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Geodetically‐derived deformation rates are sometimes used to infer seismic hazard, implicitly assuming that short‐term (annual‐decadal) deformation is representative of longer‐term deformation. This is despite geological observations indicating that deformation/slip rates are variable over a range of timescales. Using geodetic data from 2016 to 2021, we observe an up to 7‐fold increase in vertical deformation rate in mid‐2019 across the Pisia‐Skinos normal fault in Greece. We hypothesize that this deformation is aseismic as there is no temporally correlated increase in the earthquake activity (M > 1). We explore four possible physical mechanisms, and our preferred hypothesis is that the transient deformation is caused by centimeter‐scale slip in the upper 5 km of the Pisia fault zone. This is the first observation of shallow tectonic (i.e., not related to human activities) aseismic deformation on a normal fault globally. Our results suggest that continental normal faults can exhibit variable deformation over shorter timescales than previously observed, and thus care should be taken when utilizing geodetic rates to quantify seismic hazard. Plain Language Summary: Slip rates of active faults are used in seismic hazard assessment to infer the frequency of damaging earthquakes. However slip rates are known to be variable when measured using different methodologies (e.g., geodesy, geomorphology) and timescales (years to millennia) for many types of faults in a range of tectonic settings. Hence we need to observe slip rates for different time scales and try to understand the mechanism(s) that cause slip rates to vary. In this paper, we present observations of vertical deformation around an active normal fault in the Gulf of Corinth, Greece. We see that the deformation is non‐uniform during 2016–2021 (the time period of data availability), and we analyze the data to find that the deformation rate increased in mid‐2019. To try and understand the physical mechanism that may have caused this increase in deformation rate, we use simple modeling to test four different hypotheses. We find that the best fit to the observations is for centimeters of slip occurring on shallowest few kilometers of the fault. Our results highlight the importance of understanding short‐term (annual‐decadal) processes happening on active faults, and the potential pitfalls of using slip rates measured over short‐timescales to infer seismic hazard. Key Points: Spatial variations in vertical ground motion (uplift/subsidence) correlate with the mapped trace of the Pisia‐Skinos fault in GreeceThe ground deformation is non‐uniform over the 6 years studied, and shows an up to 7× increase in rate, lasting at least 3 yearsThis transient interseismic deformation is hypothesized to be caused by shallow aseismic centimeter‐scale slip on the Pisia‐Skinos fault [ABSTRACT FROM AUTHOR]

To assess whether continental extension and seismic hazard are spatially-localized on single faults or spread over wide regions containing multiple active faults, we investigated temporal and spatial slip-rate variability over many millennia using in-situ 36Cl cosmogenic exposure dating for active normal faults near Athens, Greece. We study a ~ NNE-SSW transect, sub-parallel to the extensional strain direction, constrained by two permanent GPS stations located at each end of the transect and arranged normal to the fault strikes. We sampled 3 of the 7 seven normal faults that exist between the GPS sites for 36Cl analyses. Results from Bayesian inference of the measured 36Cl data implies that some faults slip relatively-rapidly for a few millennia accompanied by relative quiescence on faults across strike, defining out-of-phase fault activity. Assuming that the decadal strain-rate derived from GPS applies over many millennia, slip on a single fault can accommodate ~ 30–75% of the regional strain-rate for a few millennia. Our results imply that only a fraction of the total number of Holocene active faults slip over timescales of a few millennia, so continental deformation and seismic hazard are localized on specific faults and over a length-scale shorter than the spacing of the present GPS network over this time-scale. Thus, (1) the identification of clustered fault activity is vital for probabilistic seismic hazard assessments, and (2) a combination of dense geodetic observations and palaeoseismology is needed to identify the precise location and width of actively deforming zones over specific time periods. [ABSTRACT FROM AUTHOR]

Shale gas could help address the insatiable global demand for energy. However, in addition to risks of environmental pollution, the risk of induced seismicity during the hydraulic fracturing process is often considered as the major showstopper in the public acceptability of shale gas as an alternative source of fossil fuel. Other types of subsurface energy development have also demonstrated similar induced seismicity risks. This article presents an interdisciplinary review of notable cases of suspected induced seismicity relating to subsurface energy operations, covering operations for hydraulic fracturing, wastewater injection, conventional gas extraction, enhanced geothermal systems and water impoundment. Possible causal mechanisms of induced seismicity are described and illustrated, then methods to mitigate induced seismicity, encompassing regulations, including so‐called traffic light systems, monitoring and assessment, and numerical modeling approaches for predicting the occurrence of induced seismicity are outlined. Issues relating to public perception of energy technologies in regards to induced seismicity potential are also discussed. This article is categorized under:Photovoltaics > Climate and EnvironmentFossil Fuels > Climate and EnvironmentEnergy Infrastructure > Economics and PolicyEnergy and Development > Systems and Infrastructure A mechanism for seismicity induced by fluid injection operations is pore pressure increase via a diffusional process (left). This leads to a reduction in the effective normal stress on preexisting faults; allowing frictional resistance to fault sliding to be overcome. This mechanism requires a high permeable pathway from the fluid injection point to the preexisting fault. Even in the circumstances when injection fluids may be hydraulically isolated from any preexisting faults, the fault may be activated through perturbations in the stress field brought about by changes in volume or mass loading transmitted to the fault poroelastically (right: hydraulic fracturing example). [ABSTRACT FROM AUTHOR]