GPS Monitoring and New Data on Slope Movements in the Maratea

Physics and Chemistry of the Earth 27 (2002) 1535–1544 www.elsevier.com/locate/pce

GPS monitoring and new data on slope movements in the Maratea Valley (Potenza, Basilicata) Vincenzo Rizzo * CNR-IRPI, Via Cavour, Roges di Rende, 87030 Cosenza, Italy Accepted 23 July 2002

Abstract Large-scale earth movements in the Maratea valley involving the inhabited area (Basilicata, Italy) have already been the object of scientific studies. These dislocate the outcropping clayey formations and the superimposed masses made up of detritus, carbonate units and large blocks, especially on the left side of the valley. Initial data on earth movements were obtained by the variation in distances monitored by an infrared distance-meter instrument (EDM), between 1983 and 1996. The present study brings out the results obtained by three successive high precision GPS monitoring campaigns undertaken between 1997 and 2000, on a grid of approximately 50 bench-marks. This process was supplemented by EDM monitoring carried out on a wider network of bench-marks than previously imposed. A comparison of different maps and other historical measurements complete the picture. The presence of sustained movements in correspondence with the outcropping clays in the lower part of the valley was confirmed, while such movements are drastically reduced on the detritus and large dislocated carbonate units and blocks, which occupy almost uninterruptedly the left side and the upper parts of the valley. Overall, the arrangement of the vectors allows us to achieve a first model of the on-going gravitational processes in the valley which appear to be attributable to a composite landslide: a spreading evolving lower-down into a large and deep flow. These processes should affect the dislocation of Carbonate Units on the so-called Sackung of Maratea, whose instability and causes are still being studied. Ó 2002 Elsevier Science Ltd. All rights reserved.

Keywords: GPS monitoring; Map and topographic data comparison; Landslide movements; Maratea valley

1. Introduction detrital covers (Amelio et al., 1997). Inclinometric data show that the relevant slip planes are located at the base Large-scale earth movements in the Maratea valley of the first 50 m; indeed, the movements monitored develop into impressive Sackung phenomena in the upper on the surface seem close to those monitored in depth (2– part on the left side (Guerricchio and Melidoro, 1979). 4 cm year1) (Rizzo and Limongi, 1997). The consistency With the aim of characterising these, interdisciplinary of these movements, along with their relative spatio- studies (stratigraphic, hydrological, morphological, mor- temporal homogeneity, in relation to the limited damage phodynamic, gravimetric, geotechnical, geophysical and, to buildings has led to the hypothesis of deeper psue- above all, systematic displacement monitoring) have dotectonic gravitational movements (Guerricchio and been carried out over the last 20 years. From the ob- Melidoro, 1981). More recent studies, based on the sub- tained results it is evident that the instability is favoured marine seismic profiles in front of the study area, have by the geological structure, that is the superimposition shown the presence of a significant active fault, located of permeable and rigid masses (carbonate units, breccia on the direction of right flank of the valley (Colantoni and detritus) upon plastic ones (clayey flysch). Conse- et al., 1997). As a consequence, it has been assumed that quently in the first 50 m of substratum, the clayey flysch tectonics plays an active role in the morphodynamic is highly saturated (Cotecchia et al., 1990). This flysch is processes even if the gravitational ones appear more dislocated everywhere and often overlaps more recent important in determining the morphology of the slopes (Rizzo, 1997; Di Filippo et al., 1997). * Fax: +39-984-835-319. One relevant aspect of these studies has been the E-mail address: [email protected] (V. Rizzo). monitoring of surface movements, which began in 1983.

On account of the lack of adequate equipment and the The present work shows new data on slope move- orographic difficulties, these movements were monitored ments, obtained by a comparison of: as variations in distance between carefully chosen points measured by an infrared distance-meter (AGA (1) EDM monitoring on a widened network of bench- 120). The measurements were continued annually on a marks, between 1996 and 1999; grid of approximately 50 bench-marks (Guerricchio et al., (2) GPS monitoring on a new network, between 1997, 1994). The results were of limited use because they were 1999 and 2000; not associated with vector information and the extent of (3) GPS measurements on pre-existing quoted bench- unstable area (about 6 sq km). On the basis of the data marks; collected between 1983 and 1985 the valley was subdi- (4) Detailed cartographic data from different periods. vided into different cinematic zones (Rizzo, 1997); the same data, moreover, showed the absence of significant movements on the so-called ÔSackungÕ and on the thick 2. Updating landslide monitoring covers of detritus and breccia which occupy the left side of the valley (Rizzo, 1997). The record of damage to 2.1. EDM distance measurements (June 1996–October several buildings over time led to the hypothesis of two, 1999) relatively recent, accelerations; the first between 1973 and 1976 and the second between 1992and 1994. Even The network consisting originally of about fifty up to 1995 these accelerations did not seem to be sup- bench-marks was installed in 1983 and later updated in ported by the topographical data (Rizzo, 1997). 1986. This net was widened in 1996 with a further 73

