Hydrothermal Alteration As a Trigger Mechanism for Earthquake Swarms 3

Geophys. J. Int. (2009) 178, 1–13 doi: 10.1111/j.1365-246X.2009.04138.x

Hydrothermal alteration as a trigger mechanism for earthquake eophysics swarms: the Vogtland/NW Bohemia region as a case study g

1 2 3 4 1 5 pplied J. Heinicke, T. Fischer, R. Gaupp, J. Gotze,¨ U. Koch, H. Konietzky a and K.-P. Stanek6 and 1Sac¨ hsische Akademie der Wissenschaften zu Leipzig, Ofﬁce TU BAF, Institut fur¨ Angew. Physik, Freiberg, Germany. E-mail: [email protected] 2Charles University in Prague, Faculty of Science and Institute of Geophysics, Academy of Sciences, Prague, Czech Republic ﬁeld 3Friedrich Schiller Universitat¨ Jena, Institut fur¨ Geowissenschaften, Jena, Germany 4TU Bergakademie Freiberg, Institut fur¨ Mineralogie, Freiberg, Germany 5TU Bergakademie Freiberg, Institut fur¨ Geotechnik, Freiberg, Germany 6TU Bergakademie Freiberg, Institut fur¨ Geologie, Freiberg, Germany otential p ,

Accepted 2009 February 3. Received 2009 February 2; in original form 2008 May 13 desy Geo

SUMMARY GJI Earthquake swarms occur mostly in regions with CO2-enriched pore fluids. It is generally accepted that both geodynamic stress accumulation and critical pore fluid pressures act as a triggering mechanism for most seismic events. The new thesis presented here is that hydrothermal alteration processes in fault zones help facilitate the shear failure propagation due to mechanical weakening and dissolution of the wall rock, in addition to the normal shear stress and fluid overpressure. The basic idea that stress corrosion cracking results from chemical weakening and comminution has been discussed for many years. However, it has not yet been applied to explain the earthquake swarm phenomenon. Studies of extensive alteration as well as the latest investigations of CO2 sequestration give evidence that these high dissolution rates of wall rock in contact with an acid fluid phase exist in seismogenic fault zones. Several indications support the assumption that in the Vogtland/NW Bohemia region, the weakening of stressed fault zones by hydrothermal alteration could take place at seismogenic depths and could generate earthquake swarms. Investigations of quartz samples from the fracture zones by means of cathodoluminescence as well as spatiotemporal analysis of seismicity and numerical modelling of alteration-induced earthquake swarms support this hypothesis. Key words: Gas and hydrate systems; Hydrothermal systems; Fracture and flow; Earthquake source observations; Fractures and faults.

larger fractures from developing, and as a consequence, a swarm- 1 INTRODUCTION like seismicity is observed. Earthquake swarms are most frequent Earthquake swarms are seismic sequences with a strong time–space near active volcanoes (Roman & Cashman 2006), where both mag- clustering that have no dominant single event and relatively small matic activity and hydrothermal CO2 emission take place, as well magnitudes. Earthquake swarms occur mainly in volcanic areas, as in previously active volcanic regions (or regions of unrest), for geothermal fields and ocean ridges (e.g. Wyss et al. 1997; Lee example, the Long Valley Caldera/Mammoth Mountains (Hill et al. 1998). Intraplate earthquake swarms without active volcanism are 1990), the Alban Hills (Feuillet et al. 2004) and the Izu Peninsula unusual. They occur mainly in the areas of enhanced crustal fluid (Koizumi et al. 2004), as well as in geothermal regions such as activity, particularly in regions with the Quaternary volcanism. The Larderello, Italy. Hill (1977) and Rubin & Gillard (1998) discuss moderate size of swarms is characterized by the size of the largest dyke intrusions associated with active volcanoes as a stress factor events only rarely exceeding the magnitude level of 5. responsible for generating earthquake swarms. The occurrence of swarms is explained by intrusion of crustal In intraplate regions, the occurrence of swarms is usually ex- fluids into the fault zone or by a seismic slip (Vidale & Shearer plained by simultaneous activity of tectonic stress and the existence 2006). Fluid activity can be due to either a periodic dyke intrusion of highly pressurized pore fluids in subcritically loaded rocks. Arti- that generates stress heterogeneity near the fracture zone (the swarm ficially increased pore pressure in the KTB borehole was the trigger model of Hill 1977) or an injection of high-pressure fluid that de- mechanism of many microevents at a few kilometres’ depth (Zoback creases the effective normal stress. Yamashita (1999) has shown by & Harjes 1997; Shapiro et al. 2006) or in hot dry rock projects numerical modelling that if the seismic slip is accompanied by the (Evans et al. 2005). Many studies show that the stress perturbation creation of new pores, the resulting increase in pore space prevents responsible for triggering earthquakes is rather low, down to some

