the course of the Mizunashi valley which marks the path of debris flows that occurred within the time interval between

* Corresponding author. Tel.: +81-977-22-0713; Fax: +81-977-22-0965. E-mail address: [email protected] (Y. Yusa).

0377-0273 / 03 / $ ^ see front matter G 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-0273(03)00112-4

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the two SAR images used in the construction of the interferogram. Overall, these results demonstrate the utility of interferometric SAR-derived surface height and coherence measurements for monitoring the pyroclastic- and debrisflow regimes of active volcanoes. G 2003 Elsevier Science B.V. All rights reserved. Keywords: pyroclastic £ow; SAR interferometry; Unzen volcano; debris £ow

1. Introduction Active microwave remote sensing systems employing the Synthetic Aperture Radar (SAR) technique possess a number of advantages over passive remote sensing methods in the visible or infrared wavebands. One such advantage is the capability of microwave radar for all-weather ob-

servations conducted at any local time. Another advantage derives from the fact that a SAR system receives complex backscattered echoes from each point on the Earth’s surface. The amplitude data from these echoes can be used to generate terrain images in the normal way. However, the phase data from two SAR images can be combined through the interferometric SAR (InSAR)

Fig. 1. Aerial view of Unzen volcano taken on 29 August 1992, looking roughly westward. Fresh pyroclastic-£ow deposits are clearly visible as white aprons within the Akamatsu (AK), Mizunashi (MN) and Oshigadani (OS) valleys. Key to other features: CR, crater rim; MY, Mayuyama; KK, Kita^Kamikoba; RC, course of the Mizunashi river; OR1, OR2, over£ow reservoirs; SH, outskirts of Shimabara city.

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technique to form the basis of a powerful new remote sensing tool. For a satellite SAR platform which nearly repeats its orbital path, phase data gathered at two di¡erent times can be used in the construction of an interferogram if the separation baseline between the e¡ective imaging points does not exceed a critical value beyond which phase coherence is destroyed. The form of the resulting interference pattern is in£uenced in principle by a ‘£at earth’ component governed by the orbital geometry of the twin observations, a component related to the local topography of the planetary surface being imaged and a third term produced by di¡erential surface motion between the times of the two SAR images. Evaluation of the latter term reveals differential surface motion in the line-of-sight direction to centimetric accuracy. This capability of InSAR has been employed in a wide range of geophysical research applications (e.g. Gabriel et al., 1989; Massonnet et al., 1993; Goldstein et al., 1993). For the method to work e¡ectively, a number of constraints must be satis¢ed which stem from the need to maintain coherence between the two images (Zebker and Villasenor, 1992). Germane amongst these, from the perspective of the present paper, is the fact that any resurfacing taking place between the acquisition times of the images (such as that due to pyroclastic £ows, lava £ows, snowfall, etc.) will essentially cause the system to decorrelate (Zebker et al., 1996). Given the rather stringent conditions required for coherence, the success of InSAR in detecting small surface motions has been remarkable. One striking demonstration of the power of this technique has been in the ¢eld of volcanology, through the observation of pre- and post-eruptive volcano distortions. Prior to the onset of activity, the accumulation of large volumes of magma generally cause an in£ationary distortion of the

Fig. 2. (a) Location of Unzen volcano (southwest Japan) in relation to the Quaternary Volcanic Front (QVF), the Nankai Trough and the islands of Kyushu (K), Shikoku (S) and Honshu (H). (b) The Shimabara Peninsula of Kyushu (southwest Japan). The Unzen study area enclosed in the black rectangle represents the area covered by Fig. 3a,b.

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£anks of the volcano, whereas discharge of the magma on eruption can result in a contractile de£ation. Typically, the line-of-sight displacements involved in these processes are considerable and lie well within the InSAR measurement capability. As a result, both volcano in£ation and de£ation episodes have been identi¢ed by SAR interferometry (Massonnet et al., 1995; Lu et al., 2000a). In addition, Jonsson et al. (1999) reported local surface deformations caused by lava intrusion into a subsurface dike. Other surface manifestations of active volcanism have also been investigated using the InSAR technique. In particular, lava £ows and their related e¡ects have been considered in a number of studies (e.g. Zebker and Villasenor, 1992; Coltelli et al., 1996; Rosen et al., 1996; Zebker et al., 1996; Lu and Freymueller, 1998). Attention here has often centered on radar coherence, or rather the absence of it. Zebker et al. (1996) monitored the motion of active lava £ows on Kilauea volcano, Hawaii, by analyzing correlation coe⁄cients derived from the SIR-C Shuttle Imaging Radar. They were able to follow the progress of fresh lava breakouts over a 3-day period by identifying new regions of decorrelation caused by the resurfacing. More recently, Lu and Freymueller (1998) used ERS-1 data to plot the seasonal and temporal changes of radar coherence for a number of volcanic surface types including fresh and weathered lavas. InSAR observations of di¡erential surface height change due to the post-emplacement compaction of lava were reported by Briole et al. (1997) and Sigmundssen et al. (1997). In the former case, the rates of subsidence measured at Mt. Etna in ERS-1 interferograms maximized at 4.7 cm/yr, though not all of this signal could be attributed to compaction within the lava itself. Using a model based on a 1-D Maxwell viscoelastic medium, Briole et al. (1997) concluded that 25^

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50% of the height decrease could be the result of relaxation of the substrate taking place on a predicted timescale of V3.5 years. For Kra£a volcano in Iceland, the ERS-1 interferograms analyzed by Sigmundssen et al. (1997) indicated compaction rates of V6 mm/yr in lava ¢elds more than 8 years old. The deformation in this case could be explained by the contractive e¡ect of cooling in (50 m) thick lavas at a rate of 12‡C per year. Investigation of active pyroclastic-£ow regimes by the InSAR method has, in contrast to the study of lava £ows, received relatively little attention. One obvious area of application would make use of the capability of InSAR for generating updated digital elevation models which are important input elements in models of pyroclastic-£ow and lahar prediction (e.g. Iverson et al., 1998). However, as we show in this article, the InSAR approach can also provide a more direct means of detecting active pyroclastic-£ow paths through the identi¢cation of zones of decorrelation caused by resurfacing for events that take place within the InSAR time window. In addition, the study of the processes of compaction, ash deposition and lahar development can bene¢t from the height change and coherence data obtained by this technique. Within the vicinity of an active volcano, it would normally be di⁄cult (in terms of time, expense and danger) to obtain comparable levels of information by conventional ¢eldwork. Here we report the results from a satellite-based InSAR study of Unzen volcano, southwest Japan (Figs. 1 and 2a,b), which covers a period when pyroclastic £ows, generated through the collapse of gravitationally unstable out£ows from a growing lava dome, were descending the mountain. The study compliments and extends earlier work using aircraft-borne SAR in this region (Ishikawa et al., 1994). All radar data used here were ob-

Fig. 3. (a) Height contours with 100-m intervals in the vicinity of Unzen volcano as derived from the Nippon III topographic map database. Note the rotation relative to Fig. 2b, with north now to the left. A distortion e¡ect has been included here to mimic the typical range foreshortening found in JERS-1 SAR images. Key: a, active dome region; CR, crater rim; MY, Mayuyama; NK, Nakao valley; OS, Oshigadani valley; MN, Mizunashi valley; AK, Akamatsu valley. (b) JERS-1 SAR amplitude image of the area covered by the range-foreshortened map of Fig. 3a. These data were acquired on 1 December 1993 and are presented for comparison with Fig. 3a.

