Within polymetamorphic crystalline basement rocks of the Eastern Alps deep-reaching gravitational slope deformations (DGSD) are phenomena consequently affecting overdeepened portions of glacial modelled valleys. In some regions up to 70% of total slope area indicate morphological features of instable slope conditions. Three general modes of DGSD, which differ in slope morphology as well as physical parameters and which one occur interdependent on host rock lithology, can be distinguished. Each mode consequently indicate increasing ratio of slope denudation through accompanying secondary mass movements. Mode one DGSD indicate high level of linear erosion within relaxed host rock as well as the generation of block flow masses and rock fall at lateral transition zone towards stable host rock. Mode two DGSD, which one are of typical bulging slope morphometry, do provoke both large scale landslides as well as rockfall. Those especially occur in lateral extension of main scarp, where host rock completely is distorted. Land behind single scarp also do represent potential source of rockfall, when there is a step higher than 50 meters. Planar plaike structures in upper slope portions result in large debris cones at the toe of instable slope. Those are source of torrent floods of first order drainage system in several cases. Mode three DGSD consequently are connected with "Talzuschub" geometry. This mode often shows secondary landslides of rotational type and in general do increase torrentiality of local drainage systems. Along the Enns- valley in Upper Styria district (Austria) more than 35 localities were mapped. This means that DGSD are wide spread phenomena in the Eastern Alps, but up till now these phenomena are not detected systematically. We think that they do play a dominant role for postglacial morphodynamic evolution of mountainous valleys. There is clear evidence that important processes of hazardous denudation in alpine regions are triggered by the action of DGSD.
The aim of this paper is the study of the landslides in the tectonic trough (graben) of Cuautepec, North Mexico City Area. Landslides are one of the most common geomorphic expression of earthquakes in this area. The classification of landslides is based on the materials comprising for landslides, the character of movement, the degree of internal disruption of comprising materials, the velocity of emplacement and the deposit depth. Landslides are divided in: rock falls, rock slides, rock slumps, rock block slides, soil falls, disrupted soil slides, soil slumps, soil block slides, liquefaction and rapid soil flows. Five generations of landslide are identified in this region. Relative ages are assigned based on: 1) the topographic and geological position, 2) degree of the stream dissection and the depth of the stream dissection, 3) geomorphic expression of the loosening scarp (exposed rupture surface) and landslide deposit, and 4) historic earthquakes (period 1455 - 1995).
Within the Indonesian German MERAPI project the seismic structure of the Merapi Volcano is investigated with an active seismic experiment. First seismic profiling was performed along 4 profiles arranged radially with respect to the summit. Each profile consists of 30 3-component seismometers with a station spacing of 100 m. As a seismic source, a 2.5 l mudgun was placed in 3 different water basins at a distance of 5 km from the summit. This special seismic source has a well defined spectral characteristic and high repetition accuracy.
Along the 3 km long near-source profiles we observed refracted waves. The corresponding 1D depth profiles show low-velocity surface layers of some hundred m/s P-wave velocity. A greater depth the P-wave velocity increases to approximately 3000 m/s.
The volcanic strata leads to strong scattering and absorption effects. A significant decrease of the main signal frequency from approximately 10 to 5 Hz between 0 and 3 km offset can be observed. This frequency shift coincides with the formation of a long coda with maximum amplitudes several seconds after the first break. The coda wavefield is mainly incoherent, which suggests some sort of random character of signal formation. We use randomly distributed scatterers to model the seismogram envelopes and to determine scattering parameters as well as Coda-Q-values.
An analysis in the frequency-wavenumber domain shows, however, that besides the incoherent energy, the interference of coherent waves travelling back and forth along the profile also contibutes to the coda.
We conclude that to understand the mechanisms of the natural volcanic seismic sources of Merapi, scattering and absorption effects along the propagation path must be considered.
Four deformation stations have been installed since 1995 along the hillsides of Merapi Volcano (7.54S, 110.45E, 3000 m) at altitudes between 1280 m and 2020 m. Each station includes a 100 m wide array of 3 tiltmeters, placed in boreholes with depths of 3 m to 4 m, a permanent DGPS receiver and various sensors recording environmental parameters. The experiment is ingrained in the interdisciplinary research project MERAPI, an Indonesian-German cooperation among the Volcanological Survey of Indonesia, the Geo Forschungs Zentrum Potsdam, Germany, and several universities in both countries.
