Tectonic Stress Regime Research Articles

Three-dimensional inversion analysis of an in situ stress field based on a two-stage optimization algorithm

Establishing an accurate in situ stress field is important for analyzing the rock-mass stability of the underground cavern at the Huangdeng hydropower station in China. Because of the complexity and importance of the in situ stress field, existing back analysis methods do not provide the necessary accuracy or sufficiently recognize nonlinear relations between the distribution of the in situ stress field and its formative factors. Those factors are related to the geological structures of high compressive tectonic stress regimes, including geological faults and tuff interlayers. The new two-stage optimization algorithm proposed in this paper is a combination of stepwise regression (SR), difference evolution (DE), support vector machine (SVM), and numerical analysis techniques. Stepwise regression is used to find the set of unknown parameters that best match the modeling prediction and determine the range of parameters to be recognized. Difference evolution is used to determine the optimum parameters of the SVM. The SVM is used to create the DE-SVM nonlinear reflection model to obtain the optimal values of the parameters from measured stress data. We compare the new two-stage optimization algorithm to other two popular methods, a multiple linear regression (MLR) analysis method and an artificial neural network (ANN) method, to estimate the in situ stress field for the actual underground cavern at the Huangdeng hydropower station. The two-stage optimization algorithm produces a more realistic estimate of the stress distribution within the investigated area. Thus, this technique may have practical applications in realistic scenarios requiring efficient and accurate estimations of the in situ stress in a rock-mass.

The present-day state of tectonic stress in the Darling Basin, Australia: Implications for exploration and production

Knowledge of the full present-day stress tensor and pore pressure has significant applications in the exploration and production of conventional and unconventional hydrocarbon reservoirs. The Darling Basin of New South Wales, Australia, is an old sedimentary basin (Late Cambrian/Silurian to Early Carboniferous) in which there was limited information about the present-day stress field prior to this study. In this paper we evaluate the contemporary stress field of the Darling Basin using a dataset from recent exploration wells and perform a geomechanical risk assessment with respect to borehole stability, fracture/fault generation and reactivation. Our interpretations of borehole failures in borehole image logs reveal a prevailing east-west orientation of the maximum horizontal stress throughout the Darling Basin. The estimates of the magnitudes for the vertical, minimum and maximum horizontal stress in the studied wells indicate a transition between thrust and strike-slip faulting stress regime at 600–700 m depths, where the magnitude of vertical stress and minimum horizontal stress are close to each other. However, the presence of borehole breakouts and drilling-induced tensile fractures, that we observe in the image logs at greater depths (900–2100 m) indicate a transition into a strike-slip tectonic stress regime below a depth range of approximately 700–900 m. These findings are in agreement with overcoring stress measurements east and west of the investigated wells. Furthermore, there are several Neogene-to-Recent geological structures in the study area that indicate thrust faulting with an east-west oriented maximum horizontal stress orientation around this old sedimentary basin. The consistency between the orientation of maximum horizontal stress determined from wellbore data and neotectonic structures is significant, and implies that horizontal stress orientations derived from very recent geological features may be valuable inputs to geomechanical models in the absence of wellbore or other data. However, the recent surface geological structures suggest a thrust faulting stress regime that is in slight contrast to the transition between thrust and strike-slip stress regime (SH > Sh ∼ Sv) indicated by petroleum data, and highlights a potential pitfall of using neotectonic structures in geomechanical models. In particular, careful attention and verification should be made when using neotectonic structures for input, calibration or confirmation of geomechanical models, especially in intraplate tectonic settings such as Australia.

Effects of gas generation on stress states during burial and implications for natural fracture development

The complexity of fracture networks in shale gas reservoirs may result from the superimposition of several fracture sets forming in temporally modified stress states during burial. In addition to tectonic evolution, this modification can also be caused by the overpressure generated by gas generation. Compared with undercompaction, gas generation shows a different overpressuring process and may cause different effects on stress states. This paper addresses these effects during burial in terms of pore pressure stress coupling. A series of finite element simulations for this investigation are designed with different tectonic stress regimes using a simplified model with a homogeneous poroelastic medium. The results show that during burial, gas generation not only significantly decreases effective stresses but also changes differential stress by decreasing it in the normal faulting regime and increasing it in the thrust faulting regime compared with undercompaction. Furthermore, gas generation during burial may locally transform the given tectonic stress regime from normal faulting to strike slip or even thrust faulting regimes. The investigation implies that gas generation rather than undercompaction in burial history can significantly diversify the patterns of natural fractures by affecting the differential stress and even the local stress regime in addition to influencing their spatial distribution due to its spatial heterogeneity and its different effects on stress states in different stress regimes. As a result, the contribution of gas generation to overpressures should be differentiated from that of undercompaction before analysing the effects of overpressures on natural fracture development.

