Seismogeological effects on rocks during the 12 April 1998 upper Soča Territory earthquake (NW Slovenia)

Tectonophysics 330 (2001) 153–175 www.elsevier.com/locate/tecto

Seismogeological effects on rocks during the 12 April 1998 upper Socˇa Territory earthquake (NW Slovenia) R. Vidrih a,1, M. Ribicˇicˇ b,2, P. Suhadolc c,* a

Geophysical Survey of the Republic of Slovenia, Ministry of Environment and Spatial Planning, Kersnikova 3, Ljubljana, Slovenia b Civil Engineering Institute ZRMK, University of Ljubljana, Dimicˇeva 12, Ljubljana, Slovenia c Department of Earth Sciences, University of Trieste, Via Weiss 1, I-34127 Trieste, Italy Received 17 November 1999; accepted 14 September 2000

Abstract On 12 April 1998, the strongest earthquake (ML 5.7), with an epicentre in Slovenia, in the last 100 years shook the upper Socˇa Territory (NW Slovenia). Its maximum intensity was between VII and VIII according to the European Macroseismic Scale (EMS-98). Its epicentre is located in the area SE of the town of Bovec in the Krn mountain range (Julian Alps). Apart from substantial material damage to nearby settlements, the earthquake caused considerable changes to the landscape through many rockfalls and landslides. In this paper, we try to assess the effects of this event on the natural surroundings. The presented results are useful to understand the relation between “seismogeological” effects and the intensity scale EMS-98 for this particular event, but might also be a first step towards a future statistically meaningful assessment of the intensity scale on the basis of “seismogeological” effects. 䉷 2001 Elsevier Science B.V. All rights reserved. Keywords: earthquake; seismogeological effects; intensity scale; rockfall; slope failure; Slovenia

1. Introduction The area of NW Slovenia is one of the most seismically active parts of the country. Earthquakes may reach intensities higher than IX on the MCS scale (Ribaricˇ, 1980, 1987), while the maximum observed historical event was evaluated to have MLH ˆ 6:8 (Lapajne et al., 1997). Its seismicity is controlled mainly by the margin between the Adriatic microplate and the Eurasian plate, since it lies on the north-east* Corresponding author. Tel.: ⫹39-40-676-2122; fax: ⫹39-40676-2111. E-mail addresses: [email protected] (R. Vidrih), [email protected] (M. Ribicˇicˇ), [email protected] (P. Suhadolc). 1 Fax: ⫹386-61-432-7067. 2 Fax: ⫹386-61-136-7451.

ern rim of the Adriatic microplate (e.g. Anderson and Jackson, 1987; Platt et al., 1989). On 12 April 1998, the strongest earthquake (ML 5.7), with an epicentre in Slovenia, in the last 100 years shook the upper Socˇa Territory in NW Slovenia (Fig. 1a). 3 This event has been referred to either as the Bovec or Krn earthquake (e.g. Bajc et al., 1999; Gosar and Zupancˇicˇ, 1999). The earthquakes effects occupied and caused damages all over the upper Socˇa Territory, so we have decided to denote it as the upper Socˇa Territory event. Its epicentral area lies right on the contact between the thrust units of the Alps (striking EW and verging to the south) and the Dinarides right-lateral transpressive zone (striking NW–SE). 3

All photos were taken by R. Vidrih.

0040-1951/01/$ - see front matter 䉷 2001 Elsevier Science B.V. All rights reserved. PII: S0040-195 1(00)00219-5

154

EUROPE

IA STR U A

Maribor

Bo ve c a

O L S

VE

A I N No vo Me sto

CR OA TI A

Ljubljana

ITALY

Tolmin

ea

s tic ria d A

Ko p e r

Fig. 1. (a) NW Slovenia with the epicentral area of the 1998 upper Socˇa Territory earthquake. (b) Epicentral area of the 1998 upper Socˇa Territory earthquake with the locations of aftershocks (after Bajc et al. 2000) plotted on a digital elevation model (DEM) of the area.

R. Vidrih et al. / Tectonophysics 330 (2001) 153–175

Y AR NG HU

Obser ved area : Upper Posocje

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155

Bovec

Fig. 1. (continued)

