We hindcasted the seismicity rates and the next largest earthquake magnitude using seismic and hydraulic data from two hydraulic stimulation campaigns carried out in adjacent (500 m apart) ultra-deep wells in Finland. The two campaigns performed in 2018 and 2020 took place in the frame of St1 Helsinki project produced stable, pressure-controlled induced seismic activity with maximum magnitudes of MW 1.3 and 1.7, respectively. The seismicity rates were modeled using simplified physics-based approaches tailored to varying injection rates. This is the first time that this framework was applied to a cyclical injection protocol. The next largest earthquake magnitude was estimated using several existing models from the literature. Despite the close proximity of the two hydraulic stimulations and associated seismicity, we obtained strongly different parameterization of the critical model components, questioning the use of a-priori seismic hazard analysis tools in the planning of a neighboring stimulation. The differences in parameterization were attributed to the contrasting hydraulic energy rates observed in each stimulation, small differences in the structural inventory of the reservoir and resulting seismic injection efficiency, and potentially to variations in the injection protocol itself. As far as the seismicity rate model is concerned, despite a good performance during the 2018 campaign, the fit during the 2020 stimulation was suboptimal. Forecasting the next largest magnitude using different models led to a very wide range of outcomes. Moreover, their relative ranking across stimulations was inconsistent, including the situation whether the best performing model in 2018 stimulation was the worst performing one in the 2020 stimulation.

A document by Grzegorz Kwiatek. Click on the document to view its contents.

On November 23rd 2022, a MW 6.0 earthquake occurred in direct vicinity of the MW 7.1 Düzce earthquake that ruptured a portion of the North Anatolian Fault in 1999. The Mw 6.0 event was attributed to a small fault portion of the Karadere segment that did not rupture during the 1999 sequence. We analyze the spatio-temporal evolution of the MW 6.0 Gölyaka-Düzce seismic sequence at various scales and resolve the source properties of the mainshock. Modelling the decade-long evolution of background seismicity of the Karadere Fault employing an Epistemic Type Aftershock Sequence model shows that this fault was almost seismically inactive before 1999, while a progressive increase in seismic activity is observed from 2000 onwards. A newly generated high-resolution seismicity catalog from 1 month before the mainshock until six days after created using Artificial Intelligence-aided techniques shows only few events occurring within the rupture area within the previous month, no spatio-temporal localization process and a lack of immediate foreshocks preceding the rupture. The aftershock hypocenter distribution suggests the activation of both the Karadere fault which ruptured in this earthquake as well as the Düzce fault that ruptured in 1999. First results on source parameters and the duration of the first P-wave pulse from the mainshock suggest that the mainshock propagated eastwards in agreement with predictions from a bimaterial interface model. The MW 6.0 Gölyaka-Düzce represents a good example of an earthquake rupture with damaging potential within a fault zone that is in a relatively early stage of the seismic cycle.

Aseismic slip may occur during a long preparatory phase preceding earthquakes, and what controls it remains poorly understood. In this study, we explored the potential dependencies of the slow slip during the preparatory stage prior to stick-slip instabilities on two main factors, namely the loading rate and surface roughness. To that end, we conducted shear stress-driven friction experiments by imposing varying loading rates on sawcut granite samples with different surface roughness at confining pressure of 35 MPa. We measured the average slip along the fault using far-field displacements and strain changes, while using acoustic emission sensors and local strain gages to capture local slip variations. We found that the average aseismic slip during preparatory stage increases with roughness, whereas its duration decreases with increased loading rate. These results also evidence a complex slip pattern on rough faults which leads to dynamic ruptures at high loading rates.

We investigate induced seismicity associated with a hydraulic stimulation campaign performed in 2020 in the 5.8 km deep geothermal OTN-2 well near Helsinki, Finland as part of the St1 Deep Heat project. A total of 2,875 m3 of fresh water was injected during 16 days at well-head pressures <70 MPa and with flow rates between 400-1000 l/min. The seismicity was monitored using a high-resolution seismic network composed of 10 borehole geophones surrounding the project site and a borehole array of 10 geophones located in adjacent OTN-3 well. A total of 6,121 induced earthquakes with local magnitudes were recorded during and after the stimulation campaign. The analyzed statistical parameters include magnitude-frequency b-value, interevent time and interevent time ratio, as well as magnitude correlations. We find that the b-value remained stationary for the entire injection period suggesting limited stress build-up or limited fracture network coalescence in the reservoir. The seismicity during the stimulation neither shows signatures of magnitude correlations, nor temporal clustering or anticlustering beyond those arising from varying injection rates. The interevent time statistics are characterized by a Poissonian time-varying distribution. The calculated parameters indicate no earthquake interaction. Focal mechanisms suggest that the injection activated a spatially distributed network of similarly oriented fractures. The seismicity passively responded to the hydraulic energy input rate, with the cumulative seismic moment proportional to the cumulative hydraulic energy and maximum magnitude controlled by injection rate. The performed study provides a base for implementation of time-dependent probabilistic seismic hazard assessment for the project site.