INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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The paper was published in the proceedings of the 7th International Conference on Earthquake Geotechnical Engineering and was edited by Francesco Silvestri, Nicola Moraci and Susanna Antonielli. The conference was held in Rome, Italy, 17 – 20 June 2019. Earthquake Geotechnical Engineering for Protection and Development of Environment and Constructions – Silvestri & Moraci (Eds) © 2019 Associazione Geotecnica Italiana, Rome, Italy, ISBN 978-0-367-14328-2

Seismic swarms in South Limburg (The Netherlands): Tectonic or induced as coal mining lagging effect?

C. Sigarán-Loría Royal HaskoningDHV, Nijmegen, The Netherlands

S. Slob Witteveen+Bos/Cohere Consultants, Amsterdam/Amersfoort, The Netherlands

ABSTRACT: Tectonic earthquakes in The Netherlands are usually related to the extensional graben along the Peelrandbreuk in South Limburg. The strongest earthquake recorded in is the M 5.8 earthquake in Roermond on 13 April 1992. In Germany induced earthquakes are recorded related to on-going coal mining activities in the Ruhr area with magnitudes mostly smaller than M 3.0. There is a strong correlation in time and space between the seismic events and mining activity. Other mechanism that induce seismicity are known, such as waste water injection from the shale gas activities in America, some reservoirs from hydropower projects as well due to the increase of water pressure along the stressed faults will reduce the normal stress and thus the friction along the fault and this could induce fault movement, resulting in an earth- quake. Another mechanism may be the increase in mass due to the rising ground water. This increase in mass could be an additional driving force behind fault movement and thus the devel- opment of seismicity. Two earthquake swarms have occurred at the Southern edge of the former coal mining area, around the village of Voerendaal. The first swarm occurred in 1985 and the second swarm at the end of 2000 until the beginning of 2001. These two swarms could be related to the rising mine water. A temporal and spatial analysis on groundwater level devel- opment, seismic data, ground uplift and fault data to determine a possible relation between the occurrence of two earthquakes swarms around Voerendaal and the mine water level increase was performed. The study looked at two possible mechanisms that could have triggered the earthquake swarms: (1) exceedance of the critical state of the active faults and (2) the increase and shift in mass due to ground water level rise as a driving (energy) source.

1 INTRODUCTION

The Netherlands has a long history of underground coal mining. Coal mining gradually decreased in the sixties and stopped entirely in the early seventies of the last century. For a long period, the mines were drained, to ensure dry working conditions which resulted in a lowering of the ground water table. Since the closure of the mines, the groundwater pumping decreased in stages. This has resulted in a step-wise rise in groundwater, which is now levelling towards the original level before the coal mining started. There is tectonic and induced seismicity in The Netherlands. The induced seismicity is related to reservoir compaction due to gas extraction in the North of The Netherlands. The tectonic occurs in the south-east of the country, related to extensional movements along the Roer Valley Graben, which is an extension of the Rhine Valley Graben structure. An example is the ML 5.8 Roermond earthquake of 13 April 1992, strongest registered tectonic event that caused some damages. There is induced earthquakes related to coal mining activities in western Germany, close to the border with The Netherlands. In the Ruhr area, every year about 1400 seismic events with magnitudes up to ML 3.0 are measured. There is a strong correlation in time and space between the seismic events and mining activity. Significantly less earthquakes occur at the weekend when there are no mining activities (Bischof et. al., 2005).

5019 Seismicity can be triggered as well by raise in the groundwater level. These earthquakes are often generated by faults that are under shear stress due to regional tectonics or due to past mining (or other human-related) activities. An increase of water pressure along a stressed fault can reduce the normal stress and friction along the fault and this can induce the fault move- ment. The rising ground water as earthquake trigger can be understood as well as an energy shift. The increase in mass or mass shift could be an additional driving force of fault move- ment or seismicity trigger. In South-Limburg, two earthquake swarms have occurred at the southern edge of the former coal mining area, around the village of Voerendaal. The first swarm occurred in 1985 and the second swarm at the end of 2000 until the beginning of 2002. It is unclear if these two swarms can be related to the rising mine water or are purely tectonic. The coal was mined from Carboniferous rocks, at varied depths ranging from 0 to 800 m below ground surface (De Vent and Roest, 2013). The top level of the Carboniferous rocks has a downward trend towards the northwest. These rock formation is dissected by various faults, which are part of the southern boundary of the Roer Valley Graben. This research aimed to identify a possible relationship between the increase in groundwater and seismicity in South Limburg and determine its possible impact on the existing seismic risk in the region. To understand and identify possible relations, the following input data were compiled: – Earthquake catalogue; – Ground water level measurements; – Ground deformation from interferometric synthetic aperture radar (InSAR) data; – Scientific literature from the region and other induced seismicity cases. The approach followed to analyse the data comprised different steps: – Documentation of reference cases; – Geological and tectonic setting and seismicity; – Identification of swarms or anomalous trends in the seismicity catalogue; – Multivariate analysis (temporal variation in mine groundwater, seismicity, ground uplift); – Stress-state analysis at different depths of the active faults to evaluate slip potential; – Energy-balance analysis of the system.