C 2009 The Authors 1 Journal compilation C 2009 RAS 2 J. Heinicke et al. bars (King et al. 1994; Hardebeck et al. 1998; Ziv & Rubin 2000; Fein & Walther (1987) and Bischoff & Rosenbauer (1996). These Brauer¨ et al. 2004). Similar low values are reported for earthquake reports show the dissolution of feldspar, calcite and quartz, as well swarms (Liu et al. 2003; Rothert & Shapiro 2007). The resulting as alteration of granite or metamorphic rocks in first experiments fracturing of the heterogeneous fault zone and extensive flow of under supercritical CO2–H2O conditions. May (1998) modelled the mineralized fluids is followed by precipitation and gradual healing progressive alteration of silicates by CO2-rich waters including the of the fault (Sibson et al. 1975). influence of different water types, temperatures (≤150◦C), pressures Here we show that besides these effects, the circulating hydrother- (<500 bar) and salinity. mal, possibly CO2-saturated, fluids can act aggressively on the host The fluid–rock interaction has been given more attention in the rock by dissolving and altering its minerals, which could induce past decade owing to the increased interest in mineral precipitations earthquake swarm generation. In the first part, we review the results (mining industry) and the sequestration of CO2. Parry (1998) dis- of the recent investigations of the underground CO2 sequestration to cusses the influence of pressure, temperature and fluid composition get an idea of the process kinetics. Next, we analyse the hydrogeo- as the main factors determining the solubilities of the fracture-filling chemical and seismological observations from the Vogtland/NW minerals calcite and quartz, and the formation of alteration minerals Bohemia earthquake swarm region with abundant CO2 emission (e.g. clay minerals) that severely influence the mechanical behaviour to find indications for hydrothermal alteration taking part in earth- of the rock. quake swarm generation. Laboratory experiments at 400–800◦C and 1–9 kbar (Shmulovich et al. 2006) provide evidence for the solubility of quartz in brines with H O–CO admixtures under crustal constraints. The most im- 2 HYDROTHERMAL ALTERATION 2 2 portant results come from the latest investigations of geological DUE TO CO - ENRICHED AQUEOUS 2 CO sequestration. Kaszuba et al. (2003) report a significant re- CRUSTAL FLUIDS: AREVIEW 2 active behaviour of supercritical CO2 to silicate minerals (quartz, plagioclase, microcline and biotite) at 200◦C and 200 kbar, which 2.1 Thermodynamic aspects corresponds to the conditions at the hypocentre depth of about The presence of aggressive CO2-enriched fluids under hydrother- 8 km. The presented experiments show a short-term reaction of the mal conditions suggests that alteration processes take place in pores mineral assemblages: increased mineral dissolution due to CO2 in- and fracture zones. Although the geochemical influence of a CO2– jection into the experimental chamber after 80 days. Gunter et al. H2O mixture on the rock matrix is not yet perfectly understood, (2000) performed studies of CO2 trapping and mineral dissolution many theoretical models and experimental investigations have shed under model conditions with glauconitic sandstones and carbonatic some light on this continuous process in crustal compartments, aquifers. In most of their runs, muscovite is the only newly formed including the influences of temperature and pressure conditions. phase increasing with temperature (muscovite/mica-like clay min- The fluid separation into a complex multiphase CO2–H2Ofluid erals as major authigenic phases that reduce rock strength). Xu et al. according to its thermodynamic conditions in the upper crust is (2004); Gaus et al. (2005) and Suto et al. (2007) give an overview of important. Thermodynamic models to calculate the solubility of wall rock minerals and their reaction kinetics affected by carbonic CO2 in aqueous solutions, partly with additional ions of Na, K acid. Collettini & Holdsworth (2004) show Miocine faults with or Mg, were calculated by Duan & Sun (2006); Hu et al. (2007); long-term alteration and discuss the weakening of faults because Portier & Rochelle (2005) and Li & Duan (2007). According to of CO2-rich pore fluids causing dissolution-precipitation processes these results, CO2 exists as a fluid in a two-phase system of liq- followed by a shear zone deformation. uid and vapour phases and their transitions occur under condi- All these investigations support the concept of the alteration tions above the critical temperature (31◦C) and critical pressure of primary host rock minerals like quartz and feldspar into sheet (74 bar). In deeper reservoirs, water is in a supercritical state silicates (mica, clay) by aggressive fluids containing considerable ◦ (>374 C, >218 bar). The thermodynamic models of phase sep- amounts of CO2. Hereby, the secondary mineral assemblage can in- aration show two main phases: an H2O-rich liquid with a few per fluence the porosity and permeability, thus sealing fractures by pre- cent CO2 and a CO2-rich liquid/vapour with a few per cent water cipitation, or can be transported to the surface via mineral springs, (Shyu et al. 1997). These phases can coexist as a binary system which contain in general a minimum of 1 g of dissolved minerals and will be immiscible over a broad range of temperature and pres- per litre, or other groundwater sinks. sure within the shallow crust (Kaszuba et al. 2003, 2006). Li & Duan (2007) show their chemical properties in model calculations 2.3 Seismogenic aspects with a pressure up to 1 kbar: the pH of the carbonic acid decreases with increasing dissolved CO2 content, with decreasing temper- Critical stress conditions and fluid pressure perturbations are sup- ature (250–50◦C) and with increasing salinity (NaCl). Therefore, posed to be the main trigger mechanism for brittle faulting (Sibson we must consider on the one hand CO2-dominated fluids in mag- 2000; Rothert & Shapiro 2007). However, the associated weakening matic and metamorphic CO2 reservoirs with a negligible content of the host rock due to alteration appears to be crucial for the seis- of water (‘dry’ mofettes on the Earth’s surface give evidence for mogenic process. The consequence is a reduced Coulomb fracture this), and on the other hand water-filled fracture systems with only criterion caused by a reduced friction coefficient. The first experi- a few per cent dissolved CO2. The occurrence of these fluids can be ments to investigate the stress corrosion cracking of quartz because recognized by surface expression, for example, sparkling acidulous of hydrolytic weakening were carried out by Atkinson & Meredith springs (Sauerlinge).¨ (1981). Scholz (1990) and Dunning et al. (1994) follow this method of slow crack and fracture growth as well as of comminution in seismically active fault zones. 2.2 Aspects of fluid–rock interaction The application of this complex interaction of alteration, rock The reactive behaviour of supercritical CO2 under shallow crustal softening and reduced friction strength to microseismogenic pro- conditions was first investigated by Pacesˇ (1973); Lagache (1976); cesses is a logical consequence.