Fig. 1. Displacements monitored by EDM (the vectors are only a displacement component): from June 1996 to October 1999. Bench-marks Vr are pre-existing points, belonging to old networks. The reported displacements were taken from Madonna degli Ulivi Stations (NS and VS for the new and the old network respectively); they represent change in distance along the measure direction. The aereophoto shows the study area and the large slope instability interesting the left flanc of the Maratea valley. V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544 1537 bench-marks placed in the inhabited area of the valley, and the variations in distance between successive readings were obtained, from June, 1996 to October, 1999 (a period of 40 months) from the ‘‘NS’’ Station (at ‘‘Ma- donna degli Ulivi’’, see Fig. 1). The instrument used was an AGA 12, mounted on a Salmoiraghi theodolite, mod 4180. The procedures employed and related errors have already been treated in previous studies (Guerricchio et al., 1994). Each distance was measured six times in order to obtain the average value (DIST) and the standard deviation (LT). The position of the points and the subsequent displacements were not calculated in terms of vector components, in order to avoid further errors. Fig. 2. Effective and casual data on distance variations monitored by The difference between the distances in 1996 and EDM equipment (electronic distance-meter, AGA12). ‘‘LT1’’, ‘‘LT2’’, those in 1999 represents, with acceptable simplifications, ‘‘LT3’’ represent the absolute value of standard deviation between a group of six consecutive readings, acquired in the campaign of June the displacement in alignment. In Fig. 2, the overall 1996, July 1996 and October 1999 respectively; they could be consid- values relating to the three periods when readings were ered as casual errors. ‘‘V ’’ could be seen as a larger accidental error, taken are reported: LT1, LT2and LT3 represent the due to stationing and weather conditions (because it represents the standard deviation of the measurement in June, 1996, difference in distance between very close monitoring campaign). ‘‘D’’ is the bench-mark displacement from June 96 to October 1999.

Fig. 3. Displacements obtained by GPS monitoring (static mode; 2h sampling at rover station; receivers 4000 and 4400 SSi TRIMBLE), comparing in time (June 1997–March 2000) the diﬀerential data referred to Reference Station (benchmark 0). The vectors show consistent gravitational movements oriented as a large ﬂow-slide. 1538 V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544

July, 1996 and October, 1999 respectively; V represents difference is of the same order as the standard deviations the difference between the average values DIST1 (June, obtained in the single monitoring campaign, with an 1996) and DIST2(July, 1996); while D represents the average value of about 1 cm. The displacements ob- difference (displacement) between DIST3 (October, tained are several times larger than the mean error. 1999) and DIST1 (June, 1996). The difference between It can be seen that displacements are between 1.5 and the first two periods can be considered largely due to 4.5 cm year1 in the inhabited area of the valley. These accidental errors- the time gap was only one month and values are similar to those previously monitored on the the work was undertaken by different operators. This existing bench-marks of the same zone, which, in 1996,

Fig. 4. Spatial distribution of GPS displacement vectors: components in the horizontal plain (above) and the vertical one (below). On the left the comparison of results of November 1999 campaign with respect to those of June 1997; on the right those from March 2000 to June 1997.

