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Active tectonic fault microdisplacement analyses: a comparison of results from surface and underground monitoring in western Slovakia.

1. INTRODUCTION

Over the past five years a network of extensometric crack gauges has been established in western Slovakia. This region represents a significant contact zone between the Eastern Alps and the Western Carpathians. The network was constructed in order to observe the behaviour of rock massifs during seismic events and to establish the long-term tectonic trends that affect the region. Twelve significant fault structures have been selected for monitoring. Of these, eleven are located in the Male Karpaty Mts and one is located in the Povazsky Inovec Mts. Ten of the monitored structures are sited underground with just two sited at the surface.

The monitoring of microdisplacements in natural underground spaces is comparatively rare. Some attempts to measure tectonic displacements have been undertaken in Italy (Mocchiutti, 2004; Mocchiutti and Dandrea, 2002) and Belgium (Tshibangu et al., 2004). More locally, certain studies have examined tectonic block stability within the caves of the Western Carpathians (e.g. Zacharov, 1984) or in the Bear Cave in Poland (Cacon et al., 1994; Makolski et al., 2008). Unfortunately, many studies only measure one of the displacement components. The methods most commonly used in caves were described by Zacharov (1984). Of these methods, the most comprehensive instrument was developed by geologists working in Czechoslovakia. The device is an extensometric crack gauge, known as a TM71. It allows subsurface displacements to be measured in relation to three components (vertical, strike-slip, and crack opening or closing) and indicates rotations in two transversal planes. The first of these gauges to be installed in a karstic cave system was located within the Strochy abyss in the Velka Fatra Mts, Slovakia. It was installed in 1981 and continues to record displacements at the present day. In contrast to sites located at the surface, underground sites record almost stable temperatures throughout the year and therefore the effects on rock massif displacement are minimised (Kostak and Rybar, 1978).

[FIGURE 1 OMITTED]

We have searched for active tectonic structures over the past five years in order to measure fault movements within the most seismic part of western Slovakia. These movements are associated with the western Slovak neotectonic block. Twelve fault structures have been selected and equipped with extensometric gauges (TM71s). The first was installed in the epicentral area around Dobra Voda in 2004, as reported in Briestensky (2005). Ten of these gauges are situated underground in natural caves whilst two are situated at the surface.

2. THE GEOLOGICAL AND CLIMATIC SETTING

The western part of Slovakia represents the country's most seismically active region (Hok et al., 2000; Hrasna, 2002; Cipciar et al., 2010). The region forms part of the important contact zone between the orogenic Eastern Alps & Western Carpathians and the cratonic North European Platform represented by the Bohemian Massif. The geological history of this region is complicated and this situation persists to the present day. The most significant tectonic structure in the region is the Mur-Murz-Leitha-Dobra Voda (MMLDV) sinistral strike-slip fault. This is, in fact, a continuation of the Vienna Basin Transfer Fault (Hinsch and Decker, 2003). It represents the topographic expression of the tectonic contact between the Eastern Alps and Western Carpathians. In the study area, the Vienna Fault Zone coincides with the Pieniny Klippen Belt (PKB) as a sinistral fault zone that forms the border between the Inner Western Carpathians and the stable North European Platform (Ratschbacher et al., 1993; Hok et al., 2000; Lenhardt et al., 2007). These authors have proposed that the Western Carpathians recently rotated to the northeast in an arc, in addition to the northward progression of the Alps. If this is the current situation, faults that trend NE-SW should presently be active. The aforementioned MMLDV and PKB both follow this NE-SW trend. Therefore these fault zones form the basis of this study.

Situated within central Europe, western Slovakia is characterised by a continental climate. In July the mean temperature reaches up to 21 [degrees]C whilst in January the mean average temperature drops to -3 [degrees]C. Such annual temperature variations are thought to affect rock massif dilation at the surface.

3. METHODOLOGY

To investigate three-dimensional microdisplacements in the field, we have employed the extensometric gauge TM71 (Fig. 1). This instrument has been used in numerous previous studies including Kostak (1991), Stemberk and Stepancikova (2003), Sebela et al. (2005, 2009, 2010), Briestensky et al. (2007), Gosar et al. (2007, 2009), Stemberk and Kostak (2007, 2008), Briestensky and Stemberk (2007, 2008), and Stemberk et al. (2010). The gauge is an optical-mechanical instrument. It does not have any electrical components and is known to survive for tens of years in the field without any maintenance. The data are either collected manually or by camera approximately once a month. The gauge records three fault displacement components (vertical, strike-slip, and crack opening) and indicates rotations in two transversal planes. It is accurate to 0.01 mm per year.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

It has frequently been observed that fresh shear cracks in sinter decoration (Fig. 2) were associated with rock failures, as documented by slickensides (Briestensky and Stemberk, 2008). This association allowed us to identify suitable sites for installing the gauges required for fault microdisplacement monitoring.