Changes in natural surroundings caused by earthquakes can help us in providing a more reliable definition of the macroseismic epicentre and earthquake intensity. An intensity of VII–VIII EMS (European Macroseismic Scale, EMS-98) was determined in Mala vas (Bovec) and the villages Spodnje Drezˇnisˇke Ravne and Magozd located in the epicentral area of the upper Socˇa Territory event. The same intensity level was ascribed to damage located at Krn village, at a “kota 1776 m”, Javorsˇcˇek, the Krncˇica ridge, in the area along the Tolminka river between its source and the Polog mountain, and in the mountains above the Lepena valley — Lemezˇ, Sˇija, Lipnik. The area where the most frequent natural seismogeological phenomena are concentrated runs from Bovec (small rockfalls), along the south-western ridges above the Lepena valley, across the Krn mountain area to the Tolminka river spring and the Polog alm (Ribicˇicˇ and Vidrih, 1998a,b; Godec et al., 2000). For intensity levels higher than VI, the EMS almost exclusively takes into account the damage caused to buildings. In sparsely inhabited areas, as is the area

affected by the 1998 upper Socˇa Territory earthquake, where only the valleys are inhabited, whereas in the alpine parts there are only alpine dairy farms, hunting lodges, mountain huts and individual mountain farms, the use of this scale is difficult, making the results unreliable. The question arises whether assessing an earthquake’s intensity would not be more precise if it also took into account landscape changes such as rockfalls and landslides. Such additional information, if interpreted correctly, could increase the reliability of an earthquake’s intensity assessment. To enable such a possibility, a comparison of seismogeological effects on the natural surroundings with damage to buildings must be made wherever possible, and the correlation extended to the entire area under study. Three different approaches are possible when including seismic effects on natural surroundings in the definition of a macroseismic scale. Under the first approach, a description of seismic effects (like rockfalls and landslides) is gradually included in the description of intensity levels, i.e. in the original scale. According to the second approach, as used in

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Table 1 Location and magnitudes of the main shock on Sunday, 12 April 1998 at 10:55 (UTC) and of its strongest aftershocks …M ⱖ 3:0†: Except for the last two aftershocks, the locations are from Bajc et al. (1999) Date

Origin time (UTC)

Lat, deg N

Lon, deg E

Depth (km)

ML

12.04.1998 12.04.1998 12.04.1998 12.04.1998 15.04.1998 15.04.1998 06.05.1998 11.05.1998 13.05.1998 10.06.1998 30.08.1998 24.11.1998

10:55:32.6 13:35:27.3 16:15:39.2 22:13:47.7 19:40:30.0 22:42:09.7 02:52:59.8 23:30:48.3 01:58:53.2 23:32:40.9 01:18 13:49

46.308 46.259 46.309 46.318 46.272 46.302 46.279 46.281 46.285 46.301 46.251 46.235

13.629 13.553 13.601 13.610 13.722 13.647 13.693 13.702 13.703 13.629 13.684 13.664

7.6 12.1 7.3 4.5 5.0 3.8 5.3 4.7 4.5 6.4 / /

5.7 3.2 3.0 3.2 3.4 3.2 4.2 3.2 3.1 3.2 3.1 3.4

Chapter 7 of the EMS-98 (Gru¨nthal, 1998), seismogeological phenomena are treated more precisely, taking into account both the recently gained expert knowledge regarding rock mechanics and spatial analyses concerning an area’s vulnerability made using GIS technology. The third and most recent approach is to create a separate scale for seismogeological phenomena linked only to the European Macroseismic Scale through comparative tables (Gru¨nthal, 1998). Analyses of the damage caused to natural surroundings by the upper Socˇa Territory earthquake show that it could be useful to include such seismogeological effects, whenever possible, in the systematic assessment of earthquake intensity of events in similar mountainous regions.

2. The upper Socˇa Territory earthquake on 12 April 1998 On 12 April 1998, the strongest earthquake, with an epicentre in Slovenia, in the last 100 years shook the upper Socˇa Territory in NW Slovenia. Its magnitude is about 5.7 (Md 5.6, ML 5.7, MWA 5.8, MW 6.0) and its maximum intensity (Cecic´ et al., 1999) between VII and VIII according to EMS-98. Its epicentre is located in the area between the Lepena valley near the town of Bovec and the Krn mountain range (Julian Alps), the focal depth being around 8 km (Bajc et al., 1999). It appears that the earthquake nucleated along the Dinaric trending fault

system running from the Rombon mountain (NE of Bovec) towards the Krn mountain (N of Tolmin) and across the village of Tolminske Ravne towards the Cerkno area. Some authors (e.g. Buser, 1986) call it the Knezˇje Ravne fault, others (Bajc et al., 2000) prefer not to make any association and delimit the activated part of the fault by two structural barriers. The Dinaric trend is confirmed by both the trend of observed maximum damage elongated along a NW– SE direction and extending from Bovec, the Lepena valley, Drezˇnisˇke Ravne to Krn village (Vidrih and Godec, 1998) and by the pattern of the relocated aftershocks (Bajc et al., 1999, 2000) that cover a 12-km-long narrow strip along the damaged area (Fig. 1b) and the strike-slip character of the focal mechanism of the main shock as obtained from waveform inversion (Kravanja et al., 1999). The upper Socˇa Territory earthquake was felt all over Slovenia and in nine neighbouring countries: Croatia, Bosnia and Herzegovina, Hungary, Czech, Slovakia, Austria, Germany, Switzerland and Italy. The parameters of the main event and its strongest aftershocks are given (Bajc et al., personal communication, 1999) in Table 1. The origin time according to local time was at 12:55, right at the time when people were having the traditional Easter lunch, giving rise to some panic. The Geophysical Survey of Slovenia installed in the epicentral area at first three, then five and finally six portable stations, which have recorded more than 400 aftershocks during the first 20 h and more