State of tectonic stress in Shillong Plateau of northeast India

Tectonic stress regime in the Shillong plateau, northeast region of India, is examined by stress tensor inversion. Some 97 reliable fault plane solutions are used for stress inversion by the Michael and Gauss methods. Although an overall NNW-SSE compressional stress is observed in the area, the stress regime varies from western part to eastern part of the plateau. The eastern part of the plateau is dominated by NNE-SSW compression and the western part by NNW-SSE compression. The NNW-SSE compression in the western part may be due to the tectonic loading induced by the Himalayan orogeny in the north, and the NNE-SSW compression in the eastern part may be attributed to the influence of oblique convergence of the Indian plate beneath the Indo-Burma ranges. Further, Gravitational Potential Energy (GPE) derived stress also indicates a variation from west to east.

Evolution of the Late Cenozoic Tectonic Stress Regime in the Tianshui Basin, Northeast Tibetan Plateau

Evolution of the Late Cenozoic Tectonic Stress Regime in the Tianshui Basin, Northeast Tibetan Plateau

Debris entrainment and landform genesis during tidewater glacier surges

AbstractThe englacial entrainment of basal debris during surges presents an opportunity to investigate processes acting at the glacier bed. The subsequent melt‐out of debris‐rich englacial structures during the quiescent phase produces geometrical ridge networks on glacier forelands that are diagnostic of surge activity. We investigate the link between debris entrainment and proglacial geomorphology by analyzing basal ice, englacial structures, and ridge networks exposed at the margins of Tunabreen, a tidewater surge‐type glacier in Svalbard. The basal ice facies display clear evidence for brittle and ductile tectonic deformation, resulting in overall thickening of the basal ice sequence. The formation of debris‐poor dispersed facies ice is the result of strain‐induced metamorphism of meteoric ice near the bed. Debris‐rich englacial structures display a variety of characteristics and morphologies and are interpreted to represent the incorporation and elevation of subglacial till via the squeezing of till into basal crevasses and hydrofracture exploitation of thrust faults, reoriented crevasse squeezes, and preexisting fractures. These structures are observed to melt‐out and form embryonic geometrical ridge networks at the base of a terrestrially grounded ice cliff. Ridge networks are also located at the terrestrial margins of Tunabreen, neighboring Von Postbreen, and in a submarine position within Tempelfjorden. Analysis of network characteristics allows these ridges to be linked to different formational mechanisms of their parent debris‐rich englacial structures. This in turn provides an insight into variations in the dominant tectonic stress regimes acting across the glacier during surges.

Evolution of the late Cenozoic tectonic stress regime in the Shanxi Rift, central North China Plate inferred from new fault kinematic analysis

The Shanxi Rift, an intracontinental belt along the eastern margin of the Ordos Block, North China Plate, has undergone multi-stage extensional deformation during the late Cenozoic. In this study, new fault kinematic analysis of boundary faults in the Shanxi Rift, constrained by geochronological data, defines a three-stage tectonic stress regime since the late Miocene, providing new structural evidence for deciphering the northeastward growth of the Tibetan Plateau. The earliest stage involved a transtensional stress regime of NW–SE extension and NE–SW compression in the late Miocene–early Pleistocene (ca. 10–1.8Ma) and characterized by mean principal stress axes of σ1 at 344.7°/87.2°, σ2 at 217.6°/01.1°, and σ3 at 128.1°/01.0°. The early transtensional stress regime was responsible for the opening of the Shanxi Rift and was very likely associated with intensive right-lateral pull-apart activity in the eastern Ordos Block due to the northeastward growth of the Tibetan Plateau. During the second stage, the stress regime changed to less intense NE–SW extension, with mean stress orientations of 323.3°/85.2° for σ1, 127.5°/02.9° for σ2, and 214.9°/02.6° for σ3 in the early late Pleistocene (ca. 1.8–0.11Ma). This stage is marked by a distinct shift from fluvio-lacustrine to fluvio-piedmont sedimentary facies at the end of the early Pleistocene, and the widespread eruption of basalt in the Datong Basin north of the Shanxi Rift. This short-lived extension appears to be the result of relaxation in the midst of crustal shortening during growth of the Tibetan Plateau. The third stage was transtensional, characterized by NNW–SSE extension and WNW–ESE compression (σ1 at 060°/88.9°, σ2 at 074.9°/03.9°, and σ3 at 164°/00°) since the late Pleistocene (ca. 0.11Ma), causing dextral shear deformation in the Shanxi Rift and the disappearance of its paleo-lake. This stage resulted from regional ENE–WSW shortening due to northeastward growth of the Tibetan Plateau.