Three classes of tilt signals with amplitudes ranging from 20 µrad to 20 µrad are observed, each one of them potentially containing information about exogene or endogene processes that are symptomatic for Merapi's activity. a) Significant tidal tilt variations validate the successful coupling of the instruments to the ground. Deviations of the local tidal tilts from the body tides possibly reflect the influence of discontinuities in the elastic properties within the edifice of Merapi. A 4% decrease in the local tidal tilt response functions during 1997 may indicate temporal changes in the rheology of Merapi's interior. This hypothesis, however, needs to be verified by a careful study of tidal tilt disturbances that are generated by ocean tides in the Indonesian Sea. b) Transient tilt anomalies are detectable down to an amplitude of 0.2 µrad. The high resolution makes the bore hole installations sensitive to small poro-elastic deformations caused by rain. The purely elastic part of this effect has been quantified empirically and analytically by rain-tilt coupling coefficients of 0.03 µrad/mm. An anomaly of 1.2 µrad in the rain-corrected tilt residuals, coinciding with the first pyroclastic flow of the eruption on August 9, 1996, is compatible with a pressure release of a source located within the volcano's edifice. A cumulative tilt step of 8 µrad, observed only at the western flank, is attributed to a local loading effect caused by the debris of the pyroclastic flows of July 11 and 19, 1998. The lack of significant co-eruptive tilt anomalies during another 3 events indicates that the eruptions of Merapi between 1996 and 1998 were associated with minor pressure changes in its interior. c) Long term trends of the order of 10 µrad/year in tangential directions, i.e. roughly perpendicular to the local slope, are observed by all the tiltmeter arrays located at the southern and western flanks. Possible sources are (i) the continuous deposit of debris in the south-western sector of the edifice due to rockfalls, and/or (ii) spatially inhomogeneous deformations of the volcano's edifice due to a gradual change of internal pressure.
Meteorological Doppler radar offer the potential for measuring velocity, turbulence and, possibly, the particle load of volcanic jet and/or plumes. However, they are usually too heavy to be easely set up on volcanoes. So, a new UHF Doppler radar, specially adapted to volcanic sounding, has been developed at OPGC within the frame of both EC and CNRS-INSU research contracts. Here we report on the first testing of this radar during an eruptive phase at Etna (Sicily) in October 1998. VOLDORAD consists of 4 electronic units (total weight less than 100 kg), a steerable tripod mounted antenna and a PC. Its operating frequency is 1238 Mhz and peak power 1 kW ; pulse period as well as length can be selected. Either the raw time series, or the real time computed power spectra of the received signals can be sent to the PC for display and storage. A worthwhile feature of VOLDORAD is its ability to provide volcano's jet measurements (reflectivity and speed) with high time resolution (down to 0.1 s). VOLDORAD was set up on Etna from october 8 to 13, 1150 m from Southeast summit crater. During the first days volcanic activity was restricted to continuous emission of gas steam plume from the craters. The vertical rise of the plume was limited and the radar data were affected from strong echoes from the volcanic topography. In contrast, extremely good radar soundings were obtained during a phase of intense strombolian activity with lava fountains at SE crater during the night of October 11 to 12. Strong Doppler signals were continuously recorded during almost 7 hours. Different parts of the eruptive column were initially explored across several range gates (150 m wide each), by varying the viewing angle of the antenna. Subsequently, continuous recording of Doppler signal was made at selected angle of 30°. The very strong echoes (106 times those typical of rainfall) were measured all during the eruptive phase with a clear temporal correlation between the echo characteristics (intensity and velocity), the observed evolution of eruptive activity and the sequence of seismic tremor recorded by IIV. These results verify the capability of VOLDORAD to monitor temporal variations of volcanic dynamism, even under cloudy conditions. The strength of volcanic echoes recorded at Etna indicates that the radar transmitted power can be substantially reduced thus allowing to further reducing the weight of the equipment. An improved version of VOLDORAD could thus be operated on different volcanoes and such an instrument could be useful for continuous monitoring of active volcanoes.