R. Vidrih et al. / Tectonophysics 330 (2000) 153–175

Fig. 2. Generalised geological structure of the epicentral area with marked rockfalls (Ribicˇicˇ and Vidrih, 1998a,b) (Geological map 1: 25 000, author M. Poljak, 1998).

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than 7000 in the following months (Sincˇicˇ and Vidrih, 1999). The strongest aftershock occurred on 6 May 1998 at 02:52 (UTC) with a magnitude of 4.2. The main event had a focal depth of about 8 km and the aftershocks spanned a depth range from 4 to 12 km (Bajc et al., 1999). The mechanism of the main event as given by the EMS is a pure strike-slip on an essentially vertical plane.

3. Observed damage in the epicentral area The broader epicentral area of the 1998 earthquake is a mountainous region formed by carbonate rocks, mainly limestones and dolomites. The valleys are filled with alluvium of fluvial and glacier origin and alluvial cones. The Socˇa river and its affluents form alluvial terraces made of gravel, sand and, more rarely, conglomerates (Vidrih et al., 1991). It is well known that local geology can have enormous effects on the amplification of seismic waves (e.g. Borcherdt, 1970; Aki, 1993) leading to intensity increments of the order of one or two degrees. The areal extent of the observed damage has fully confirmed these findings (Ribicˇicˇ and Vidrih, 1998; Ribicˇicˇ et al., 2000). The strongest damage was observed in Mala vas (Bovec), and the villages of Spodnje Drezˇnisˇke Ravne, Magozd, Lepena and on the Polog alm. The estimated intensity in these places reached values between VII and VIII degrees on the EMS scale. Damage of degree VII (EMS) was estimated for the localities Kal-Koritnica, Zgornje Drezˇnisˇke Ravne, Jezerca, Krn, Tolminske Ravne (Vidrih and Godec, 1998b; Cecic´ et al., 1999). The greatest effects on the natural surroundings (VII– VIII EMS-98) have been observed in the Lepena valley, the SW slopes of Krn, Krncˇica, Javorsˇcˇek, mountain “kota 1776 m”, Lemezˇ, the Tolminka river spring and the Osojnica alm. The many rockfalls that have changed the landscape are a consequence of the alpine mountainous terrain and its inherent instability (Fig. 2). When compared to the 1976 Friuli earthquake that in Slovenia mostly affected the Breginj area, the 1998 upper Socˇa Territory event mainly damaged an area lying east of Breginj between the localities of Bovec, Kobarid and Tolmin. The 1976 event was felt by the population as an undulation, while the 1998 one has mostly felt as an upward directed thrust. This is well

161

confirmed by the type of damage and by the effects on the natural surroundings. In the Lepena valley and the Krn area, the seismic waves lifted and overthrew massive rocks, some of which cracked or rolled down the slopes. 4. Changes in the natural surroundings caused by earthquakes During an earthquake, we usually only register the natural phenomena endangering people and buildings or phenomena having very large dimensions. However, as it will be shown below, the diversity of damage to nature is very large. In the Alpine area, damage is connected mainly with rock. If only these are considered, the earthquake-related phenomena may be classified into: • damage to the rock itself (appearance and opening of joints on the surface and in rock faces); • loss of natural equilibrium (rockfalls, planar and wedge failures); • falls of pieces of rock (stones, rock blocks); • slides of talus and scree; • rock block movements and splitting. There are probably many more types of damage, and these will have to be identified through systematic work. For any earthquake-related change in nature, relationships can be determined, also connected with the earthquake intensity. The stronger the earthquake, the higher is the number and intensity of the related phenomena. 5. Causes of instability in natural surroundings during an earthquake The most frequent and visible changes occurring in nature in the Alpine mountain environment are instabilities of rocks. The extent to which rock is liable to landslides depends more on the level and characteristics of their cracks than on the geomechanical properties of the material composing the rock (Hoek and Bray, 1981). The principal characteristics of cracks reflecting the possibility of a landslide are as follows: • the direction of joints with regard to the direction of the slope;

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Fig. 3. Planar rockslide along a crack or at bedding of limestone inclined in the direction of the slope.

Fig. 4. Wedge-shaped rockslide at two cracks crossing each other where the intersecting line is inclined in the direction of the slope.