Thrust‐type focal mechanisms of tectonic tremors in Taiwan: Evidence of subduction

AbstractUnlike tectonic tremors in subduction zones and along transform faults, the hosting structure for tremors in Taiwan remains debated. Tectonic tremors in Taiwan have been discovered at ~30 km depth beneath the southern Central Range, which is a young and active collisional mountain belt. Here we provide the first evidence for the focal mechanism of tremor using moment tensor inversion in the very low frequency band, employing broadband seismograms stacked relative to the hypocentral time of tremor. The best solution corresponds to low‐angle thrust faulting, suggesting the subduction of the Eurasian plate. This mechanism is consistent with strong tidal modulation of tremor activity but differs from the normal‐type faulting that dominates regional shallow earthquakes. This result suggests vertical variations in the tectonic stress regime. Thrust faulting may be facilitated by a decrease in normal stress due to the buoyant roots of the mountain belt and local high fluid pressure.

Could a Sumatra-like megathrust earthquake occur in the south Ryukyu subduction zone?

A comparison of the geological and geophysical environments between the Himalaya-Sumatra and Taiwan-Ryukyu collision-subduction systems revealed close tectonic similarities. Both regions are characterized by strongly oblique convergent processes and dominated by similar tectonic stress regimes. In the two areas, the intersections of the oceanic fracture zones with the subduction systems are characterized by trench-parallel high free-air gravity anomaly features in the fore-arcs and the epicenters of large earthquakes were located at the boundary between the positive and negative gravity anomalies. These event distributions and high-gravity anomalies indicate a strong coupling degree of the intersection area, which was probably induced by a strong resistance of the fracture features during the subduction. Moreover, the seismicity distribution in the Ryukyu area was very similar to the pre-seismic activity pattern of the 2004 Sumatra event. That is, thrust-type earthquakes with a trench-normal P-axis occurred frequently along the oceanward side of the mainshock, whereas only a few thrust earthquakes occurred along the continentward side. Therefore, the aseismic area located west of 128°E in the western Ryukyu subduction zone could have resulted from the strong plate locking effect beneath the high gravity anomaly zone. By analogy with the tectonic environment of the Sumatra subduction zone, the occurrence of a potential Sumatra-like earthquake in the south Ryukyu arc is highly likely and the rupture will mainly propagate continentward to fulfill the region of low seismicity (approximately 125° E to 129° E; 23° N to 26.5° N), which may generate a hazardous tsunami.

Evidence for exhumation of a granite intrusion in a regional extensional stress regime based on coupled microstructural and fluid inclusion plane studies – An example from the Velence Mts., Hungary

A new approach, the fluid inclusion plane technique coupled with fluid inclusion microthermometry and field measurements have been applied to demonstrate the exhumation and relative vertical displacements of an allochthonous, Permian granite intrusion (Velence Mts.) which was situated in the Middle Triassic at the western end of the Neotethys rift system. The pressure and the temperature conditions during the Permian fluid flow (>350 °C and ∼2 kbar) in the granite were considerably higher than during the Triassic fluid flow (<250 °C and ∼0.5 kbar), which indicates the exhumation of the granite intrusion in a primarily extensional tectonic stress regime. The fluid inclusion planes with NE–SW and NW–SE strike prove stress field permutation that can be explained by the exhumation of the host granite during the Middle Triassic fluid flow.It is suggested that the exhumation of the Velence Mts. along with the regional fluid flow can be connected to the passive pre-rift phase of the northern Adriatic Block, during the Middle–Late Triassic. The variations in homogenization temperatures of Triassic regional fluid inclusion assemblages between the blocks of the granite, as well as variations in the fluid inclusion plane orientations, suggest a post-Triassic vertical segmentation and relative block rotation of the granite.

