Local Stress Field Correction Method Based on a Genetic Algorithm and a BP Neural Network for In Situ Stress Field Inversion

The in situ stress field is the fundamental factor causing deformation and damage in geotechnical engineering, so it is the main basis for underground engineering design and excavation. However, it is difficult to accurately obtain the in situ stress through most existing inversion methods in areas with complex geological conditions. For the problem of a relatively discrete and nonlinear relationship of measured stress in the Yebatan Hydropower Station area, a new in situ stress inversion method called the local stress field correction (LSFC) method combining a genetic algorithm (GA), backpropagation (BP) neural network, and submodel method is proposed. The inverted in situ stress results produced by this method show that the distribution of in situ stress is greatly influenced by tectonic movements in the Yebatan area, there is no obvious linear relationship with depth, and the stress release phenomenon occurs at the faults. By comparison with the multiple regression method, it is found that the method still has high inversion accuracy under complex geological conditions, and the average relative error of LSFC inversion results is 17.05%, which is much lower than the value of 43.58% via the multiple regression method. Therefore, the LSFC method can be used for the inversion of in situ stress in complex geological regions and provide a reference for engineering design and construction.

Обоснование основных положений деформационной модели подготовки очага корового землетрясения

The article reasonably shows that the uppermost layer of the Earth's crust up to 25 kilometers is seismogenic. Aim. The article provides the evidence that crustal seismicity is generated not by regional stress fields of a homogeneous shear, as it was adopted in the strategy for solving the problem of earthquake forecast, but by local fields of exponential elastic stress. Such fields arise in one or another section of a seismogenic fault due to the occurrence of a stress concentrator in this section. According to the Saint Venant principle, such a stress concentrator (an additional load in the system) generates a local stress field of an exponential form. In this field the maximum stress is localized in the areas of an increment load application (in the fault) and decreases very quickly (exponentially) on both sides of the fault. Such stress concentrators arise in those areas of a seismogenic fault, where displacements along the fault stop due to various reasons. G.A. Gamburtsev foresaw this situation and very precisely called such concentrators as “seams”. The origin of a local stress field at the place, where a seam appears, is caused by the following fact: the power impulse generated by the seam is small compared to the linear momentum of the entire system of blocks of the considered fault and, therefore, it will stop the displacement of blocks only within the seam; but the displacements of blocks outside the seam will continue in the same mode. One can single out the following reasons causing stress concentrators in the fault: variations in different stress fields, changing the value of the friction coefficient in the fault; variations in fluid processes; the influence of temperature and pressure; mechanical “hooks” of blocks due to irregularities of their contacting surfaces, etc. Methods. The fact of the existence of the considered local stress fields is confirmed by geodetic studies, i.e. the results of repeated geodetic measurements in the epicentral zones of strong earthquakes. Results.These results allow drawing the following conclusions: 1) the sign of the preparation of a crustal earthquake source was reliably determined. This sign means the increasing deformation of the elastic bending of rocks in the