• • • • • • •

example of a planar failure, very clearly visible from Bovec (if looking towards Podsocˇa). When the secant of two-joint systems has the same direction as the slope and is inclined downwards, a wedge failure is possible (Fig. 4). Apart from the failures described above, rockfalls appear in those areas where the slope is vertical or almost vertical. These are a special type of rock failures that appear when there is a weak plane in the rock mass of the slope, approximately parallel to the slope and more or less vertical. Figs. 5 and 6 show the various types of rockfall. During the upper Socˇa Territory earthquake, there were very many rockfalls that were noticed everywhere in the Krn range, including extraordinarily large rockfalls, like the one from the top of Lemezˇ, the rockfalls in the surroundings of the Tolminka source, those on Krn, Krncˇica, etc. Apart from rockfalls, sliding, falling or rolling of rock blocks and stones can appear on very steep slopes. Most frequently, the sliding of an unstable block appears in the first stage, changing into rolling and bouncing along the slope. After the earthquake, many roads were covered by stones, released during the earthquake, including several large rock blocks, some with a cubic dimension of more than 200 m 3. One of them even completely crushed a car. The above description does not take into account that earthquakes are dynamic processes. The dynamics caused by successive oscillations lasting several seconds during an earthquake produces additional effects. Earthquake waves in the rock can induce oscillations of a potentially unstable block, so that first the cohesive strength along a crack weakens (the cohesion drops to zero) and then the block leaps over the jagged edges that form the

the number of joint systems; the frequency of joints and the size of blocks; the spatial extent of joints; the level of roughness and undulation of joints; the shear strength along joints; the strength of walls along joints; the openness and filling of joints.

A joint is any planar weakness in the rock, including stratification, small calcite veins, etc. Joints in rocks were caused by tectonics, which apart from large faults, also cause a higher or lower frequency of joints in the rock. Rocks composing the Julian Alps are mostly limestones originated as sea sediments elevated as mountains during the Alpine orogenetic phase. During the uprising and thrusting of rock masses, the tectonic processes damaged the rocks, creating joint systems of intersecting and parallel cracks that split the rock masses into blocks of various sizes. Three joint systems are theoretically required by fundamental rock mechanics in order to have the whole system in equilibrium. The Slovenian Alpine area is normally characterised by three-joint systems almost perpendicular to each other with the average size of blocks being from one decimetre to half a metre. One system of discontinuity in rocks is usually the stratification of the rock due to its sedimentation origin. Therefore, failures in rocks can appear where a certain joint system is unfavourably oriented with respect to the direction of the slope. The most frequent and unfavourable type of sliding appears when the direction of the slope and the direction of a certain joint system are approximately the same. In such cases, when the shear resistance of a crack on the surface is less than the gravity upon it, planar failures are possible (Fig. 3). On the slope above Jablenica, there is an

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163

P P T

P PT PN

PN

T T' W

N

α

T'

T W N α

Fig. 5. Different types of rockfalls near steeply inclined cracks.

Fig. 6. The mechanism of forming a landslide influenced by additional earthquake forces with different inclinations of cracks.

roughness of the block. At the location of such a leap, the oscillation amplitude is larger than the size of the jagged edges and the block slides into a potentially lower position due to the gravitational force in the direction of the sliding plane. During these actions, the jagged edges also break off and the sliding plane becomes “smoother”. If the shear resistance decreases so much that the component of the block weight in the direction of the plane is larger than the shear resistance, the final slide of the block results. Where the process of leaping over a rough plane does not cause an irreversible process, one can find an open crack extending perpendicular to the direction of the slope. A good example of such a crack is visible on the ridge of Krncˇica.

scale have been:

6. EMS-98 and seismogeological phenomena

However, even with the latest version of the EMS-98 scale, the descriptions in Annex C on effects on natural surroundings are still too general. The main phenomena affecting the rock environment that might appear in the Alpine region during earthquakes and that can be used to estimate the local earthquake intensity are:

To parameterise an earthquake, magnitude and intensity are most frequently used. Intensity scales have evolved in time. Developments in science, especially in civil engineering, and the recurrent tragic experiences with earthquakes around the world when reinforced concrete buildings, as well as other brick or stone buildings, collapsed, have required updates in intensity scales. The need to make changes was so extensive that a scale now called the EMS (European Macroseismic Scale) has been proposed. The scale was formulated in 1992 (Gru¨nthal, 1993) and was then followed by a period of testing and adjustments. The main reasons for the introduction of the new

1. the need to include new types of buildings and materials (special stress on buildings with earthquake-resistant design); 2. elimination of non-linearity between the degrees VI and VII of the MSK scale; 3. the need to generally improve the clarity of the definitions; 4. the need to define earthquake effects on high-rise buildings; 5. to design a scale that not only meets the needs of seismologists alone, but also those of civil engineers; 6. to design a scale that would also be suitable for evaluating historical earthquakes; 7. the need to critically revise usage of macroseismic effects in the ground.