2 REFERENCE CASES

In Germany and Belgium, there is a similar history of coal mining as in The Netherlands. In the former coal mine region Campine in Belgium mines have been flooded, but no earth- quakes have been measured (Rosner, 2015). In Germany, parts of the Ruhr and Saar mining areas are being flooded. In the Ruhr region, no flooding-induced seismicity has been observed so far (Fritschen, 2015). Active mining in the Saar area came to an end in June 2012. Since May 2013 mines have been flooded and on 15 September 2014 the strongest flooding-induced seismic event so far was measured with M2.7 (Saarland, 2014). Another example of flooding-induced seismicity in France is the Gardanne coal mining region in the Provence. Seismicity measurements started in 2008 (Matrullo et al., 2015) and seismic events were measured in November 2012 and in December 2014 (M>2.5). Goldbach (2009) describes experiences with flooding-induced seismicity in South African gold mines. Here mining- induced seismicity was measured in the active part of the mine and flooding-induced seismicity in the abandoned part of the mine. Comparing mine-water level increase with seismicity, it was found that the majority of events took place about 14 months after the start of flooding.

3 TECTONIC SETTING AND SEISMICITY

3.1 Tectonic setting, faults The former Dutch coal mining district is situated at the southwestern boundary of the Roer Valley Graben, south of the Feldbiss Fault. At a regional scale, the Roer Valley graben is part

5020 Figure 1. Active faults and top surface level of the Carboniferous (bed)rock. of the Rhine Valley Graben. Along the Roer Valley Graben, the governing active faults are the Feldbiss Fault and Peel Boundary Fault. These faults are active since late Oligocene (Geluk et al., 1994) and have an estimated slip rate of 0.05–0.01 mm/year (Giardini et al., 2013) and are dominantly normal with a small strike-slip component in depth (Dost & Haak, 2007). Locally, the active faults found in the area correspond to the southern boundary of the Roer Valley Graben: the north-west-south-east faults Feldbiss, Heerlerheide, Benzenrade and the west-northwest Kunrader Fault. These have a steep NE dipping direction and tens of meters vertical offset in the geological formations (Dinoloket, DGMdiep v4.0). The source mechanism is normal but Camelbeeck (1994) reports a strike-slip movement in the Kunrader fault. Dost & Haak (2007) propose two hypotheses: decoupling of the crust, where the upper crust is more brittle and presence of deeper faults with different orientation. Until now, the seismicity registered in the Limburg area has been related to tectonics only. The Dutch seismological survey (KNMI) reports the nearest induced seismicity only in the north of The Netherlands or the western part of Germany due to gas extraction and coal mining. The largest tectonic earthquakes registered in the area occurred in April 13, 1992 in Roermond (The Netherlands) and in March 14, 1951, in Euskirchen (Germany), 80 km SE from Roermond. Both earthquakes had a magnitude ML=5.8 and an intensity Mercalli VII, (Dost & Haak, 2007). The Roermond earthquake and most of the seismicity from the region is associated to the Peel Fault, the northern boundary of the graben. The Kunrader Fault, at the south-western side of the graben, has associated seismic swarms during 1985–1986 and 2000–2002 (Dost & Haak, 2007). This fault diverts in strike from the general NW-SE trend of the Roer Valley Graben (Figure 1). The seismic catalogue was retrieved from the KNMI. It consists of 238 events between 1906 and 2014, with focal depths shallower than 20 km. Deeper events were removed because their source mechanism differs from the shallower seismicity, which is the focus of this study. (B. Dost, oral comm., 16-03-2015). The magnitudes (ML) range between -0.1 and 3.9. The accur- acy in the measurements from the catalogue is considered for the analysis and summarized as: – Before 1985: location: ~ 5 km, depth ~ 5 km; – After 1985: location ~1 km, depth: ~2 km.

The first swarm registered 9 events with ML 1.4–3.0 at 2.3–8.1 km depth between 7 December 1985 and 6 January 1986. The second registered 145 events between 20 December 2000 and 31 August 2002, with ML up to 3.9 and depths shallower than 10.4 km. From the largest event associated to this swarm (ML 3.9 on 23 June 2001 at near 2.2 km depth), 400 damage cases were reported, consisting mainly to chimneys collapse, wall cracks and interior damages. A normal- faulting mechanism is better constrained for these events (Dost & Haak, 2007). KNMI (2014b)

5021 considered a possible relation between those events with the former mining activities unlikely due to their deeper location in relation to the depth of the abandoned mines.