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3 POSSIBLE ALTERATION IN THE The marginal faults of the Eger Graben cross-cut the Variscan base- EARTHQUAKE SWARM REGION ment. In the westernmost part of the graben, the marginal faults were VOGTLAND/NW BOHEMIA: A CASE cut by still-active faults (Bankwitz et al. 2003). These young, NNW- STUDY to N-trending faults belong to the Regensburg–Leipzig lineament, a zone of increased seismic activity. Along the Regensburg–Leipzig 3.1 Geodynamics of the area lineament, the most active region comprises the northwestern Bo- hemia and the Vogtland. The Vogtland/NW Bohemia region comprises the western extent The Vogtland/NW Bohemia region is characterized by the fre- of the Erzgebirge metamorphic dome (Fig. 1). The marginal part quent occurrence of earthquake swarms. The increased occurrence of the metamorphic complex consists of Palaeozoic greenschist of this low-magnitude activity at the end of the 19th century initi- facies, metasedimentary and metavolcanic sequences intruded by ated the first scientific description of this phenomenon: the so-called post-orogene granites of the Eibenstock–Karlovy Vary massif. The Erdbebenschwarm (Credner 1900). The long-term trend and peri- Variscan tectonometamorphic structure was uplifted and eroded odicity of swarms was analysed by Neunhofer¨ & Meier (2004), during the Late Carboniferous and Permian. The brittle deformation who found a statistically significant recurrence interval of strong during and after the uplift of the Erzgebirge complex was dominated swarms of about 72 months. The improved localization accuracy in by NW-trending strike-slip and normal faults. These faults were past years indicates temporal variability of the position of different reactivated several times during the Mesozoic and Cenozoic, fa- seismically active compartments. The minimum focal depths are cilitating the hydrothermal mineralizations of the West Erzgebirge about 5 km, whereas the maximum depths change from less than ore district. Starting in the Eocene, the subsidence of the ENE- 12 km in the central Novy´ Kostel (NK) area down to 22 km in the trending Eger Graben was accompanied by extensive volcanism. NNE direction (Babuskaˇ et al. 2007). Spiˇ cˇak´ & Horalek´ (2001) speculated about a stepwise intrusion of fluids into the fractured volume. Several numerical models and statistical analyses of the triggering of the swarm activity were presented (Parotidis et al. 2003; Hainzl 2004; Fischer & Horalek´ 2005; Hainzl & Ogata 2005) showing that not only fluid injection but also the continuous interaction between the consecutive events (similar to elastic stress transfer) are responsible for initiation and further driving of the continuing swarm activity. The brittle failure mode (shear, tensile and their transition) is highly sensitive to the acting fluid pressure (Sibson 2000). The vary- ing stress directions during the Variscian Orogeny formed highly fractured crystalline bedrock, which resulted in the development of extensional fault-fracture meshes. Local seismicity induced by the regional stress field generated many small fault planes running in several directions, which is a typical feature of this area.

3.2 Origin and distribution of CO2 emission sites and mineral springs The Vogtland/NW Bohemia region is well known for its spas and many acidic springs and mofettes. Contrary to the gas-bearing springs with significant water discharge, mofettes are dry CO2 out- lets, even if they are often filled with near-surface groundwater. The high content of CO2 is the result of a relatively young Quaternary volcanism with a supposed recent magmatic body associated with a local Moho updoming up to 27 km depth (Geissler et al. 2005). Delta 13Cdataof−4.5...−1.7‰ and high 3He/4He ratios up to 6.2 Ra are evidence for the magmatic origin of the CO2 emission (Weinlich et al. 1999; Brauer¨ et al. 2005). The location of the magmatic reservoir was proposed in the centre of our area of interest as a mantle/crust updoming detected by seismic investigations of re- ceiver function studies (Heuer et al. 2006). The over-pressurized hydrothermal fluids from the magmatic CO2 reservoir are distributed along the existing fault zones and migration pathways, which were created during ancient geological/tectonic processes. Babuskaˇ et al. (2007) proposed that boundaries of three mantle lithosphere units identified by different orientation of seismic anisotropy could pro- 3 vide open paths for the ascent of He- and CO2-rich fluids directly from the asthenosphere. The CO2-dominated gas phase escapes to the surface mainly via Figure 1. Geological map of the region of interest. mofettes. The local magmatic gas reservoirs have probably only

