Acta Geodyn. Geomater., Vol. 8, No. 1 (161), 17–27, 2011 GROUNDWATER LEVEL VARIATIONS IN THE SEISMICALLY ACTIVE REGION OF WESTERN BOHEMIA IN THE YEARS 2005–2010 Renata GAŽDOVÁ *, Oldřich NOVOTNÝ, Jiří MÁLEK, Jan VALENTA, Milan BROŽ and Petr KOLÍNSKÝ Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, v.v.i., V Holešovičkách 41, 182 09 Prague, Czech Republic *Corresponding author‘s e-mail: [email protected] (Received December 2010, accepted February 2011) ABSTRACT The western part of the Bohemian Massif (Vogtland/West-Bohemia region at the Czech-German border) is characterized by relatively frequent intraplate earthquake swarms and by other manifestations of current geodynamic activity, such as mofettes, mineral and thermal springs. In this study we analyze variations of groundwater level in four hydrological wells in the region during the years 2005–2010. Monitoring during the previous time interval of 2000–2004 is also mentioned and used for comparison. Two of the wells are located in the epicentral region of Nový Kostel, and the other wells are more distant. The time interval includes the 2008 earthquake swarm when all the wells displayed a noticeable drop in the water level. This effect was observed up to epicentral distances of nearly 30 km, which exceeds the distances of hydrological changes observed during previous earthquake swarms. Moreover, it seems that a small rise in the water level preceded the intervals of increased seismic activity, which could represent a certain precursory phenomenon. On the other hand, the hydrological changes in the Nový Kostel area were relatively small, indicating that this epicentral area is not hydrologically linked with the seismically active fault at depth. Consequently, more suitable localities for hydrological monitoring should be sought in a broader vicinity of Nový Kostel. KEYWORDS: Western Bohemia, earthquake swarm, groundwater level 1. INTRODUCTION The strong earthquake swarm at the turn of the The Vogtland/West-Bohemia region in the years 1985 and 1986 aroused great interest in this western part of the Bohemian Massif is known for the seismically active region. Two seismic arrays were occurrence of e arthquake swarms with earthquakes then installed in the Czech part of the region. Thus, of intermediate magnitude (ML < 5). The active part of the subsequent earthquake swarms in the years 1997, the region is located in the western part of the Krušné 2000 and 2008 have already been well documented hory Mountains (Ore Mts., Erzgebirge) at the Czech- (Fischer and Horálek, 2000; Slancová and Horálek, German border. The seismically active area is 2000; Fischer and Michálek, 2008; Horálek and delimited approximately by the towns of Kraslice, Aš, Fischer, 2008; Horálek et al., 2009; Fischer et al., Cheb and Sokolov (Fig. 1). Swarm earthquakes are 2010). Moreover, the 1985/1986 earthquake swarm usually recognized as a common feature of continental also initiated various studies of hydrological, volcanic regions and mid-ocean rifts (Hill, 1977; geochemical and geodetic variations, and their Sigmundsson et al., 1997; Dahm et al., 2008). relations to seismic activity. In the following, some Historical macro-seismic reports confirm seismic measurements of water and soil gas parameters in the events in the Vogtland/West-Bohemia region as far earthquake area are presented. b ack as 1198 (Kárník, 1963). Since the end of the 19th During the strong 1985/1986 earthquake swarm, century, the most intensive earthquake swarms significant changes in mineral spring parameters were exceeding the local magnitude of 4.0 occurred there in observed at the spa of Františkovy Lázně, about the years 1896/1897, 1903, 1908/1909 and 1985/1986 15 km distant from the epicentral area. Several (Bormann, 1989; Neunhöfer and Hemmann, 2005). changes in discharge were very distinct, amounting up Since 1985/1986, most hypocentres have been to 40 %, and had a co-seismic character (Novotny and concentrated near the village of Nový Kostel Matyska, 1988). The changes in temperature were less (50.24°N, 12.44°E). This seismically active zone is noticeable, but preceded the beginning of the swarm located at the intersection of the N-S-trending Nový by several months (Stejskal et al., 2008). Some Kostel-Počátky-Zwota zone with the NNW-SSE- hydrological changes persisted for nearly two years striking Mariánské Lázně Fault zone (Bankwitz et al., after the earthquake swarm. The character of the 2003; Fischer and Horálek, 2003; Neunhöfer and observed changes seemed to support the hypothesis of Meier, 2004; Geissler et al., 2005). an injection of mantle fluids as the main triggering 18 R. Gaždová et al. Table 1 Parameters of the wells: code, locality, northern latitude and eastern longitude in degrees, altitude h in metres, depth d of the well in metres, r is the distance from well NK-1 in kilometres. Code Locality Latitude