A giant blast deposit covering much of central and western Tenerife (Canary Islands) represents the terminal explosive event of the Las Cañadas volcano ca. 0.18 million years ago. The thickness of the deposit is some 8 m near source and ca. 1 m at near-coastal exposures. Its large areal extent and volume of solidclasts expelled (several km3) are much larger than previously recognized blast deposits. The blast expanded radially and concentrically from the source area unlike in restricted sectors as in Mt. St. Helens and Bezymianny. The deposit consists of a basal unit generally <50 cm thick and fine-grained and poorly sorted proximally but well-sorted and in the coarse ash to fine lapilli range in medial and distal outcrops. It is also generally discontinuously layered with minor dune and mound structures reflecting unsteady flow. In the distal and medial outcrops locally abundant grooves subparallel to transport direction suggest scouring by longitudinal vortices. The bulk of the blast deposit is more massive, almost invariably inversely graded, a significant fraction of its rock content reflecting locally scoured lithology. High turbulence is inferred to explain the formation of the basal layer over most of the area. The massive central part may have formed by deposition from suspension. The paucity of juvenile material (except locally) is thought to indicate fragmentation of a suddenly depressurized hydrothermal system during slope failure. The abundance of fresh miarolitic syenite xenoliths and local presence of very crystal-rich, slightly pumiceous phonolite suggests that the upper part of an active hot magma resovoir system was depressurized and fragmented as well. Lahars are associated with the blast deposit especially along the canyon such as Barranco de Orchilla. The association of the blast deposit with the Abrigo ignimbrite of the same age is still uncertain. Several major types of hazard were associated with the event: sector flank collapse, lahar, concentric high speed blast and tsunamis.
The islands of Nisyros, Yali, Kos, Santorini, Milos, Poros, Methana and Aegina constitute the South Aegean volcanic island arc, which is a result of northward-directed subduction of the African plate beneath the Aegean microplate. The islands of Nisyros, Santorini, Milos and Methana are considered today the most active areas in terms of a potential volcanic reactivation. Therefore, these islands were chosen for a detailed noble gas investigation. The combination of noble gas ratios, such as 3He/4He and 20Ne/22Ne compared to major and trace gas compositions will allow an appropriate discussion on magma degassing, the amount of atmospheric, meteoric and hydrothermal contamination, as well as the determination of equilibrium temperatures in the hydrothermal systems. During the past year priority was given to gas and water analyses for the following reason. On Nisyros island, high-seismic activity occurred from June to September 1997 and were accompanied by increased tectonic and fumarolic activity in the hydrothermal crater field. In this respect, the scheme of events is comparable with the violent hydroclastic and gas explosions in 1873 and 1888. The observed 3He/4He and 4He/Ne ratios in gas condensates and hydrothermal waters from the islands of Nisyros, Santoririni, Milos and Methana range from 1.419 x 10-6 to 7.5 x 10-6 and from 0.274 to 12.634, respectively. They fit well into the array of active island arc samples which mark a mixing line between atmospheric 3He/4He ratio of 1.4 x 10-6 and mantle derived helium with a maximum ratio of 10 x 10-6. The 3He/4He ratios in the Nisyros condensates as well as in some geothermal waters from Milos reflect well the high amount of mantle derived primordial 3He, which in the case of Nisyros, may be related to magma degassing.
The largest Plinian and ignimbritic eruption in historical times (VEI 6) in the Andes took place in AD 1600 at Huaynaputina, a small eroded stratovolcano located on a high volcanic plateau in southern Peru, in the Central Andean Volcanic Zone, 75 km ESE of Arequipa (16°37'S, 70°51'W). According to chronicles, the eruption began on February 19th with a 13-19 hour-long Plinian stage following 4 days of intense seismic activity, and included at least nine distinct events until March 6th. The erupted tephras totalling 10.35-12.10 km3 bulk volume stratigraphically include eight deposits: 1) a widespread (1-cm-isopach area ~115 000 km2) and voluminous (~7 km3) pumice-fall deposit which includes an ash-rich pyroclastic-flow deposit interbedded in some proximal sections; 2) ignimbrites 1.6-2 km3 from channeled pumice-rich flows and from unconfined ash-flows that travelled ~50 km from the vent across barriers ~1,000 m high; 3) companion ground-surge and ash-cloud surge deposits; 4) several widespread co-ignimbrite ash layers; 5) base-surge deposits with accretionnary lapilli interbedded in the ignimbrite sequence; 6) a crystal-rich flow and surge deposit which overlaps the plinian fallout area to the WNW; 7) unconfined ash flows and ash-cloud surges, and; 8) late ashfall and ash-cloud surge deposits. Mass flows and 'secondary ignimbrites' eposits amounts 5-20 m thick on the steep edges of the plateau and in the high-gradient radial channels showing that the removal of the tephra was topographically-controlled. In addition, pyroclastic flows choked the Rio Tambo canyon and formed two temporary lakes whose catastrophic breaching released large-scale debris flows which swept down the 120-km-long valley to the Pacific Ocean. Disruption of a hydrothermal system and hydromagmatic interactions have triggered or enhanced the large-scale 1600 AD eruption. Although the voluminous eruption did not involve caldera collapse, ring-fractures cut the floor of the pre-1600 AD horseshoe-shaped caldera and of the 1600 AD vents, pointing to the onset of a funnel-type caldera collapse. Finally, the paroxysmal eruption generated a severe regional aftermath and global climatic effects over the early seventeenth century.