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Because of hydrodynamic dispersion, the CO2-oversaturated water will be ‘diluted’ or mixed with further CO2-free groundwater along its flow path. The existence of these mixed fluids has been recognized by field surveys. Fig. 2 shows a map with some of the 290 existing acidic springs and mofettes [see also Weinlich et al. (1999)]. The geographic coordinates are based on data by Geissler et al. (2005) and our own GPS mapping results. Interest- ingly, more or less all springs with CO2 are located at, along or near rivers or creeks that can be used as geomorphological sig- natures of fault zones. The local strike is marked by short blue lines in Fig. 3. Accordingly, the gas emission sites, the hydrological regime and the morphology reflect the ancient fracture/fault system that has been used as a transport pathway for these fluids to the surface.

3.3 Indications of alteration in the studied region The result of recent alteration processes can easily be recognized in the mineral content at the springs on the surface. Egerter et al. (1984) and the web pages of the local spas give an overview of the main chemical components of the regionally distributed springs. On the basis of the host rock, the mineralization of the waters is different, reflecting a kind of geological signature. Springs perco- Figure 2. Spatial distribution of mofettes (green points) and acidulous min- lating through the local granitic and metamorphic host rock show a eral springs (blue points) in the Vogtland/NW Bohemia region. typical enrichment in Na, Ca, Mg, Fe2O3,SiO2 and HCO3. Dissolution of minerals leads to their deficit in their host rock. a few conduits to the surface filled with a pure CO2-dominating Transformation of host rock minerals, for example, the argillitiza- phase, marked by six mofettes (Fig. 2). The three areas of mofettes tion of feldspar, changes mechanical rock properties and alteration, are located on the top of the proposed mantle updoming (Heuer et al. can contribute chemical species to the water involved in the reac- 2006) and above the junction of the three mantle blocks (Babuskaˇ tions. We show a simple estimation for the Vogtland/NW Bohemia et al. 2007). spring area on the basis of the mineral transport during the period Besides these CO2-dominated fluids, water-filled pores and frac- of the past 100 yr, for which relatively reliable data are available. ture zones will dissolve CO2 according to the mixture kinetics. The calculated mineral deficit amounts to about 111 kton of total

Figure 3. All mofettes and springs of Fig. 2 are located on creeks, rivers or valleys. Short blue lines indicate the local strike of creeks and river valleys. These lines also indicate local faults or geomorphological weak zones. Their distribution reﬂects the local geomorphology according to the main tectonic elements trending in the N–S, SW–NE and NW–SE directions. The partly shaded relief supports the direction of the tectonic lineaments. Blue circles: degassing areas of the Soos natural park area. Red squares indicate cities. The most important local fault zone, the ‘Marienbader Stor¨ ungszone,’ is marked as a black line.

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Table 1. Estimation of the total dissolved solids (TDS), CO2 (dissolved) and free CO2 transported via springs in Vogtland and NW Bohemia during the last 100 yr

Region TDS CO2 dissolved CO2 free Total Remarks t/100a t/100a t/100a t/100a

Vogtland including Bad 58 900 16 000 600 75 600 CO2 underestimated because of Brambach and Bad Elster lacking gas ﬂow data

NW Bohemia including Frantiskˇ ovy 52 800 58 300 343 400 454 500 TDS and CO2 dissolved underestimated and MarianskeL´ azn´ eˇ because of lacking discharge data at Frantiskˇ ovy and MarianskeL´ azn´ eˇ Total 111 700 74 300 344 000 530 100

Figure 4. Borehole picture in 111 m depth of a well drilled at Bad Brambach. The open width of the fracture is approximately 1–2 cm here.

