Linking strike directions to invariant TE and TM impedances of the magnetotelluric impedance tensor

Estimation of the traditional transverse electric (TE) and transverse magnetic (TM) impedances of the magnetotelluric tensor for two-dimensional structures can be decoupled from the estimation of the strike direction with significant implications when dealing with galvanic distortions. Distortion-free data are obtainable by combining a quadratic equation with the phase tensor. In the terminology of Groom–Bailey, the quadratic equation provides amplitudes and phases that are immune to twist, and the phase tensor provides phases immune to both, twist and shear. On the other hand, distortion-free strike directions can be obtained using Bahr's approach or the phase tensor. In principle, this is all that is needed to proceed to a two-dimensional (2D) interpretation. However, the resulting impedances are strike ignorant because they are invariant under coordinate system rotation, and if they are to be related to a geological strike, they must be linked to a particular direction. This is an additional ambiguity to the one of 90° arising in classic strike-determination methods, which must be resolved independently. In this work, we use the distortion model of Groom–Bailey to resolve the ambiguity by bringing back the coupling between impedances and strike in the presence of galvanic distortions. Our approach is a hybrid between existing numerical and analytical methods that reduces the problem to a binary decision, which involves associating the invariant impedances with the correct TE and TM modes. To determine the appropriate association, we present three algorithms. Two of them require optimizing the fit to the data, and the third one requires a comparison of phases. All three keep track of possible crossings of the phase curves providing a clear-cut solution. Synthetic and field data illustrate the performance of the three schemes. Graphical Abstract

PETROGENETIC MODELING OF THE GRANITIC ROCKS IN AND AROUND LODHMA, DIST RANCHI, JHARKHAND, INDIA

The Lodhma area, in the Ranchi district, constitutes the central segment of Chotanagpur Granite Gneissic Complex ( CGGC) and falls in the northeastern segment of Survey of India toposheet number 73E/4. The CGGC is mainly composed of variants of granitic rocks, amphibolites and metasedimentaries. The granitic rocks find good exposure in the area. They comprise predominantly of granite gneisses, crystalline massive granites and migmatites showing noticeable variations in mineralogy, texture, structure and associations. The regional strike of the lithounits of the area show almost E-W trend. However ambient structural controls have brought about local changes in the strike direction. The area has witnessed at least three phases of deformations which is reflected by three generations of folds, which are generally isoclinals. Petrogenetic characterization of granitic rocks and its variants in the area has been attempted using various geochemical tools involving major/ trace element studies and Niggli values. S –type parentage of these Granites have been established on the basis of distribution behavior of various chemical components present in it, thereby indicating role of pelitic schists in the generation of anatectic melt, which ultimately crystallized into granites. S-type lineage of these granites has also been further cemented with the help of several well established binary and ternary variation diagrams

Linking strike directions to invariant TE and TM impedances of the magnetotelluric impedance tensor

The traditional transverse electric (TE) and transverse magnetic (TM) impedances of the magnetotelluric tensor can be decoupled from the strike direction with significant implications when dealing with galvanic distortions. Distortion-free impedances are obtainable combining a quadratic equation with the phase tensor. In the terminology of Groom-Bailey, the quadratic equation provides amplitudes and phases that are immune to twist and the phase tensor provides phases immune to both, twist and shear. On the other hand, distortion-free strike directions can be obtained using Bahr’s approach or the formula provided by the phase tensor. In principle, this is all that is needed to proceed to a two-dimensional (2D) interpretation. However, the resulting impedances are strike-ignorant because they are invariant under rotation and, if they are to be related to a geological strike they must be linked to a particular direction. This is an extra ambiguity beside the classical of 90 degrees which must be resolved independently. In this work we use the distortion model of Groom-Bailey to resolve the ambiguity by bringing back the coupling between impedances and strike in the presence of galvanic distortions. Considering that most quantities are already known, fitting the responses of the model to the data requires minimizations only over the single variable of twist, instead of the original approach of having to minimize not only twist, shear and strike, but also the impedances themselves. Our approach is a hybrid between existing numerical and analytical approaches that reduces the problem to a binary decision. The fusion of the two approaches is illustrated using synthetic and field data.

