Ocean-bottom seismometers (OBSs) are equipped with seismic sensors that record acoustic and seismic events at the seafloor, which makes them suitable for investigating tectonic structures capable of generating earthquakes offshore. One critical parameter to obtain accurate earthquake locations is the absolute time of the incoming seismic signals recorded by the OBSs. It is, however, not possible to synchronize the internal clocks of the OBSs with a known reference time, given that GNSS signals are unable to reach the instrument at the sea bottom. To address this issue, here we introduce a new method to synchronize the clocks of large-scale OBS deployments. Our approach relies on the theoretical time-symmetry of time-lapse (averaged) crosscorrelations of ambient seismic noise. Deviations from symmetry are attributed to clock errors. This implies that the recovered clock errors will be obscured by lapse crosscorrelations' deviations from symmetry that are not due to clock errors. Non-uniform surface wave illumination patterns are arguably the most notable source which breaks the time symmetry. Using field data, we demonstrate that the adverse effects of non-uniform illumination patterns on the recovered clock errors can be mitigated by means of a weighted least-squares inversion that is based on station-station distances. In addition, our methodology permits the recovery of timing errors at the time of deployment of the OBSs. This error can be attributed to either: i) a wrong initial time synchronization of the OBS or ii) a timing error induced by changing temperature and pressure conditions while the OBS is sunk to the ocean floor. The methodology is implemented in an open-source Python package named OCloC, and we applied it to the OBS recordings acquired in the context of the IMAGE project in and around Reykjanes, Iceland. As expected, most OBSs suffered from clock drift. Surprisingly, we found incurred timing errors at the time of deployment for most of the OBSs.@en

Lack of data and urban noise pose significant challenges to monitoring anthropogenic seismicity in densely populated areas such as the Randstad region in the Netherlands. We deployed a temporary seismic array to monitor a geothermal doublet located at Kwintsheul, Netherlands. We implement innovative array processing, beamforming, and ML automatic-picking techniques to detect low-magnitude microseismic events that could be obscured by urban noise. Additionally, we propose a novel method to incorporate model uncertainties into hypocenter estimations based on the open-access subsurface information of the Netherlands. Contrary to previous studies, our analysis clearly shows that local low-magnitude seismicity does exist and highlights the value of denser seismic arrays and novel detection techniques for monitoring anthropogenic seismicity. The proposed hypocenter localization aids in avoiding under- or overestimation of location uncertainties, which is crucial for informed decision-making. This study advances seismic monitoring techniques in urban geothermal settings, providing critical data for informed decisionmaking and risk assessment of geothermal operations.@en

The timing of the recordings of ocean-bottom seismometers (OBSs) is critical for accurate earthquake location and Earth model studies. GNSS signals, however, cannot reach OBSs deployed at the ocean bottom. This prevents their clocks from being synchronized with a known reference time. To overcome this, we developed OCloC, a Python package that uses time-lapse cross-correlations of ambient seismic noise to synchronize the recordings of large-scale OBS deployments. By simultaneously quantifying deviations from symmetry of a set of lapse cross-correlations, OCloC recovers the incurred clock errors by means of a least-squares inversion. In fact, because non-uniform noise illumination patterns also break the symmetry of (lapse) cross-correlations, we introduce a distance-based weighted least-squares inversion. This mitigates the adverse effect of the noise illumination on the recovered clock errors. Using noise recordings from the IMAGE project in Reykjanes, Iceland, we demonstrate that OCloC significantly reduces the time and effort needed to detect and correct timing errors in large-scale OBS deployments. In addition, our methodology allows one to evaluate potential timing errors at the time of OBS deployment. These might be caused by incorrect initial synchronization, or by rapidly changing temperature conditions while the OBS is sunk to the sea bottom. Our work advances the use of OBSs for earthquake studies and other applications.@en