• • • • • • • •

falling of individual stones; opening of fresh short cracks in rock; falling of individual rock blocks; small rockfalls; slides of talus; crumbling of stones in large amounts; small planar and wedge failures; movements of rock blocks on slightly sloping or

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• • • • • • • • •

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level ground; large rockfalls; opening of long fresh cracks; splitting and turning over of rock blocks; falling of rock blocks in large amounts; large planar failures; large wedge failures; rockfalls of regional dimensions; planar failures of regional dimensions; large wedge failures of regional dimensions.

If a more detailed and systematic study were possible, an even larger variety of seismogeological phenomena in rock could be established. Above all, it would be possible to distinguish between different types of minor phenomena, like the appearance, opening, extension and type of joints in rock, movements of rock blocks, various types of rockfall, etc. Of course, all these phenomena depend on the intensity level of an earthquake and the pre-existing local situation. The EMS divides buildings into six vulnerability classes. Likewise, we have determined vulnerability classes for the terrain consisting of various rock types and morphologically varied. To maintain the general applicability and also because of the lack of information, we have only determined vulnerability classes and gave general directions on how to define the vulnerability of the terrain.

7. Classification of terrain in vulnerability classes Regarding the appearance of rock failures, mostly those factors that are used in calculating terrain stability in rock mechanics are decisive. Apart from the basic characteristics of rock in terms of strength and frequency of joints, failures due to instability also depend on the morphology of the terrain and local conditions like the level of tectonic damage to the rock. When assessing the vulnerability of larger areas, the local conditions are neglected and only typical characteristics of the rock and the form of the terrain are taken into account. Above all, the level of vulnerability is defined on the basis of the analysis of instabilities in the terrain. Where the amount of instability in natural surroundings is high, losses of natural equilibrium will also appear during an earth-

quake, while other terrains will not see much new instabilities appearing during an earthquake. One should know the connection between the type of natural phenomenon and its vulnerability in terms of its occurrence during an earthquake. Through such an analysis, the instabilities in rocks become seismogeological phenomena, the study of which can result in useful information on earthquake intensity (Fig. 7). At the moment, it is not possible to form a valid scale of terrain vulnerability with regard to an earthquake involving different types of rock, as there are no sufficient analyses of seismological phenomena appearing during earthquakes. Therefore, the following starting points are proposed to be used in formulating a seismic vulnerability scale. We propose, in analogy with the buildings vulnerability that is determined using classes A–E, the fivestage scale for natural phenomena given in Table 2. Classification into one of the five vulnerability classes can be made on the basis of rock characteristics, its morphology and other influential factors that can cause the appearance of seismogeological phenomena during earthquakes. In our opinion, the characteristics that were used in the formulation of the presently generally valid RMR (Rock Mass Rating) geotechnical classification (Bieniawski, 1974) on the basis of extensive analyses of rock mechanics can be used for defining the main rock characteristics: • • • • •

strength of material; RQD; distance between joints; roughness and filling of joints; presence of required general preconditions.

Like in RMR, classification parameters of rock vulnerability would be defined with regard to their importance for the appearance of seismogeological phenomena. For the first four parameters, a point numbering should be defined characterising the progressive seismic vulnerability of rocks. The fifth parameter should take into account the influence of the required general preconditions as well as the influence of water. To this we should add the morphological characteristics of the terrain structure. Clearly, the more a terrain is mountainous and the steeper is its slopes,

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Fig. 7. Map of rockfall risk during a maximum intensity earthquake according to the seismic map of Slovenia for an earthquake with a return period of 500 years and Legend (Vidrih and Ribicˇicˇ, 1994).

the greater the possibility of losing the natural equilibrium and also the occurrence of some other seismogeological phenomena. Therefore, it is more appropriate to link the definition of vulnerability to the terrain, not only to the type of rock that it is composed of. The upper Socˇa Territory earthquake occurred in an Alpine region with very steep, even vertical, slopes in rock with all the characteristics favouring most the appearance of instabilities in rock, according to RMR. Regarding vulnerability, the Alpine region clearly belongs to very vulnerable or extremely vulnerable terrain (classes A and B). Consequently, after the earthquake, we were able to observe many seismogeological effects, whose frequency and size depended on the local intensity of the earthquake (Ribicˇicˇ et al., 2000). To illustrate the procedure of forming an assessment of terrain vulnerability, let us describe the seismogeological effects occurring in the area of the Lepena valley, where they were strongest. The

steep slopes of the valley are composed of bedded Dachstein limestone. On both edges of the valley, under the steep slopes, there are terrace and morainic formations of carbonate talus and gravel, occasionally interrupted by alluvial fans. The core of the valley is filled with torrent and the Lepena alluvia, and the remains of the front moraine of a glacier. Before the earthquake, one could observe a very old huge rockfall on the right side at the mouth of the valley and many smaller rockfalls on the steep slopes on the left side of the valley. The appearance of all these instabilities Table 2 Terrain vulnerability classes for seismogeological phenomena Class

