Post-5 Ma rock deformation on Alonnisos (Greece) constrains the propagation of the North Anatolian Fault

The localization of the North Anatolian Fault in the northern Aegean Sea (North Aegean Trough) is an intriguing example of continental transform fault propagation. Understanding this process critically depends on the quantification of strike-slip displacement and the superposition of normal and strike-slip faulting in the region, which is the aim of this study. In particular, we unravel and quantify normal and dextral faulting along the Alonnisos fault system, at the south-western margin of the North Aegean Trough (Sporades Basin). We present detailed structural data collected from Messinian strata of Alonnisos to infer the amount of post-5 Ma tilting and shortening on the island, and relate them to normal and dextral faulting along the Alonnisos fault system through simple analytical half-space models of dislocations. The Messinian rocks of Alonnisos record significant (13.5°) tilting and gentle folding close to the termination zone of the main fault segment. The tilting of the Messinian rocks was related to footwall uplift during normal faulting (in the order of 6–7 km vertical displacement) along the Alonnisos fault system, which implies that the deepening of the Sporades Basin occurred post-5 Ma. The post-Messinian folding accommodated ∼1 km shortening along the footwall termination zone of the Alonnisos fault and was related to 3–4 km dextral slip, possibly during the last 100–200 kyr. This is the first clear indication of major dextral displacement along the Alonnisos fault system. Our results support interpretations of currently distributed dextral strain in the North Aegean in response to the propagation of the North Anatolian Fault. However, similarities with the evolution of the Sea of Marmara might suggest that dextral shear could yet become fully localized in the NAT

Double-difference relocation of the 29 January 2011 ML 4.5 Oroszlány earthquake and its aftershocks and its relevance to the rheology of the lithosphere and geothermal prospectivity

In the central part of Hungary, an earthquake with the local magnitude of 4.5 occurred near the town of Oroszlány, on 29 January 2011. The main shock and its more than 200 aftershocks were recorded by a significant number of three-component seismic stations, which enabled us to perform multiple event location on the event cluster. We applied the double difference, HypoDD method to relocate the aftershock sequence in order to identify the pattern of active faulting. We used the extended International Seismological Centre location algorithm, iLoc to determine the initial single event locations for the aftershock sequence and applied multiple event location algorithm on the new hypocenters. To improve both location precision and accuracy, we added differential times from waveform cross correlation to the double-difference multiple event location process to increase the accuracy of arrival time readings. We show that both HypoDD collapses the initial, rather diffuse locations into a smaller cluster and the vertical cross-sections show sharp images of seismicity. Some of the relocated events in the cluster are ground truth quality with a location accuracy of 5 km or better. Having achieved accurate locations, we further examined the extent of the seismogenic zone. We investigated the relationship between geothermics and seismicity through strength profiles constructed for the study area. The aftershocks of the Oroszlány earthquake are dominantly in the range of 5–10 km, fitting well to the extent of the thin brittle part of the crust. It shows that the events are well in accordance with a thermally attenuated lithosphere and elevated geothermal gradient in the upper crust and basin sediments. These findings underline the geothermal prospectivity of the Panonian Basin. © 2017, Akadémiai Kiadó

Post-5 Ma rock deformation on Alonnisos (Greece) constrains the propagation of the North Anatolian Fault

The localization of the North Anatolian Fault in the northern Aegean Sea (North Aegean Trough) is an intriguing example of continental transform fault propagation. Understanding this process critically depends on the quantification of strike-slip displacement and the superposition of normal and strike-slip faulting in the region, which is the aim of this study. In particular, we unravel and quantify normal and dextral faulting along the Alonnisos fault system, at the south-western margin of the North Aegean Trough (Sporades Basin). We present detailed structural data collected from Messinian strata of Alonnisos to infer the amount of post-5 Ma tilting and shortening on the island, and relate them to normal and dextral faulting along the Alonnisos fault system through simple analytical half-space models of dislocations. The Messinian rocks of Alonnisos record significant (13.5°) tilting and gentle folding close to the termination zone of the main fault segment. The tilting of the Messinian rocks was related to footwall uplift during normal faulting (in the order of 6–7 km vertical displacement) along the Alonnisos fault system, which implies that the deepening of the Sporades Basin occurred post-5 Ma. The post-Messinian folding accommodated ∼1 km shortening along the footwall termination zone of the Alonnisos fault and was related to 3–4 km dextral slip, possibly during the last 100–200 kyr. This is the first clear indication of major dextral displacement along the Alonnisos fault system. Our results support interpretations of currently distributed dextral strain in the North Aegean in response to the propagation of the North Anatolian Fault. However, similarities with the evolution of the Sea of Marmara might suggest that dextral shear could yet become fully localized in the NAT

An effective method for paleo-temperature correction of 3D thermal models: A demonstration based on high resolution datasets in the Netherlands