4 ANALYSIS

4.1 Relation between seismicity and uplift Radar interferometry (InSAR) has been used to study ground movements in the Zuid-Lim- burg coal district during 1992–2009 (De Vent and Roest, 2013; Caro Cuenca, 2012). A relation was established between ground deformation and mining activity, but not with seismicity.

4.2 Relation between seismicity and faults Two cross sections perpendicular to the strike of the active faults were made to verify the spa- tial correlation between the Voerendal seismic swarms to the Kunrader fault, proposed by Dost & Haak (2007). Despite the location uncertainty form the events, these correlate well to the fault (Figure 2).

Figure 2. Left: Location of the swarm epicenters and active faults (upper: 1985–1986, lower: 2000–2002). Right: Cross sections along yellow lines from maps, showing the hypocenters from the same events with their uncertainty in location. Left fault is the Kunrader Fault. Shadowed area is the coal mining area.

5022 Figure 3. Ground water level increase per coal mining basin (Rosner, 2011) with accumulated yearly seismic energy (red dashed line) from KNMI catalogue.

4.3 Spatial and temporal analysis Possible spatial and temporal relations between ground deformation (uplift or subsidence), ground water variation and seismicity were combined to identify possible induced seismicity by the mining lagging effects related to the groundwater increase. Location from the faults were provided by TNO (Dutch Geological Survey), groundwater variation (local and regional aquifers) by IHS (Ingenieurbüro Heitfeld-Schetelig) and the seismic catalogue was retrieved from the KNMI (Dutch seismological survey) website. The earthquakes magnitude was translated into seismic energy (in Joules) with the relation from Gutenberg and Richter: log Es = 11.8 + 1.5M; where Es is the energy in ergs (1 erg=10e- 7 joules) and M is the (local) earthquake magnitude. The accumulated seismic energy vari- ation in time is compared to the temporal variation of the mine water measured per basin in Figure 3. Two non-stationary phases are recognized: the first in 1982 and the second around 2000. The latter can be associated with the second Voerendaal swarm. The first Voerendaal swarm is not noticeable because likely because it comprised relatively few and small events. In 1982 occurred a larger seismic event with ML 3.5 (Sittard, 2 March 1982, Depth 6.6 km), but this event occurred north of the Feldbiss Fault, just outside the mining area. This event influ- ences the accumulated energy curve strongly. The Voerendaal swarms seem to occur after some years of important groundwater rise in the mine basins. The first swarm (1985–1986) appears about 11 to 12 years after the main groundwater rise of 1974–1975. The second Voerendaal swarm (2000–2002) appears about 5 to 6 years after the main ground water level increase of 1995. A certain time delay is expected. However, the large time delay between the first groundwater rise and first Voerendaal swarm makes a possible relation less apparent. The second swarm appears to have a stronger tem- poral relation with the second groundwater rise. It may be possible that a certain critical threshold value is required to be exceeded for more fault movement (seismicity) to occur. Des- pite there appears to be a temporal relation between the second swarm and the ground water rise, the groundwater rise from the period 1970–1975 was more substantial than in 1994–1995.

4.4 In situ stress analysis The stresses along a fault can be expressed with the Mohr–Coulomb failure criterion, where the failure envelope is defined by the strength parameters: cohesion (c) and friction angle (φ) of the material. In this case the failure envelope is defined in terms of shear stress (τ) by the strength parameters along the fault, which is the surface along which failure is assumed to occur:

5023 Table 1. Geological profile and material properties layer top [m NAP] thickness [m] unit weight [kN/m3] Poisson’s ratio [-] sand/clay 86 36 18 0.25 marl 54 79 21 0.25 sand/clay -25 75 20 0.25 Carboniferous -100 700 25 0.35 End of borehole -635 - - -

Figure 4. Stress state analysis in relation to groundwater rise.