Scale

A B C D E

Extremely vulnerable terrain Very vulnerable terrain Vulnerable terrain Slightly vulnerable terrain Non-vulnerable terrain

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Table 3 Vulnerability classes of the terrain in the Lepena valley Class

Scale

Area

A B C D E

Extremely vulnerable terrain Very vulnerable terrain Vulnerable terrain Slightly vulnerable terrain Non vulnerable terrain

Entire left slope of the valley (Sˇija and Lemezˇ) Right edge at the beginning of the valley (above Pristava) Right slope of the valley, steep slopes of terraces Terrain with rock blocks in the upper part of the valley Fluvial and morainic alluvia in the valley

was connected with the slides in the direction of the stratification; only a few smaller rockfalls were connected with cracks resulting from tectonic processes. The assessment of terrain vulnerability, based on information about the lithological structure and mechanism of slides, shows the classifications according to vulnerability given in Table 3. During the earthquake, the extremely vulnerable terrain (A) on the left side of the valley experienced many cases of smaller rockfalls and individual large planar and wedge failures. In the area of the large old rockfall (vulnerability B), the damage was already considerably lower. One could observe smaller landslides and falls of individual larger rock blocks. The terrain of the right steep slopes of the Lepena valley (vulnerability C) only suffered some smaller rockfalls, being considerably apart from each other, and falls of individual stones. On the steep slopes of the terraces (vulnerability D), individual cracks opened, indicating the beginning of a sliding process. In the valley itself (vulnerability E), characteristic terrain damage was not noticed, except in the upper part of the valley where a glacier (or possibly large lateral rockfalls) deposited rock blocks, which moved for a few decimetres during the strong shocks of the earthquake. In assessing terrain vulnerability, one must also distinguish between seismogeologic effects defining the level of damage and the actual level of damage to the environment. A clear example is the splitting of a rock block appearing during an earthquake of EMS level VII or more, while the effect on the environment is minimal. On the other hand, rockfalls already triggered on level VI EMS damaged a slope in its full length from the place of failure to the valley bottom, where rock blocks and talus are piled in the form of an alluvial fan.

8. Determination of the level of damage for seismogeological phenomena As in other definitions, in defining the level of damage one also tries to set similar criteria both for damage to buildings and for damage to the environment. However, the difference in terms of how damage appears requires a different approach. With buildings, objects and people, one speaks of individual, many and majority. With damage to the environment, we only propose individual and many, as it is difficult to speak of majority when assessing damage to the environment. In the environment, the definition of the level of damage is much more difficult, since one does not know where in the threatened area certain phenomena will appear, while with buildings we know their location. After an earthquake, one can also establish the percentage of damage in a certain vulnerability class according to the total number of buildings placed in this class, while this is not possible in the case of seismogeological phenomena. One characteristic of seismogeological phenomena in rocks is that they are more intense and frequent the stronger the earthquake is. Therefore, it is very important to define the extent of each such phenomenon, since on this basis alone can one assess the earthquake intensity. The scale formulated for the phenomena connected with rock instabilities in the example of the upper Socˇa Territory earthquake shows how to define their size (Tables A1 and A2). In preparing the scale, we did not follow statistical calculations, but placed in the class individual terrains where only rare, i.e. individual, phenomena were recorded; in the class many we have placed terrains where an observed phenomenon appearing during the earthquake was more frequent. We are aware that such a

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Table 4 Classification of damage during seismogeological phenomena appearing in the environment

c)

b)

a)

e)

d)

g)

Grade 2: moderate damage (d) Small rockfalls (e) Slides of talus (f) Crumbling of stones in large amounts

f)

u)

j) i)

l) See other sketches

See other sketches

Grade 1: negligible to slight damage (a) Falling of individual stones (b) Opening of fresh short cracks in rocks (c) Falling of individual rock blocks

k)

Grade 3: substantial to heavy damage (g) Small planar failures (h) Small wedge failures (i) Movements of rock blocks on slightly sloping or level ground (j) Large rockfalls (k) Opening of long fresh cracks Grade 4: very heavy damage (l) Splitting and turning over of rock blocks; falling of rock blocks in large amounts, large planar failures, large wedge failures

Grade 5: destruction Rockfalls of regional dimensions, planar failures of regional dimensions, large wedge failures of regional dimensions

division according to frequency is relative and possibly subjective, however, for the area in question this partition proved to be very effective. A clear example is the movement of rock blocks on level or slightly sloping ground, which were very frequently close to the epicentre, as almost all rock blocks moved to some extent. On the contrary, in areas further away from the epicentre we only observed movements of individual rock blocks located in critical places. In the future, the analysis of earthquake effects on the environment and the definition of the intensity level will have to pay more attention to these “tiny” and more frequent effects in the environment and not only the rockfalls and slides visible from far away. Such an approach requires very detailed and time-consuming fieldwork, for which it is difficult to find funds.

