376 PRELIMINARY ASSESSMENT OF LANDSLIDES

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Dhfere, V015, NO. 4, pp. 376 - 383, (1981)

preliminmy assessment of larzdslides resdtingJi.om the earthquake of 23rd November 1980 in S o u t h Italy

David Alexander

Department of Geology and Geography University of Massachusetts at Amherst MA 01003, U.S.A.

This paper examinea the hazards, mechanisms and &ecb of landsliding provoked by the 1980 earthquake in CMpanlr and Basillcata Regions, Southem Italy. The &ecb of selsmidy-induced mass-movement are ass- d with respect to slope stabiuty and damage to both settlements and roads. Whereas the mechanism of cyclic lording of solls, which can give rise to landslides, is different h m the pore-pressure, gravity loading and strength-reduction mechanisms that normally cause slope failure, the morphology of slidcs is often indistinguishable and thja made it difficult to identi€y which slides were directly caused by the earthquake. However, creep in potential shear planes andoubtably became more wideapread, and the incidence of small, bowl-shaped slidea increased as B direct result of the earthquake. Although variations in the detailed stress- pattern within individual slopes meant that some very

mobile sou and rock masses did not move, 36 settlements reported landslide damage and 29 road9 were rffected by landslides o c a d n g during the earthquake and its immediate aftermath. A fall assessment of the dieaster, together with an explanation of the geography of the disaster area, can be found in Alexander (lW2).

THE PROBLEM OF LANDSLIDES

The three most prominent afflictions of Southern Italy were identified by the eminent nineteenth century Writer Giustino Fortunato as malaria, earthquakes and landslides. In the limestone, clay and alluvial terrains of Irpinia (Campania Region) and Basilicata seismic activity and mass-movement have both had a long and complex history in which the ground shaking caused by earthquakes has often stimulated fresh landsliding or reactivated existing slides. The earthquake which occurred at 18.34.52 GMT on 23rd November 1980 was of Richter magni ude 6.5-6.8 and its probable energy

profound modifications to the geomorphology of the epicentral zone, where the maximum intensity observed was X on the MSK scale (cf. Tocher, 1958). It is also difficult to distinguish by retrospective observation of form (rather than by direct observation of process) between landslides provoked or reactivated by the 1980 earthquake and those that would have occurred in any case from non-seismic causes. Landsliding has neverthe- less been an extremely important consequence of the earthquake, has directly affected at least 36 comuni (basic administrative ‘parishes’), including the central area of several towns, and has caused the deaths-of four people by derailing an express train on the Naples- Calabria main line.

generation of 10 2% ergs was not sufficient to cause many

PRELIMINARY ASSESSMENT OF LANDSLIDES 377

Yet mass-movement is only one of the alterations to surface geology to have been provoked by the tremors (cf. Youd and Hoose, 1978; Ambraseys, 19811, and Ortolani (1981) has recognized the following range of effects:

Uplift to a maximum of 14 cm in the epicentral area, especially in the Ofanto Valley near Conza di Campania on the Ariano formation (neritic* con- glomerates, sands and sandy clays - see Ortolani and Torre, 1981).

Small surface cracks in clay and ,alluvium, which although they are not very deep may extend as far as 1 km, especially in the Calore, Ofanto, Sele and Agri Valleys in Irpinia and Basilicata.

Fractures in rock formations which are underlain at depths of 30 to 60 m by clays at locations in the Calore, Ofanto and Sele Valleys.

Surface movements of faults at the margins of the Sele Valley and elsewhere.

The reactivation of deep or extensive landslides, for example at Calitri and Senerchia (AV).""

Rockfalls and rockslides on the dolomite limestones and other rocks of the Campano-Lucanian platform (MonteMarzano, Monte Ogna and Monti Picentini structural units - see Pescatore. 1981).

Liquefaction on alluvial plains, occurring up to 70 km from the epicentre (Dragone, Serino and Sarno Plains; Agri Valley in Basilicata and Lake Laceno in the Picentini Mountains).