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tained by JERS-1, a spacecraft operated by the Japanese National Space Development Agency (NASDA). The L-band SAR carried by JERS-1 has provided data suitable for interferometry in a number of geophysical applications (Rossi et al., 1996; Murakami et al., 1996; Fujiwara et al., 1998). The use of L-band o¡ers a major advantage in that the observations are much less susceptible to temporal decorrelation than at C-band. It is interesting to note that Zebker et al. (2000) found the quality of the ERS-1 C-band interferometric data taken over a 70-day period from 1 September 1992 to be too poor for reliable analysis of Unzen surface deformations, although coherence was maintained within a limited region on the north £ank of the volcano. In contrast, the L-band InSAR data used here have provided useful information on a number of aspects of Unzen pyroclastic £ows and their related e¡ects.

2. Geological background Unzen, a dacitic volcano located on the Shimabara Peninsula of the island of Kyushu, lies about 70 km behind the volcanic front of the Southwest Japan Arc (Fig. 2a). It is one of the most active of Japan’s volcanoes, with eruption products characterized by lava domes, lava £ows and pyroclastic material covering an area of at least 400 km2 . Unzen has been active for about 0.5 Ma. The latest eruption began on 17 November 1990 as a series of phreatic explosions following a period of precursory seismic activity (Nakada et al., 1999). By May 1991, continuous exogenous growth from an active dome (‘a’ in Fig. 3a) was taking place and the ¢rst pyroclastic £ows, generated by dome collapse, descended to the east via the Mizunashi, Akamatsu and Oshiga valleys (Figs. 1 and 3a). This eastward bias in descent paths continued for most of the eruption and thereby posed an ongoing threat to the southern suburbs of the city of Shimabara. It may, incidentally, in part explain the observation of radar coherence solely on the north £ank of Unzen by Zebker et al. (2000). Towards the end of 1991, when lava e¡usion rates peaked at V4U105 m3 d31 (Ohta et al.,

1995) numerous Merapi-type pyroclastic £ows caused by the gravitational collapse of lava blocks were observed and subsequent debris £ows were triggered by heavy rainfall. This pattern of lava discharge followed by pyroclastic and debris £ows continued late into 1992, when e¡usion rates decreased signi¢cantly. By this time, broad landform alterations within the Mizunashi, Akamatsu and Oshiga valleys had taken place. In early 1993, a second phase of activity commenced as lava discharge rates suddenly increased and pyroclastic £ows and associated debris £ows once more became frequent (Nakada et al., 1999). Pyroclastic £ows entered the Nakao valley (Fig. 3a) for the ¢rst time in May and June 1993 and destroyed both residential and agricultural areas. On 19 July 1993, a large pyroclastic £ow took place which ran out to a distance of 5.3 km eastward along the Mizunashi valley. This event is of particular interest here since it took place only 2.6 days before the acquisition date of the 22 July 1993 SAR image used in this study. It is important to note that following this large event, no further resurfacing from pyroclastic £ows took place along the lower stretch of the Mizunashi river, so that any radar decorrelation recorded there must be attributed to other processes such as resurfacing caused by debris £ows. Towards the end of 1993, with lava e¡usion rates once more waning, most of the dome growth had become endogenous in nature (Nakada et al., 1995). Activity during 1994 was at a relatively low level while during 1995 it had, in e¡ect, ceased. Overall the eruption caused some $2 billion worth of damage to property and claimed the lives of 44 people. Descriptions of the pyroclastic- and debris-£ow deposits laid down during the current eruption have been presented in Ui et al. (1999), Miyabuchi (1999), and Fujii and Nakada (1999). The latter paper provides a detailed investigation of the large pyroclastic £ow of 15 September 1991 (volume V106 m3 ), which travelled down the Oshigadani valley and into the lower reaches of the Mizunashi valley. For the present purposes, the characteristics of this £ow are assumed to be generally similar to those of the somewhat smaller event of 19 July 1993 (volume V5U105 m3 ),

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which ran out to roughly the same distance. Fujii and Nakada (1999) identi¢ed a block-and-ash component as typical of ¢lled valley regions and an ash-cloud surge component in distal parts of the £ow (see their ¢g. 1). There was, however, considerable local variation in the sizes of large blocks within the valley (see Fujii and Nakada, 1999, ¢g. 3), with some central valley areas appearing to be relatively boulder-free. Also of relevance here is the fact that the ash-cloud surge was observed to detach itself from the main body of the £ow in order to move straight ahead when the local topographic guidance changed direction. For the large 15 September 1991 event, Fujii and Nakada (1999) reported resurfacing to depths of 9 10 m for deposits in the central valley region considered in detail in this article. This is consistent with the V2 m average depth of resurfacing caused by the major pyroclastic £ows in this part of the central Mizunashi valley as calculated from the total in¢lling. Fujii and Nakada (1999) described three principal units within the pyroclastic-£ow deposits examined at a location further down the valley close to Kita^Kamikoba. Here, a layer of fall-out ash of a few centimeters thick capped a main layer of poorly sorted block-and-ash component up to 2 m in thickness which in turn rested on a 20-cm-thick bottom layer of better sorted ash. The upper V30 cm of the block-and-ash component was generally composed of a coarse-depleted better sorted ash of grain size PMd V0.2 at maximum. A discontinuous transition took place into the ¢ner top layer where values of PMd s 4 were measured. The larger blocks, when present, were composed of nonvesiculated to poorly vesiculated dacite showing mainly plagioclase, hornblende and biotite phenocrysts and ranged in diameter from several centimeters to more than 10 m (Miyabuchi, 1999). Debris-£ow deposits were moderately ¢nes-depleted and of a gray or brownish gray color. Several features such as carbonized wood fragments, gas segregation pipes and steam release attested to the elevated temperatures of the fresh ash deposits. Although blast pyrometers recorded emplacement temperatures of over 660‡C in the Akamatsu valley (Taniguchi et al., 1996), for events in the Mizunashi valley, which invariably result from col-

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lapse of the cooler eastern dome (Wooster and Kaneko, 1998), emplacement temperatures were generally in the range of 300^500‡C (e.g. Ishikawa et al., 1993a; Nishida and Mizuyama, 1998).

3. Data and data analysis The results presented here were mainly derived from an interferogram formed from images gathered on 22 July 1993 and 1 December 1993, which, of a restricted number of possible image combinations, was found to be the best in terms of overall quality (coherence) and volcanological relevance. Use has also been made of an interferogram constructed from the 18 October 1993^22 October 1996 image pair. In gathering these images, the satellite viewed the mountain from a direction close to ESE while moving towards the equator on a descending path. Since the JERS-1 SAR is an L-band device which observes the ground at a relatively large angle of incidence (V40‡), the critical baseline distance (taken perpendicular to the look direction) beyond which coherence is lost (Zebker et al., 1994) is also relatively large at V5.4 km. For the 22 July 1993^ 1 December 1993 image pair, the perpendicular baseline distance of 92.6 m lies well below the value of this key parameter. The corresponding perpendicular baseline distance for the 18 October 1993^22 October 1996 combination is 38.4 m. A second key parameter involved in the analysis procedure is the altitude of ambiguity (Massonnet and Rabaute, 1993), which corresponds to the topographic elevation change required to increment the interferometric phase by 2Z radians. Ideally, for the analysis of di¡erential surface motions, this parameter should be large (e.g. s V100 m) so that the fringes contributed by the background topography can be e¡ectively removed. For the July 1993^December 1993 and October 1993^October 1996 combinations, the values of the altitude of ambiguity calculated from the satellite orbital data are 566 m and 960 m, respectively. In contrast, only 11.8 cm of di¡erential surface motion in the line-of-sight direction is required to increment the phase by 2Z radians. The software used to combine the SAR image