Fig. 5. Network of differential comparison and compensation (triangular grid) of rover stations with the reference ones, utilised in the different monitoring campaigns. V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544 1539 showed a total displacement of about 50 cm for the vember 1999 campaign, three receivers and antenna of previous 15 years (Rizzo, 1997). the same kind were used. The campaign was repeated for further information in March, 2000, this time using 2.2. GPS Measurements (June 1997–November 1999– compensation modes and four Trimble 4400 receivers March 2000) equipped with similar antennae. The obtained data shows notable movement (Fig. 3). The differences were In June 1997, a network of almost forty bench-marks compared with those from 1997 (referring time) and are was set up; this grid was used for differential static GPS relative to different time intervals (after 30 month in monitoring, two hour standings for each point. The November 1999 and after 34 months in March 2000; reference station was located outside of the valley, on Figs. 4 and 5). In the central part of the valley the the western carbonate slope (Station 0, see Fig. 3). In the monitored velocity is 1.9–4.5 cm year1. The obtained first campaigns two Trimble 4000SSi receivers equipped vectors are clearly oriented following a gravitational with ground plate antenna were employed; in the No- flow.

Table 1 Comparison of horizontal and vertical displacements in the GPS campaigns of November 1999 and March 2000 Bench mark May 1997/November 1999 May 1997/March 2000 November 1999/March 2000 n° d1ðxÞ mm d1ðyÞ mm d1ðzÞ mm d2ðxÞ mm d2ðyÞ mm d2ðzÞ mm d2 d1ðxÞ d2 d1ðyÞ d2 d1ðzÞ mm mm mm MAR1 )93 )32 )41 )112 )28 )10 )19 4 31 MAR2 )106 )14)126 4 3 )205)1 MAR3 )100 )11 )37 )118 )16 )20 )18 )517 MAR4 )87 )20 )31 )94 )22 )17 )7 )214 MAR5 )45 )27 )13 )52 )23 )55 )74)42 MAR6 )63 )43 17 )84 )24 )47 )2119)64 MAR7 )55 )66 14 )63 )61 3 )85)11 MAR8 )46 )64 )24 )53 )70 )39 )7 )6 )15 MAR9 – – – – – – – – – MA10 )46 )17 5 )55 )15 )8 )92)13 MA11 )89 )5 )13 )101 14 )46 )1219 )33 MA12 )46 )10 )11 )59 )11 11 )13 )122 MA13 )86 37 )71 )104 37 )66 )18 0 5 MA14 )113 )39 8 )131 )43 )26 )18 )4 )34 MA15 – – – – – – – – – MA16 )42 )733)59 )10 )10 )17 )3 )43 MA17 )288)37 )41 13 1 )13 5 38 MA18 )128 25 )23 )137 27 )60 )92)37 MA19 – – – – – – – – – MA20 )39 37 )34 )37 33 )252)49 MA21 )61 66 )21 )69 83 )80 )817)59 MA22 – – – – – – – – – MA23 )111 )83 )51 )125 )101 )60 )14 )18 )9 MA24 )122 )98 )39–––––– MA25––––––––– MA26 )75 44 )73 )76 17 )79 )1 )27 )6 MA27 )32 )84 )39 )42 )89 )17 )10 )522 MA28 1 )13 )15 )1 )15 )35 )2 )2 )20 MA29––––––––– MA30 )93 )90 )7 )111 )95 )11 )18 )5 )4 MA31 )31 15 )31 )4228 )5 )11 13 26 MA32 )56 19 )34 )69 47 )97 )13 28 )63 MA33 )47 )6 )50 )75 )7 )74 )28 )1 )24 MA34 )110 17 )51 )141 10 )28 )31 )723 MA35 )110 2 )32 )146 4 )8 )36 224 MA36 )86 )43 )7 )88 )50 )27 )2 )7 )20 MA37 )46 )43 )5 )53 )53 )22 )7 )10 )17 MA38 16 )38 )68)34 )33 )84)27 MA39 )20 )22 )14 )9 )21 )17 11 1 )3 MA40 0 )7 )14 )14 )5 )22 )14 2 )8 MA41 )9 )93)32 )8 )3 )231)6 MA42 )11 )11 12 )31 )83)203)9 MAT1 )16 )7 )23 )35 )912)19 )235 1540 V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544

The campaign in March, 2000 using more than two racy furnished by receiver manufacturers and of the receivers was conducted on a different arrangement of same amount of monitoring data show in other articles the rover station and utilising the best data to ensure (Gili et al., 2000); it also appears that angular variations maximum precision (Fig. 5). The information obtained of the movement vectors are so small as to be insigni- from the November, 1999 campaign and the March, ficant (Table 1). 2000 campaign is very similar, with greater differences along the Z axis (Table 1). The differences are, to a 2.3. GPS measurements on pre-existing bench-marks certain extent, referable to variations in satellite visibil- (October 1993–March 2000) ity between the different standings and partly to the landslide movements (near to 0.8–1 cm in four months) In 1993 public works engineers (Genio Civile) placed occurring between the two readings. Taking away such a series of bench-marks in the inhabited area and its an amount, the errors can be considered less than 1 cm immediate surroundings for a topographic monitoring. for the measurement of horizontal modules and 1.5 cm Traditional methods were used to quote their relative for the vertical ones. These errors are near to the accu- position, setting up a system with its axis X passing

