Initiation of Triangle Zones by Delamination, Shear, and Compaction at the Front of Fold-and-Thrust Belts

The interest of this paper is to investigate the initiation of triangle zones at the front of fold-and-thrust belts by analyzing the virtual velocity fields in triangle wedges. It allows achieving five collapse mechanisms by delamination, shear, and compaction of competing for the formation of triangle zones as follows. The first mechanism is the classical Coulomb shear thrust. The second is delamination at the frontal part of the décollement with straight back thrust, while the third is delamination with curvy back thrust. The fourth is the combination of ramp with Coulomb shear and shear-enhanced compact fault, while the fifth is the combination of the exchanging motion on the ramp and thrust. The dominating mechanism in the formation of triangle zones relies on the competition of the least upper bound of each mechanism when subjected to tectonic force. The controlling factors of the competition are discovered as follows: (1) the frictional characters and cohesion of horizontal décollements and thrust, (2) the slope of the topography of accretion wedge, and (3) the thickness and rock density of the front toe of accretion wedge.

The Evolution of Stress and Strain around the Bayan Har Block in the Tibetan Plateau

With the constraint of GPS observation, the tectonic deformation of the Bayan Har block and its periphery faults is investigated based on an elastoplastic plane-stress finite element model. The results show that the elastic model cannot explain the current GPS observation in the Bayan Har block. When East Kunlun fault and Yushu-Xianshuihe fault are under plastic yield state or high strain localization, the calculated velocities fit well with the observation values. It indicates that most of the current shear deformations or strain localizations are absorbed by these two large strike-slip faults. In addition, if the recurrence intervals of large earthquakes are used to limit the relative yield strength of major faults, the order of entering the plastic yield state of the major faults around Bayan Har block is as follows. The first faults to enter the yield state are Yushu-Xianshuihe faults and the middle segment of East Kunlun faults. Then, Margaichaka-RolaKangri faults (Mani segment) and Heishibeihu faults would enter the yield state. The last faults to enter the yield state are the eastern segment of East Kunlun faults and Longmenshan faults, respectively. These results help us to understand the slip properties of faults around the southeastward moving Bayan Har block.

Weighted Residual Method for Diffraction of Plane P-Waves in a 2D Elastic Half-Space Revisited: On an Almost Circular Arbitrary-Shaped Canyon

Scattering and diffraction of elastic in-plane P- and SV-waves by a surface topography such as an elastic canyon at the surface of a half-space is a classical problem which has been studied by earthquake engineers and strong-motion seismologists for over forty years. The case of out-of-plane SH-waves on the same elastic canyon that is semicircular in shape on the half-space surface is the first such problem that was solved by analytic closed-form solutions over forty years ago by Trifunac. The corresponding case of in-plane P- and SV-waves on the same circular canyon is a much more complicated problem because the in-plane P- and SV-scattered-waves have different wave speeds and together they must have zero normal and shear stresses at the half-space surface. It is not until recently in 2014 that analytic solution for such problem is found by the author in the work of Lee and Liu. This paper uses the technique of Lee and Liu of defining these stress-free scattered waves to solve the problem of the scattering and diffraction of these in-plane waves on an on an almost-circular surface canyon that is arbitrary in shape.

Dynamics of an Earthquake under Magma Thrust Strength

This paper deals with the study of the behaviour of a one spring-block model subjected to the strengths due to the motion of the tectonic plates and the upflow of magma during volcanism. Using the direct integration method, we show that the sound velocity decreases (or increases) with the amplitude of the block’s oscillation when the external frequency is zero (or not). It appears that this amplitude grows with the amplitude of the magma thrust strength. For the resonance case, where this external frequency equals the pseudofrequency of the block’s oscillation, we establish that the slip which occurs during the periodic movement of a block is a stick-slip motion instead of a creep motion as usually known; therefore, the transition does not occur. We also obtain that the event probability decreases with the amplitude of the magma thrust strength while the power of the earthquake increases with it.