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couplets interferometrically was developed by M. Ono of the Remote Sensing and Technology Center of Japan (RESTEC) and is distributed free of charge by NASDA. Following removal of the orbital fringe component, the resulting compound interferogram contains fringe contributions from both the background topography and from di¡erential surface displacements taking place between the acquisition times of the two images. Spatial averaging and/or ¢ltering are not applied at this time (or later) in order to avoid the danger of masking or even removing the subtle ¢ne scale features associated with the pyroclastic £ow activity. Using the above values for the altitude of ambiguity, it is a straightforward task to derive a simulated interferogram from a digital terrain model that can then be subtracted from the compound interferogram. The result leaves only the di¡erential surface displacement contribution together with residual noise and any e¡ects due to the atmosphere or ionosphere. The digital terrain model employed here was derived from the Nippon III database (50 m horizontal resolution) of the Geographical Survey Institute of Japan which was updated on the basis of aerial photography taken in March 1995, by which time the eruption had ceased. The vertical accuracy of these data can be considered to be better than 10 m, which for the 22 July 1993^1 December 1993 couplet would correspond to less than one part in 56.6 of a cycle of interferometric phase. This level of accuracy is quite su⁄cient for our purposes. In the error analysis budget for JERS-1 interferometry in an urban area presented by Murakami et al. (1996), an identical error in the digital elevation model gave rise to V2 cm of range error for a baseline separation distance of 1.4 km. In the cases considered here, the errors resulting from inaccuracy in the digital elevation model are

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smaller ( 6 1 cm) due to the shorter baseline distances involved. Murakami et al. (1996) determined errors from tilt and o¡set bias within their interferograms by making a comparison with height data derived from a network of GPS stations. They derived a tilt-plus-o¡set residual of V3.5 cm. If a similar value for this error is adopted here, together with a non-systematic range error caused by spatial, temporal and thermal decorrelation of V2 cm, the overall error estimate for our height data in the Unzen valleys would then become V4 cm. An alternative approach to the estimation of this uncertainty, based on simple analytic considerations, would yield a maximum value of V6 cm for the error. With the contribution from the background topography removed, the resulting interferogram formed from the 22 July 1993^1 December 1993 image combination can be displayed as in Fig. 4a, in which di¡erential surface height increase follows the direction blue to green to red. At point ‘b’ to the northwest of the dome complex (‘a’), for example, there is an indication of a slight de£ation of the volcano £ank by V6 cm within the 22 July 1993^1 December 1993 time window, when lava e¡usion rates were waning. This feature appears as a blue to red to green change which signi¢es a line-of-sight height decrease of V10 cm. In fact, a marked deformation was recorded by tiltmeters at this location at an early stage (May 1991) of the eruption by Yamashina and Shimizu (1999), which they linked to the ascent of the magma column and the lateral intrusion of a dike. However, as modeling studies to simulate this type of deformation have already been performed (e.g. Massonnet et al., 1995; Lu et al., 2000b), we do not intend to consider this small de£ation e¡ect further here. Rather, the focus in the following sections of the paper is on pyroclastic £ows and their related e¡ects and particularly

Fig. 4. (a) JERS-1 SAR interferogram of the Unzen region constructed from images acquired on 22 July 1993 and 1 December 1993. The diagram shows di¡erential surface height change in the absence of £at earth or topographic contributions. Key: a, active dome region (as in Fig. 3a); b, small de£ation event on volcano £ank; c,d,e, di¡erential surface height decreases in Nakao, Mizunashi and Akamatsu valleys, respectively; f, decorrelation along the course of the Mizunashi river. The white rectangle de¢nes the area represented in Fig. 3a,b. (b) Heavily smoothed contours of radar correlation C (see Eq. 1) overlain onto the di¡erential surface height change data presented in Fig. 4a. Contour interval = 0.3. Areas of decorrelation lie within C 6 0.3.

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the coherence and di¡erential surface height changes present in the 22 July 1993^1 December 1993 interferogram.

4. Active descent paths for pyroclastic £ows The interferogram shown in Fig. 4a contains a rich source of information on the e¡ects related to Unzen pyroclastic £ows. The analysis of this image focuses initially on the important aspect of radar coherence, since it is anticipated that resurfacing caused by pyroclastic £ows active within the 22 July 1993^1 December 1993 time window should destroy coherence locally. Coherence represents the extent to which, at any location, the interaction between the radar signals and the surface is replicated at the times of the two images selected for interferometry. This can be represented in mathematical terms by the correlation parameter, C, which is de¢ned in the normal way: C ¼ GS 1 S
2 fðGS 1 S
1 fGS2 S
2 fÞ30:5

ð1Þ

Here S1 and S2 are the two complex signals derived from the two data takes, the asterisk represents a complex conjugate, and the brackets G f represent spatial averaging (in this case over a 5U5 pixel box). The correlation values derived in this way from the SAR data were converted into a well-smoothed contour map using contour intervals of 0.3 and overlayed onto the height change data of Fig. 4a. The resulting image is shown in Fig. 4b. In discussing this image it is important to note that a completely incoherent surface, such as that produced by the moving waters of the Ariake Sea (shown at the top of Fig. 4a,b), will nevertheless yield a non-zero value of the correlation parameter C. As discussed in Zebker et al. (1996), this can be explained by recognizing the presence of a bias in the data derived from Eq. 1. The mean value of the correlation parameter for regions covered by water is 0.26 and hence the contour at 0.3 is taken as an approximate boundary marking the transition from coherent to incoherent regions. Comparison of Fig. 4a and Fig. 4b shows that areas covered by the Ariake Sea, together with

other zones of decorrelation (C 6 0.3), assume an easily recognizable speckled multi-colored appearance. Regions located in the (partial) radar shadow of mountains, such as the area below and to the right of the letter ‘b’ in Fig. 4a (which lies on the £ank of the old crater wall), also clearly ¢t into this category. A similar zone of decorrelation is present in the shadow region to the west of Mount Mayuyama (above and to the right of the letter ‘c’ in Fig. 4a). The speckled multi-colored signature can also be identi¢ed across the active dome of the Unzen complex ‘a’. The radar decorrelation here is presumably the result of the large and often chaotic surface deformations that took place at this location within the interferometric time window, a period of transition from exogenous to endogenous dome growth regimes. The region of decorrelation centered on the dome complex ‘a’ extends out in three directions which, through comparison with the digital terrain model, are seen to correspond to the pyroclastic-£ow descent paths that were visited by events within the period of analysis (radar shadowing is not involved here). The broadest and most obvious extension is oriented toward the northeast (from ‘a’ to ‘c’ in Fig. 4a) and represents the descent path feeding both the Nakao and Oshigadani valleys (see Fig. 3a). Ui et al. (1999) reported that these valleys were visited by pyroclastic £ows with runout distances of V3 km within the time window encompassed by the two SAR images discussed here. For the Nakao valley, these events brought fresh material into the upper reaches of the valley but did not completely resurface the more distant deposits laid down earlier in June 1993, when the pyroclastic £ows ran out further ( s V4 km). Other extensions to the zone of decorrelation centered on the dome ‘a’ can also be readily identi¢ed as pyroclastic-£ow descent paths that were active during the 22 July 1993^1 December 1993 period. One of these branches out in an ESE direction toward the feature labelled as ‘d’ in Fig. 4a, and corresponds to the Mizunashi valley descent path. As with the Nakao valley, pyroclastic£ow runout distances here were relatively short within the period of analysis so that only the