source in the course of time; 2) from the standpoint of solving the problem of earthquake forecast, the main and decisive result of these studies is that the deformation processes occurring in the impending source also capture the Earth's surface, because this is precisely what opens up great opportunities in solving this problem; 3) with the help of special geodetic systems (forecast profiles), one can detect the places of the impending earthquake source preparation, i.e. make an accurate forecast of the site of a future earthquake; 4) since the energy of the earthquake source is functionally related to its size, one can realize the correct prediction of the maximum possible intensity of the future earthquake by determining the length of the seismogenic fault section, elastically deformed by the preparation of the earthquake using the forecast profiles В статье обоснованно показано, что сейсмогенным является самый верхний слой земной коры мощностью до 25 километров. Цель работы. В статье приведены доказательства того, что коровая сейсмичность порождается вовсе не региональными полями напряжений однородного сдвига, как это было принято в стратегии решения проблемы прогноза землетрясений, а локальными полями упругих напряжений экспоненциального вида. Такие поля возникают в том или ином участке сейсмогенного разлома из-за появления на этом участке концентратора напряжений. Согласно принципу Сен-Венана такой концентратор напряжений (дополнительная нагрузка в системе) порождает локальное поле напряжений экспоненциального вида. Максимальная величина напряжения в этом поле расположена в месте приложения дополнительной нагрузки (в разломе) и очень быстро (экспоненциально) убывает в обе стороны от разлома. Такие концентраторы напряжений возникают на тех участках сейсмогенного разлома, на которых в силу тех или иных причин прекращаются смещения по разлому. Г.А. Гамбурцев провидчески предвидел данную ситуацию и очень метко такие концентраторы назвал «спайками»». Возникновение локального поля напряжений в месте появления спайки обусловлено тем, что импульс силы, порождаемый спайкой мал по сравнению с количеством движения всей системы блоков рассматриваемого разлома и, следовательно, он остановит смещение блоков лишь в пределах спайки, но смещения блоков вне спайки будут продолжаться в прежнем режиме. Среди причин, порождающих концентраторы напряжений в разломе можно назвать следующие: вариации различных полей напряжений, изменяющие величину коэффициента трения в разломе; влияние температуры и давления; вариации флюидных процессов; механические «зацепы» блоков из-за неровностей их соприкасающихся поверхностей и др. Методы исследования. Факт существования рассматриваемых локальных полей напряжений подтвержден геодезическими исследованиями – результатами повторных геодезических измерений в эпицентральных зонах сильных землетрясений. Результаты работы. Эти результаты позволяют сделать следующие выводы: 1) достоверно определен признак подготовки очага корового землетрясения, которым является нарастающая во времени деформация упругого изгиба горных пород в его очаге; 2) с позиций решения проблемы прогноза землетрясений главным и определяющим результатом этих исследований является то, что происходящие в готовящемся очаге деформационные процессы захватывают и земную поверхность, ибо именно это открывает большие возможности в решении этой проблемы; 3) с помощью специальных геодезических систем (прогнозных профилей) можно обнаруживать места подготовки очагов готовящихся землетрясений, т.е. осуществлять точный прогноз места будущего землетрясения; 4) так как энергия очага землетрясения функционально связана с его размерами, то определив с помощью прогнозных профилей длину участка сейсмогенного разлома, упруго деформированного подготовкой землетрясения, можно осуществить и точный прогноз максимально возможной силы будущего землетрясения