Pore pressure stress coupling in 3D and consequences for reservoir stress states and fault reactivation

The spatio-temporal changes of the stress state in a geothermal reservoir are of key importance for the understanding of induced seismicity and planning of injection and depletion strategies. In particular the poro-elastic effects on the stress state due to re-injection or depletion of water are of interest for both geothermal projects and hydrocarbon exploitation. In addition to the conventionally used effective stress concept, poro-elasticity affects the stress tensor components differently as a function of changes in pore pressure. Here, we provide an analytical base for the long-term changes of the 3D stress tensor components as a function of pore pressure changes. Results indicate that for a constant rate of injection or depletion the coupling between pore pressure and all stress tensor components depends on the location in the reservoir with respect to the re-injection/depletion point as well as the time since the beginning of pore pressure changes. Our systematic analysis suggests that poro-elastic stress changes can even locally modify the given tectonic stress regime. Furthermore, the results predict that localized changes of maximum shear stress can lead to different fracture orientations than those expected when poro-elastic effects are not considered. These results indicate a need for 3D geomechanical-numerical studies of more realistic reservoir settings in order to study the 3D effects of pore pressure/stress coupling. Our generic 3D geomechanical-numerical study shows that less than two years of production of a single well changes shear stresses by 0.2MPa. Thus, in reservoirs with decades of production shear stress change can reach sufficiently high values to re-activate pre-existing faults or even generate new fractures with unexpected orientations.

Microseismic Monitoring, Event Occurrence, and the Relationship to Subsurface Geology

Abstract The Illinois Basin – Decatur Project (IBDP) is one of the US Department of Energy National Energy Technology Laboratory-funded carbon dioxide (CO 2 ) sequestration projects that is approaching the goal of injecting one million tonnes of CO 2 within three years. Since mid-November 2011, IBDP has maintained the target injection rate of approximately 1,000 tonnes of CO 2 per day into the Mt. Simon Sandstone at a depth of approximately 7,000 ft (2134 m). Several measurement, monitoring, characterization, data integration, and modelling technologies have been implemented on this project, including real-time continuous microseismic monitoring which commenced several months prior to start of injection. Much of the extensive site characterization effort at IBDP has been motivated by the desire to understand the source mechanisms for observed microseismicity toward the ultimate goal of developing predictive capability. A rich dataset of microseismic observations has been acquired over nearly 4.5 years of monitoring to-date. These observations form semi-linear clusters in space, supporting the interpretation of a structural source mechanism. However, corresponding structural features are not observed in the existing 3D seismic data. This lack of direct observation of a structural source feature prompted a multi-disciplinary geoscience based approach to understanding the source mechanism for the observed microseismicity. Relationships between microseismic event occurrence and subsurface geology are observed in multiple domains and at multiple scales. Geomechanical characterization efforts show that the orientations of clusters are consistent with the in-situ tectonic stress regime. Microseismic event first motion analysis suggests focal mechanisms also consistent with the tectonic stresses. While no faults or other structural discontinuities may be unambiguously interpreted from the seismic data using conventional amplitude interpretation methods, investigation of specialized edge detection seismic attributes reveals a directional fabric in the rock mass that is consistent with the orientation of microseismic clusters. At a macro scale, some microseismic clusters appear to be associated with topographic features in the Precambrian basement interpreted from 3D seismic data. At the opposite end of the scale, the fractures observed in grains are suggestive of a tectonic stress regime consistent with both the geomechanical analysis and the microseismic cluster orientation. Through these multi-disciplinary, multi-scale studies, an understanding of the relationship between subsurface geology and observed microseismicity is evolving. While no single observation set supports unambiguous correlation, the consistency between multiple lines of investigation supports an interpretation of in-situ stresses, rock fabric anisotropy, grain scale failure, and relationships to basement structure, all consistent with realistic hypotheses for the microseismic source event mechanisms. One such source mechanism hypothesis has been tested using sophisticated numerical fluid flow and geomechanical modeling followed by seismological calculations, preliminary results of which are presented here.