dissolved solids (Table 1). The mass of dissolved CO2 is about (Neuser et al. 1995). The system was operated at 14-kV acceler- −2 74 kton and that of transported free CO2 gas at least 340 kton. ating voltage and a current density of about 10 µAmm .Lu- The transport of dissolved minerals takes place in open fractures minescence images were captured ‘on-line’ during CL operations of much less than a millimetre up to a centimetre width. An image using a Peltier cooled digital video camera (KAPPA 961-1138 CF of an open fluid-filled fracture at a depth of 111 m in a drill hole at 20 DXC). The use of a CL microscope permits the comparison of Bad Brambach shows an example of conduits with high diffusivity photomicrographs obtained in transmitted polarized light with those (Fig. 4). from CL. Luminescence studies of quartz (SiO2) and other silica modifica- tions are of great interest because they can provide information not 3.4 Cathodoluminescence measurements on altered rock available from other analytical methods. The close relationship be- samples from the shallow crust tween crystal chemical properties and luminescence characteristics Fortunately, when the springs at Bad Brambach were first estab- is the basis for recognizing different growth generations, internal lished around 1913, three rock samples were collected from the structures or the distribution of trace elements within quartz. The spring shaft near this borehole. The study of thin sections from defects causing the different CL emissions in quartz often reflect the this previously hydrothermally altered host rock gave insights into specific physicochemical conditions of crystal growth and therefore a repeated precipitation and dissolution of quartz, calcite and chal- can be used as a signature of genetic conditions of mineral forma- cedony along open microjoints during the existence of this fracture tion. This close connection between primary conditions of quartz for more than 3 Ma. The open joints are accompanied by discrete formation, namely defect structures (incorporation of defects) and zones of cataclastic fracturing in quartz crystals. To evaluate the characteristic CL emission, can be used to interpret primary ori- different generations of hydrothermal mineralization, we applied gin and secondary alteration processes of natural quartz. Therefore, cathodoluminescence (CL) imaging to the thin sections. different generations of quartz from primary crystallization and/or The CL measurements were performed on carbon-coated, pol- from secondary alteration processes can be distinguished by their ished thin sections using a ‘hot cathode’ CL microscope HC1-LM different luminescence colours.

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Figure 5. CL image of a quartz crystal (primary hydrothermal generation with blue CL) of the Bad Brambach rock samples showing strong alteration features; banded chalcedony represents a secondary mineralization event.

Figure 6. Euhedral quartz crystal (blue CL) from Bad Brambach; ﬁssures and cracks resulting from secondary alteration are only visible under CL as brown lines.

In most of the Bad Brambach quartz samples, the first quartz gen- chalcedony with different luminescence properties. It is presumed eration is characterized by a typical transient blue CL. The primary that the silica rapidly precipitated from oversaturated silica-bearing blue quartz CL colour disappears under the electron beam after sev- solutions, probably partly via a non-crystalline precursor. These pro- eral seconds of irradiation. This behaviour is typical for hydrother- cesses altered and mechanically cracked the primary quartz (Fig. 6) mal alpha-quartz crystallized from aqueous solutions (Gotze¨ et al. and formed agate-like silica structures (Fig. 7). 2001). The relatively homogeneous primary internal structure indi- All these observations lead to the conclusion that the silica min- cates that the crystals formed under more or less constant conditions eralization in the Bad Brambach samples results from multiple (Fig. 5). processes. During Mesozoic times, hydrothermal euhedral quartz However, the euhedral hydrothermal quartz shows distinct fea- crystallized first from aqueous solutions in open veins at depths tures of secondary alteration in the outer part. The primary quartz of several kilometres. CO2-enriched fluids altered these crystals grain was corroded and secondary generations of silica precipi- at the surface of the veins and at the same time, these aggressive tated. These generations partially crystallized as microcrystalline fluids penetrated into microfissures of the crystal generated by the