Semi-automated fault extraction and structural analysis from DEM data of the Magadi and Natron basins, East African Rift System

<p>Continental rifts show surface expressions of deep crustal processes, such as faulting and volcanism. The East African Rift System (EARS) is one of the most prominent examples of an active continental rift driven by tectonics and magmatism. Nonetheless, we still struggle to quantify the amount of extension due to these processes on a kyr- to Myr-time-scale. In particular, the distribution of extension within low-offset normal fault networks within rift basin interiors is challenging to determine.</p><p>To address these issues, we develop a semi-automated workflow to extract normal faults from the TanDEM-X science DEM data (12 m horizontal resolution, 0.4 m average height error) of the Magadi-Natron Region of the Eastern branch of the EARS, limited to the north by the Suswa caldera (1.15°S) and to the south by Gelai and Oldoinyo Lengai volcanoes (2.75°S). This data allows us to quantify brittle surface deformation that occurred since the last deposition of widespread volcanic lavas in  these basins. Our workflow consists of five steps: (1) gradient calculation, (2) thresholding, (3) skeletonization, (4) Hough transformation, and (5) clustering. Because the surface faults appear as topographic discontinuities, we first calculate the gradient of the DEM to detect them. Then we use an adaptive threshold (Otsu) to distinguish faults from unfaulted areas. Next, we skeletonize the threshold to extract line segments and perform a Hough transformation to determine the orientation of these segments. Finally, we use a density-based clustering algorithm (DBSCAN) to group these segments into faults. This algorithm is considering proximity between the segment, similarity in dip and strike direction.</p><p>A strike analysis applied on the fault data of the whole basin shows four main directions from distinct fault populations. Each direction cluster corresponds to a geological layer and a time interval. For example, the azimuth N20°, corresponds to present and recent rift direction, mostly on the ~1Myr old Magadi trachyte. A direction of N170° is mostly represented in earlier,  Mio-Pliocene volcanic units of the rift. Moreover, we derive the fault displacement distribution throughout the basin.This allows us to calculate the total extension of each geological unit and to compute the overall amount of extension of the region during geologically recent times.</p><p>We provide a new high-resolution fault map that depicts strike direction and throw even of small-offset normal faults. This characterization helps us increase our understanding of recent brittle deformation within the Magadi-Natron region and thus the propagation of rifting in the eastern branch of the East African Rift System.</p>

The Effects of Sub-Faults Strike Direction and Landslide Energy on the Propagation of the 2011 Tohoku Earthquake and Tsunami

We carried out a tsunami simulation of the 2011 (Mw 9.0) Tohoku earthquake. We analyze the tsunami run-up modeling by applying additional variables to seismic moment and moment magnitude equation to find out what extent it affects of sub-faults strike direction and landslide energy to tsunami propagation. To investigate the accuracy of run-up and inundation of the tsunami, we processed and analyzed the mainshock and aftershocks by applying scaling law method and inundation equation. We applied the aftershocks data to determine the wide area of the fault. The fault is divided into several sub-faults to make simulation design and scaling formulation adjustment. Each of sub-faults strike direction on simulation design has a different energy one another, which is determined by the strike direction of each fault position. Furthermore, we calculated the affects of submarine landslides on tsunami propagation. To obtain the variable of resultant energy of earthquake and landslide it performed by using the law of mechanical energy conservation. We applied both L-2008 and ComMIT tools for processing tsunami simulation modeling. The result presents that the sub-fault strike direction and landslide energy can increase the propagation energy of the tsunami waves.

Reliability assessment of slopes with three-dimensional rotated transverse anisotropy in soil properties

The influence of soil variability on three-dimensional (3D) probabilistic slope stability analysis has been previously investigated for soils that display isotropic spatial variability features or anisotropic horizontal fabric patterns. However, due to various soil deposition processes, weathering, filling or tectonic movements, the assumptions of isotropy or horizontal layering may not always be realistic. This study presents 3D analyses of slopes with spatially variable soils associated with rotated transverse anisotropy features. The results show that for cross-dip slopes where the strike direction of soil strata is perpendicular to the out-of-plane direction of the slope, the reliability depends on various factors including strata rotation angle and autocorrelation distances, and differs significantly from slopes with horizontally deposited soil fabric. The influence of strata orientation is also pronounced for dip slopes and reverse dip slopes, and these are presented in terms of reliability indices of the slopes and statistics of the length of sliding mass, and elaborated by considering the failure mechanism under different scenarios. Through these analyses, this paper discusses the key features of slope reliability considering rotated transverse anisotropy in soil properties, and their major differences from situations involving horizontal soil layers or two-dimensional probabilistic assessments.