It appears that relatively few cases of faulting, fracturing, surface cracking, subsidence and uplift gave rise directly to landslides, and that liquefaction slides (Peacock and Seed, 1968; Youd, 1973) were not an important consequence of the earthquake. Perhaps the main landsliding effects were rockfalls in weathered dolomite limestone and other hard rock outcrops, and the disturbance of potential shear planes and zones within the profiles of slope soils. Failure of the disturbed shear planes tended to occur when rainfall subsequently increased soil moisture to field capacity and thus increased pore - water pressure (cf. Rice and Foggin, 1971).

*Neritic conglomerates are formed in an environment or depth zone between low tide level and 100 fathoms (or the edge of a continental shelf): see Gary et d. (1974). **The Provinces of Southern Italy are denoted in this paper according to standard Italian abbreviations, as follows: Campania Region - Avellino (AV), Benevento (BN), Caserta (CE), Naples (NA), Salerno (SA); Basilicata Region - Matera (MT] Potenza (PZ).

Types of landslide

Landslides which could be described as catastrophic - particularly large, deep seated rotational movements resulting in the diversion of drainage by blocking stream channels with debris (Hadley, 1%4) - were infrequent and localized. However, a notable example near Torella dei Lombardi (AV) involved the creation of a small lake by damming the basal channel with slumped material. In most other cases the earthquakegave rise to landsliding only where there were detached masses of unstable material which would sooner or later have collapsed, or where potential planes of shearing already existed.

Nevertheless, the widespread instability of soil and rock masses throughout the disaster area and over much of the upland periphery gave ample scope for mass-movement as a result of 'cyclic mobility' (Chowd- hury, 1978, p. 2771, or increased shear stress provoked by the tremors. It appears that the range of slides included both shallow and deep-seated, circular and non-circular, and simple and complex types, with or without regression headwards or the development of chute morphologies beneath the head-scarp (see the classification of landslide form by Skempton and Hutchinson, 1969).

The pattern of deeper mass-movement was distin- guishable from that of shallow slides. Large rotational and deep-seated slides tended to exploit existing planes of shear and therefore usually involved the reactivation of landslides which had been created long ago by previous earthquakes or, more likely, by normal patterns of stress and erosion in the slopes (Radina and Vignola, 1981). Such slides frequently had a long history or movement: that of Calitri in Avellino Province (to be described below) had undergone substantial movements in 1890 and 1923 (Fig. 1). Many smaller rotational and deep-seated slides were newly created by the earthquake, especially in steeper slopes perhaps exceeding about 25 degrees. Such slides commonly occurred in the deeper parts of soils on limestone bedrock and in flysches (such as those of the Trias-Lower Miocene Lagonegrese formation north-east of Potenza), where recent stream incision or excavation by man had oversteepened slopes.

Shallow landsliding consequent upon the earthquake tended to exploit planes or zones of soil creep or transitions within the profiles of thin upland soils such as rendzinas, which form on limestones with an abrupt transition from A to C horizon (i.e. from fully structured topsoil to subsoil or bedrock). Such sliding, of which the largest examples occurred near San Fele (PZ), could involve very extensive areas in the sporadic movement of sheets of material, or could be restricted to shallow, bowl shaped scars (Fig. 2).

Mechanism of landsliding

Although blockfalls, debris avalanches and other dry slides occurred as a direct and immediate result of the earthquake (for example, in the Quaternary conglomer- ates to the north and north-east of Eboli (SA)), most

378 PRELIMINARY ASSESSMENT OF LANDSLIDES

Fig. 1. Calitri (AV): seismically generated landslide in urban area.

landslides followed the torrential rain that began on 27th November 1980, four days after the earthquake, and continued to fall spasmodically until it turned to snow on 29/30th November. Thus the town of Calitri was praised on 26th November for the resistance of its modern anti-seismic buildings @Z Mattino - Naples daily newspaper; 27th November 19801, but during the first week of December many of them suffered substantial damage when the complex series of slides in the clay subsoil on which the town is built became re-activated by rainwater infiltration Ul Mattino, 6th November 1980).* Calitri is built along a ridge 530 m above sea level between the Torrente Cortino and the River Ofanto. The slope of the Ofanto Valley rises 162 m in less than a kilometre from the river to the summit of the ridge; and the combination of unstable slopes and high relief causes the main road, the strada statale No. 399, to take 5 km to cover this distance, traversing the toe and body of the landslide at a number of points. The slide is clearly of the