The largest explosive eruption in historical times in the Andes took place in AD 1600 at Huaynaputina, a small eroded stratovolcano of southern Peru (Central Andean Volcanic Zone) (Thouret et al., 1997; 1999; de Silva and Zielinski, 1998). This eruption lasted 15 days and devastated an area 60 km around the vent and shook Arequipa city 75 km away. Bulk rocks ICP-AES analyses indicate that the erupted magma is a medium to high-K calc-alkaline dacite. The mineral assemblage encompasses plagioclase (An68-43)+ biotite (Mg# = 56-71) + amphibole (Mg# = 61-82) + magnetite + apatite ± ilmenite. The pumices from the plinian fallout deposit as well as from the pumice-rich pyroclastic flow deposits are porphyritic with a modal crystal contain (recalculated to 100% vesicles-free) between 25 and 30 in volume percent. Major and some highly incompatible trace elements bulk rock compositions through the deposits shows a slight evolution during the eruption. The Plinian fallout magma is slightly less differenciated (SiO2% = 65.4 ± 0.3; MgO% = 1.8 ± 0.1; Th (ppm)= 5.1 ± 0.3) than the magma produced during the following ignimbritic phase of the eruption (SiO2% = 66.3 ± 0.4; MgO% = 1.6 ± 0. 1; Th (ppm)= 6.0 ± 0.3). A slight average evolution in the composition of the mineral assemblage correlates this general trend as well as the composition of the rhyolitic glassy matrix of the pumices: SiO2% = 72 ± 1; MgO% = 0.5 ± 0.1 in the Plinian fallout deposit for SiO2% = 76 ± 1; MgO% = 0.15 ± 0. 1 in the ignimbrites. However the presence of a few more calcic plagioclase (>An75) indicates a differenciation from a more primitive magma. Furthermore, ongoing investigations on the volatiles content of the erupted magma seems to show a higher initial volatiles content in the Plinian phase than during the ignimbritic phase of the eruption. These observations are thought to represent the eruption from a zoned dacitic magma chamber formed by differenciation from a more basic magma. The initiation of the eruption might have been triggered by the high volatiles content of the magma after the differenciation of a new magma batche inside a superfial magma chamber, even if hydromagmatic interactions with a superficial hydrothermal system (as shown from the large amount of oxidized or hydrothermaly altered lithic fragments throughout the Plinian fallout) might have enhanced the explosivity of the initial phases of the eruption.
Thouret JC, Davila J, Rivera M, Eissen JP, Gourgaud A, Le Pennec JL, Juvigné E, Comptes Rendus Acad. Sci. Paris, 325, 931-938, (1997).
Thouret JC, Davila J, Eissen JP, Geology, in press, (1999).
de Silva SL, Zielinski GA, Nature, 393, 455-458, (1998).
We emphasise the importance of the use of Digital Elevation Models (DEM's) derived from digital stereo imaging of aerial photographs in studies on volcanoes. Relative to most types of remotely sensed data, the resolution of the DEM's derived from this process are higher and therefore prove more valuable in some aspects of volcanic studies that implore remote sensing techniques. Problems that concern detail and which cannot be resolved by more advanced digital remote sensed imagery may have a solution with digital stereo processed images of aerial photographs. Available aerial photographs, which can date back up to several decades, can be used and its potential value maximised. When three-dimensional images of topography that pre-date a volcanic event are required, and images taken from new and more advanced imaging techniques are not available, aerial photographs become important. This particularly applies for volcanic terrain where rapid changes in topography have occurred due to volcanic activity.
We present two cases on the successful use of digital stereo processed aerial photographs for volcanic studies. The first case is with the study of the small volcanic crater of Mayon Volcano, Philippines. The dimensions of its vent and crater were determined by digital stereo imaging for the creation of the correct geometry of the summit. This geometry was used in the Computational Fluid Dynamic modelling of the volcanic jet for the first and second phases of the 1984 and the 1993 eruptions. The second case is on the study of the Iriga debris avalanche deposit. The DEM derived from the aerial photographs were used to calculate the volume of the debris avalanche deposit and for the analysis of its distribution and structure. In the analysis of the morphology of Mayon (post 1984) and Iriga, the aerial photographs were used to complement radar data that were available on the area. These were important in regions that were not well represented by the radar image due to shadowing and foreshortening effects.
