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Induced seismicity is one of the main factors that reduces societal acceptance of deep geothermal energy exploitation activities, and felt earthquakes are the main reason for closure of geothermal projects. Implementing innovative tools for real-time monitoring and forecasting of induced seismicity was one of the aims of the recently completed COSEISMIQ project. Within this project, a temporary seismic network was deployed in the Hengill geothermal region in Iceland, the location of the nation's two largest geothermal power plants. In this paper, we release raw continuous seismic waveforms and seismicity catalogues collected and prepared during this project. This dataset is particularly valuable since a very dense network was deployed in a seismically active region where thousand of earthquakes occur every year. For this reason, the collected dataset can be used across a broad range of research topics in seismology ranging from the development and testing of new data analysis methods to induced seismicity and seismotectonics studies.

Abstract. The rapid increase in energy demand in the city of Reykjavik has posed the need for an additional supply of hot water from deep geothermal energy. The deep hydraulic (re-)stimulation of well RV-43 on the peninsula of Geldinganes (north of Reykjavik) is an essential component of the plan implemented by Reykjavik Energy to meet this energy target. Hydraulic stimulation is often associated with fluid-induced seismicity, most of which is not felt on the surface, but which, in rare cases, can cause nuisance to the population and even damage to the nearby building stock. This study presents a first of its kind pre-drilling probabilistic induced-seismic hazard and risk analysis for the site of interest. Specifically, we provide probabilistic estimates of peak ground acceleration, European microseismicity intensity, probability of light damage (damage risk), and individual risk. The results of the risk assessment indicate that the individual risk within a radius of 2 km around the injection point is below 0.1 micromorts, and damage risk is below 10−2, for the total duration of the project. However, these results are affected by several orders of magnitude of variability due to the deep uncertainties present at all levels of the analysis, indicating a critical need in updating this a-priory risk assessment with in-situ data collected during the stimulation.

In this article, a high-resolution acoustic emission sensor, accelerometer, and broadband seismometer array data set is made available and described in detail from in situ experiments performed at Äspö Hard Rock Laboratory in May and June 2015. The main goal of the hydraulic stimulation tests in a horizontal borehole at 410 m depth in naturally fractured granitic rock mass is to demonstrate the technical feasibility of generating multi-stage heat exchangers in a controlled way superiorly to former massive stimulations applied in enhanced geothermal projects. A set of six, sub-parallel hydraulic fractures is propagated from an injection borehole drilled parallel to minimum horizontal in situ stress and is monitored by an extensive complementary sensor array implemented in three inclined monitoring boreholes and the nearby tunnel system. Three different fluid injection protocols are tested: constant water injection, progressive cyclic injection, and cyclic injection with a hydraulic hammer operating at 5 Hz frequency to stimulate a crystalline rock volume of size 30 m × 30 m × 30 m at depth. We collected geological data from core and borehole logs, fracture inspection data from an impression packer, and acoustic emission hypocenter tracking and tilt data, as well as quantified the permeability enhancement process. The data and interpretation provided through this publication are important steps in both upscaling laboratory tests and downscaling field tests in granitic rock in the framework of enhanced geothermal system research. Data described in this paper can be accessed at GFZ Data Services under 10.5880/GFZ.2.6.2023.004. [ABSTRACT FROM AUTHOR]

In this paper, an underground experiment at the Äspö Hard Rock Laboratory (HRL) is described. Main goal is optimizing geothermal heat exchange in crystalline rock mass at depth by multistage hydraulic fracturing with minimal impact on the environment, that is, seismic events. For this, three arrays with acoustic emission, microseismicity and electromagnetic sensors are installed mapping hydraulic fracture initiation and growth. Fractures are driven by three different water injection schemes (continuous, progressive and pulse pressurization). After a brief review of hydraulic fracture operations in crystalline rock mass at mine scale, the site geology and the stress conditions at Äspö HRL are described. Then, the continuous, single-flow rate and alternative, multiple-flow rate fracture breakdown tests in a horizontal borehole at depth level 410 m are described together with the monitoring networks and sensitivity. Monitoring results include the primary catalogue of acoustic emission hypocentres obtained from four hydraulic fractures with the in situ trigger and localizing network. The continuous versus alternative water injection schemes are discussed in terms of the fracture breakdown pressure, the fracture pattern from impression packer result and the monitoring at the arrays. An example of multistage hydraulic fracturing with several phases of opening and closing of fracture walls is evaluated using data from acoustic emissions, seismic broad-band recordings and electromagnetic signal response. Based on our limited amount of in situ tests (six) and evaluation of three tests in Ävrö granodiorite, in the multiple-flow rate test with progressively increasing target pressure, the acoustic emission activity starts at a later stage in the fracturing process compared to the conventional fracturing case with continuous water injection. In tendency, also the total number and magnitude of acoustic events are found to be smaller in the progressive treatment with frequent phases of depressurization. [ABSTRACT FROM AUTHOR]