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upper reaches of the valley above feature ‘d’ were resurfaced. A third (and much shorter) zone of decorrelation emanating from the dome corresponds to the Akamatsu valley pyroclastic-£ow descent path. This is directed towards, but stops short of, the red-colored regions labelled as ‘e’ in Fig. 4a. Within this interferometric time window, relatively few pyroclastic £ows descended into the Akamatsu valley and the runout distances of the events that did take place were short ( 6 V1.5 km). As expected therefore, the branch-like extensions to the zone of decorrelation centered on the active dome correspond to the pyroclastic£ow paths that were active within the interferometric time window. Outside the regions of decorrelation caused by shadowing or resurfacing by pyroclastic £ows, the remaining data show moderate coherence with the well-smoothed correlation levels lying in the range of 0.3^0.6, except for the urban areas towards the top left of Fig. 4a, where values exceed 0.6 in magnitude. In general, the coastline derived from the 0.3 correlation contour agrees well with that obtained from the digital terrain model. However, near the mouth of the Mizunashi river (top arrow emanating from the letter ‘f’), the coastline is strongly indented. This feature relates to decorrelation resulting from resurfacing by debris £ows that took place within the interferometric time window, as will be discussed later.

5. Surface height decreases in fresh pyroclastic-£ow deposits The results of the careful monitoring program carried out jointly by the Japan Self Defense Force and the Shimabara Police Department (reported in Ui et al., 1999) show that Unzen pyroclastic £ows occurring within the interferometric time window of 22 July 1993^1 December 1993 had relatively short runout distances. For the lower reaches of the Nakao, Mizunashi and Akamatsu valleys in particular, therefore, some areas covered by fresh pyroclastic deposits from events prior to 22 July 1993 maintained the same surface throughout this time window and, as a result, ex-

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hibit varying degrees of coherence (C s 0.3) in the SAR interferogram of Fig. 4a. This is in contrast to regions covered by the very youngest material brought down within the InSAR time window (for which C 6 0.3). The InSAR di¡erential data of Fig. 4a show that, in some regions where the radar returns from fresh deposits of the recent Unzen eruption remain coherent, well-de¢ned surface height decreases take place. Considering ¢rstly the Nakao valley, the deposits here were largely laid down during the May^ June 1993 series of pyroclastic events which followed the in¢lling of the Oshigadani valley and were initiated by collapse of the north side of lobe 11 of the Unzen dome (Ohta et al., 1996). The area in the vicinity of the letter ‘c’ in Fig. 4a maintained the same surface throughout the interferometric time window and here bright red regions are present to the lower left and lower right of ‘c’, with the color change corresponding to line-of-sight height decreases of V3 cm (V4 cm in the vertical). Similar features of comparable magnitude are present in the lower Akamatsu valley to the left of the letter ‘e’ in Fig. 4a. Heavy deposition from pyroclastic £ows caused major landform alterations within the Akamatsu valley during 1992, though resurfacing of the lower valley in the latter half of 1993 did not take place. The well-de¢ned feature visible to the right of the letter ‘d’ in Fig. 4a exhibits the largest line-ofsight surface height decrease recorded in the interferogram at V9 cm, corresponding to V12 cm in the vertical direction. This is located in a region where the Mizunashi valley broadens (Fig. 3a,b) and veers gently to the right for a descending £ow. The surface deposits here were laid down by the large pyroclastic £ow of 19 July 1993 that ran out to a distance of 5.3 km along the Mizunashi valley. The delay of only V2.6 days before the young deposits from this event were imaged is signi¢cant. The fact that no rainfall fell in this period means that JERS-1 ¢rst imaged these deposits when they were in a dry, powdery state (Matsuwo, pers. commun.) which very likely corresponds to the ‘£uid’ state reported by Hoblitt et al. (1985) for young Mount St. Helens dacite £ow deposits. Hence, the InSAR-derived vertical surface height decrease of V12 cm for

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Fig. 5. JERS-1 SAR interferogram of the Unzen region constructed from images acquired on 18 October 1993 and 22 October 1996 and showing di¡erential surface height change. For key, see Fig. 4a. The white rectangle de¢nes the area represented in Fig. 3a,b.

the Mizunashi valley ash includes a contribution from the earliest phase of compaction and adjustment. Apparent surface height change signatures caused by atmospheric or ionospheric irregularities of the type considered by Massonnet and Feigl (1995) or Fujiwara et al. (1998) cannot be invoked to explain the di¡erential surface height changes under discussion here. One reason for this is that the observations of Fig. 4a show height decreases within all the major valley systems for deposits lying beyond the reach of pyroclastic £ows taking place within the 23 July 1993^ 1 December 1993 time window. For the case of the Mizunashi valley, in particular, the regions of surface height decrease are located, as might be expected, at a turn in the valley where the local gradient levels o¡ and their spatial distribution conforms to the local topography. Attempts to explain these features by events unrelated to the ground would be unrealistic. An alternative way of excluding the possibility

of an atmospheric or ionospheric in£uence is to compare interferograms spanning di¡erent time windows. Though the number of possible JERS1 interferogram pairs for Unzen is limited, a combination based on images acquired on 18 October 1993 and 22 October 1996 is suitable for the present purpose and is shown in Fig. 5. The coherence levels across most of this interferogram are low. However, a well-de¢ned di¡erential height decrease signature of V8 cm in the lineof-sight direction is present in the Mizunashi valley to the right of the letter ‘d’ (positioned as in Fig. 4a) where no resurfacing from pyroclastic £ows took place within the 3-yr time window covered by the interferogram. The height decreases at this location are the largest in the ¢gure and they take place within the same region as the changes observed near point ‘d’ in Fig. 4a. In addition, a height change signature of a few centimeters in magnitude can also be identi¢ed at this site in the Mizunashi valley in the interferogram formed from images acquired on 14 January 1994 and 30

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May 1997, although the data in this case are not shown owing to the generally heavy degradation of phase coherence. This repeated observation of surface de£ation at the same location enables us to rule out explanations based on ionospheric or atmospheric irregularities. For other valley systems emanating from Unzen, the interferogram of Fig. 5 shows relatively little by way of surface height change. During 1994, the Akamatsu valley was visited by pyroclastic £ows and so resurfacing (leading to decorrelation in the interferogram) was extensive there. However, a number of spatially small height decrease signatures of V3 cm in magnitude are present in the Nakao valley, located close to the features labelled as ‘c’ (positioned as in Fig. 4a). The runout distances of the pyroclastic £ows which descended into the Nakao valley during late 1993 and in 1994 did not exceed those of the June 1993 events and thus the areas near the letter ‘c’ did not resurface during the 18 October 1993^22 October 1996 interferometric time window. Numerical results derived from the interferograms are summarized in Table 1. The rates of surface height decrease were obtained by crudely dividing the vertical height change values by the number of days encompassed by the relevant InSAR time window. Average rates of surface height decrease of V1 mm/day or more are possible in the ¢rst few weeks after deposition, whereas on timescales of a few months to V1 year after deposition, the rates fall to 6 0.5 mm/day. Subsequently, over timescales of several years, the rate of subsidence falls to 6 0.1 mm/day. Though

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a number of factors such as seasonality of rainfall and local variations in substrate strength can bias these data, they are nevertheless valuable in the study of the post-emplacement behavior of pyroclastic-£ow deposits since their synoptic coverage facilitates comparison between the ash sheet adjustments taking place in adjacent valley systems. In addition, the capability for repeated imaging provides an indication of temporal variations in the rates of surface height change. A discussion of the data summarized in Table 1 is presented in Section 6.