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Figure 7. CL micrograph of secondary agate-like chalcedony which crystallized within the free pore-space under near-surface conditions. ancient tectonic activity. These microfissures are visible in Figs 5– the existence of extensional components in the source mechanisms 7 as brown lines in comparison to the undisturbed blue primary of the 1997 swarm in the NK area. However, none of the existing quartz crystal. Such destroyed crystals reduce the asperities and the seismological studies have shown direct evidence for the presence friction coefficient and lead to a latent mechanical instability. At of pressurized fluids in the seismogenic zone; they have remained present, under near-surface conditions, several generations of sec- somehow hidden and have been identifiable only indirectly as a ondary silica filled the open pore space of the veins to produce the force responsible for bringing the focal zone to a subcritical state. present formation, also visible in Fig. 7. The chemical and physical interactions between the CO2- These investigations give us the opportunity to get a realistic saturated pressurized fluids and the host rock may either create insight into the recent chemical and physical conditions of the hy- new fissures and planes of weakness or heal previously ruptured drothermal fracture zones, which are identical with the recent seis- fault surfaces and reactivate them later. To get some insight into mogenic crustal volume because these crystals grew under the same these processes, we analysed the spatiotemporal relations of the conditions some millions of years ago. seismic activity in the NK area. We used the relative locations of the 1991–2007 seismicity of Fischer & Michalek´ (2008) amounting to about 13 000 events in the magnitude range from −0.5 to 3.2 with the mean location error of about 100 m. Fig. 8 shows the locations 3.5 Indications of possible alteration in swarm seismicity along with their magnitude–time plot colour-coded by their origin The distribution of West-Bohemia/Vogtland seismicity is clustered time. The hypocentres occurred at depths from 6 to 13 km along both in time and space. The time occurrence is manifested in a va- an almost vertical, 8-km-long fault belt striking 169◦ east. More riety of forms, including fast and slow swarms that last from hours than 8000 events occurred during the 2000 swarm. It is apparent to months, as well as solitary events. The lateral distribution is lim- that during the observation period the activity migrated from north ited to a few focal zones among which the NK area dominates with to south and to greater depths. Simultaneously, new sections of 85 per cent of energy release during the past 15 yr. Although the rea- the fault area were activated. Interestingly, according to Fischer & sons for the strongly heterogeneous lateral distribution of seismicity Michalek´ (2008), a variety of focal mechanisms indicate activation are poorly understood, many seismological studies have provided of a complex fracture system. some basic understanding of the possible forces that could trigger To quantify the migration of activity, we examine, on the basis the individual swarm and drive its activity. Most of the existing of the location accuracy, the number of events activating new intact studies are based on data from the earthquake swarm of 2000 in areas termed below fresh ruptures. Our approach is based on the the NK area and aim to trace the effects of pore pressure diffusion premise that two earthquake ruptures are separate if their mutual and elastic stress changes in the swarm seismicity. According to distance is larger than their location error. Accordingly, we have the studies of Hainzl & Fischer (2002); Parotidis & Shapiro (2004); subdivided the focal zone into more than 240 000 cells with the Hainzl & Ogata (2005) and Fischer & Horalek´ (2005), see also length of their size equal to the mean location error in x, y and z.Ina Horalek´ & Fischer (2008) for review, the 2000 swarm was probably spreading time window starting at the beginning of the investigated initiated by an injection of high-pressurized fluids at the bottom of interval, we calculated the number of ruptures occupying newly the focal zone. Its further development was probably driven by a activated cells and the number of ruptures that occur within already combination of pore-pressure diffusion along newly open fractures activated cells. Note that occurrence of two ruptures within one cell and elastic stress transfer. Independent evidence for the presence does not necessarily imply rupture reactivation. This depends on the of high-pressurized fluids in the seismogenic zone comes from the relation between the size of the rupture and the size of the cell. Here studies of Horalek´ et al. (2002) and Vavrycukˇ (2002), which indicate should be noted that only the sizes of the largest events exceeding

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Figure 8. Space–time distribution of hypocenters in the Novy´ Kostel focal zone in the period 1991–2007 (from Fischer & Michalek´ 2008). The different panels show the map view (a), the depth view from south to north (b) and from east to west (c) and a magnitude–time plot (d). The horizontal coordinates are rotated to the strike of the fault of 349◦.

magnitude 2.5 are comparable to the location error. Thus, for most tial reactivation. However, despite our relatively short observation of the events one cannot infer if they occurred as a reactivation period of 17 yr, the area of fresh ruptures (full curve) is found to of previously existing ruptures or as an activation of an adjacent, increase continuously, which also agrees with the migration of seis- so far intact fault patch. However, one can (up to the limit of the micity apparent in Fig. 8. Another striking outcome of Fig. 9 is that observation period) identify events that have activated so far intact the fast growth of the cumulative seismic moment during Novem- areas. To account for the rupture sizes, we have converted the event ber 2000 for other ruptures is not observed for the fresh ruptures. magnitudes to the seismic moment M 0 using an empirical formula This indicates that the large events responsible for this fast growth of Hainzl & Fischer (2002). occurred close to the previously ruptured patches and not in the The time dependence of the cumulative seismic moment of the intact fault area. The curve of growth factor (Fig. 9 bottom) shows fresh ruptures and of the remaining ruptures is shown in Fig. 9, the contribution of fresh fractures to the total seismic moment cal- top. The ruptures that occur closer to their neighbours than their culated in a moving window. It illustrates that a repeated growth of location error (dashed line) strongly prevail compared to the fresh the fractured area takes place during the whole observation period. ruptures, which account for only 9 per cent of the total released Two long-term maxima of the growth rate occur: the first one from seismic moment. However, the fresh ruptures make up 18 per cent the period 1991–1997 is clearly a result of an incomplete data set of the number of events. The half contribution of the fresh ruptures at the beginning of the monitoring. The second one in the period to the released seismic moment indicates their significantly smaller from 2001 to 2006 shows the rearrangement of the seismic activity magnitudes compared to the events that have reactivated already after the 2000 swarm when new fault zones at the boundary of the ruptured areas. 2000-swarm patch were activated. The rate of fresh ruptures is, from its definition, dependent on We conclude that the continuous growth of the fresh ruptures the observation period and is expected to decrease with time as the in the NK focal zone and their small sizes could be a result of area of already ruptured fractures increases, along with the proba- chemical alteration by CO2-saturated fluids that create new planes bility of their reactivation. This effect is probably manifested in the of weakness, reduce their friction and finally lead to their subsequent fast growth of seismic moment of the possible reactivation (other healing. As a result, a heterogeneous system of small fractures of ruptures in Fig. 9) in the last period of the 2000 swarm (November different strength is created that cannot sustain a large stress load 2000), which created a large ruptured area as a candidate for poten- (Mogi 1963) and tends to generate a swarm type of seismicity.