τ ¼ c þðσn À σppÞÃtan ’ ð1Þ

Where: σn=normal stress along fault (total stress perpendicular to fault plane). The normal stress is composed of the minimum σ3 and maximum σ1 total stress vectors at depth and depends on the dip angle of the fault; σpp=pore pressure (or total water pressure) at depth. The friction angle corresponds to the friction angle of the fault, determined by the roughness and infill material of the fault. The amount of pore pressure that can be added to the system before slip occurs, corresponds to a water column of a certain height. This is the water level increase at which slip occurs along the fault. For the Roer Valley Worum et al. (2004) deter- mined the ratios between the major and minor principal stress at which faults were expected to be activated. Worum et al (2004) estimated a ratio of σ3/σ1, at which slip occurs to be between 0.33 to 0.59. The geological stratigraphy near Voerendaal was interpreted using the log from borehole VRD-01, 636 m deep, available via NLOG from TNO website (Table 1). The depth was extrapolated to -800 NAP (Dutch reference height) referred to assume the bottom from the Carboniferous. With a Poisson’s ratio assumed as 0.35 for the Carboniferous, the stress ratio σh/σv at the site is 0.54. This is in the range of 0.33–0.59 interpreted by Worum et al. (2004) for normal faults. Fault properties of c=0 kPa and φ=17–30° were assumed for the Mohr Coulomb ana- lysis, after Worum et al. (2004). Different groundwater levels were considered for different fault conditions (φ=17°, 20° and 30°) at different stress levels or depths. Mohr circles and fail- ure envelopes were calculated and the maximum water pressure required to reach the failure envelope (converted into water column) determined. The estimated critical positions per fric- tion angle of the fault are shown in Figure 4, together with the measured groundwater level from shaft II of Orange Nassau I (“ONI”) mine. For a fault with φ=17° the failure envelope was already reached at all target depths. Therefore, calculations were made for φ=20° and 30°. Results are presented in Figure 4. The fault was assumed permeable.

4.5 Relation of seismic energy with ground water rise This approach, proposed by Klose (2013), calculates the total energy balance of the system and to what extent a mass shift, e.g. rising mine water, could be the driving force behind seismic activity. The increase in mass (potential energy) due to mine water rise was calculated taking into account

5024 Figure 5. Comparison of the mass change and observed (moment) magnitudes (MW) of the two Voer- endaal Swarms with data from induced and triggered earthquakes after Klose (2013). The data points with symbol ▼ are related to normal faults. The data points with symbol ▲ are related to reverse faults. The data points with symbol ● are related to strike-slip faults (Klose, 2013). the size of the relevant mining basin, the porosity in the Carboniferous rocks and the porosity in the overburden material. Then, the release of energy due to seismic events is calculated. Klose (2013) proposed empirical relations between induced and triggered earthquake magnitudes and the mass shift. An important limitation for the applicability of these relations is that they are valid for magnitudes larger than 3.0. For the Voerendaal swarms, most of the events have M<3.0. However, the concept was considered valid for this research. The data points from the two swarms fall on the lower range of data points found by Klose (2013) for other normal faults Figure 5).

5 CONCLUSIONS

Two seismic swarms have been felt and registered in the Voerendaal region, associated to the Kunrader fault, between 1985–1986 and 2000–2002. In this region, underground coal mining took place for a long time. During the sixties and seventies mining stopped and the ground- water started to rise. This study aimed to identify a possible relationship between the increase in groundwater level and the occurrence of the seismic swarms and determine the possible impact of this on the existing seismic hazard in the region. A temporal and spatial analysis was done by comparing the temporal variation in the groundwater level with the seismicity, fault data and ground movement from InSAR data. No clear relation was found between movement and seismicity. The temporal variation from the groundwater rise shows that the first Voerendaal swarm (1985–1986) appears about 11 to 12 years after the main groundwater increase of 1974–1975. The second Voerendaal swarm (2000–2002) appears about 5 to 6 years after the main ground water rise of 1995. Two approaches to assess the mechanism that could have induced the seismic swarms con- sidered are: a) exceedance of critical state of the fault; b) mass shift translated as a potential energy increase, due to the increase in groundwater level, as driving force of the seismicity. The stress state analysis shows that the Kunrader Fault is close to its critical state. This means that an increase in ground water level as experienced in the past could cause the fault to exceed its critical state and result in slip. However, the analysis does not provide an explanation for the time at which the (swarms of) earthquakes occur. It does provide an explanation for the location of the swarms, since the Kunrader fault near Voerendaal has an apparent dip with respect to the general stress field which makes it more prone to slip than the other faults in the area.

5025 Comparing the seismic energy released during the swarms with the potential energy added with groundwater increase (mass shift), as well as ground uplift, there appears to be a delayed reaction between the water increase and the seismic energy that is released by the swarms. Nevertheless, the amount of energy added by the water is larger than the released in the seismicity. The considered approaches can explain the occurrence of the seismic swarms. Theoretically, a similar amount of seismic energy release could be expected from the estimated potential energy. Nevertheless, the rate of increase (expected to be about 2 m per year) is less than was experienced in the past. A fast rate in groundwater level increase could have been the trigger- ing factor for the occurrence of the registered swarms. A new seismic swarm is possible but the amount of seismic energy that could be released and the expected magnitudes are not expected to increase the actual seismic hazard. Considering that the ground level increase is slowly levelling off to a steady state, the likelihood of another swarm also decreases with time.

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