Six caves in the Male Karpaty Mts. (Sedmicka, Plavecka, Driny, Zbojnicka, Slopy, and Cachticka) were selected, as well as one cave in the Povazsky Inovec Mts. (Beckovska). The majority of these caves were equipped with only one gauge. However, two gauges were installed in Plavecka Cave and three were installed across an orthogonal fault system in Driny Cave. The gauges are located at a range of depths from the surface. The deepest is at Beckovska Cave, at a depth of 45 m, whilst the shallowest is at Slopy Cave, at a depth of 8 m. In addition, two gauges were installed across surface fault outcrops at the sites of Dobra Voda and Prekazka (Fig. 3).

[FIGURE 4 OMITTED]

Seismic activity in the area has been monitored by the local seismic network, Male Karpaty (MKNET). From these data, moment tensors of the events were calculated using amplitude inversion from the vertical components of the P waves. Moment tensors were decomposed into the double-couple (DC) and non-double-couple (ISO, CLVD) components (Vavrycuk, 2001; Vavrycuk, 2009; Fojtikova et al., 2010).

4. RESULTS

The results obtained from surface measurements at Dobra Voda show significant fault displacements influenced by annual temperature dilations (Fig. 4). Movement variations appear as sinusoidal changes (changes in peak-to-peak amplitude) in all three fault displacement components, i.e. vertical, strike-slip, and crack opening. The annual peak-to-peak amplitude of these sinusoidal changes reaches a maximum of 1 mm (Fig. 4). This figure is comparable to the results registered at Spis Castle by Jezny et al. (2007). It is, however, considerably less than the 2.5 mm registered between adjacent blocks of thickly-bedded sandstones in the Cretaceous Bohemian Basin (Zvelebil, 1995). The latter example is accompanied by significant slope deformation.

The dominant pattern of seasonal displacement obtained from surface measurements at Dobra Voda does not allow us to monitor tectonic displacements. Kovac et al. (2001) calculated that the mean amount of fault displacement along significant tectonic structures within the study area has been ~0.01-0.1 mm/yr during the Pliocene-Quaternary.

Such low values suggest that tectonic trends would not be visible in a graph acquired from this surface locality, given that seasonal peak-to-peak amplitude ganges are far greater and reach up to 1 mm (or possibly more) during the course of a year. At Dobra Voda, disturbances were registered in the smooth sinusoidal course of crack opening during a significant earthquake period during 2006 (Fig. 4). The effects of the tremors were reflected more sensitively in the rotation results than the displacement results at the site (Briestensky et al., 2007).

The results obtained from surface measurements at Prekazka also show fault displacements influenced by annual temperature massive dilations (Fig. 5). However, movement variations appear only as sinusoidal changes in the crack closing component. These seasonal variations reach a maximum of 0.25 mm. The other components are not sensitive to seasonal effects. Instead, these show some important tectonic relationships. Locally significant earthquakes in the region are well reflected in the course of the graphs (cf. e.g. Briestensky et al., 2007). The most remarkable quake, which caused significant displacement, occurred close to the town of Vrbove on 13th March 2006. This had a magnitude of [M.sub.L] = 3.2 and almost certainly represents the most significant local event to have occurred during our observation period. The effect of the quake breaks the smooth course of the seasonal sinusoidal strike-slip component at the site of Dobra Voda (Fig. 4). The seismic disturbances continued until the earthquakes registered around the village of Trstin between 5-8th August 2006. The maximum magnitude recorded was [M.sub.L] = 2.2. Both quake periods also significantly affected the displacements at Prekazka (Fig. 5) (Briestensky et al., 2007). The final quakes terminated an overall increase in the strike-slip displacement trend (0.08-0.16 mm/yr). This can be explained either as a reversal in the sense of fault displacement or as increased movement on the opposite fault block (Fig. 5).