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Figure 9. Time dependence of the growth of seismicity for the data set from Fig. 8. The top panel shows the cumulative seismic moment released by newly created fresh ruptures (full line) and remaining ruptures (broken line). The growth factor in the bottom panel is calculated as the ratio between the seismic moment released by the new ruptures and the total seismic moment in a moving window of 100 events width. The horizontal axis shows time in format month-year; note the axis is linear with regard to the event number.

Al is one or two orders lower than that of the other elements. The 4 DISCUSSION formation of secondary minerals as clay (kaolinite, smectit) and One feature of earthquake swarms is their occurrence in fluid-filled carbonates (calcite, dolomite) is a typical result of these alteration fracture zones under mostly hydrothermal conditions, and this is processes. Nevertheless, the fluid pressure is lower in these exper- relevant for the Vogtland/NW Bohemia region with hypocentres at iments than in the real hypocentres of the Vogtland/NW Bohemia 5 to 12 km depth. To estimate the temperature range in the hypocen- region, giving further evidence for an increased alteration potential. tre depths, we use the geothermal gradient of 28◦Ckm−1 measured Moreover, the solubility will be increased under crustal conditions in the KTB borehole, which is located 50 km SW (Wilhelm 2000). by the contemporary influence of further aggressive fluids: H2Sand Thereby, the expected temperatures in the hypocentre depths range H2SO4 of as yet unknown concentrations. between 130 and 350◦C, at a minimum hydrostatic pressure of 0.5– Such water–rock interaction processes become much more com- 1.2 kbar. These P–T conditions appear optimal for an alteration plicated if one considers the influence of mineral contents, mineral process of granites or metamorphic rocks according to the experi- transformations, argillitization, precipitation, retrograde dissolution mental results of Suto et al. (2007). These experiments carried out and ion exchange. Too many free parameters as well as many open over a temperature range of 100–350◦C, and a pressure up to 250 questions preclude a simple common solution applicable to all cases. bar showed that the dissolution of granite minerals and the deposi- We believe that the reviewed laboratory experiments, field geologi- tion of secondary minerals were encouraged by the addition of CO2 cal observations and seismicity analyses suggest that alteration with to the water-filled autoclave. Table 2 shows an example of the total a remarkable velocity is an enduring process in the seismogenic dissolved elements in millimoles per kilogram during an experiment fracture zones. However, its real extent cannot be determined by the of only 7 days. The concentration of dissolved elements depends surface observations; a deep drilling experiment in the epicentral on the temperature of a CO2-free and a CO2-saturated granite–H2O area could possibly disclose the real processes taking place in the system. fault zones, like the SAFOD experiment (Schleicher et al. 2006). The major dissolved components are silica, Na, Ca, Fe and K. Accordingly, we propose the following conceptual scheme: At lower temperatures (100 and 200◦C), the dissolved plagioclase exceeds the silica content in the solution. The dissolution of Mg and (1) Hydrothermal alteration in the fluid-filled fracture zone reduces the friction of asperities and fissures by dissolving crystal edges and/or argillitization. Recent tectonic activity can reduce the mechanical stability of the quartz and feldspar crystals, which will Table 2. Total dissolved elements in millimoles per kilogram after 7 days be altered quickly by the aggressive fluids coming into the microfis- depending on temperature (according to Suto et al. 2007). sures (Figs 5–7). The subcritical stress load leads to seismic slip, Temperature (◦C) 100 200 300 350 crack propagation and comminution of the rock matrix (Dunning et al. 1994). CO -free system 2.26 5.58 16.44 13.31 2 (2) These first slippages along the fluid-filled microfractures in- CO -saturated system 5.60 15.51 18.47 13.71 2 crease the pore volume and the permeability of the fractures, thereby

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Figure 10. Model set-up with indication of random generated fracture system (yellow lines) in 10 km depth. The model size is 300 × 500 m. allowing the acceleration of the pore pressure diffusion process (Parotidis et al. 2003, 2005). (3) As a result, most adjacent, critically altered fracture zones are activated and reactivated by fluid pressure perturbation. When there are no alteration processes, the friction is not reduced, and an increased stress must be accumulated before the failure of pre-existing ruptures occurs. As a result, fewer earthquakes with higher magnitudes will be generated. To validate the assumed hydrothermomechanical mechanism of earthquake swarms, one can use numerical modelling. A conceptual model with some preliminary results is presented below. The used numerical approach is based on the discrete element method, which was recently successfully applied to simulate microtremors observed during tunnelling in Switzerland (Hagedorn & Konietzky 2007). The basic idea is as follows: at a depth of ∼10 km, a highly fractured system exists under initial stress conditions and additional tectonic lateral forces. As an example, a random joint set generator was used with the mean value and standard deviation for angle of joint track (45 ± 10◦ and 135 ± 10◦, respectively), gap length between joints (2 ± 2 m), spacing normal to joint tracks (5 ± 2m)and trace length of joint segments (30 ± 10 m). The material behaviour of the fractures (open or closed) is governed by strain-softening behaviour. Fluid flows through fractures causing continuous alteration and softening of wall rock takes place, which reduces strength Figure 11. Three examples of different stages of crack growing. The time and later triggers randomly distributed seismic events. Because of steps are of a relative unit. The velocity vectors (green) at different points in time indicating the occurrence of microseismic events randomly distributed stress redistributions, a whole series of small earthquakes can be in the rock volume. The assumption that a reduced friction due to alteration triggered. If, and how many, seismic events are triggered depends stimulates the activation of microevents can be confirmed.