Fig. 6. Displacements of pre-existing bench-marks (placed in 1993 by «Genio Civile» Public Works Administration), today quoted by topographic (a theodolite mounting EDM) and GPS equipment. Below: The movements (over 77 months) show coherence as regard the present GPS and topographic monitoring (similar in modulus and differing of only 11°–17° in orientation); the reference system of these monitorings (line ‘‘S1 S2’’, origin in S1) is placed on the southern and more stable slope of the village. Consistent differences in orientation could be observed in comparison to 1997– 2000 GPS data (over 34 months), monitored on close bench-marks in the examined area; these differences could be explained as a consequence of the S1 S2reference system displacement. V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544 1541 through two of them (S1andS2; Fig. 6). The instrument The results of the GPS campaign in March, 2000 used in this campaign was an electronic theodolite show consistent displacement in the order of 1.4–4.3 (TWILD2002). To the zero measurement undertaken cm year1; this is in line with the GPS results in the in October, 1993, a second campaign was undertaken in period 1997 to 2000 (Fig. 6). However, the direction December of the same year and a third and final one obtained is more to the north compared with the pre- in March, 1994. The bench marks showed little dis- vious ones and, for this reason, a number of bench- placement with variable orientation (data supplied by marks were reset using traditional topographic methods Genio Civile). Probably, this was partly due to acci- (Fig. 6; by using a theodolite TWILD2002). The latest dental error and partly to the short time interval be- results confirm the data already acquired and open tween the readings. question on the causes of such differences. The positioning of the existing bench-marks was re- calculated using static GPS monitoring. The reference 2.4. Comparison between detailed maps from different stations were placed on S1 and S2and the rover ones on times (1903–1999) the network point, monitoring for two hours. There was a slight modification in the vertical position of receivers Considering the amount of displacements the prob- because the supports were different. For this reason the lem arose whether it would be possible to make use of variation in quota was not taken into consideration; comparisons between old and recent maps in order to operating in this way errors can be regarded as negligible. have wider information over long time. The town of

Fig. 7. Displacements over a long time, obtained by superimposing an original cadastral map, (realised on undeformable support) from the be- ginning of this century (1903; scale 1:1000) to a recent aerophotogrammetry (1999; scale 1:2000). In legend: A ¼ displacement vector obtained by overlapping the structures sited in the southern and more stable slopes; B ¼ displacement vector, obtained by setting in coincidence the referring systems. Vectors A are comparable with other acquired data and are similar in amount to those today monitored (2–5 cm year1). Aligning being the south-western buildings of the village the other structural elements are considerably shifted in the north-western direction. The vector distribution shows––as in the case of Fig. 6–– more northern orientation than GPS ones. This conﬁrms a possible displacement of southern relief, where the referring system is placed, by tectonic movement. Invalid vectors B are exorbitant: they are aﬀected by errors in updating and superimposing the considered reference systems. 1542 V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544

Fig. 8. Iso-displacements map of the Maratea inhabited area, elaborated on the basis of the data as explained in Fig. 7 (A vectors); it is shown in the frame of geological and morphostructural setting of the valley (below). The displacements increase from the southern carbonate rocky slopes to northern clayey flysch covered by detritus and breccia (A ¼ boundary line). In legend: 1—Carbonatic rocks (Verbicaro Unit); 1a—as before, dislocated by gravitational and tectonic processes; 2—Clayey flysch (Crete Nere Formation); 3—Carbonatic rocks (Alburno-Cervati Unit); 3a—as before, dislocated by gravitational and tectonic processes; 4—Detritus and breccias; 5—Eluvium; 6—alluvial fan: recent and actual; 7—boundaries between masses having different velocity and rupture processes; 8—main morphostructural dislocations; 9—thrusting line; 10—directions of gravitational movement; 11—spring; 12—geological section or seismic line.