Aguila, LG, Newhall, CG, Miller, CD, & Lianco, EL, Phil. J Volcanol, 3, 54-72, (1986).
Campbell, JB, Introduction to Remote Sensing, 206-209, (1996).
Cruz, JB, Solidum, RU, & Corpuz, EG, Phil J Volcanol, 2, 68-93, (1985).
Lagmay, AMF, Pyle, DM, Dade, B, & Oppenheimer, C, EOS, AGU Fall 98 Abs, 78, (1997).
Ui, T, & Catane, S, Monbusho technical report, 35-45, (1993).
In volcanic active districts such as an Alpine orogenic belt, there are many steep mountains and earthquakes occur frequently. An earthquake triggers many shallow landslides and occasionally a huge landslide in these mountain areas. To reveal the mechanism of these landslides, it is important to clarify the relationship between geological properties and the distribution of seismic acceleration caused by earthquakes. However, field observation of landslides triggered by earthquakes in mountain areas is difficult and there have been only a few cases. This study investigates the characteristics of the distribution of seismic acceleration by dynamic simulation of a numerical mountain model.
This mountain model is simulated for Yui Landslide Area in Japan. This area is distributed Tertiary clay and sandstone and could suffer a huge earthquake in future. The model is 1000 m long (N-S) and 900 m wide (E-W) and is constructed of 5 layers that were determined by several seismic explorations. It is 3-dimensional model that includes 2,394 nodes and 1,800 elements. A Sinsweep wave of maximum amplitude of 200 gal as a typical earthquake is input to the bottom nodes of this model. This model was analyzed by dynamic 3-dimensional elasto-plastic FEM.
The result showed that seismic acceleration in a shallow layer of mountain is greater than in a deep part, which agrees with the result of field observation. An acceleration is greatly amplified on a convex slope compared with on a concave slope at the same altitude, and in particular seismic acceleration is high in mountain ridges during an earthquake. This result shows that many landslides triggered by earthquakes occur concentrically in ridges of mountains and it reveals geotopography that is susceptible to earthquakes.
Since slope stability analysis included simulated seismic acceleration, it is clear that the major direction of wave and topology of the surface and slip-surface of a landslide have a great effect on the stability of the slope.
On the basis of analyses of satellite imagery, aerial photographs, and field observations, we describe the occurrence of one of the largest paleolandslides ever recognized in an intracontinental domain (20 x 15 km2). This gigantic landslide occurred in the Gobi-Altay mountains in Mongolia, at the northern flank of the Baga Bogd massif, which is bounded by an active thrust. This thrust corresponds to the easternmost segment of the Bogd fault, a strike-slip fault that ruptured most recently on December 4, 1957 during a Mw 8.1 earthquake. From morphological, structural, and lithological aspects, the Baga Bogd paleolandslide may be defined as a gigantic soil block slide (Varnes 1978). It occurred along a 3° slope and affected poorly-cemented conglo-breccias underlain by evaporate sediments. Cross sections of the landslide area show that the slide alluvial cover is thicker upstream (~ 200 m) than downstream (~50 m), and the displacement decreases from the head (3-4.5 km) to the toe, where the alluvial cover is stacked in numerous imbricated slices. The basal slip surface, probably located in evaporates appears very flat. The age of the landslide is inferred to be comprised between Early and Late Quaternary. The relationship between this large-scale landslide and the active Bogd fault system suggests that it was triggerred by strong shaking during an earthquake (e.g. Keefer, 1984; Jibson, 1996), yet its amplitude requires an additional mechanism: a condition of no frictional or cohesive strength along its basal shear plane, possibly achieved by fluidization in basal evaporate sediments. In conclusion, pre-existing surface fracturing, strong shaking, peculiar stratigraphic conditions (coherent alluvial cover underlain by gypsiferous clays) and perhaps wet climate were combined to produce this extraordinary mass movement.
Varnes, DJ, Slope movement types and processes, in Landslides-Analysis and Control, R. L. Schuster and R. J. Krizek (Editors), National Academy of Science, Transportation Research Board, Washington, D. C., Special Report, 176, 11-33, (1978).
Keefer, DK, Landslides caused by earthquakes: Geological Society of America Bulletin, 95, 406-421, (1984).
Jibson, RW, Use of landslide for paleoseismic analysis: Engineering Geology, 43, 291-323, (1996).