6. Surface height decrease mechanisms Although the InSAR data discussed above provide new information on the surface height adjustments of freshly deposited pyroclastic-£ow deposits, the paucity of interferogram pairs and the lengthy time intervals ( s 4 months) spanned by each interferogram mean that it is not possible to use them to discriminate among the speci¢c mechanisms responsible for the subsidence, particularly since these may be acting simultaneously. Nevertheless, some useful deductions can be made on the basis of the surface height change measurements presented in Table 1. When interpreting InSAR di¡erential height change measurements, due consideration must ¢rst be given to an important restriction on surface adjustment that arises from the need to maintain radar coherence between the times of the two SAR images used in the interferometry. Given that the scattering from an individual pixel can

Table 1 Valley name

Vertical height change (cm)

Surface layer agea (days)

InSAR time window (days)

Rate of surface height Decrease (mm/day)

Mizunashi (1)b Mizunashi (2) Nakao (1) Nakao (2) Akamatsu (1)

12 V10 4 V4 4

2.6 90 28 116 95

132 1096 132 1096 132

0.9 V0.09 0.3 V0.04 0.3

a Surface layer ages are calculated from the known or estimated deposition date to the time of acquisition of the ¢rst image of the InSAR image pair. b Label (1) refers to the 22 July 1993^1 December 1993 image pair, (2) refers to the 18 October 1993^22 October 1996 image pair (symbol V indicates less reliable values).

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be viewed as the vector sum of the responses from many small scattering cells, this coherence requirement amounts, in e¡ect, to a limit on the random motion of scattering cells over the InSAR time window. To investigate this further, the treatment of interferometric decorrelation given by Zebker and Villasenor (1992) is used here, with the random motions of the scattering cells assigned standard deviations of cy and cz in the surface and vertical planes, respectively. However, in our case a mean displacement N, representing the joint vertical translation of the surface elements, is introduced. We can then derive a correlation function b as: b ¼ exp ðð38Z2 V32 ðcy 2 sin2 a þ cz 2 cos2 aÞ 3ð4ZjNV31 cosaÞÞ

ð2Þ

where V is the radar wavelength and a is the radar angle of incidence. The imaginary exponent in the third term re£ects the fact that coherence is maintained if the scattering cells move in unison in the vertical direction. The case of random motion in the surface plane alone was simulated in detail by Zebker and Villasenor (1992) who obtained the decorrelation condition cy V10 cm for the L-band SEASAT SAR (V = 23.5 cm, a = 23‡). For a situation in which the random component lies solely in the vertical plane, the condition becomes more stringent owing to the cos2 a dependence. For the similar case of JERS-1 (V = 23.5 cm, a = 35‡), some degree of independent motion between the individual scattering cells is therefore permitted so long as its magnitude is restricted, roughly speaking, to within a few centimeters. In terms of possible surface de£ation mechanisms relevant to this paper, this strict condition would rule out localized e¡ects causing substantial di¡erential motion between scattering cells, e.g. the irregular settlement of blocks in a boulder ¢eld. As a ¢rst example, consider surface de£ation resulting from the slow post-emplacement relaxation of a substrate loaded by layers of block-andash from repeated pyroclastic £ows. This type of subsidence would clearly satisfy the above coherence restriction, since all the scattering cells in a pixel would be translated together. The scenario can be examined in more detail using a simple

1-D Maxwell viscoelastic model (Turcotte and Schubert, 1982). Following Briole et al. (1997), the maximum rate of line-of-sight surface displacement due to relaxation is written as: ½dR=dtmax Vð0:25c0 h cosKÞ=ðEdÞ

ð3Þ

where c0 is the loading caused by ash deposited on a viscoelastic medium of depth h, K is the angle between the vertical and line-of-sight directions, E is Young’s modulus and the parameter d = (9W/2E) represents the Maxwell relaxation time, with W the viscosity. The value of c0 can be calculated by taking the ash deposits to be of depth V40 m, in line with estimates for the total in¢lling from the recent eruption in this part of the Mizunashi valley (Ishikawa et al., 1996). Taking a mean density of bV1500 kg m33 , this leads to a loading of cV0.6 MPa. If, for example, the relaxation takes place within a viscoelastic substrate of thickness h = 1 km and viscosity W = 6U1016 Pa, as considered by Briole et al. (1997) for the loading caused by fresh lava £ows at Mt. Etna, the maximum subsidence rate then becomes [dR/dT]max V0.04 mm/day. This is attained after the corresponding Maxwell relaxation time of dV1^9 yr, assuming a value for E in the range 109 ^1010 Pa. Primarily, the total depth of the deposits determines a relatively slow rate of subsidence for this process. The mean rate of surface height change of V0.09 mm/ day, derived from the 18 October 1993^22 October 1996 interferogram for point ‘d’ in the Mizunashi valley, may therefore include a V44% contribution from substrate relaxation. The average height change signature of V0.04 mm/day recorded in the Nakao valley within the 18 October 1993^22 October 1996 interferogram (Fig. 5; Table 1), however, can include a relaxation contribution of only V12% since the deposits at this location are of depth V5 m (Ishikawa et al., 1996). Also, in the Akamatsu valley, where successive pyroclastic £ows during 1992 created ash deposits to a total depth of V60 m, the 0.3 mm/day of surface height decrease recorded within the 22 July 1993^1 December 1993 interferogram (Fig. 4a; Table 1) is roughly equal to that recorded in the same interferogram for the shallow Nakao valley deposits. This is unexpected

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from the substrate relaxation viewpoint. In addition, this rate is considerably greater than the V0.06 mm/day anticipated from substrate relaxation calculated for loading caused by a 60 m layer of ash. These facts provide a strong indication that, within V1 year after deposition, other processes (operating most likely within the surface layer) cause enhanced rates of subsidence. This impression is greatly reinforced by the fact that the 12 cm of vertical height decrease recorded for the fresh Mizunashi valley deposits in the 22 July 1993^1 December 1993 interferogram is three times greater than that for the Nakao valley (Fig. 4a), even though the surface deposits at the latter site were emplaced only V3 weeks earlier. The fresh Mizunashi valley deposits must therefore have undergone an ‘extra’ surface height decrease of V8 cm within the time interval spanned by the interferogram. Conversely, if surface changes involving fresh ash alone are involved, the Nakao valley deposits must have undergone the ‘extra’ 8 cm of subsidence within V3 weeks following emplacement. A clue as to how such relatively rapid, shortterm surface adjustments may possibly develop can be obtained from some perceptive ¢eld observations made shortly after pyroclastic £ows. In the course of their work on the Mount St. Helens deposits, Hoblitt et al. (1985) noted a dramatic change in the rheological properties of the ash surfaces after the ¢rst rainfall following deposition, when the loosely-packed ‘£uid-like’ nature of the fresh deposits was transformed by the action of the rainwater into a more rigid structure. Similar observations were made at Unzen (Matsuwo, pers. commun., 2001). Typically, a large stone thrown onto the freshly deposited surface would sink into the body of the ash and trigger a release of gas and/or steam. Following the ¢rst rain, however, it then became possible to walk on the ash surfaces and their overall appearance in terms of color and texture approached that of the older deposits. Hoblitt et al. (1985) suggested that this change might be the result of hydrocompaction (referred to here by the alternative name of hydroconsolidation; Rogers et al., 1994), a process causing volume reductions on wetting in some high-porosity soils as the medium collapses

