Full-Waveform based methods for Microseismic Monitoring Operations: an Application to Natural and Induced Seismicity in the Hengill Geothermal Area, Iceland

Adv. Geosci., 54, 129–136, 2020 https://doi.org/10.5194/adgeo-54-129-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Full-Waveform based methods for Microseismic Monitoring Operations: an Application to Natural and Induced Seismicity in the Hengill Geothermal Area, Iceland Camilla Rossi1 , Francesco Grigoli2 , Simone Cesca3 , Sebastian Heimann3 , Paolo Gasperini1 , Vala Hjörleifsdóttir4 , Torsten Dahm3 , Christopher J. Bean5 , Stefan Wiemer2 , Luca Scarabello2 , Nima Nooshiri5 , John F. Clinton2 , Anne Obermann2 , Kristján Ágústsson6 , and Thorbjörg Ágústsdóttir6 1 Department of Physics and Astronomy (DIFA), University of Bologna, Bologna, 40127, Italy of Earth Sciences, ETH Zurich, Zurich, Switzerland 3 German Research Center for Geosciences (GFZ), section Physics of Earthquakes and Volcanoes, Potsdam, Germany 4 Reykjavìk Energy (OR), Reykjavìk, Iceland 5 Dublin Institute for Advanced Studies (DIAS), Dublin, Irland 6 Iceland GeoSurvey (ÍSOR), Reykjavìk, Iceland 2 Department Correspondence: Camilla Rossi () Received: 11 June 2020 – Revised: 27 August 2020 – Accepted: 20 October 2020 – Published: 19 November 2020 Abstract. Geothermal systems in the Hengill volcanic area, SW Iceland, started to be exploited for electrical power and heat production since the late 1960s. Today the two largest operating geothermal power plants are located at Nesjavellir and Hellisheiði. This area is a complex tectonic and geothermal site, located at the triple junction between the Reykjanes Peninsula (RP), the Western Volcanic Zone (WVZ), and the South Iceland Seismic Zone (SISZ). The region is seismically highly active with several thousand earthquakes located yearly. The origin of such earthquakes may be either natural or anthropogenic. The analysis of microseismicity can provide useful information on natural active processes in tectonic, geothermal and volcanic environments as well as on physical mechanisms governing induced events. Here, we investigate the microseismicity occurring in Hengill area, using a very dense broadband seismic monitoring network deployed in Hellisheiði since November 2018, and apply sophisticated full-waveform based method for detection and location. Improved locations and first characterization indicate that it is possible to identify different types of microseismic clusters, which are associated with either production/injection or the tectonic setting of the geothermal area. 1 Introduction The Hengill volcanic system is located in Iceland in the southern end of the western volcanic zone (WVZ), at the triple junction between the WVZ, the Reykjanes Peninsula (RP), the landward extension of the Reykjanes spreading ridge, and the South Iceland Seismic Zone (SISZ), i.e. the left-lateral transform zone (Saemundsson, 1979; Einarsson, 2008). Therefore, the area is characterized by a complex local geology and tectonic setting, and intense seismicity. The Hengill complex is primarily composed by three main volcanic systems, which are, from SE to NW with decreasing age, Grændalur (0.3–0.5 My), Hrómundartindur (Ölkelduháls) in decline, and Hengill, with the present-day volcanic activity (Arnason et al., 2010). The dominant tectonic trend of the area is extensional, with the distribution of major faults and eruptive fissures oriented NNE, parallel to the accretionary zones (Foulger and Toomey, 1989). South of 64◦ N, in the SISZ, the area is characterized by a transform faulting with the main tectonic structures striking N–S. The Hengill geothermal system has been exploited for electrical power and heat production since the late 1960s (Gunnarsson et al., 1992). The natural geothermal activity is expressed by numerous hot springs and fumaroles spread throughout the area around the volcanic system (Saemunds- Published by Copernicus Publications on behalf of the European Geosciences Union. 130 C. Rossi et al.: Full-Waveform based methods for Microseismic Monitoring Operations Figure 1. Seismic network considered here is composed by stations from IMO (light blue), ISOR (blue) and COSEISMIQ (green). The two Nesjavellir and the Hellisheidi geothermal field are marked with white squares. son, 1995; Arnórsson et al., 2008). In this region, the two largest operating geothermal power plants, respectively at NE and SW of Hengill area, are the Nesjavellir and the Hellisheiði geothermal plants, where electricity and hot water are extracted. Due to its complex tectonic setting, this area is highly seismically active with several thousand earthquakes located yearly. In this region, earthquakes have M>6 in the neighbouring SISZ, and M>5 in the Hengill area (Rögnvaldsson et al., 1998; Árnadóttir et al., 2001; Vogfjörd and Slunga, 2003; Pedersen et al., 2003; Jakobsdóttir, 2008; Hreinsdóttir et al., 2009; Decriem et al., 2010). According to previous studies (Julian et al., 1997; Miller et al., 1998; Foulger, 1988a, b; Foulger and Toomey, 1989; Sigmundsson, 1997), the seismic activity at the Hengill triple junction can be mostly divided in two groups. First, infrequent intense episodes, occurring along the accretionary plate boundary and the transform zone (SISZ), outside the high temperature geothermal area. Second, background of small-magnitude earthquake activity that occurs more frequently a potentially related to geothermal energy exploitation activities. Since both anthropogenic and natural seismicity occur at the Hengill area, it is important to understand the relationship between the seismic activity and geothermal exploitation, as well as discriminating between natural and induced seismicity. There are already a few reported cases of induced seismicity such as the M 4.0 induced events in 2011 (Bessason et al., 2012) and the Hverahlíð cluster (Kristjánsdóttir et al., 2020). The 2011 earthquakes occurred during rapid changes in the injection rates, but their triggering mechanism is still disputed. It may either Adv. Geosci., 54, 129–136, 2020 be related to Coulomb stress changes, due to the depletion effects associated to the geothermal production operations, or to pore pressure transients from fluid injection. The analysis and characterization of microseismicity requires a seismic monitoring infrastructures allowing to record a massive number of low SNR events. However, the analysis of microseismicity is challenging since recorded seismic signals are often characterized by low amplitude, high-frequency content and strong seismic noise contamination, with low signal-to-noise ratio. Therefore, to improve the performance for the analysis of large microseismicity dataset, alternative methods (i.e. detection, location, magnitude and source mechanisms determination) have been recently proposed (e.g. Cesca and Grigoli, 2015, and references therein). This is particularly true for induced seismicity applications in seismically active areas, where seismic events can have natural origin or can be linked to several industrial act (...truncated)