It was reported that slides also resulted from * broken drainage and water pipes.

complex, deep-seated variety and some planes of failure extend under the summit of the ridge and have caused the top of the Cortino Valley slope to move several centimetres towards the Ofanto Valley. Movement is now being intensively monitored by borehole, piezometer and inclinometer experiments at several points in the silt- clays, flysches and terrace deposits in which the sliding is occurring.

In the Sele Valley the town of Senerchia (AV) has suffered a complex series of multiple slides which have clearly been augmented by the earthquake, although to what extent is difficult to determine (Fig. 3). Senerchia occurs on a dissected and segmented alluvial fan crossed by a surface fault at the junction between Cretaceo- Miocene variegated clays and dolomite limestones of the Campano-Lucanian platform (Trias-Lower Miocene - see Ortolani and Torre, 1981). The earthquake caused normal displacements of about 0.5 m in the fault, rock slides on the mountain slopes above the town and bowl-shaped mass-movements with long, lobate debris tongues in the material of the alluvial fan. It is, however, difficult to decide whether strong motions per se, faulting, landsliding under non-Seismic duresses or factors connected with the morphology of the material should take most of the blame for the 80’70 damage and destruction of the urban fabric of Senerchia.

In the headwaters of the Ofanto catchment the comune of San Fele (PZ) has suffered excessive shallow landsliding as a result of the tremors. Sheets of soil have been set in motion above and below the town, involving perhaps 1,500 hectares of land, and all roads on the flanks of the Costa Squadro mountain (1,342 m) have suffered from small, bowl-shaped slides (Fig. 2). In the former case faulting and deformation within the thin, poorly developed soils of the dolomitic limestones are to blame, and in the latter case these factors have been augmented by the slope-steepening role of dissection during post-Pleistocene stream erosion.

There is little indication that those landslides which were consequent upon the rain following the earthquake moved at very high speeds (in contrast to certain slides in the Ofanto Valley which occurred during or directly after the earthquake and apparently involved fluidization of the shear plane): the typical maximum rate of movement was about a metre a day, which is more than four orders of magnitude greater than mean rates prevailing before the earthquake. During the month following the earthquake landslides threatening the town of San Fele moved at about 1 ni a day with occasional surges at up to 3 m an hour, according to local observers. Movement continued for months and the general instability of the soils above the town will probably never be fully rectified.

It is probable that the principal direct effect of the earthquake on land surface stability was to accentuate soil creep in shear zones (rather than increasing initial stresses to the point at which internal resistance is overcome and shearing takes place on a large scale). ‘Rapid’ creep - of millimetres or centimetres per day - had the effect of opening tension cracks in the soil fabric (especially near the top of slopes) and infiltration

PRELIMINARY ASSESSMENT OF LANDSLIDES 379

Fig. 2. San Fele (PZ): small- scale landslide with bowl- shaped scar.

by rainwater later created positive pore-pressures of the magnitude necessary to induce sliding (see Anderson, Richards and Kneale, 1980; Alexander, in press, a). The shear stress within saturated slopes would undoubtably have been increased by cyclic loading created during aftershocks such as those that occurred at 02.52 on 29th November and 08.42 on the 30th, which both created effects exceeding intensity VII on the MSK scale.

Several towns situated on conglomerateor indurated sands and gravels suffered damage as a result of the earthquake (for example, Tricarico (W), which is located on dipping beds of strongly coherent but jointed

calcarenites). As the urban fabric of these towns tends to bear no relationship to the pattern of vertical joints in the underlying rock, many buildings straddle fissures which are gradually enlarging as erosion dissecting the margins of the deposit releases the confining pressure. Such a process appears to have led to the destruction of Craco (MT) in 1966, after the conglomerates on which the town is built has been disturbed by landsliding in the adjacent and underlying clays. Whilst the earthquake has not created such processes it has undoubtably accelerated them tu the extent of provoking fresh damage in buildings.