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to a new particle packing regime of lower void ratio (e.g. Dudley, 1970; Lutenegger and Hallberg, 1988). In certain instances, clay particles are thought to play a central role in this type of structural change (Assallay et al., 1998). Since in situ experimentation on (or attempts to remove samples of) a hot ash body under the threat of further pyroclastic £ows would be extremely hazardous, a controlled investigation of water-induced adjustments in fresh pyroclastic£ow deposits has not so far been performed. Our current understanding of hydroconsolidation is largely derived from laboratory experiments made using materials such as loess soil (e.g. Assallay et al., 1997) and under very di¡erent conditions to those existing within hot ash in the ¢eld. To illustrate this point, consider that the clay content of 2.68% found in the Unzen blockand-ash deposits (Miyabuchi, 1999) is much lower than that normally present in collapsible soils (Assallay et al., 1998) and that the respective shapes of rhyolitic glass shards and loess particles are totally di¡erent. The results from the experimental work on loess cannot, therefore, be applied with con¢dence in the present case. Clearly, new laboratory investigations are required to determine whether rain-induced hydroconsolidation indeed takes place in freshly deposited ash. Perhaps the closest approach we can presently make is to rely on the detailed laboratory work of Haruyama (1969, 1973, 1977), who investigated the properties of the collapsible ‘Shirasu’ soil, a Quaternary ash deposit found over a wide area of southern Kyushu, Japan. From the perspective of water-induced structural change through adjustments in particle packing, it is of interest to speculate whether, in spite of its temporal modi¢cation, the behavior of historical Shirasu might indicate a similar (as yet unproven) e¡ect in the fresh Unzen ash since both deposits are composed largely of volcanic glass (80 wt% for Shirasu compared to s 70 wt% for Unzen (Nakada and Motomura, 1999)). Also, for the adjusted Shirasu sample examined in Haruyama (1969), the clay content was V3%, close to the 2.68% found at Unzen, and the cumulative grain size distribution was similar to that of the ¢ner components of the Unzen ash deposits. Starting

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from an air-dried sample, Haruyama (1969) reported a maximum void ratio of 1.525 corresponding to a porosity of V60%, a value commonly assumed for fresh pyroclastic-£ow deposits (Friedman et al., 1963; Wilson, 1984). The subsequent addition of water brought on volume reductions so that, at 24 wt% water content, the void ratio had decreased to a minimum value of 0.8, corresponding to a porosity of 44.5%. This latter value is close to the porosity of 41% measured with 20-month-old Unzen deposits by Ishikawa et al. (1993b). If we make use of these limiting values of void ratio for the purposes of a primitive calculation and consider a V2-m-thick layer of fresh ash in which only the ¢ner top V30 cm can be e¡ectively involved in hydroconsolidation (Fujii and Nakada (1999), their Fig. 5), then the predicted surface height decrease becomes V10 cm. This level of surface height decrease compares with the (‘extra’) short-term change of V8 cm determined from the InSAR data if the re£ection process within each SAR pixel was dominated by returns from ash rather than from large boulders embedded within the ash. Also, the small V10cm downward translation of the scattering elements anticipated as a result of this theorized hydroconsolidation would probably take place with su⁄cient uniformity to satisfy the coherence condition discussed in connection with Eq. 2, so long as the deposits were emplaced in an even manner. A weakness with this argument is that if the short-term surface height decreases became considerably larger and/or the emplacement of the deposit took place in an irregular fashion, we may then expect to see greater levels of decorrelation as a result of the larger concomitant di¡erential motion between the scattering cells. In this sense the surface height change phenomenon reported herein may not be generally observable as the conditions for coherence are strict. Future InSAR investigations of the active pyroclastic-£ow regimes of other volcanoes will show whether this is in fact the case. An obvious prerequisite is that su⁄cient rainfall should fall to chill the newly formed ash sheet so that water-induced processes such as hydroconsolidation can then ensue. At Unzen, the

amount of rainfall required to cool a 1-m-thick sheet lies in the range of 18^66 cm (Nishida and Mizuyama, 1998). Substantial chilling must have taken place quite rapidly in the V2-m-thick deposits from the 19 July 1993 Mizunashi valley pyroclastic £ow since a total of 32 cm of rain fell between 27 July and 2 August 1993 alone and a further 30 cm fell before the end of August. Chilling must have been similarly rapid for the deposits laid down in the Nakao valley some 3 weeks earlier, since 67.3 cm of rainfall fell during the period 24 June^19 July 1993. Hence, as temperatures decrease, a possible progression of water-induced e¡ects through the surface layer of a freshly laid ash sheet could be envisaged. However, a much higher temporal resolution in the InSAR-derived surface height measurements than that available here would be required to examine this possibility in detail. The qualitative ¢eld observations made at both Mount St. Helens and Unzen also make mention of a release of gas from the fresh ash sheet in response to a disturbance of the surface. This suggests the further possibility that, with decrease of the intergranular gas pressure with time, the body of the ash could quite literally de£ate (J. Riehle, pers. commun., 2001). To investigate the magnitude of this e¡ect within the Unzen deposits, we used the ‘Compact’ program (a component of the Ashpac computational model; Miller and Riehle, 1994) with input parameters relevant to the present situation. In order to retain su⁄cient overpressure after 2.6 days to support an V8-cm increment in surface height for a freshly deposited 2-m-thick ash sheet emplaced at V500‡C, a value of gas permeability in the region of V1 md is required, compared to the nominal input value for permeability in the Compact program of 21 md. Recent experimental work at Yucca Mountain, Nevada, measured the permeability of a historical non-welded tu¡ of V30% porosity to be V10 md (Ahlers et al., 1999), indicating that at the higher porosities of fresh ash, the required permeability of V1 md would be rather low and that in fact the gas would tend to escape too rapidly for this purpose. However, formation of a con¢ning crust on the fresh deposit (A. Hurst, pers. commun.) or possibly an e¡ect asso-

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ciated with the details of the sorting could easily alter this conclusion. Here again, new laboratory measurements on freshly deposited ash are required. Surface de£ation has been known to take place at other locations through post-emplacement viscous deformation and welding in hot volcanic ash deposits. This mechanism has received considerable attention from the geological community since it has an important bearing on the interpretation of large, ancient ash-£ow deposits (Smith, 1960; Friedman et al., 1963; Riehle, 1973; Riehle et al., 1995). The surface compaction caused by such viscous deformation can be substantial, particularly in deep ash sheets emplaced at high temperatures. However, at the relatively low emplacement temperatures of the Mizunashi valley ash ( 6 500‡C), calculations made on the basis of Compact show that the extent of surface compaction caused by this process is negligible. Emplacement temperatures in excess of V600‡C would be required to generate compaction at the levels observed here. This type of viscous deformation may have played a role in the surface adjustments of ash sheets deposited earlier in the Unzen eruption, particularly for those originating from collapse of the hotter western dome (Wooster and Kaneko, 1998), though it seems unlikely to have contributed in the Mizunashi valley in July 1993. In the medium term of within V1 year after deposition, a number of other water-induced processes such as selective leaching of minerals into the intergranular water ¢lm and basal erosion might also cause surface subsidence. In spite of the research carried out at Mount St. Helens, Unzen and elsewhere (e.g. Collins and Dunne, 1986; Hinkley et al., 1987; Jitousono et al., 1997; Risacher and Alonso, 2001), the overall e¡ect of such processes on surface adjustments remains di⁄cult to quantify to the level of accuracy required here. As noted above, the synoptic coverage of the InSAR data reveals levels of subsidence in the Akamatsu and Nakao valleys in the 22 July 1993^1 December 1993 interferogram that are similar, even though the ages of the surface layers and deposit depths di¡er greatly at these locations. Within the coherence criterion, this could be construed as indicative of subsidence related

263

to leaching within surface layers, if, in the months following deposition, such changes depended primarily on rainfall levels.