Activation of optimally and unfavourably oriented faults in a uniform local stress field during the 2011 Prague, Oklahoma, sequence

SUMMARY The orientations of faults activated relative to the local principal stress directions can provide insights into the role of pore pressure changes in induced earthquake sequences. Here, we examine the 2011 M 5.7 Prague earthquake sequence that was induced by nearby wastewater disposal. We estimate the local principal compressive stress direction near the rupture as inferred from shear wave splitting measurements at spatial resolutions as small as 750 m. We find that the dominant azimuth observed is parallel to previous estimates of the regional compressive stress with some secondary azimuths oriented subparallel to the strike of the major fault structures. From an extended catalogue, we map ten distinct fault segments activated during the sequence that exhibit a wide array of orientations. We assess whether the five near-vertical fault planes are optimally oriented to fail in the determined stress field. We find that only two of the fault planes, including the M 5.7 main shock fault, are optimally oriented. Both the M 4.8 foreshock and M 4.8 aftershock occur on fault planes that deviate 20–29° from the optimal orientation for slip. Our results confirm that induced event sequences can occur on faults not optimally oriented for failure in the local stress field. The results suggest elevated pore fluid pressures likely induced failure along several of the faults activated in the 2011 Prague sequence.

The Time-Dependent Failure Mechanism of Rocks and Associated Application in Slope Engineering: An Explanation Based on Numerical Investigation

In this study, a numerical model for long-term deformation and progressive failure of rock slope is presented. The model accounts for both rock heterogeneity and the initiation, activation, nucleation, and coalescence of cracks in rock slope through a stochastic local stress field and local rock degradation by using an exponential softening law. The time-dependent behaviour of rocks is taken as a macroscopic consequence of damage evolution and strength degradation in microstructure. A series of demonstrative slope cases containing preexisting joints are constructed and investigated. The slope instability occurs at a particular point in time when the rock strength is reduced to a certain value. The temporal and spatial evolution of joint linkage structures is numerically obtained, which clearly shows how the local stress field and damage evolution within the joint network contribute to the fracture pattern and the long-term instability. Then, a practical slope case in jointed and layered rock formations in Yunyang city is studied. The prevailing failure phenomena of the slope, including gradual surface scaling, sliding collapses, and block falling, are numerically reproduced, with an emphasis placed on the slope failure process and development tendency. There is a good agreement on the failure mode and instability time between the numerical simulations and the field observations.

Seismic anisotropy time variations at Mt Etna

SUMMARY The aim of this paper is to study the temporal variations in the seismic wavefield associated with the stress changes in the dynamic features of the Mt Etna volcanic activity. We used shear wave splitting analysis on a huge data set of local earthquakes, in order to identify changes of the local stress field at Mt Etna during the time interval from 2006 to 2011. This analysis allows us to obtain two parameters: the polarization direction of the fast shear wave (φ) and the time delay of the slow shear wave (Td,time delay between the split shear waves). Orientation of φ generally provides information about the anisotropic symmetry and stress direction whereas Td provide information about the average crack density along the ray path. Based on our findings it is possible to divide Etna Volcano in three different sectors, each one distinguished by typical fast wave polarization direction. We find that the western part of the volcano is controlled by the regional tectonic stress field having a NS and EW directions. Instead, the eastern part of the volcano is mainly controlled by the local volcanic stress, particularly an EW local stress field in the NE sector (Pernicana), and a quasi NS local stress field in the SE sector (Mascalucia, Timpe), where previous studies evidenced: (i) some low-Qp anomaly regions between 0 and about 6 km depth, probably associated with high pore pressure and the intense faulting and (ii) by magnetotelluric surveys, several high conductivity zones, up to 8 km depth, related to a diffuse presence of hydrothermal activity and fluid circulation. Temporal variations in time delay, mostly before the 2008–2009 lateral eruption, can be interpreted as stress accumulation increase with a consequent release of stress due to coalescing of microcracks in the conduit for the eruption of magma.

Instruments and methods: a case study of ice core bubbles as strain indicators

Measurements of a sample from ~580 m depth in the WAIS Divide (WDC06A) ice core reveal that bubbles are preferentially elongated in the basal plane of their parent grain, as expected if bubble shape preserves the record of dominant basal glide. This suggests that a method using bubbles as strain gauges could provide insights to grain-scale ice deformation. We introduce a technique using fabric and image analyses of paired thin and thick sections. Comparison of the crystallographic orientations of 148 grains and the shape orientations of 2377 intragrain bubbles reveals a strongly preferred elongation of bubbles in the grain basal planes (R2 = 0.96). Elongation magnitudes are consistent with a balance between ice flow deformation and diffusive restoration, with larger bubbles more elongated. Assuming bubbles record ice strain, grains with greater resolved stress on their basal planes from the far-field ice flow stresses show greater deformation, but with large variability suggesting that heterogeneity of the local stress field causes deformation even in unfavorably oriented grains. A correlation is also observed among bubble elongation, grain size, and bubble size, explaining a small but significant fraction of the variance ( P< 0.05), with implications for controls on ice deformation, as discussed here.

Reduction of the Local Stress Field around Holes through Porous Shaped Structures

Geometrical discontinuities in mechanical components are detrimental for the mechanical properties of the product itself. Actually, in proximity of such features, the stress increases due to the stress concentration factor, that in the case of a circular hole is equal to 3. Several solutions have been proposed to reduce the stress concentration value. In the present article, the application of a particular porous pattern that can be obtained by laser cutting with the appropriate finishing requirements is introduced in order to modify the local stress field and reduce the stress concentration value near the hole boundary.