Fault Process and Broadband Ground-Motion Simulations of the 23 October 2011 Van (Eastern Turkey) Earthquake

Abstract On 23 October 2011 an M w 7.1 earthquake occurred in eastern Turkey, close to the towns of Van and Ercis, causing more than 600 casualties and widespread structural damage. The earthquake ruptured a 60–70 km long northeast–southwest fault with a thrust mechanism, in agreement with regional tectonic stress regime. We studied the fault process of the event and the recorded ground motions using different sets of data. Regional records (0.005–0.010 Hz) are used to constrain the centroid moment tensor solution. Near‐regional data, 100–200 km from the fault, are used for relocation of the hypocenter and, in the frequency range 0.05–0.15 Hz, for inversion of the rupture propagation by two methods: multiple point‐source model (ISOLA) and multiple finite‐extent (MuFEx) source model. MuFEx also provides an estimate of the model uncertainty, which is quite large due to unfavorable station distribution. We arrive at several plausible scenarios (equally well fitting the observed data including Global Positioning System coseismic displacements) with different styles of the rupture propagation. A few alternative source models are used for broadband (0.1–10 Hz) ground‐motion simulations by means of the hybrid integral‐composite source model. Only models comprising source complexities, such as a delayed rupture of shallow asperities, enable explanation of the acceleration record at the only available near‐fault station, which exhibits a long duration and two prominent wave groups. These complex rupture models are used to simulate the ground motion in the near‐fault area, specifically, at Van and Ercis, where records of the mainshock were missing, providing reasonable agreement with the observed spatial distribution of damage.

Stress magnitude and orientation in the Potiguar Basin, Brazil: Implications on faulting style and reactivation

AbstractWe analyzed borehole breakout data and drilling‐induced tensile fractures derived from resistivity image logs run at 10 oil wells to derive the orientation of the maximum horizontal stress SHmax from the Potiguar Basin in the continental margin of Brazil. Stress magnitudes are derived from density logs for the vertical stress, mini‐frac tests for the minimum horizontal stress Shmin, and rock strength laboratory analysis to estimate the SHmax magnitudes. We compared these results with the stress regime and SHmax orientation derived from nine earthquake series located in the crystalline basement, where seismicity is concentrated, and previous breakout data from the basin. In the basin, the SHmax gradient is 20.0 MPa/km, and the SHmax/Shmin ratio is 1.154, indicating a normal tectonic stress regime from 0.5 to 2.0 km, whereas the SHmax gradient of 24.5 MPa/km and SHmax/Shmin ratio of 1.396 indicate a transition from a normal to strike‐slip stress regime at 2.5 to 4.0 km. The deeper stress regime in the basin is similar to that in the basement at 1–12 km depth. This transition of the tectonic stress regime is consistent with an incipient tectonic inversion in the basin. We note that the SHmax orientation rotates from NW–SE in the western part of the Potiguar Basin to E–W in its central and eastern parts, roughly following the shoreline geometry, indicating that local features such as flexural stresses influence the local (scale &lt; 100 km) stress pattern. We also conclude that the basement is critically stressed, but not the basin.

Regional Tectonic Transformation in East Kunlun Orogenic Belt in Early Paleozoic: Constraints from the Geochronology and Geochemistry of Helegangnaren Alkali‐feldspar Granite

Abstract:The Helegangnaren feldspar granite exposed in the eastern part of East Kunlun, is characterized by high concentrations of SiO2 and alkaline, low abundances of Fe, Mg and Ca, metaluminous‐weak peraluminous. Trace elements analysis shows that the granite is depleted extremely in Ba, Sr and Eu, and rich in some large‐ion lithophile elements and high field strength elements. Besides, the granite has high Ga contents, the values of 104(Ga/Al) vary from 2.50 to 2.77, which is mainly greater than the lower limit of A‐type granites (2.6), and is higher than the I‐ and S‐type granites’ average (2.1 and 2.28, respectively). Rare earth element (REE) is characterized by relatively high fractionations of light REE (LREE) and heavy REE (HREE) (LREE/HREE=9.3–13.60, (La/Yb)N=10.92–18.02), pronounced negative Eu anomalies (δEu=0.08–0.13), and exhibits right‐dipping gull pattern. Major elements, rare elements and trace elements features show the granite is ascribed to A‐type granite and A2 subtype in tectonic genetic type. They are plotted into post‐collision or within‐plate area in a variety of tectonic discriminations. Geological and geochemical data comprehensively suggest that the granite is formed in a post‐collision extensive tectonic setting. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) zircon U‐Pb dating yields a weighted mean age of 425 Ma, belonging to Middle Silurian, which is similar to the age of the post‐collision geological events in the region. The differences of magmatic rocks in formation age, rocks assemblage and rocks series systematically indicate that the regional tectonic stress regime in the East Kunlun orogenic belt experienced a major transformation from compress to extension in Middle Silurianin, and the Helegangnaren feldspar granite intruded in the early stage of tectonic transformation.