[FIGURE 5 OMITTED]

The gauge at Dobra Voda was installed six metres above a narrow ravine or trench cut for a local railroad. The gauge at Prekazka was installed in a small quarry (width: 40 m) which exhibit relatively fresh outcrops. It was sited at the base of a conglomerate excavation 0.5 m below the surface, which causes the gauge to be permanently submerged underwater. Data are recorded once the water has been removed from the excavation. These two sites highlight the importance of both the location of the gauge in relation to the amount of sunlight it receives and the condition of the rock massif itself. With regard to the former, the site at Dobra Voda is exposed more-or-less continuously to sunlight whereas the site at Prekazka is affected for just a short period during the day. Once a site has been monitored for at least three years, seasonal variations can be identified with confidence. It is then possible to remove these variations from the results to reveal tectonic displacement trends (Kostak et al., 2007).

Conversely, the analysed seismic data (Table 1) from this area show significant non-DC components (Fojtikova et al., 2010). These may reflect the physical properties of the earthquake source in the centre of the focal area. In addition, the ISO and CLVD components show positive correlations that may indicate of presence of tensile faulting. The positive values of ISO and CLVD are produced by the opening of faults while the negative values of ISO and CLVD are produced by the closing of faults (Vavrycuk, 2001, 2002).

In addition to the surface sites, subsurface displacements have been recorded at sites that were supposed to be less affected by seasonal massif dilation. In Slopy Cave, the peak-to-peak amplitude of seasonal dilation reaches a maximum of 0.1 mm. At this site, the gauge is installed only 8 m below the surface. This dilation appears to only affect the horizontal crack opening component. Therefore, the vertical and strike-slip components are able to reflect the prevailing tectonic regime along the structure: increasing NW subsidence (0.04 mm/year) and growing dextral strike-slip (0.05 mm/year) during the earthquake period (Fig. 6). Moreover, tremors also affect the vertical displacements at this site. During the earthquake of August 2006 there was a noticeable change in the seasonal summer peak of fault opening (Fig. 6).

In general, the horizontal crack opening component shows a decreasing seasonal effect with increasing depth (Fig. 7 and Table 2). A similar relationship is also visible in the vertical component (Table 2).

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

In Slopy Cave and the nearby Zbojnicka Cave, a direct relationship has been noted between the recorded microdisplacements and the latest earthquakes to occur hereabouts during 2006-2007 (Briestensky et al., 2007). The effect is most clearly seen in the vertical displacements (Fig. 6). In particular, 2006 was a time of comparatively high earthquake activity. This activity decreases during the final third of 2007 (Fig. 8). The energy of the earthquakes was calculated using the following formula: log(E) = 9.05 + 1.96[M.sub.l] (Energy in ergs) (Kanamori et al., 1993). The strike-slip components in both caves reflect this decreasing regional tectonic activity.

The long term measurements from Driny Cave, characterised by strike-slip components, allow trends in the data to be defined (Fig. 9). From these, it is then possible to constrain the local stress field orientation. Two gauges in Driny Cave showed sinistral strike-slip displacements along NNE-SSW striking faults, whilst a third indicated dextral strike-slip movements along a NW-SE striking fault (Figs. 9 and 10) (Briestensky and Stemberk, 2008). The resultant major principal horizontal stress ([sigma]1) has an approximately NNW-SSE orientation (Fig. 10), as defined by a regional 2D deformation model (McClay, 1987; Bahat et al., 2005). This stress field orientation, in addition to the rate of strike-slip calculated for the various faults, is in full agreement with previous geological ideas (e.g. Hok et al., 2000; Kovac et al., 2002). However the orientation of the major principal stresses determined from recent earthquakes in and around the Male Karpaty Mts has recently been calculated to have an azimuth/plunge of [sigma]1 = 210-220[degrees]/5-25[degrees] (Fojtikova et al., 2010). These results are in direct contrast to the orientations calculated from microdisplacement observations in Driny Cave. The movements in Driny Cave were also indicated by damaged speleothems along the faults. Almost everywhere within the cave, freshly healed cracks cutting the sinter decoration appear in the close vicinity of fault outcrops (Briestensky and Stemberk, 2008).

[FIGURE 9 OMITTED]

5. DISCUSSION

The decision of where to locate gauges, either at the surface or underground, is of fundamental importance for studies of active fault movement. It is clear that caves and other underground sites are preferential as temperature is almost constant and the seasonal dilations that interfere with the identification of tectonic displacements are minimised. However, many caves around the world are associated with significant fluctuations of air temperature. Therefore, each cave system should be considered individually. Caves are also able to preserve a record of fault movement through slickensides and damaged speleothems along fault planes. Such properties are helpful when selecting the most suitable sites for monitoring rock massif behaviour over protracted periods. Nonetheless, certain problems do exist. Underground monitoring is often strenuous for operators. For example, six of the localities in western Slovakia require climbing and the use of specialist caving equipment. Fortunately, suitable fault sites can also be found at the surface. The site at Prekazka shows that surface measurements may be viable once the record is sufficiently long to identify seasonal dilations. In contrast, the site at Dobra Voda poses a number of interpretation problems.