Mapping Sediment Thickness in Shillong City of Northeast India through Empirical Relationship

Modified form of Nakamura method, H/V ratio, is used to assess the site response through estimation of fundamental resonant frequency at 70 sites using three component digital seismographs in Shillong city, capital of Meghalaya in northeast India. With available borehole information, an attempt is made to develop an empirical relationship between sediment thickness and resonant frequency estimated from H/V ratio technique. Simultaneously, shear wave velocities are computed entailing resonant frequency and sediment thickness for these boreholes. We also endeavored building another empirical relation between sediment thickness and VS. With the help of this, the probable VS values for other sites were also evaluated. It is observed that shear wave velocities range from 200 to 550 m/s while sediment thickness ranges from 10 to 80 m, implicating the heterogeneity prevailing in the soil layers of the Shillong city.

Characteristics and Prevention of the Debris Flows following Wenchuan Earthquake in Jushui River Basin, An County, China

After analysing the catastrophic debris flows on August 18, 2012, and on July 9, 2013, in Jushui River basin, An County, the Wenchuan Earthquake seriously striken areas, it was found that they were characterized by the clay soil content of 0.1~1.2%, the density of 1.68~2.03 t/m3, the discharges of 62.2 m3/s to 552.5 m3/s, and the sediment delivery modulus of 1.0~9.4 × 104 m3/km2. Due to intense rainstorm, many large debris flows produced hazard chain, involved in flash flood, debris flow, dammed lake, and outburst flood, and rose Jushui River channel about 1~4 m as well as amplified flood. The hazards and losses mainly originated from the burying and scouring of debris flows, flood inundating, and river channel rise. The prevention of debris flows is facing the intractable problems including potential hazard identification, overstandard debris flow control, control constructions destructing, and river channel rapid rise. Therefore, the prevention measures for the basin, including hazard identification and risk assessment, inhabitants relocating, monitoring and alarming network establishing, emergency plans founding, and river channel renovating, and the integrated control mode for watershed based on regulating the process of debris flow discharge, were recommended for mitigation.

Ratio of the Dead to Wounded (D/W) Indicators and Associated Factors in Major Earthquakes of America from 1960 to 2011

The paper presented deals with the casualties, mortality, and morbidity occurred during the major earthquakes of America during a period of 51 years. The work provides statistical evidence that the deaths/wounded (D/W) ratio used for many agencies in the planning of the preparation and response activities to earthquakes does not fit the relation 1 : 3. In addition, a model is presented in order to evaluate the possible association between different analysis variables such as the subregion of the American continent affected, population density, HDI, and the time and magnitude of the earthquake and the effects of these on the death toll, the number of the wounded, and the D/W indexes. Although the model generated it is not robust enough for decision making, it could be useful and improvable in order to apply it in the planning and management of these kinds of natural disasters. For these reasons, we think that it would be interesting to do further progress in this line of research by making a more comprehensive study of the variables associated with mortality and morbidity, using a more representative sample of earthquakes that sure will confirm the results presented in this work.

Seismic Behaviour of Buried Pipelines: 3D Finite Element Approach

This paper presents a numerical investigation on six pipeline models to study the seismic response of single and double buried pipelines using finite element method. Different depth and spacing of pipes are considered to investigate their prominent role in the seismic response of buried pipelines under an earthquake loading having PGA of 0.2468 g. In case of single pipeline, the maximum magnitude of final displacement as well as the stress at the end of the seismic sequence is found at the burial depth equal to the pipe diameter. In case of double pipeline, the maximum magnitude of final displacement is found when the spacing between pipes is equal to half the pipe diameter and there is an increasing tendency of developed stress with increase in spacing between pipes. In addition to the above results, the response of the buried pipelines with a particular bend angle (artificially induced bend/buckle) to the permanent ground deformation which is assumed to be the result of seismic wave propagation has also been studied. Remarkable differences in these results are obtained and with these results the designers can reduce seismic risk to their buried pipelines by taking proper precautionary measures.