Longitude h d r Measured since NK-1 Nový Kostel 50.2299 12.4430 535 23 0.0 June 2000 NK-2 Nový Kostel 50.2318 12.4472 595 23 0.4 June 2000 HM-1 Krásno 50.1067 12.7721 805 20 27.2 June 2000 S-4 Lázně Kynžvart 50.0077 12.6346 718 97 28.3 November 2006 mechanism of the earthquake swarm and the main can cause pre-, co- and post-seismic groundwater cause of the observed anomalies (Novotný et al., level changes. From the earthquake prediction point of 1987; Babuška and Plomerová, 2008; Horálek and view, the pre-seismic changes play the most important Fischer, 2008; Stejskal et al., 2008). role as they could represent possible earthquake During the 1985/1986 earthquake swarm, some precursors. Such effects have been reviewed, e.g., by observers also described sudden transient changes of Kissin (1982), Roeloffs (1988), Kissin and Grinevsky the water level in wells at several other sites of the (1990), King et al. (2006). These authors have region. Both the water level rises and drops were summarized the characteristics of pre-seismic reported; see the references in Stejskal et al. (2008). groundwater level changes, such as the size of the All these phenomena were classified as co-seismic anomaly, lead time of the occurrence, and possible effects. Some authors explained them by changes in relations between earthquake magnitude or epicentral the mechanical properties of water reservoirs during distance and the amplitude of the anomaly. Possible ground shaking. The hydrological changes were not mechanisms of earthquake triggering by increased observed farther than 20 km from the epicentre. pressure of deep fluids (after critical stress Accurate measurements of some hydrological accumulation) have been analyzed, e.g., in the most and geochemical parameters were carried out during recent paper by Koch and Heinicke (2011). the earthquake swarms in the years 1994 and 2000 The present paper deals with monitoring where anomalous variations were observed (Koch and the groundwater level in Western Bohemia during Heinicke, 1999; Heinicke and Koch, 2000; Weise et the years 2005–2010. As this time interval includes al., 2001; Koch et al., 2003; Geissler et al., 2005; the occurrence of the earthquake swarm in 2008, some Stejskal et al., 2005; Bräuer et al., 2007). Bräuer et al. relations between earthquakes and hydrological (2003) assumed a captured fluid trap zone above the changes will be discussed. N ový Kostel hypocentre, which could be responsible for the fact that noticeable gas outlets and mineral 2. GEOLOGICAL SETTING AND LOCATION OF springs do not exist in the Nový Kostel area. Soil gas THE WELLS monitoring was carried out at three localities during The Institute of Rock Structure and Mechanics of the Academy of Sciences of the Czech Republic the 2008 earthquake swarm, which has been the most intensive earthquake activity since 1986 (Faber et al., monitors the groundwater level in Western Bohemia 2009). For preceding soil gas monitoring we refer to in four observation wells (Fig. 1). Two wells (denoted the paper of Weinlich et al. (2006). as NK-1 and NK-2) are situated in the vicinity of the The groundwater level is affected mainly by village of Nový Kostel. The third well (HM-1) is hydrological, meteorological and anthropogenic located near the town of Krásno and the last one (S-4) is set in the area of the Pediatric Spa at Lázně factors. Moreover, it is also influenced by deformation processes in the Earth’s crust including tidal forces Kynžvart. Groundwater levels in wells NK-1, NK-2 and changes in stresses resulting from seismic and HM-1 have been measured since June 2000 and activity. In the literature, one can find many reports the results of these observations for the period 2000- describing effects of earthquakes on hydro-geological 2004 were described by Stejskal et al. (2005). structures in various seismically active regions (e.g., Additional measurements in well S-4 started in Novemeber 2006. The main parameters of the wells Thomas, 1988; Igarashi and Wakita, 1990; Roeloffs and Quilty, 1997; Grecksch et al., 1999; Gavrilenko et are summarized in Table 1. al., 2000; Chadha et al., 2003; Montgomery and Manga, 2003; Stejskal et al., 2009). One of the best 2.1. NOVÝ KOSTEL examples is the Great Geyser in Iceland whose Two of the wells, NK-1 and NK-2, were drilled activity depends significantly on earthquakes for scientific purposes in the epicentral area of Nový Kostel. This area is situated at the NW margin of (Rinehart, 1968). Kümpel (1992) also concluded that changes in the Svatava crystalline block, which is a part of the groundwater level in wells reflect the stress the Krušné hory (Erzgebirge) crystalline unit. The variations in the Earth’s crust. The stress variations Svatava crystalline block is further divided into two GROUNDWATER LEVEL VARIATIONS IN THE SEISMICALLY ACTIVE REGION OF … 19 blocks by the Krušné hory fault.