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 measure- ments 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 yearÀ1) (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. 1474-7065/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S1474-7065(02)00174-2 1536 V. Rizzo / Physics and Chemistry of the Earth 27 (2002) 1535–1544 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 read- ings 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 stan- dard 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 yearÀ1 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).