Compared with seismic waves, near-field static deformation can provide more robust constraints on earthquake size and slip distribution because it is less sensitive to the rupture process and Earth structure. The static deformation data are now obtained using space-geodetic measurements. For early warning and rapid hazard assessment, such geodetic measurements are less useful because they are usually available only with a time delay of days to weeks or even longer. Recent studies have shown that coseismic static displacements can be estimated from modern seismometer records after an appropriate correction for baseline errors that may be caused by rotational motion, tilt, instrument, and other effects. For this purpose, several empirical baseline correction methods have been proposed. Algorithms, in which an acceleration or other acceleration record derived threshold is used to determine the timing of baseline shift, can be easily implemented. In practice, however, the baseline shift is not necessarily accompanying the strongest ground shaking; methods based on the threshold approach tend to lead to an over- or underestimation of the true baseline shift. Other correction schemes, which can be performed by manual calibration, rely on subjective decisions for the choice of correction parameters. In this paper, an automatic scheme is presented, in which the method used to determine the baseline shift has a stronger physical basis. Case studies on the 1999 Chi-Chi, 2007 Tocopilla, 2008 Wenchuan, and 2010 Maule earthquakes, where the strong-motion data were obtained from different types of accelerometers, show that this automatic scheme is more robust than the previously suggested ones. In all test cases, the coseismic displacements recovered from the strong-motion records agree within 10%-20% with direct GPS measurements or indirect model predictions. In addition, the automatic scheme was also successfully used to correct the strong-motion records of a low-cost sensor obtained from a laboratory test. [ABSTRACT FROM AUTHOR]

Aftershocks of the magnitude Mw 6.4 Bingöl earthquake of 1 May 2003 (eastern Turkey) were monitored by a local temporary seismic network. The spatial distribution of the aftershocks shows that the earthquake did not occur on the left-lateral East Anatolian fault, but perpendicular to it along a north-northwest-striking right-lateral fault. The event ruptured an area of 20 km by 8 km, but did not reach the Earth's surface. The aftershocks of the 2003 Bingöl earthquake and those of the 1992 Erzincan earthquake seem to indicate the existence of a new seismically active right-lateral shear zone. Indeed, we show that historical earthquakes along both the North and East Anatolian fault led to positive changes in Coulomb failure stress for north-northwest-oriented right-lateral strike-slip faults in the Bingöl epicentral area. Moreover, the existence of such faults is supported by remote-sensing data. [ABSTRACT FROM AUTHOR]

By analysing several years of discharge and electrical conductivity data from an artesian well in Kajaran, Armenia, we found a significant sensitivity of this well to distant large earthquakes. In general, the discharge increases co-seismically and the conductivity decreases post-seismically with a time delay of about 1 hr. The post-seismic trends of the conductivity reach a minimum after about 3 weeks and then need several months to recover to the pre-seismic level. For instance, the Izmit earthquake in Turkey on 1999 August 17, at a distance of 1400 km, led to an increase of 25 per cent in discharge and a decrease of up to 6 per cent in conductivity. The discharge also shows tidal fluctuations of amplitude roughly 5 per cent (peak-to-peak) of the mean well production, whereas the tidal signal in the conductivity data is less significant and unstable. The maximum co-seismic static strain estimated for 11 earthquakes that induced an anomaly during the monitoring period is below 10−9, or at least one order smaller than the tidal strain. Therefore, the well-water anomalies related to the distant earthquakes are believed to be induced by seismic ground shaking rather than co-seismic deformation. We suggest mixing of groundwater as an explanation for the observations and present a model which considers specific conditions for this particular groundwater system: a confined aquifer with a high contrast in the hydrogeochemical composition between two different groundwaters and a macrofracture as the mixing zone which is hydraulically connected to the artesian well. The earthquake-related anomalies are believed to result from a local head increase near the artesian well, induced by the passage of seismic waves. Possible mechanisms are discussed, and the time histories of the anomalies are modelled. [ABSTRACT FROM AUTHOR]