Structural style and fault kinematics of the Lower Eocene Rus Formation at Jabal Hafit area, Al Ain, United Arab Emirates (UAE)

The current contribution presents aspects of the structural style and fault kinematics of the Rus Formation that expose at Jabal Hafit, Al Ain, United Arab Emirates. Although the major structure of Jabal Hafit is an anticlinal fold, fractures (joints and faults) are the prominent structure of the study area. The fractures can be interpreted as the distributed effect of deep-seated basement fault reactivation or to be as reactivation of deep-seated basement faults. These fractures were created during two main tectonic stress regimes. The first is a WNW–ESE S Hmax strike-slip stress regime, responsible for producing E–W to ESE–WNW joints and E–W dextral strike-slip and NNE–SSW reverse faults. This stress is interpreted to be post-Early Eocene in age and related to the second phase of thrusting in the Oman Mountains in the Miocene. The second stress regime is a NNE–SSW S Hmax transtensional (strike-slip extensive) stress regime that was responsible for N–S to NNE–SSW striking joints and NE–SW sinistral strike-slip and N–S normal faults. This regime is interpreted to be post-Middle Eocene in age. This stress was the response to the collision of the Arabian–Eurasian Plates which began during the Late Eocene and continues to the present day.

Present-day tectonic stress regime in Egypt and surrounding area based on inversion of earthquake focal mechanisms

Stress field inversion is performed in Egypt on the basis of 219 focal mechanism solutions in the period from 1955 to 2007. For this purpose Egypt is divided into six seismotectonic zones: the northern part of the Gulf of Suez, southern Gulf of Suez, and Gulf of Aqaba, Cairo-Suez district, Dahshour zone and the Aswan Zone. The entire Gulf of Suez is currently under extensional stress field, with NE–SW trending horizontal extension. In the Gulf of Aqaba, the strike-slip regime predominates with sub-horizontal σ1 and σ3 axes trending NNW and ENE, respectively. A normal dip slip with small strike-slip component due to a nearly sub-vertical σ1 and sub-horizontal NNE striking σ3 characterizes Cairo-Suez district and Dahshour zone. Aswan seismic zone shows mainly strike-slip stress regime with a slight extension component (horizontal NW σ1 and NNE σ3). The stress field derived in this study indicates a prevailing tension stress (σ3 horizontal) which agrees well with the general tectonic frame of northeastern African, which is subjected to tensional stresses. Generally, extensional and/or extensional-strike slips are dominating the Egyptian territory. These regimes are compatible with the kinematics of the Red Sea–Gulf of Suez rift and Gulf of Aqaba transform plate boundary. Furthermore, the inferred stress in the present study (SHmin directed NNE–SSW) for the Cairo-Suez, Dahshour, and Aswan areas is similar to the East African Rift stress fields “Congo and Sudan” especially (Bosworth et al., 1992; Delvaux and Barth, 2010), whose origin is attributed to the far field effects of ridge push in the Atlantic and Indian Oceans (Zoback, 1992).