Fig. 3. Senerchia (AV): complex landslide and mud- flow morphology.

380 PRELIMINARY ASSESSMENT OF LANDSLIDES

Although research elsewhere has demonstrated a strong spatial association between frequency and size of landslide scars and distance from the epicentre of an earthquake (Simonett, 1%7), it is doubtful whether such a generalization can be applied to the present case, as substantial mass-movements occurred in the south and east of Basilicata 70 - 80 km from the epicentre, where maximum seismic intensities were only of the order of VI or VII MSK? Engineers have shown that the probability depends not only on the velocity, acceler- ation, duration, frequency, amplitude and number of strong motionsy*but on local site conditions, geological details, soil characteristics and stress histories (Sherard, 1967). As many of these characteristics are seldom known with respect to individual slopes, it is difficult to predict which slopes will fail in the event of a strong earthquake. Chowdhury (1978) and Seed and Idriss (1%7) have noted that the complexity involved in considering all of these factors simultaneously makes seismically-generated landslid es virtually inpredict- able; although others, notably Nilsen and Brabb (1975) and Nilsen et d. (1975) argue to the contrary, that the seismic landslide hazard can be mapped according to the distribution of known geological variables. Neverthe- less, a complex series of slides in disturbed flysches at Bella (PZ) which has been incised by a tributary of the Fiumara di Muro were not reactivated by the tremors, presumably because accumulated stresses did not bring them at that particular moment sufficiently close to yield stress (cf. Costa and Baker, 1981, p. 265). Similarly, at Albano di Lucania (Pa , further to the east, normal fault displacements of almost a metre in depth did not succeed in re-activating old slides in deeply folded variegated clays and mudstones of the Cretaceous-Lower Miocene.

Both the Cretaceous limestones of the Puglian murge plateau and the Plio-Calabrian over-consolidated silt- clays of the Fossa Bradanica (Fossa Premurgiana) in eastern Basilicata strongly resisted against landsliding (Alexander, in press, a). However the clays, mark and limestones of the Tammaro Valley in the Sannio zone of Benevento Province were severely affected by the re-activation of old landslides.

Damage to roads

As a result of the 1980 earthquake many minor roads throughout Basilicata and Campania were damaged by the accumulation of landslide debris where they are ~~ ~ ~~ ~ ~ ~ _ _ _ _

Harp et d. (1981) found that a seismic intensity of VI was necessary to generate landsliding in the neighbour- hood of Guatemala City in 1976, but above this threshold there was very little spatial relationship between seismic intensity and landslide occurrence - at least at the level of microseismic zoning. **The probability of slope failure will also depend on the propensity of different combinations of rock formation and soil mass to amplify or transmit strong motions - see Dobry et al. (1971).

*

narrowly enclosed in cuttings. Other sections of road were damaged by sliding from beneath, which in many cases involved reactivation of previous slides onto which hardcore had been laid after the last movement in order to restore the level surface of the road. Such repairs are common on the minor roads of Basilicata and Campania because they are a quick and cheap expedient way to repair landslide damage, even though they simply create further problems of overburden weight to landslides that have only been stabilized by their own frictional, cohesive and pore-pressure constraints and are therefore highly susceptible to further movement (systematic drainage and stabilization of roadside landslides is rare in central Southern Italy). The prevalence of such instances in the earthquake zone made them a prime target for the tremors.

There are no major state roads through Irpinia and Upper Basilicata, where the worst damage to settlements occurred. The main highway (the A3 Naples-Salerno- Reggio Calabria motorway and its feeder route from Sicignano (SA) to Potenza) suffered rockfalls where it passes through cuttings in fractured limestone under- lain by clays. Near Eboli (SA) boulders several metres in diameter landed on the carriageway, but structural damage was limited and mass-movements did not close the motorway. Of the minor state roads in the earthquake zone at least 29 - in other words virtually all - suffered some form of landslide damage. Information on the nature and severity of the damage is extremely hard to obtain, but field surveys by the present author suggest that only a handful of roads became impassable immediately after the earthquake as a result of collapsed bridges, fallen rocks or other obstructions." As few settlements in the disaster area are approached by only one road landslide damage posed very little problem to the distribution of relief and aid after the disaster (snowfall at the beginning of December posed a greater threat to access within Irpinia and upper Basilicata). Nevertheless, no less than 12 km of road between Bella and San Fele (PZ) suffered severe damage from rockfalls and rotational slides, and access was temporarily restricted to cross-country vehicles.