[FIGURE 10 OMITTED]

Numerous factors dictate site suitability. Well defined and important fault structures may sometimes be inappropriate for monitoring depending on, for example, the condition of the rock massif, sun exposure, underground water table, depth of freezing, and gravitational slope deformations. The TM71 gauge is clearly capable being submerged underwater more-or-less continuously, as seen at Prekazka. The absence of electrical components and an anticorrosive finish allows the gauge to be used under extreme monitoring conditions.

Our results show that fault displacements do not develop consistently but are easy modified by stress changes. For example, in Slopy Cave two of the observed tremors affected vertical displacement components. Conversely, the second quake caused initiated only a change of the peak-to-peak amplitude of crack opening (see Fig. 6). Similar behaviour was also observed at Prekazka. If a sufficiently dense network of gauges can be constructed, these changes could be used in the future to discover the underlying earthquake mechanism.

The difference between the orientation of the major principal stress axes calculated from recent earthquakes in the Male Karpaty Mts. (Fojtikova et. al., 2010) and the orientation calculated from our microdisplacement observations in Driny Cave probably reflects the tectonic complexity of Male Karpaty Mts.: the local stress in the Driny Cave area can be quite different from the average stress computed from the large number of earthquakes scattered all over the Male Karpaty Mts.

6. CONCLUSIONS

From our data relating to the monitoring of fault microdisplacements in western Slovakia, the follow conclusions can be drawn:

* Sinter damage can be induced by active fault movements and therefore provides a useful indicator of ongoing tectonism. This damage enables localities for microdisplacement monitoring to be readily identified.

* Fault displacements seem to correspond to recent tectonic activity and probably to significant earthquakes in the vicinity of the study sites. The observed microdisplacements along differentially striking fault systems can provide data for determining local stress field orientations.

* The effects of seasonal dilation within the rock massif decrease rapidly with depth. In underground settings, the peak-to-peak amplitude of these seasonal displacements are either small or cannot be recognised. This minimises problems associated with data interpretation.

* Seasonal dilations affect the horizontal crack opening component most clearly both at surface and underground. The ongoing monitoring program should provide further opportunities to evaluate these trends.

ACKNOWLEDGEMENTS

The authors wish to gratefully acknowledge the financial support provided by the Czech Ministry of Education, Youth, and Physical Culture (Project COST OC 625.10), the Czech Science Foundation (Projects Nos. 205/05/2770, 205/06/1828, & 205/09/2024), the Institute of Rock Structure and Mechanics at the Academy of Sciences of the Czech Republic (Code A VOZ30460519), the Grant Agency of the Academy of Sciences of the Czech Republic (No. IAA300120801), the Slovak Grant Agency (VEGA Grant 2/0072/08), the Slovak Research and Development Agency (No. APVV-0158-06 Project NEOTACT "Neotectonic activity of the Western Carpathians"), and the European Union (No. 230669 FP7 Consortium Project AIM "Advanced Industrial Microseismic Monitoring").

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Milos BRIESTENSKY (1) *, Blahoslav KOSTAK (1), Josef STEMBERK (1), Lubomir PETRO (2), Jozef VOZAR (3) and Lucia FOJTIKOVA (4)

(1) Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, v.v.i., V Holesovickach 41, 182 09 Prague, Czech Republic

(2) Geological Survey of the Slovak Republic, Jesenskeho 8, 040 01 Kosice, Slovakia, Telephone: +421 556 250 045

(3) Geological Institute, Slovak Academy of Sciences, Dubravska cesta 9, 840 05 Bratislava, Slovakia, Telephone: +421 259 203 602

(4) Geophysical Institute, Slovak Academy of Sciences, Dubravska cesta 9, 845 28 Bratislava, Slovakia, Telephone: +421 259 410 611

* Corresponding author's e-mail briestensky@irsm.cas.cz

(Received January 2010, accepted October 2010)
Table 1 Descriptions of significant earthquake events registered
during the study period and their focal mechanisms. The focal
mechanisms were calculated using the polarities of P waves, the
amplitudes of P waves, and complete waveforms. The lower-hemisphere
equal-area projection is used.