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Although this ﬁrst approach is promising, further work is neces- sary to include the hydromechanical coupling and to calibrate the model.

5 CONCLUSIONS Earthquake swarm areas show a typical phenomenon: many microearthquakes and no single main events with magnitudes greater than5–why?Weconsiderthisphenomenon as a result of mechanical instability or weakness generated by hydrothermal alteration of wall rock due to an acidic fluid environment in the interconnected fracture zones. These continuous alteration processes are generated by aqueous CO2 solutions that lower the friction of crystals and their asperities. Because this induced weakness on many small fault planes reaches a critical level, many microearthquakes in a large volume can occur that are induced mainly by stress accumulation or triggered by pore pressure perturbation. These processes take place in many fault planes at the same time, which could result in almost simultaneous release of energy on critically stressed microfaults during earthquake swarm. The principal points of our study could be summarized as follows: (1) In our model the occurrence of earthquake swarms is bound to the existence of aggressive pore fluids, in particular a mixture of CO2,SO4,H2SandH2O in highly fractured wall rock. This is not the case, of course, for hydro-fracturing or injection tests. (2) The mixture of CO2 and H2O acts in crustal fracture zones in two ways: as a chemical solvent (alteration) and as a physical element (fluid pressure transmitter/pore pressure perturbation). It makes no difference whether the fluid is in a CO2-orH2O- Figure 11. (Continued.) dominated phase. (3) The seismogenic activation of fresh ruptures as well as reactivation of previous shear planes is considered as one result of continuously acting alteration. on several factors – orientation of fractures, secondary stress field, strength parameters (may be influenced by fluid processes) and fluid Here, we have considered the influence of alteration in earthquake pressure – and is therefore a random process. swarm regions. The relevancy of this effect in ‘normal’ seismogenic The principles of the proposed modelling strategy were tested us- areas with tectonic events of high magnitudes and CO2-enriched ing a pure mechanical 2D model with an extension of 300 × 500 m. fluids, as shown by Irwin & Barnes (1980) for the worldwide dis- A random fracture system generates 3719 individual bocks, which tribution of seismicity and CO2 gas emission sites is a subject for form the fracture system (Fig. 10). An elastoplastic strain-softening further investigations. constitutive law was applied to the fractures, which lowers the friction angle from 30◦ to 25◦ and sets cohesive and tensile strength ACKNOWLEDGMENTS to zero, if the strength of the joints is exceeded. The virgin stress state corresponds to a depth of ∼10 km. The tectonic force was sim- We appreciate the valuable suggestions of two anonymous review- ulated by a superimposed small lateral displacement velocity. For ers. The work was partially supported by the research projects the blocks, an internal elastic behaviour is assumed. Figs 11(a)–(c) MSM0021620855 and AV0Z30120515. show the fracture system superimposed by velocity vectors at se- lected different points at different times. The locally huge velocity vectors indicate the occurrence of seismic events. The extensions REFERENCES of the seismic sources are from a few metres up to some 10 m, Atkinson, B.K. & Meredith, P.G., 1981. Stress-corrosion cracking of the stress drops reach values of a few 104 bar or smaller and the quartz—a note on the influence of chemical environment, Tectonophysics, dislocations are on the order a few millimetres up to a few centime- 77(1–2), T1–T11. tres. Both shear and tensile failure, including mixed-mode rupture, Babuska,ˇ V., Plomerova, J. & Fischer, T., 2007. Intraplate seismicity in the are observed. The observed mechanism is as follows: locally the western Bohemian Massif (central Europe): a possible correlation with a strength is exceeded and because of the strain-softening behaviour, paleoplate junction, J. Geodyn., 44(3–5), 149–159. a sudden stress drop takes place (microseismic event), again leading Bankwitz, P., Schneider, G., Kampf,¨ H. & Bankwitz, E., 2003. Structural characteristics of epicentral areas in Central Europe: study case Cheb to stress redistributions, which trigger further dynamic fracturing. Basin (Czech Republic), J. Geodyn., 35(1–2), 5–32. This continues until a quasi-stable situation is reached and a phase Bischoff, J.L. & Rosenbauer, R.J., 1996. The alteration of rhyolite in CO2 of quiescence occurs. During that phase, because of the tectonic charged water at 200 and 350 degrees C: the unreactivity of CO2 at higher forces, stress accumulation takes place until again the strength is temperature, Geochim. Et Cosmochim. Acta, 60(20), 3859–3867. exceeded at a certain point and the next swarm of earthquakes takes Brauer,¨ K., Kampf,¨ H., Niedermann, S., Strauch, G. & Weise, S.M., place. 2004. Evidence for a nitrogen flux directly derived from the European

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