HISTORICAL AND SOCIAL FACTORS

The damage to upland settlements and roads highlights a general problem in which mass-movements provoked by the earthquake have played an important part. Many settlements in the highlands of Campania and Basilicata have traditionally suffered damage from landsliding because they are situated on the tops of hills, which are less stable than valley floors. Many sites consist of the remnants of marine or fluvial terraces - beds of

*The Naples daily newspaper I2 Mattino (26th November 1980) listed only 4 strade statali, or state roads, as impassable for long stretches, and the newspaper Avunti (27th November 1980) listed 11 which were partially blocked as a result of earthquake damage.

PRELIMINARY ASSESSMENT OF LANDSLJDES 381

sand, gravel, calcarenite or conglomerate perched on flysches or marine clay deposits. These outcrops once provided a good source of flat or gently inclined building land, containing groundwater and isolated by steep slopes from the attacks of marauders or malarial disease (both of which prevailed in the valleys). These settlements have repeatedly fallen prey to deep-seated rotational landslides. Mori (1968) estimated that 104 settlements are at risk in Basilicata alone: at Pisticci (MT), for example, 400 people died when an entire section of the town collapsed in a seismically-generated landslide in 1688, and sliding occurred on a similar scale - fortunately without loss of life - after heavy rains in March 1976. Relocation has traditionally been pro- hibited by the risk of malaria and flooding, the need for land reclamation and the vulnerability to attack of valley floor sites. Nevertheless, it has occurred: two years after the earthquake of 5th June 1688 Cerreto Sannita (BN) was rebuilt in a valley site 1 km from its former hilltop site. Uggiano, which was destroyed by the earthquake of 1490 and rebuilt on a nearby hilltop site as Ferrandina (MT), did not subsequently escape the problems of erosional dissection, landsliding and seismic damage; and Melfi (PZ), which was reconstructed on the same hilltop site after the 1851 earthquake, did not entirely withstand the 1930 earthquake, or the secondary shocks of the 1980 disaster.

Grassano (MT) was founded by the inhabitants from the towns of Irso (modern Irsina) and Altojanni (Grottole), which were both devastated by an earthquake in 1168. Unfortunately, the town is situated on a multiple regressive palaeoslide of substantial dimensions and declivity (Del Prete, 1981), and 35 70 of its older central buildings have been damaged by movements in the subsoil provoked by the earthquake of 1980. The damage created by the landslide has cut right across the range and quality of buildings: of 334 buildings which have been evacuated as a result of seismically-activated landsliding (leaving 1,500 people homeless), 103 occur in the historical centre, 119 in a zone of pre-war housing and 112 in a zone that was constructed in 1950 (Radina and Vignola, 1981). In understanding the slope movements which have led to the damage at Grassano the post-Pliocene pattern of fluvial terrace formation and dissection in the adjacent Basento Valley must be taken into account, as must the role of other recent earthquakes. The earthquake of 1930 provoked damage at Grassano; and in 1956 an earthquake of MSK intensity VII had its epicentre underneath the town, and both tremors reputedly gave rise to slope instability.

Although it is clear that the problem of seismically- activated landslides in urban areas is a recurring one, refounding communities in supposedly safer locations is not necessarily a successful policy. Although not linked to seismic events, two examples from Basilicata serve as an important warning. Campomaggiore (PZ), which was destroyed in 1885 by a rockslide, was refounded on top of a palaeo-landslide; and Craco (MT), which was ruined after a landslide in 1966, was refounded as Stazione Craco at a stable valley floor site but has not been very

successful as a community, perhaps because its architecture lacks the traditional feel which has assisted social cohesion under the duress of unemployment, poverty and emigration.