Alpinists who have renewed the marked and secured paths around the Alpine region (the Krn mountain range) have reported much damage, which unfortunately, has not been professionally analysed. Generally, it is possible to say that seismogeological effects in rocks only start to appear in amounts large enough to be well detected at intensity level VI. However, this depends greatly on the vulnerability of the terrain and it is a question as whether they will be in sufficient number to be analysed. Only with terrains of vulnerability classes A and B the number of phenomena and their variety is high enough to be also used for defining the earthquake intensity. Luckily, in areas where seismic effects do not appear in rock, they can be observed in soil (landslides, landslips, cracks in soil, etc.). However, these must be

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treated in a different way and are not dealt with in this article.

9. Classification according to damage intensity for seismogeological phenomena In the Alpine area most of the buildings can be divided into masonry and reinforced concrete structures. Masonry buildings are classified into five grades of damage, from negligible to slight damage, moderate damage, substantial to heavy damage and very heavy damage to destruction. Reinforced concrete buildings are also classified into five grades. On the basis of the description of damage, the grade of earthquake-caused damage to individual masonry buildings can be defined. Likewise, Table 4 classifies the seismogeological phenomena that occurred in the environment during the upper Socˇa Territory earthquake into five categories of vulnerability. We propose, in analogy with what is done for buildings, five grades of vulnerability for different types of terrain. We also propose a similar reduction of vulnerability classes for buildings (from seven classes with EMS-92 and six classes with EMS-98). The classification of vulnerability in the environment differs from the buildings classification, as it depends more on the type of seismic phenomena appearing during earthquakes than on the size of damage, as in the case of buildings.

Fig. 8. Falling of individual stones (along the path to village Magozd, the Kobarid region).

10. Conclusions The study of earthquake effects has been predominantly based (since 1923, when the Mercalli– Cancani–Sieberg intensity scale, an elaboration of the 1902 Mercalli one, was issued; since 1964, when a new scale was published by Medvedev, Sponheuer and Karnik; and since 1992, when, based on the knowledge of seismology and civil engineering available at that time, the new European Macroseismic Scale (EMS) was presented for the first time) on studying damage to buildings. In fact, for the upper intensity levels these scales are made almost exclusively with regard to damage to buildings. The study of the changes on the environment during the upper Socˇa Territory earthquake has showed that the environment has been heavily affected by many seismogeological phenomena and that these depended, as in the case of buildings, both on the

Fig. 9. Opening of short fresh cracks in the slope along the path leading to the source of the Tolminka river.

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Fig. 10. Falling of individual rock blocks (the Mali Lipnik mountain above the Lepena valley).

Fig. 11. Smaller rockfalls (the Mountain Rombon, above Bovec).

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Fig. 12. Gravel slides (the Lepena valley). Fig. 14. Smaller planar rockslides (the Osojnica mountain).

Fig. 15. Movement of rocks on gently sloping or level ground (along the path from Drezˇnica to Krn). Fig. 13. Crumbling of stones and rocks in larger amounts (the Osojnica mountain over the Tolmin).

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Fig. 16. Large rockfalls: (a) the Osojnica mountain; (b) south-western ridge of Krn; (c) “kota 1776 m” above the Lepena valley.

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Fig. 18. Rocks lifted up and lowered by the earthquake, some of which were turned around or split (along the path from Drezˇnica to Krn).

same level as for buildings is still a long one. Most important, it is necessary to collect detailed analyses of seismogeological effects on the environment in various areas around the world, in different types of rock and terrain morphology. We are sure that such an endeavour will pay off in the future by mitigating the seismic risk of mountainous valleys where some special objects such as dams might be located. Acknowledgements Fig. 17. Opening of long fresh cracks (Ridge of Krncˇica near Krn Mountain).

vulnerability of the terrain and the intensity level of the earthquake. We have shown that all well-defined features that apply to buildings can be also implemented to the environment. Therefore, many minor and major seismogeological phenomena occurring during earthquakes can testify on the seismic effects on the environment and be therefore used to better characterise the intensity level of an earthquake in sparsely inhabited mountainous regions. In Appendix A we have proposed how to use such effects in the EMS98 scale. On the basis of the material presented in this paper, we can state that a systematic and long-term research on the seismogeological effects on the environment is badly needed in order to better integrate them into the seismic intensity scales, as it was possible in this paper. The way to the establishing of a form that will allow us to define earthquake intensities on the