Geothermal energy is a cleaner and more sustainable source of power, which plays a key role in the transition to a low-carbon economy. Sustainable and safe exploitation of geothermal resources, however, depends on our ability to understand and manage the associated seismic risks. In 2018, Nature's Heat geothermal project began operations in Kwintsheul, Netherlands, aiming to supply heat to 64 hectares of greenhouses. Between July and October 2019, a temporary seismic array was installed to monitor for possible seismic activity at the site. Microseismic moment-tensor inversion is a valuable tool for understanding the mechanics and structure of geothermal reservoirs, and for optimizing their exploitation. It can be challenging, though, to apply this technique when there are high levels of ambient seismic noise, as is often the case in geothermal operations in densely populated areas. In this study, we evaluate the feasibility of inverting the centroid moment tensor of microseismic events in Kwintsheul, using probabilistic moment-tensor inversions. We first test the probabilistic inversion using synthetic recordings of ambient seismic noise, after which we apply the technique to the low-magnitude (Md=0.16) event recorded on July 14, 2019. Our results give insight into the challenges and limitations of applying moment-tensor inversion to low-magnitude events in the context of geothermal operations in the Netherlands.@en

The Mutatá earthquake is a Mw 6 earthquake which occurred in northwestern Colombia on September 14, 2016. This region is located at the junction between three tectonic plates, namely the South American, Nazca and Caribbean plates, and the Chocó-Panamá and Northern Andes Blocks. This event took place in the Murindo seismic zone, a zone characterized by a high seismic activity involving the Uramita fault zone, which defines the contact between the two blocks. In this study, we relocate the mainshock – aftershocks sequence and analyze the source characteristics of the Mutatá earthquake. Using data from the Colombian Seismological National Network, and after re-picking the 411 events, we obtain absolute locations exhibiting a NW-SE oriented cloud, with the mainshock being located 8 km away from its original location and at a depth of 17.6 km. The event cloud is situated at the intersection of three faults with different orientations, the NNW-SSE Uramita Fault, a NW-SE fault, and a NNE-SSW inferred fault. Using data coming from 8 broad-band seismographs within 300 km of the mainshock, we perform moment tensor and kinematic slip distribution inversions. The moment tensor inversion points to an event centroid at 20 km depth, with a predominantly double-couple mechanism. The fault orientations in the area, NW-SE orientation of the event cloud, and hypocenter – centroid technique, indicate that the NW-SE nodal plane likely corresponds to the fault plane giving a right-lateral strike-slip mechanism on a SW dipping plane. The rupture model estimated on this plane shows different slip patches, one being close to the mainshock centroid, and few other patches distributed around the mainshock except to the southeast where most of the aftershocks are located. The maximum slip for this model is approximately 0.16 m. The source characteristics of the 2016 Mutatá earthquake suggest then that secondary faults within the Murindo seismic zone can generate large earthquakes, potentially consisting in an important source of seismic hazard in this region.@en

In 2018, the geothermal project Nature's Heat started its operations to supply heat to 64 hectares of greenhouses in Kwintsheul, Netherlands. The operation involves the extraction and reinjection of geothermal fluids at a depth of about 2.4km. Several studies suggested that geothermal operations in these parts of The Netherlands are unlikely to generate felt seismic events (M>2.0); nevertheless, adequate seismic monitoring techniques are essential to guarantee sustainable and safe use of the Dutch subsurface. Between July and October 2019, Delft University of Technology, Seismotech (Greece), and Gastreatment Services BV installed a passive seismic network to monitor the seismic activity over Nature's Heat geothermal reservoir. The seismic network consists of 30 three-component short-period seismic sensors placed at inter-station distances of approximately 150 m along two crossing lines. A challenge for seismic monitoring systems in urban areas is the high level of background noise. In Kwintsheul, anthropogenic noise dominates the spectrograms at frequencies higher than 2 Hz. Despite these high background-noise levels, a seismic event of ML = 0.0 (duration magnitude Md 0.16) was recorded by all seismometers of the array on July 14, 2019. To understand the relation of the event and improve the safety of the geothermal operation, we are developing a probabilistic monitoring and inversion scheme. This study aims to improve the seismic network's detection and hypocentre-determination capabilities and verifies via template matching if the detected seismic event is repeating over time (possibly at the background noise level). @en