Maratea is represented on cadastral maps and reported from the year 1903; these map utilised as reference the at scale of 1:1000 in the ﬁrst land property survey map Cassini–Soldner system, later changed in Gauss–Boaca. V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544 1543

The original topographical design is preserved in the the two surveys were carried out by different operators: official archives of Potenza and it was acquired by us for it is not possible now to check the past work and it is digital scanning. The same area was also represented conceivable that serious mistakes were made. through modern aereo-photogrammetric techniques The map comparison, by considering the southern between 1998 and 1999 with the production of a digital slopes to be stable, utilises, in fact, a reference system map at a scale of 1:2000 (Gauss–Boaca system). For similar to the previous case. greater reliability the recent map was submitted to to- In conclusion, the data obtained by the different pographic checks; the data confirmed its accuracy in measuring systems show displacement modules which terms of relative positioning. It was not possible, how- are, on the whole, of similar amounts. The same data ever, to evaluate the accuracy of its reference system. shows consistent and congruent differences in orienta- By superimposing the reference systems of the maps tion, which need further study. Should these differences we could see a notable displacement of all the built up prove significant, they could be explained by a move- area (8–12m), which is 2–3times greater than the hy- ment of the relief itself where the referring systems are pothesised displacement based on available data on located, producing vectors within the valley which are movements and on their trends over time. Consequently, coherent with the reliable GPS ones. If this is this case given the possibility of an error in the positioning of the we have to presume a consistent movement of the reference system with respect to the recent map, the two southern carbonate relief (Monte S. Biagio; southern of maps were superimposed considering the minimum Maratea). With the aim of distinguishing gravitational displacement; this was obtained aligning the contours of movements from tectonic ones a new GPS network was old buildings of the southern slope which is considered, set up in March 2000 extending as far as the external on the base of available data, the more stable in the slopes of the Maratea valley. examined area. The results obtained in this way show relative displacements in the rest of the built up area and these are coherent with the GPS and EDM results (Fig. 4. Instability and cinematic interpretation 7). As in the previous case the orientation is more northward than in the GPS results. The detailed The data currently available is not sufficient to enable information generated by the considerable number of us to build up a definitive picture of the slope instability. recognised points allowed us to elaborate an ‘‘iso- Certain aspects are still not completely clear: displacement’’ map and to divide the area into homo- geneous cinematic zones (Fig. 8). (1) the effect of tectonic activity on gravitational processes; (2) the question whether or not there is deep gravita- 3. Discussion tional movement (that is below the first fifty metres in depth where inclinometric prospecting has brought The GPS measuring campaigns being of high preci- to light significant sliding surfaces). sion and having an external reference system have allowed us to build up quite a precise picture of the A complex research effort, including GPS, DiffSAR movements within the Maratea Valley, in the period (radiometric) and deep inclinometric soundings, is being from 1997 to 2000. The reference station (benchmark 0; conducted to answers these questions. ‘‘depuratore’’ in Fig. 1) is assumed to be stable; its sta- The present state of our knowledge points to a large bility over time with respect to the national GPS net- flow-type landslide in the medium-lower part of the work is being processed. area, which following the shape of the valley turns and The other measuring systems employed all have local expands towards the sea. The rotation of the movement reference systems, placed on the reliefs of the southern is particularly marked on the left side of the valley, and western flanks of the valley. where the old town is situated. There, the vectors un- Although they are internally coherent, the normally dergo a rotation of almost 180°, turning from north to reliable EDM measurements use stations placed on the south around the carbonate ridge where the Cristo di advanced unit of the Sackung which was considered Maratea stands (Monte S. Biagio). The extent of the stable in the past, but in the most recent campaigns, movement is more visible in the central part of the between 1996 and 1999, has been shown to be in valley, which is where a number of important springs movement. emerge at the point of contact of clayey masses with The repositioning of the bench-marks installed in thick detrital covers and carbonate units. The movement 1993 by public works engineers (Genio Civile) was car- diminishes slightly towards the sea, but declines drasti- ried out using different techniques and supports; the cally above that area. It shows a slow fluid-plastic measurements were related to a reference system movement. Higher up the flow, on account of the thick (S1 S2) set up on a slope considered stable. Moreover, detrital covers, the instability assumes an irregular 1544 V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544 spreading rheology, reaching the Sackung. To sum up, Cotecchia, V., DÕEcclesiis, G., Polemio, M., 1990. 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