Experimental Study of Slurry Flow in Mining-Induced Fractures during Longwall Overburden Grout Injection

Slurry flow in mining-induced overburden fractures is an important theoretical concept for the grouting design of longwall overburden grout injection engineering. In this study, a visual experimental simulation system of longwall overburden grouting was designed to study the flow, pressure distribution, consolidation, and fill thickness of fly ash slurry in overburden bedding separation. Experiments showed that the slurry generates a radial and bidirectional flow during nonpressure grouting and presents itself as an approximately elliptical dominant flow channel under pressure injection. This channel expanded horizontally along the strike direction and gradually became tabular. The slurry pressure increased as the grouting time increased. Although the pressure curves at different locations exhibited similar trends, their values did not decrease as the distance from the borehole center decreased during observations. Bleeding and consolidation occurred in the slurry as soon as it flowed out of the borehole to the fracture, and the degree of consolidation increased as a function of the distance from the injection borehole. The bleeding water gathered continually to the boundary of the bedding separation fracture and was then seeped to and stored by the underlying strata based on the injection pressure. The final injection fill is manifested as a half pace with a large thickness at the center. This research provides a theoretical basis for the design and optimization of overburden grout injection in underground longwall mining.

Overburden Migration and Failure Characteristics in Mining Shallow Buried Coal Seam with Thick Loose Layer

Quite a number of shallow buried coal seams (SBCS) are distributed in China. The overburden is easily damaged due to the mining of SBCS, resulting in water resources loss and surface damage. Taking 12403 working face of Wulanmulun coal mine in Shendong mining area as an example, this paper analyzed the overburden migration and failure characteristics after mining SBCS with thick loose layer based on actual measurement data and simulation results. The results show that the subsidence of strata in caving zone has no skewness feature along strike direction, while the subsidence of strata in fracture zone and bending subsidence zone shows skewness subsidence phenomenon. An interface exists in the overburden, and the movements of upper and lower strata at the interface have different characteristics. The cracks penetrating the whole strata exist in bedrock, not in aeolian sand. The height of water flowing fracture zone is 35.74 m–62.89 m according to the loss of fluid in the borehole and consistent with the results of numerical simulation and similar simulation. This study can provide a reference for the prediction of the height of water flowing fracture zone and the overburden migration in mining SBCS.

Identification of Source Mechanisms for the August 5 2018 Mw 6.9 and the August 9 2018 Mw7.0 Lombok Earthquakes

A series of earthquakes with magnitudes ranging from 5.8 to 7.0 occurred in Lombok in the period of July to August 2018. Two events occurred consecutively, the 6.9 on August 5, 2018 (11:46:38 UTC), and the 7.0 on August 9, 2018 (14:56:28 UTC). Those phenomena are rare because earthquakes usually require a relatively long time to accumulate their energies before being released. Because of those events, so an explanation is needed to explain what happened at the source. In this context, this study aims to determine the relations between the events based on the asperity zone and the slip distributions. Modeling was performed using teleseismic data and seismic inversion of body waves at low frequencies. The result shows that the asperity zone of 6.9 is at 0 km in a strike-direction and -18 km wide in a dip-direction with a maximum slip of 1.3 m, whereas, for the 7.0 event, the asperity zone is at -36 km in the direction of the strike and -7 km in the direction of the dip. Both events have the asperity in the up-dip section with an upward slip distribution towards the up-dip. The slip distribution of the first event and the second one has a relationship because the 6.9 earthquake slip leads to the 7.0 earthquake fault plane. The relation is suspected to be due to the weakening of rock conditions and an enlargement that is limited by space and time during the earthquake. As a result, those two earthquakes are closely related to stress distribution, forming a new asperity zone.

Detection of Deep Low-frequency Earthquakes in the Nankai Subduction Zoneover 11 Years Usingamatched Filter Technique

To improve our understanding of the long-term behavior of low-frequency earthquakes (LFEs) along the tremor belt of the Nankai subduction zone, we applied a matched filter technique to continuous seismic data recorded by a dense and highly sensitive seismic network over an 11year window, April 2004 to August 2015. We detected a total of ~510,000 LFEs, or ~23× the number of LFEs in the JMA catalog for the same period. During long-term slow slip events (SSEs) in the Bungo Channel, a series of migrating LFEbursts intermittently occurred along the fault-strike direction, with slow hypocenter propagation. Elastic energy released by long-term SSEs appears to control the extent of LFE activity. We identify slowlymigrating fronts of LFEs during major episodic tremor and slip (ETS)events, which extend over distances of up to 100 km and follow diffusion-like patterns of spatial evolution with a diffusion coefficient of ~104 m2/s. This migration pattern closely matches the spatio-temporal evolution of tectonictremors reported by previous studies. At shorter distances, up to 15 km, we discovered rapid diffusion-like migrationof LFEs with a coefficient of ~105 m2/s. We also recognize that rapid migration of LFEs occurred intermittently in many streaks during major ETS episodes. These observations suggest that slow slip transients contain a multitude of smaller, temporally clustered fault slip events whose evolution is controlled by a diffusional process.