Typical for the current activity of Merapi volcano, one of the most active and hazardous volcanoes, are long periods of dome growth accompanied by dome collapse and resulting block-and-ash flows. These travel as ground-hugging basal avalanches and overriding ash clouds, commonly preceded by surges. These surges are the most destructive and hazardous aspects of dome collapse events, but their generation is still poorly understood.
In 1998, Merapi's activity shifted from the southern to the western sector. Precursors such as seismic events, increasing deformation rates at the summit area and intensity of rockfalls started in May. Between July 11 and 19 a total amount of 128 block-and-ash flows were counted by Merapi Volcano Observatory staff. The largest of these descended on July 11 and July 19 into several valleys in the western sector of Merapi, extending up to a distance of 6,5 km from the summit. Flows resulted from collapses of old domes as well as the new 1998 dome. Ash clouds above the flows rose 6 km into the atmosphere during the July eruptions. Fallout was observed up to 60 km southwest and 50 km northwest of Merapi. Basal avalanches (~20 area%) and surges (~80%) destroyed an area of approximately 7 km2.
Four zones of destruction were identified in the destroyed area resembling those described by Kelfoun (pers. comm.) for the 1994 eruption. In the outermost zone, tree leaves were only singed. In the next zone inwards, trees and shrubs are totally stripped of their leaves, but not blown down. In the third zone, trees are either uprooted or snapped off with the number of blown down trees increasing inwards. In the fourth and largest zone of the affected area, all trees are uprooted or snapped off above the ground. Trunks are sandblasted and strongly aligned.
Surge deposits are up to 20 cm thick, well sorted (sorting coefficient = 1,4) and consist of coarse ash (median = 1,9 phi). Most surge deposits are massive or poorly laminated, cross bedding is rare. Fallout deposits are up to 7 cm thick, well sorted (sorting coefficient < 1) and consist of fine ash (median > 4 phi). Near to the valleys, up to 4 cm thick layers of normal-size-graded, rim-type accretionary lapilli up to 2 cm in diameter form the base of the fall out deposits. The block-and-ash flow deposits are poorly sorted, matrix supported and consist of blocks in a matrix of ash and fine lapilli. Maximum block diameter reach 8 m and maximum volumes 50 m3. At the surface of the deposits, more than 80% of coarse lapilli and blocks are juvenile as shown by columnar jointing and high residual temperatures.
Rock fall is one of the main geomorphologic processes in the natural rocky cliffs evolution, but its assessment involves important difficulties. GIS ability for spatial analysis, using cartographic information, allows the development of a successful methodology in rock fall activity assessment.
The study area is a mountainous region of the Cantabrian Range (NW Spain) with 250 km2 and altitudes between 500 and 2100 m.
The working method was to elaborate the rocky cliffs map, including all the rock slopes in the area. The rock fall map was made defining the boundaries of the "rock fragment basin drainage" over the rock slopes and the associated talus deposits. The rock slopes area covers 43 km2 within a 581 polygons map. Approximately a third of these polygons shows rock fall activity deduced from the presence of talus deposits at the cliff base. The basin area versus the associated talus area is a useful rock fall activity index.
The rock fall map was combined in the GIS with the litologic map and with the elevation, slope and aspect models. This analysis allows knowing the distribution of these parameters in the rock slopes.
The most active rock slopes are the siliceous ones. The 42.5% of the quartzite and sandstone cliffs shows rock fall activity which increases with the slope, reaching almost the 100% when the slope is above 70°. The siliceous rock fall are more frequent at the E and W aspect and less frequent at the S, although these differences can be related to the geological structure of the study area.
Unlike the siliceous, only the 22.5% of the calcareous rock slopes shows rock fall activity. The predominant aspects of these active slopes are the S, SW and W. In the calcareous cliffs the connection between rock fall and slope is different than in the siliceous. The most frequent rock falls are in slopes between 60 and 70° and the activity decreases a little when the angle slope is bigger than 70°. The steeper calcareous slopes are massive rock cliffs and the rock-avalanche is the most important process in their evolution.
In both litologies the rock fall increases with the rock slope elevation. This connection is more evident when the elevation is over 1700 m. For these altitudes the freeze cycle force makes rock fragmentation more important.
GIS analysis has allowed the study parameters differences quantification. The data show at the lithology and climatic conditions like the most important factors in the rock fall intensity. It is possible to develop an rock fall activity model for the analysed region which could be applied to bigger regions using only conventional cartographic information.
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