7. Radar coherence and debris £ows in the Mizunashi valley The data shown in Fig. 6a provide an enlarged version of the di¡erential surface height changes recorded near point ‘d’ of Fig. 4a, overlain with contours derived from lightly smoothed radar correlation data. The highest values of correlation ( s 0.6, max. = 0.75) are recorded in the central, green-colored portion of the ¢gure where the line-of-sight surface height decrease maximizes at V9 cm. Slightly to the north (left) of this region, correlation values drop rapidly to values below 0.4 and, though not represented in the smoothed contours, to values close to the limiting value of 0.26 discussed earlier, thereby giving rise to the mixed blue, red and green colors indicative of decorrelation. The generally east^west orientation of this region of low correlation is roughly parallel to the incoming £ow direction of the 19 July 1993 pyroclastic event that was responsible for the ¢nal surface deposition. Comparison with the visible band aerial photograph shown in Fig. 6b, which was obtained on 14 September 1994, shows that the largest height changes take place in the central region of Mizunashi valley, where boulder and rubble cover (black dots and black regions respectively) is not so dense. This is consistent with the requirement stated in the previous section of a radar re£ection process dominated by scattering cells located on the ¢ne ash surface rather than on boulders or rubble. On the other hand, the region of low coherence to the north (left) in Fig. 6a corresponds, albeit in a rather imprecise way, to an area in which large boulders and rubble are quite extensive. This may point to a decorrelating tendency caused by post-emplacement settling motion of the coarser surface component. In this sense, the settling of boulders within the fresh pyroclastic£ow deposits might lead to decorrelation in much the same way as the settling of blocks in a lava ¢eld (Zebker and Villasenor, 1992; Lu et al.,

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2000a). However, at this location (along the line of direct descent) there is also the possibility that decorrelation is caused by sporadic boulder falls or by small-scale secondary activity. Further down the valley, higher values of coherence ( s 0.4) are to be found to the east, where deposition from the 19 July 1993 pyroclastic £ow was heaviest. Study of video footage taken from helicopter and aircraft platforms just after this event con¢rms, in qualitative fashion, the correspondence between fresh ash and higher radar coherence. This result may parallel the observation of higher C-band coherence for tephras with weak water reworking by Lu and Freymueller (1998). The data of Fig. 6b can be discussed in relation to the observation reported in Fujii and Nakada (1999) that the block-and-ash component of the pyroclastic £ow that descended into the Mizunashi valley on 15 September 1991 was guided quite e¡ectively by the local topography, whereas the ash-cloud surge followed a more direct path which caused it to become detached from the main body of the £ow. This type of behavior was ¢rst documented for pyroclastic £ows from the 1976 eruption of Augustine Volcano, Alaska (Kamata et al., 1991). If £ow separation took place during the 19 July 1993 event at Unzen, then we would expect the block-and-ash unit to veer to the right (roughly southeast) as it meets the intersection with the Oshigadani valley whereas the ash-cloud surge would proceed along the incoming direction. This may explain the preponderance of large boulders and rubble corresponding to a block-and-ash deposit just beyond the right turn in Fig. 6b, whereas in the forward direction, the ¢ner surface texture is probably indicative of the presence of ash-cloud surge deposits. In this context, it is valuable to compare the spatial con¢guration of the Mizunashi valley height change signatures exhibited in the two interferograms of Figs. 4a and 5. For the latter case, the height decreases maximize within two small

265

areas separated by a region of more modest height change that corresponds closely (in the spatial sense) to the zone of low radar coherence identi¢ed in connection with Fig. 5a,b. The implication here is that the processes that gave rise to a loss of radar coherence in this region in the 22 July 1993^1 December 1993 interferogram operated on a relatively short timescale, since a comparable loss of coherence is absent in the 18 October 1993^22 October 1996 interferogram. This behavior suggests a period of several months for boulder settlement and/or for secondary collapse events to terminate. It is clear from this discussion that the nature of the deposition in pyroclastic £ows and the post-emplacement history of the ash will have an important in£uence on the signatures observed in InSAR coherence and height change data. In Fig. 7b, we next consider an image of the lower reaches of the Mizunashi valley constructed from the red, green and blue channels of a LANDSAT Thematic Mapper scene acquired on 10 January 1994. The adjacent image (Fig. 7a) is a sub-section of the JERS-1 interferogram derived from the 22 July 1993^1 December 1993 couplet (i.e. Fig. 4a) and shows di¡erential surface height change as before. Comparison of the two images shows that further down the valley from point ‘d’, the path of the Mizunashi river is clearly visible in the interferogram and is indicated by the lower two arrows emanating from point ‘f’ of Fig. 7a. The river appears as a ribbon-like feature in mixed colors (signifying decorrelation) passing through a generally blue-colored region of low surface height change. At the time of acquisition of the later (1 December 1993) image of the SAR couplet used in the construction of these data, water was present in the river channel owing to the pulse of rainfall that fell in this vicinity on the previous night of 30 November 1993. Hence, the change in surface conditions from a dry riverbed at the time of the ¢rst SAR image acquired on 22

Fig. 6. (a) Close-up of the area adjacent to ‘d’ in Fig. 4a. The colors represent di¡erential surface height change as in the 22 July 1993^1 December 1993 interferogram. The lightly smoothed contours represent radar correlation C (see Eq. 1). A contour interval of 0.1 was used here. (b) Visible band airborne photograph obtained on 14 September 1994 of the area represented in Fig. 6a. The arrows indicate the direction of a pyroclastic £ow arriving from the upper Mizunashi valley.

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Fig. 7. (a) Sub-section of the 22 July 1993^1 December 1993 interferogram of Fig. 4a showing the Mizunashi valley region. The colors represent di¡erential surface height change. For key, see Fig. 4a. In addition, ‘g’ represents a region of decorrelation associated with two over£ow reservoirs (see Figs. 1 and 7b). The lower two arrows starting at ‘f’ point to a narrow zone of decorrelation which de¢nes the course of the Mizunashi river while the upper arrow points to an expanded zone of decorrelation at the mouth of the Mizunashi river. Both features ‘g’ and ‘f’ (upper arrow) can be identi¢ed in Fig. 4b. (b) Sub-section of a LANDSAT Thematic Mapper image acquired on 10 January 1994. The red, green and blue channels used here show the same region of the Mizunashi valley and its surroundings as displayed in (a). Key: h, one of two over£ow reservoirs constructed in 1992 as mitigation measures against debris £ows; i, main channel of the Mizunashi river.

July 1993, to a moving water surface on 1 December 1993, may possibly account for the observed loss of coherence along the path of the river. Given the broad spatial width of the ribbon-like zone of decorrelation, however, it is more likely that the extent of this feature represents the integrated e¡ect of resurfacing from debris £ows that took place between 19 and 20 August 1993 (i.e. within the time window spanned by the interferogram). Closer to the coast, the zone of decorrelation associated with the river broadens considerably (as indicated by the upper arrow emanating from letter ‘f’ of Fig. 7a) and seems to bifurcate with one branch directed toward the right of point ‘g’ of Fig. 7a. These features are also represented in the contoured data of Fig. 4b. Note that the area of decorrelation around the river covers a considerably greater area than does the main course of the river itself (the bright line la-

beled as ‘i’ in Fig. 7b). This area of decorrelation is, however, smaller than that inundated by the major events of 28^29 April and 2 May 1993 which were the largest debris £ows of the recent eruption. Since these events took place outside the InSAR time window, their decorrelation signatures are not present in the data of Figs. 3a and 6a. However, the swath covered by these devastating events appears as a light gray region adjacent to both sides of the channel ‘i’ in the LANDSAT data of Fig. 7b and includes a substantial area of residential land. Judged from these data, it would appear that the e¡ectiveness of the counter-measures was severely tested by the debris £ows of April and May 1993. The bright features labeled as ‘h’ in Fig. 7b relate to two over£ow reservoirs that were constructed between March and May 1992 in an attempt to safeguard residential areas close to the main river course by divert-

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ing potentially destructive debris £ows. The debris and mud washed down into these regions following heavy rain was subsequently relocated by bulldozers. As a result of repeated resurfacing, decorrelation is present in the corresponding areas to the right of ‘g’ in the interferogram of Fig. 7a.