Mechanical and statistical evidence of the causality of human-made mass shifts on the Earth’s upper crust and the occurrence of earthquakes

A global catalog of small- to large-sized earthquakes was systematically analyzed to identify causality and correlatives between human-made mass shifts in the upper Earth’s crust and the occurrence of earthquakes. The mass shifts, ranging between 1 kt and 1 Tt, result from large-scale geoengineering operations, including mining, water reservoirs, hydrocarbon production, fluid injection/extractions, deep geothermal energy production and coastal management. This article shows evidence that geomechanical relationships exist with statistical significance between (a) seismic moment magnitudes M of observed earthquakes, (b) lateral distances of the earthquake hypocenters to the geoengineering “operation points” and (c) mass removals or accumulations on the Earth’s crust. Statistical findings depend on uncertainties, in particular, of source parameter estimations of seismic events before instrumental recoding. Statistical observations, however, indicate that every second, seismic event tends to occur after a decade. The chance of an earthquake to nucleate after 2 or 20 years near an area with a significant mass shift is 25 or 75 %, respectively. Moreover, causative effects of seismic activities highly depend on the tectonic stress regime in which the operations take place (i.e., extensive, transverse or compressive). Results are summarized as follows: First, seismic moment magnitudes increase the more mass is locally shifted on the Earth’s crust. Second, seismic moment magnitudes increase the larger the area in the crust is geomechanically polluted. Third, reverse faults tend to be more trigger-sensitive than normal faults due to a stronger alteration of the minimum vertical principal stress component. Pure strike-slip faults seem to rupture randomly and independently from the magnitude of the mass changes. Finally, mainly due to high estimation uncertainties of source parameters and, in particular, of shallow seismic events ( M6) seem to be triggered. The rupture propagation of triggered events might be dominated by pre-existing tectonic stress conditions.

Recent seismicity detection increase in the Santorini volcanic island complex

Abstract. Santorini is the most active volcanic complex in the South Aegean Volcanic Arc. To improve the seismological network detectability of the seismicity in this region, the Institute of Geodynamics of the National Observatory of Athens (NOA) recently installed 4 portable seismological stations supplementary to the 3 permanent stations operating in the region. The addition of these stations has significantly improved the detectability and reporting of the local seismic activity in the NOA instrumental seismicity catalogue. In this study we analyze quantitatively the seismicity of the Santorini volcanic complex. The results indicate a recent significant reporting increase mainly for events of small magnitude and an increase in the seismicity rate by more than 100%. The mapping of the statistical significance of the rate change with the z-value method reveals that the rate increase exists primarily in the active fault zone perpendicular to the extensional tectonic stress regime that characterizes this region. The spatial distribution of the b-value around the volcanic complex indicates a low b-value distribution parallel to the extensional stress field, while the b-value cross section of the volcanic complex indicates relatively high b-values under the caldera and a significant b-value decrease with depth. These results are found to be in general agreement with the results from other volcanic regions and they encourage further investigations concerning the seismic and volcanic hazard and risk estimates for the Santorini volcanic complex using the NOA earthquake catalogue.

In-situ stress amplification due to geological factors in tunnels: The case of Pajares tunnels, Spain

Geological and geomechanical anisotropies can significantly increase the magnitude of in-situ stress in a rock mass excavated for tunnelling purposes. Four tunnels for the new high-speed railway lines in Pajares in the Cantabrian Mountains, N. Spain, were analysed and significant deformations was found to have occurred in forty specific zones of these tunnels during excavation, requiring much more support than envisaged before construction. Local factors influencing in-situ stress have been identified in these zones of the tunnels, related to geological structures of high compressive tectonic stress regimes including thrust faults, folds with steep flank dip, or with geomechanical anisotropies in contact zones between rock formations of different strengths. A methodological procedure was applied to four tunnels to assess the influence of the geological anisotropies on the in-situ stress magnitudes. This procedure is based on the analysis of tunnel deformations and the support pressure needed to stabilise the excavations affected by geological anisotropies.The increase of in-situ stress due to local factors is expressed by the Stress Amplification Factor (SAF) defined by the ratio between the K(σH/σV)local value estimated in a particular rock mass tunnel zone and the mean K(σH/σV)regional value representative of the whole rock mass tunnel. Kregional was estimated from hydrofracture tests and from the TSI index. Klocal was calculated from the back analysis of the support pressure required to stabilise the deformed tunnel zones. SAF values for the forty specific tunnel zones ranged from 1.1 to 2.5. This significant increase in the magnitude of in-situ stress emphasises the influence of geological and geomechanical anisotropies in tunnel stability and support design. The results provide a quantitative approach for assessing structural stresses in rock masses for tunnelling excavations.