We present a new method to incorporate paleo-surface temperature effects in steady state 3D conductive temperature models. The workflow approximates the transient effects and incorporates these into steady state models, using appropriate source and sink terms for radiogenic heat production. This allows for rapid models, which can be easily used in ensemble approaches for data assimilation of high-resolution temperature models for geothermal resource assessment. The workflow is demonstrated for the Netherlands which is a sedimentary basin with a wealth of deep (temperature) data from groundwater and oil and gas wells and past studies on the 3D temperature distribution. 3D subsurface temperature models of the Netherlands ranging up to 5 km depth systematically overpredict temperatures at shallow (200,000 measurements) and deep temperature measurements (1–6 km, >1500 measurements), clearly demonstrates a shallow thermal gradient in over 200 locations of ~20 °C km−1 for the top 400 m underlain with a deep geothermal gradient of ~31 °C km−1 for the 2–4 km interval. Improvements in 3D subsurface modelling regarding the shallow part are accomplished by adding a paleo-surface temperature correction related to glaciation effects of the Weichselian glacial period. This paleo-surface temperature correction proved to be the missing link between two distinctive geothermal gradients observed and is consistent with earlier findings for limited datasets. The consistent overprediction of modelled temperatures in 74% of locations for the top 2 km which are regularly distributed over the Netherlands demonstrates that the influence of paleo-surface temperatures is rather uniform over large areas and not significantly overprinted by other effects such as groundwater flow. The updated model, marked by up to 10 degrees cooling compared to models ignoring the paleo-surface temperature effects, has major implications for assessing geothermal resource potential up to 2 km depth

Ground motions induced by pore pressure changes at the Szentes geothermal area, SE Hungary

Excessive thermal water volumes have been extracted from porous sedimentary rocks in the Hungarian part of the Pannonian Basin. Thermal water production in Hungary increased significantly from the early 1970s. Regional-scale exploitation of geothermal reservoirs without re-injection resulted in basin-scale pressure drop in the Upper Pannonian (Upper Miocene) sediments, leading to compaction. This compaction resulted in ground subsidence primarily through poro-elastic coupling. We investigated surface deformation at the Szentes geothermal filed, SE Hungary, where the largest pressure decline occurred. Subsequently, hydraulic head recovery in the western part of the geothermal reservoir was initiated in the mid-1990s. We obtained data from the European Space Agency’s Envisat satellites to estimate the ground motions for the period of November 2002–December 2006. We applied inverse geomechanical modeling to estimate reservoir properties and processes. We constrained the model parameters using the Ensemble Smoother with Multiple Data Assimilation, which allowed us to incorporate large amounts of surface movement observations in a computationally efficient way. Ground movements together with the modeling results show that uplift of the Szentes geothermal field occurred during the observation period. Since no injection wells were operated at Szentes before 2018, and production temperatures remained relatively constant through the entire production period, we explain ground uplift with pore pressure increase due to natural recharge. The estimated decompaction coefficients of the reservoir system characterizing the elastic behavior of the Szentes geothermal reservoir varies between ~ 0.2 × 10–9 and 2 × 10–9 Pa−1. Compaction coefficients of the reservoir system corresponding to the earlier depressurization period, from ~ 1970 to the mid-1990s, may be significantly larger due to the potential inelastic behavior and permanent compaction of clay-rich aquitards. The improved parametrization enables better forecasting of the reservoir behavior and facilitates the assessment of future subsidence scenarios that are helpful for the establishment of a sustainable production scheme

Source parameters of the 8 February 2016, Mw=4.2 Los Humeros earthquake by the inversion of InSAR-based ground deformation

We investigate ground movements induced by the 8 February 2016, Mw=4.2 earthquake at the Los Humeros Geothermal Field (Mexico) using Sentinel-1 radar interferometry. Previous estimated focal mechanism solution based on seismic data with a hypocentral depth of 1900 m could not resolve the measured coseismic surface deformation pattern. In this study, we applied inverse elastic dislocation models to estimate the source parameters of the seismic event. Our models suggest the reverse reactivation of the Los Humeros normal fault at a shallower depth (<1000 m), with a more significant left lateral component below ~400 m depth. The occurrence of such shallow events at Los Humeros pose increased risks for the neighboring communities and infrastructure. Therefore, continuous monitoring of seismicity and cautious planning of field operations are crucial. A NNW-SSE striking fault swarm, including the Los Humeros fault, acts as a major boundary of the subsiding area observed by InSAR time-series between February 2016 and May 2019. A potential explanation of the reverse reactivation of the Los Humeros fault and following downward movement of the eastern fault block is the depressurization of the whole hydrothermal system. Such depressurization can occur due to the exploitation of the geothermal field and/or due to natural pressure/temperature changes related to magmatic activity