The problem of damage by seismically generated landslides has come to a head for the second time in Bisaccia (AV), where the movement of three active landslide tongues has made it necessary to evacuate over a thousand homes. The 1930 earthquake reached Mercalli intensity VIII - IX in Bisaccia, killing 18 people, demolishing 137 houses and damaging 1,400 other dwellings representing 15 To of the housing stock. The structure plan for rebuilding recommended moving the town in its entirety to a safer site, but met with substantial local opposition; and it appears that the same opposition is being voiced during the aftermath of the 1980 earthquake. At Auletta (SA), where a vast, periodically active landslide borders onto the urban area, evacuation orders following the 1962 earthquake were not observed.

Economic and political factors

Poor conservation practice has ensured that the proportion of Italian soil which is under attack by landslides generated after flooding, erosion and earth- quakes has risen to 46 %, has cost 40 - 50 billion lire ($30 - 37 million) since 1975 and involves the loss of about 40,000 ha of agricultural land per annum (Alexander, in press, b). The principal risks derive from increased construction and concurrently inadequate land stabilization measures (cf. Leighton, 1976; Pomeroy, 1981). unwise ploughing techniques and inadequate upland soil conservation. In 1971 an inter-ministerial commission recommended that 10,000 billion lire ($7,400 million) should be invested over 30 years in landslide amelioration and prevention schemes. A bill submitted to parliament at the end of 1980 involved the expenditure of 3,000 billion lire ($2,200 million) over 10 years and, as a result of the earthquake and its expenditure of 1,900 billion lire ($1,400 million) over 3 years.

Whilst such schemes are being debated money from the EEC regional development schemes is available for immediate use on slope protection, and will mostly be spent on a few reafforestation and terracing schemes in a limited area (Comunita’ Montana Media Basento (Italy) - personal communication). In Basilicata the Regional Giunta has been quick to secure additional funds following the earthquake but has expresses concern that the excessively high labour input of environmental protection schemes will reduce the “productive eficien- cy” of capital returns VZ Mattino, 15th April 1981). At the time of writing, ways of encouraging farmers, land developers and young unemployed people to participate in the schemes at low labour cost are being investigated (Regione Basilicata, 1981).

382 PRELIMINARY ASSESSMENT OF LANDSLIDES

SUMMARY

The earthquake of 23rd November 1980 in central Southern Italy was not sufficiently powerful to cause widespread transformation of local geomorphology, but it nevertheless stimulated a notable quantity and variety of landsliding, ranging from soil slippages through rotational slumping to rock falls and debris avalanches. Many palaeoslides and existing landslides were re- activated, but large-scale catastrophic landsliding was not a general consequence of the tremors. Perhaps the main geomorphic effect was to initiate creep along potential failure planes and shear zones that was subsequently transformed into landslides and debris flows by infiltration resulting from the precipitation which began on 27th November, 4 days after the earthquake. Thus, although most of the rockfalls that occurred were an immediate consequence of the tremors, widespread landsliding took several days to materialize and was mostly slow moving (less than 1 m/ h).

Although the road network of the disaster zone was not seriously attenuated by landsliding, many towns suffered additional damage after the earthquake as a result of shearing in the foundations of buildings; notable examples are Auletta (SA), Bisaccia (AV), Calitri (AV), San Fele (PZ) and Torella dei Lombardi (AV), and there were at least 31 other affected settlements.

There was little spatial association between intensity of earthquake damage and size or frequency of landslides; and particular deposits and outcrops tended to be affected more than others. Shallow rendzina soils, alluvial f a t debris, outcrops of flysch, indurated terrigenous deposits overlying clays, and Holocene valley fills tended to be worst affected, with the highlands of Potenza Province in Basilicata, the middle Ofanto Valley and the upper Sele and Melandro Valleys in Capania showing the most notable landsliding. Palaeo- landslides at Auletta (SA), Calitri (AV) and Grassano (MT) were re-activated (the last of these being 70 km from the epicentre); but many landslides which had been particularly active before the earthquake - for example, at Bella (PZ) and Scalo Grassano (MT) - were not stimulated to move again.

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