In 2018, a geothermal doublet started operating in Kwinstheul, Netherlands, for supplying heat to 64 hectares of greenhouses corresponding to Nature’s Heat joint initiative. This kind of geothermal operation requires extraction, circulation, and reinjection of fluids at a depth of 2.4 km. The reservoir used for the geothermal operation has shown good hydraulic parameters which allow the circulation of the fluid. Several authors agree that this kind of geothermal operation is unlikely to generate felt seismicity, nevertheless, adequate seismic monitoring is critical to guarantee sustainable and safe use of the subsurface. To monitor the operation of Nature’s Heat project, 30 three-component short-period seismic sensors were installed by Delft University of Technology and Seismotech (Greece). A challenge for seismic monitoring in Kwinstheul is the high levels of seismic noise coming from anthropogenic and operational activities. Despite the high background noise levels, a seismic event of Md 0.16 was recorded on July 14, 2019. To understand the relation of the event and improve the safety of the geothermal operation, we are developing an optimized monitoring scheme. @en

Accurate timing of seismic records is essential for almost all applications in seismology. Wrong timing of the waveforms may result in incorrect Earth models and/or inaccurate earthquake locations. As such, it may render interpretations of underground processes incorrect. Ocean bottom seismometers (OBSs) experience clock drifts due to their inability to synchronize with a GNSS signal (with the correct reference time), since electromagnetic signals are unable to propagate efficiently in water. As OBSs generally operate in relatively stable ambient temperature, the timing deviation is usually assumed to be linear. Therefore, the time corrections can be estimated through GPS synchronization before deployment and after recovery of the instrument. However, if the instrument has run out of power prior to recovery (i.e., due to the battery being dead at the time of recovery), the timing error at the end of the deployment cannot be determined. In addition, the drift may not be linear, e.g., due to rapid temperature drop while the OBS sinks to the seabed. Here we present an algorithm that recovers the linear clock drift, as well as a potential timing error at the onset. The algorithm presented in this study exploits seismic interferometry (SI). Specifically, time-lapse (averaged) cross-correlations of ambient seismic noise are computed. As such, virtual-source responses, which are generally dominated by the recorded surface waves, are retrieved. These interferometric responses generate two virtual sources: a causal wave (arriving at a positive time) and an acausal wave (arriving at a negative time). Under favorable conditions, both interferometric responses approach the surface-wave part of the medium's Green's function. Therefore, it is possible to calculate the clock drift for each station by exploiting the time-symmetry between the causal and acausal waves. For this purpose, the clock drift is calculated by measuring the differential arrival times of the causal and acausal waves for a large number of receiver-receiver pairs and computing the drift by carrying-out a least-squares inversion. The methodology described is applied to time-lapse cross-correlations of ambient seismic noise recorded on and around the Reykjanes peninsula, SW Iceland. The stations used for the analysis were deployed in the context of IMAGE (Integrated Methods for Advanced Geothermal Exploration) and consisted of 30 on-land stations and 24 ocean bottom seismometers (OBSs). The seismic activity was recorded from spring 2014 until August 2015 on an area of around 100 km in diameter (from the tip of the Reykjanes peninsula).@en

In volcanic and hydrothermal geosystems, monitoring of mass and stress changes provide information for both volcanic hazardassessment and estimation of geothermal resources. The combined continuous recording of the gravity field and ground motionwith sufficient accuracy in an active volcano-tectonic setting allows a better understanding of the mass and stress transfermechanisms that produce short term gravity changes and local seismic activity. The aim is to gain a better understanding ofgeothermal system processes by addressing short-term mass changes within geothermal reservoirs in relation to external influencessuch as anthropogenic (reservoir exploitation) and natural forcing (local and regional earthquake activity and earth tides). Thiscontributes to knowing the reservoir properties, structure and long-term behaviour.Þheistareykir (Northeast Iceland), where the geothermal power production started in autumn 2017 (2x45 MWe) is the site chosenfor this unique experiment. The overall goal of the project is to use a network of continuously measuring gravity meters to detectsmall variations in gravity associated with managing a geothermal field (injection and extraction). The gravity changes are expectedto be small: ~5 µgal/6 months (1 µgal=10-8 ms-2). Therefore, high performance and up-to-date instrumentation such assuperconducting gravity meters (SG), spring gravity meters and broadband seismometers are used. To achieve these goals, inautumn 2017 a network of 5 relative gravity meters (3 iGravs and 2 gPhones) and 14 seismic stations were deployed. Three gravitymonitoring sites are in close vicinity to the production and injection area, and one iGrav is set up outside the geothermal field forreference. Presented in this report are the details of the infrastructure and instruments deployed and the first results of more than 18months of continuous gravity and seismicity monitoring.@en