8. Conclusions This paper discussed the use of InSAR to observe di¡erential surface height and radar coherence changes taking place with respect to freshly emplaced pyroclastic-£ow deposits at Unzen volcano (southwest Japan). Serendipitous acquisition of an image of Unzen by the L-band SAR on JERS-1 was made on 22 July 1993, only 2.6 days after a sequence of large pyroclastic £ows had descended into the Mizunashi valley and before rain had fallen on the fresh ash deposits. The image was combined with a second SAR image gathered on 1 December 1993 to form an interferogram which, after removal of orbital and topographic fringes, shows centimeter-scale surface displacements taking place between the times of the two images. Surface subsidence in the vertical direction of V12 cm (equivalent to a mean rate of 0.9 mm/day) is apparent within a region of the Mizunashi valley that was blanketed to a depth of V2 m by the 19 July 1993 event but not subsequently, so that radar coherence was maintained across the time interval spanned by the interferogram. Subsidence is also observed in this interferogram in adjacent valleys where the deposits were laid down by pyroclastic £ows taking place up to 3 months before the Mizunashi valley events of 19 July 1993, although here the height changes are smaller, i.e. V4 cm (mean rate V0.3 mm/day). A later interferometric combination of JERS-1 SAR images spanning a period of V3 years after deposition shows ongoing subsidence within the Mizunashi valley but at considerably reduced rates (0.04^0.09 mm/day). The observed height decreases probably represent the net result of several interleaving surface height change mechanisms that develop on di¡erent timescales. Possible candidate mechanisms include hydroconsolidation, de£ation through the

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release of gas, erosion, settlement and substrate relaxation. It is, however, unlikely that viscous deformation within the body of the fresh ash deposits is responsible for these changes given the relatively low emplacement temperatures of the Unzen ash (300^500‡C). As suggested on the basis of qualitative ¢eld observations made at Mount St. Helens (Hoblitt et al., 1985), short-timescale water-induced processes such as hydroconsolidation might take place with the ¢rst substantial rain following deposition. Comparison of the height changes taking place at Unzen in the three main valley systems visited by pyroclastic £ows leads to the conclusion that short-term mechanisms were responsible for V8 cm (66%) of the subsidence observed in the Mizunashi valley in the 22 July 1993^1 December 1993 interferogram. On the basis of a rather crude calculation, this modest level of height decrease could be viewed as consistent with hydroconsolidation so long as rather special conditions exist to maintain radar coherence locally. In general, however, we would expect that excessive di¡erential vertical translation between scattering elements would tend to lead to decoherence. Laboratory experimentation using fresh, unwetted samples of a pyroclastic£ow deposit is required to investigate this issue in detail. For periods of up to V1 year after deposition, it is possible that other water-induced mechanisms, such as leaching, could act to cause surface sagging, whereas in the longer term of V1 year after deposition, the data are consistent with slow de£ation caused by relaxation of the underlying substrate in response to the total load accumulated from the pyroclastic £ows of the recent eruption of Unzen. Since radar coherence is lost where pyroclastic £ows cause resurfacing within the time window covered by the interferogram, in this study the InSAR capability for measuring di¡erential surface height change has been employed at the end of a sequence of pyroclastic £ows, when a lull or cessation in activity is underway. On the other hand, the presence of decorrelation is itself an important source of information. In the 22 July 1993^1 December 1993 interferogram, zones of complete decorrelation de¢ne the descent paths for pyroclastic £ows that took place within the

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interferometric time window. This correspondence is particularly clear for the descent path that guided pyroclastic £ows into both the Nakao and Oshigadani valleys in the latter part of 1993. Several regions of decorrelation are also visible in the lower Mizunashi valley but these features are of a di¡erent origin. Comparison with a LANDSAT 4 Thematic Mapper image acquired on 10 January 1994 shows that a narrow ribbon-like feature in the InSAR data that runs along the course of the river and broadens near the river mouth probably represents the e¡ect of resurfacing from the debris £ows of 19^20 August 1993. In addition, further comparison with the LANDSAT data leads to the identi¢cation of decorrelation signatures corresponding to two over£ow reservoirs. These were constructed in 1992 with the intention of diverting debris-£ow discharge and, within the 22 July 1993^1 December 1993 interferometric time window, were subject to considerable surface disturbance as a result of bulldozing operations. In the central region of the Mizunashi valley, coherence is generally higher over regions of freshly deposited ash and is highest in a region of relatively sparse boulder cover where the largest surface height changes are recorded. Generally speaking, both the depositional and post-emplacement histories of deposits can in£uence the observed height change and radar coherence signatures. For future research using InSAR to monitor pyroclastic- and debris-£ow related e¡ects at active volcanoes, an increase in the number of interferogram pairs relative to the availability offered by JERS-1 would be of prime importance. The resulting improvement in temporal resolution would then enable the subtle changes taking place at the surface of fresh pyroclastic-£ow deposits to be monitored in detail. In an idealized case, a dual satellite system similar to that of the ERS1^ERS2 tandem mission would provide such an enhanced observational capability, although an L-band SAR rather than the C-band of the ERS satellites would be preferred for this application in order to circumvent problems associated with the more rapid decorrelation timescale at C-band. In parallel with further remote sensing research, laboratory experiments on fresh, unwetted samples of vol-

canic ash are also required to form the basis of a realistic interpretation of the imagery. This paper demonstrates that much useful information relating to the pyroclastic- and debris£ow regimes of an active volcano can be derived through application of the InSAR technique. It is likely that further advances in this direction of research will create new monitoring and predictive capabilities applicable to these lethal and damaging phenomena.

Acknowledgements We wish to acknowledge the support and advice received from many colleagues in both the Department of Geophysics and the Institute for Geothermal Science of Kyoto University. Similarly, we acknowledge considerable assistance from colleagues at the Shimabara Earthquake and Volcano Observatory of Kyushu University. J. Riehle made many valuable remarks on matters relating to the compaction of volcanic ash sheets ^ his input is warmly appreciated. Thanks are due also to R. Hoblitt for sharing a number of insights into the behavior of young pyroclastic£ow deposits and to A. Hurst for several useful comments. We acknowledge the use of rainfall data from the Japan Meteorological Agency and terrain altitude data from the Geographical Survey Institute of Japan. Information and advice were kindly provided by RESTEC. We are grateful to NASDA for providing the SAR data without which this study could not have been performed. The Japanese Self Defense Force and the Shimabara Police Department made detailed observations of Unzen pyroclastic £ows that were essential to the interpretation of the data presented in this paper. We are indebted to Kodo Umakoshi of Nagasaki University for photographs and to the Nagasaki Prefectural Government for permission to use the aircraft imagery of Unzen shown in Fig. 6b.

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