We would like to thank J. Bajc for providing the list of relocated aftershocks of the 1998 event and the DEM figure with aftershocks. This research has been financially supported by the Geophysical Survey of the Republic of Slovenia and by Italian MURST Cofinanziamento 1999 and ex 60% funds. Appendix A A.1. Relationship of seismogeological effects to intensity degree (after Gru¨nthal, 1998, EMS-98) The effects on the environment are described under the special Annex C to the European Macroseismic scale (EMS-98). As some of them are specially treated in this paper, a short description of the macroseismic effects listed in Annex C is given in Table A1. The descriptions of macroseismic effects were included in the definition of intensity levels of certain earlier scales, while in the European Macroseismic Scale

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Table A1 Relationship between intensity level and damage to the environment (EMS-98)

Seismic intensities Seismological and hydrological effects I

II

III

IV V

VI

VIII IX

X

XI XII

Hydrological effects Instrumentally detected minor changes Easily observed substantial changes in levels of well water Long period waves on standing water resulting from distant earthquakes Waves on standing water from local shaking Lake water made turbid from disturbance of bottom-lying sediment Flow of springs affected Spring stop and start Water thrown from lakes Slope failure effects Scree slopes move Small landslips Minor rockfalls Landslides, massive rockfalls Proccesses on flat ground Minor cracks in ground caused by shaking Large fissures in ground caused by tectonic proccesses Complex cases Landslides with predominant hydrological causes Liquefaction

LEGEND: most useful range as an intensity diagnostic intensities also typical of this effect possible observation range potential for extreme observations beyond the given limits

they are excluded with the explanation that their range of appearance during earthquakes of various intensities is too large to be able to define intensity levels on their basis. A brief comment on the table of seismic effects: even a brief glance reveals an extremely wide range of observations with different intensity levels, which

prevents its practical use in assessing seismic intensity. In this paper we present a different approach, reducing the intensity extent of phenomena appearances by introducing, in analogy with buildings, terrain vulnerability regarding earthquakes, the frequency of appearance and the level of damage with individual phenomena. We remind the reader

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Table A2 Relations between the intensity levels and damage to the environment regarding the upper Socˇa Territory earthquake Legend: X, most useful range as an intensity diagnostic; W, intensities also typical of this effect Slope failure effects in rock

Seismic intensity VI

Falling of individual stones Opening of fresh short cracks in rock Falling of individual rock blocks Small rockfalls Slides of talus Crumbling of stones in large amounts Small planar and wedge failures Movements of rock blocks on slightly sloping or level ground Large rockfalls Opening of long fresh cracks Falling of rock blocks in large amounts Splitting and turning over of rock blocks Large planar failures Large wedge failures Rockfalls of regional dimensions Planar failures of regional dimensions Large wedge failures of regional dimensions

that all phenomena described in this paper are connected only with seismic effects on rocks. Slope failure effects in rocks based on the analysis of the upper Socˇa Territory earthquake are shown in Table A2. The calibration of these relations has been done by comparing the seismogeological effects with the detailed macroseismic map obtained for this earthquake (Cecic´ et al., 1999) at the same localities or by interpolation between adjacent intensity points. However, an extrapolation was made to assess level IX of the scale. The table is obviously applicable to terrains appearing in the Alpine (carbonate) region in Slovenia, but might be easily adopted for other similar mountain environments. References Aki, K., 1993. Local site effects on weak and strong ground motion. Tectonophysics 218, 93–111. Anderson, H., Jackson, J., 1987. Active tectonics of the Adriatic Region. Geophys. J. Roy. Astr. Soc. 91, 937–983. Bajc, J., Aoudia, A., Suhadolc, P., Zˇivcˇic´, M., 1999. Relocation of the Bovec 1998 earthquake sequence: implication for active tectonics in NW Slovenia. IUGG 99 Birmingham Abstracts, A169. Bajc, J., Aoudia, A., Sarao, A., Suhadolc, P., 2000. The 1998 Bovec–Krn mountain (Slovenia) earthquake sequence: implication for earthquake hazard. Submitted for publication. Bieniawski, Z.T., 1974. Geomechanics classification of rock masses

X X X W W

VII

W W X X X X W W W

VIII

X X X X X X X W

Damage level

Figures

Sketch in Table 4

1 1 1 2 2 2 3 3 3 3 4 4 4 4 5 5 5

Fig. 8 Fig. 9 Fig. 10 Fig. 11 Fig. 12 Fig. 13 Fig. 14 Fig. 15 Fig. 16 Fig. 17

(a) (b) (c) (d) (e) (f) (g), (h) (i) (j) (k)

Fig. 18

(l)

IX

W W W X X X

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