Event       Place      Year    Month    Day       Time      MI

  1        V?bove      2006      3       13     8:28:38    3.2
  2        Trstin      2006      8       5      8:57:35    1.9
  3        Trstin      2006      8       5      8:58:50    1.6
  4        Trstin      2006      8       5      9:00:08    2.2
  5        Trstin      2006      8       5      23:43:18   1.4
  6      Dolna Krupa   2007      7       12     14:20:51   1.4
  7      Dobra Voda    2007      8       4      2:39:20    1.8
  8         Nahac      2008      9       3      6:03:48    0.3

Event        DC        CLVD     ISO    strike     dip      rake

  1         46.6       -42.5   -10.8    310        85      113
  2          8.7       61.8    29.5      46        39       59
  3         46.8       45.8    -7.3     360        51      -141
  4         77.8        6.4    15.8      72        58       7
  5         62.3       29.8     7.8     248        78      -48
  6           x          x       x       x         x        x
  7         82.9        1.7    15.4     340        60      154
  8           x          x       x       x         x        x

Event     Lat      Lon     Depth

  1      48.550   17.694   10.16
  2      48.520   17.475   4.25
  3      48.511   17.473   5.26
  4      48.516   17.468   5.23
  5      48.522   17.476   4.71
  6      48.470   17.520   18.1
  7      48.581   17.565   10.30
  8      48.567   17.500   10.1

Event    strike    dip     rake

  1        51       24      12
  2       264       58      113
  3       242       60      -46
  4       338       84      148
  5       351       43     -162
  6        x        x        x
  7        83       68      32
  8        x        x        x

Table 2 Description of the monitored fault structures and
the peak-to-peak amplitude seasonal effect in fault
displacements related to depth at the observed sites.

Locality               Previous fault       Fault structure
                          activity

                                            dip n      dip
                                                    direction
                                                    [[degrees]]

Dobra Voda trench   dextral /sinistral       76         136
                      strike-slip
Prekazka quarry     normal fault             75         140
Slopy Cave          thrust fault             70         315
Plavecka Cave 1     sinistral strike-slip    90         290
Zbojnicka Cave      thrust fault             65         245
Sedmicka Cave       normal fault             89         80
Driny Cave 3        strike slip              75         40
Driny Cave 1        normal fault             70         290
Plavecka Cave 2     normal fault             65         240
Driny Cave 2        normal fault             70         110
Cachticka Cave      sinistral strike-slip    80         270
Beckovska Cave      normal fault             80         310

Locality           Monitoring     Gauge situation    Peak-to-peak
                   start [year]                      amplitude [mm]

                                  surface   depth        crack
                                             [m]        opening

Dobra Voda trench      2004          x        -           0.7

Prekazka quarry        2005          -       0.5          0.25
Slopy Cave             2005          -        8           0.07
Plavecka Cave 1        2007          -        11          0.025
Zbojnicka Cave         2005          -        11
Sedmicka Cave          2007          -        12          0.03
Driny Cave 3           2005          -        20          0.05
Driny Cave 1           2005          -        21
Plavecka Cave 2        2007          -        22          0.03
Driny Cave 2           2005          -        25
Cachticka Cave         2007          -        30          0.025
Beckovska Cave         2008          -        45           0.02

Locality               Peak-to-peak
                       amplitude [mm]

                    strike-slip   vertical
                                    shift

Dobra Voda trench       0.2          0.7

Prekazka quarry
Slopy Cave
Plavecka Cave 1        0.03
Zbojnicka Cave
Sedmicka Cave          0.05          0.06
Driny Cave 3                         0.1
Driny Cave 1
Plavecka Cave 2                      0.09
Driny Cave 2
Cachticka Cave                       0.04
Beckovska Cave                       0.03

Fig. 8  The energy released by earthquakes
in western Slovakia for the period 1990-2008.
The number of quakes is shown on the peak of
columns (data provided by the Geophysical
Institute of the Slovak Academy of Sciences).

Year    log E (Erg)

1990       16
1991       77
1992       98
1993        5
1994        2
1995        7
1996        8
1997       17
1998        8
1999       27
2000       22
2001       35
2002       37
2003       79
2004       140
2005       45
2006       167
2007       91
2008       43
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Author:Briestensky, Milos; Kostak, Blahoslav; Stemberk, Josef; Petro, Lubomir; Vozar, Jozef; Fojtikova, Luc
Publication:Acta Geodynamica et Geromaterialia
Article Type:Report
Date:Oct 1, 2010
Words:4773
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