Geothermics 45 (2013) 1–17 Contents lists available at SciVerse ScienceDirect Geothermics journa l h omepa g e: www.elsevier.com/locate/geothermics Characteristics of large-magnitude microseismic events recorded during and after stimulation of a geothermal reservoir at Basel, Switzerland Yusuke Mukuhira a,∗ , Hiroshi Asanuma a , Hiroaki Niitsuma a , Markus O. Häring b a Graduate School of Environmental Studies, Tohoku University, 6-6-20, Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8579, Japan b Geo Explorers Ltd., Wasserturmplatz 1, CH-4410 Liestal, Switzerland a r t i c l e i n f o Article history: Received 23 September 2011 Accepted 27 July 2012 Available online 24 August 2012 Keywords: Basel Induced seismicity Moment magnitude Engineered geothermal system (EGS) Hydraulic stimulation Critical pore pressure Shear slip a b s t r a c t Induced seismicity with large events occurred during and after a hydraulic stimulation at Basel, Switzerland, in 2006. This paper describes a study of the characteristics of the large events (those of moment magnitude greater than 2.0) to understand their origin. The large events during the stimulation and just after bleeding off had hypocenters within the seismic cloud while the large events that occurred several weeks after shut-in were located outside of the seismic cloud. We found no evidence that either local stress concentration or increased pore pressure caused the increase of event magnitudes as no shear slip with extremely high stress drop, or a significant correlation between pore pressure and large event magnitude were identified. Our integrated analysis of the fault plane solution and rock failure mecha- nism showed unbalanced seismic activity and seismic energy release in the pre-existing fracture system. From these observations we conclude that the large events did not originate from the rupture of rigid asperities triggered by increased pore pressure. Our observations suggest instead that critical changes of the stress state or coefficient of friction on fracture planes during stimulation triggered the unstable shear slip of large events. We also conclude that the characteristics of the large events are dependent on their occurrence times and hypocentral locations. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction 1.1. Background and objective of this study Hydraulic stimulation is an essential component in the devel- opment of engineered geothermal systems (EGS) and hot dry rock (HDR) projects as it is used to increase well injectivity and system productivity. It is widely accepted that fluid injected into crys- talline or consolidated sedimentary rocks can induce shear slip on existing fractures, thus inducing microseismic events. The magni- tude of shear slip so induced has a strong impact on the enhanced permeability, so the hypocenters of induced events have been used to identify zones of improved permeability (Katagiri et al., 1980; Evans et al., 2005). It is commonly expected by EGS/HDR developers that for hydraulic stimulation at depths of around 3000–5000 m, induced microseismic events cannot be felt at the surface because the moment magnitude (M w ) of such events is less than 1.5. However, induced microseismic events that have been felt at the surface (hereafter referred to as “large events”) have been reported at commercial-scale EGS/HDR sites at Soultz (France) (Baria et al., ∗ Corresponding author. Tel.: +81 22 795 7401; fax: +81 22 795 7401. E-mail address: mukuhira@geoth.kankyo.tohoku.ac.jp (Y. Mukuhira). 1999), in the Cooper Basin (Australia) (Asanuma et al., 2005), and at Landau (Germany) and Basel (Switzerland) (Majer et al., 2007). Up to nine induced events of M w > 2.0 occurred during and after hydraulic stimulation at Basel in 2006, causing considerable dam- age to buildings (Kraft et al., 2009). The Basel project was then suspended pending the results of risk analysis and was finally dis- continued in 2009 (Baisch et al., 2009). Damage claims amounted to more than US$9.0 million (Giardini, 2009). Large events have also been reported in and around conven- tional hydrothermal reservoirs where no hydraulic stimulation has been conducted (Roger and Charles, 1982), and in various other subsurface development projects, including EOR from oil and gas reservoirs (Suckale, 2009, 2010), CCS (Evans et al., 2012), dam con- struction (Chen and Talvani, 1998), and mine development (Yabe et al., 2009). Large events such as these present a serious problem for subsurface development projects (Majer et al., 2007; Suckale, 2010). The effectiveness of the large events in the creation or extension of reservoirs is an issue of interest. It is generally believed by seis- mologists that the magnitude of large events is closely related to the size of the rupture area (Lay and Wallece, 1995). Spatiotemporal analyses of the hypocenters and source parameters of large events induced in the Cooper Basin have shown improved permeability and extension of reservoirs (Asanuma et al., 2005). However, for the Basel EGS and other subsurface projects, the relationship between 0375-6505/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.geothermics.2012.07.005