Reflection and transmission ofSH waves in an initially stressed medium consisting of a sandy layer lying over a fluid-saturated porous solid

Biot's theory is employed to study the reflection and transmission ofSH waves in a sandy layer lying over a fluid-saturated porous solid half-space. The entire medium is considered under constant initial stress. Effects of sandiness, initial stress, anelasticity and viscosity of the interstitial fluid on the partitioning of energy are studied. In the presence of initial stress the incident wave starts attenuating when incider beyond a certain angle (depending upon the amount of initial stress), even if the medium is perfectly clastic. Anelasticity of the solid layer results in the dissipation of energy during transmission. The direction of attenuation vector of incident wave affects the dissipation energy to a large extent. Effect on partitioning of energy reverse at incidence after the critical angle. A complete account of energy returmed back to the underlying half-space and that which is dissipated in the overlying layer has been discussed analytically as well as numerically.     

Steady-state migration and diffusion of hydrocarbons in complex stratigraphic sequences

A mean-field method is used to describe both average and fluctuating flow properties of water and oil in sedimentary basins. Comparison of oil and water flow shows the relative strengths of capillarity and buoyancy in controlling the mean and fluctuating components of flow. Effects of sandiness and water-wetness are studied as is the effect of decreasing porosity with increasing subsurface burial. Numerical estimates suggest that sandiness percentages between about 20 to 80% influence mean and fluctuating components of flow. Outside of this regime the end-member situations of shale or sand prevail.The extra effects of vertical buoyancy and capillarity generally make lateral diffusion less important for oil flow than for water flow.     

The global stress field in the lithosphere obtained from the satellite gravitational harmonics

In this paper, the lithosphere is considered to be a homogeneous elastic spherical shell for the sake of simplicity and the stress equations for the base of the lithosphere are taken as boundary conditions. Then the stress equations are obtained for use in the computation of the stress field in the lithosphere with the satellite gravitational harmonic coefficients. The 5 × 5° global stress field in the lithosphere is computed from harmonics of 2–30°. The directions of principal stresses of this stress field agree favourably with the directions of principal stresses indicated by mid-plate earthquake mechanisms, in situ stress measurements and sensitive geological features. This result indicates that the drag forces exerted on the base of the lithosphere, due to gravitational mantle convection, may be among the driving forces of plate motion and a major source for the stress field in the lithosphere.     

Determination of source parameters of strong earthquakes of Kamchatka,the Kurile Islands,and Japan from aftershock sequences

Aftershock sequences of some strong earthquakes of Kamchatka, the Kurile Islands, and Japan are examined. Such source parameters as the length L, along-dip width W, motion on fault D, and stress drop Δσ are determined from the aftershock sequences considered. The values of these parameters were obtained by the formal estimation of linear source parameters (lower bound estimates) and visually (upper bound estimates). The correlation dependences of the obtained parameters on the surface wave (M _S ) and seismic moment (M _W ) magnitudes are calculated.     

Effect of surface wave propagation in a four-layered oceanic crust model

Dispersion of Rayleigh type surface wave propagation has been discussed in four-layered oceanic crust. It includes a sandy layer over a crystalline elastic half-space and over it there are two more layers—on the top inhomogeneous liquid layer and under it a liquid-saturated porous layer. Frequency equation is obtained in the form of determinant. The effects of the width of different layers as well as the inhomogeneity of liquid layer, sandiness of sandy layer on surface waves are depicted and shown graphically by considering all possible case of the particular model. Some special cases have been deduced, few special cases give the dispersion equation of Scholte wave and Stoneley wave, some of which have already been discussed elsewhere.     

On the interpretation of large gravimagnetic data by the modified method of <Emphasis Type="Italic">S</Emphasis>-approximations

The modified method of S-approximations applied for processing large and superlarge gravity and magnetic prospecting data is considered. The modified S-approximations of the elements of gravitational field are obtained due to the efficient block methods for solving the system of linear algebraic equations (SLAEs) to which the geophysically meaningful problem is reduced. The results of the mathematical experiment are presented.     

On the dynamics of extensional basin

Geological and geophysical data from the North China-Bohai Basin and “Basin and Range” Province were examined and compared. They are similar to each other in many respects. Surficial geological structures are characterized by a series of half-grabens with their one flank constituted by normal fault. Those extensional structures usually extend to a depth of 6–8 km. Therefore, the stress condition in the upper 8 km can be written as $$\sigma _2 > \sigma _x > \sigma _y$$ wherex, y denote the directions of maximum compression and maximum tension on the horizontal plane, whilez signifies the vertical direction. Some people think that this kind of stress condition exists through the entire crust in the extensional basin. However, the focal mechanisms of the earthquakes in the extensional basins with focal depths usually at 12–20 km are dominated by strike-slip faults. The stress condition in the focal regions can be expressed by $$\sigma _x > \sigma _z > \sigma _y .$$ Geodetic measurements conducted before and after the Tangshan earthquake in 1976 and the Xingtai earthquake in 1966 showed that both horizontal and vertical surficial deformations with magnitudes of a similar order occurred during the earthquakes. The surficial deformations during the earthquakes can be explained by a summation of the motions produced by both stress fields in the upper crust and the middle crust. Dynamical processes other than the homogeneous horizontal regional tectonic field are required to explain the vertical variation of the stress condition in the upper and middle crusts. Evidence from the seismic refractions, reflections and the three-dimensional seismic tomography from both local earthquakes and teleseismic events provide convincing evidence that magmatic intrusions from the uppermost mantle to the middle crust occur near the hypocenters of both the Tangshan and Xingtai earthquakes. The variation from the extensional stress regime at the upper crust to the compressional stress regime in the middle and lower crusts is considered to be the common feature in extensional basins. And the magmatic intrusions from the upper mantle to the middle crust observed in the extensional basin is suggested to be its genetic cause. Numerical simulations of magmatic intrusion from the uppermost mantle to the middle crust were studied. Both the intruded compression and the thermal stress due to magmatic intrusion were considered, also the viscoelasticity of the middle and lower crusts were assumed. The results successfully explain the vertical variation of the stress condition in the crust and the process producing an extensional basin.     

Clarification of regional and local in situ stresses using the compact conical-ended borehole overcoring technique and numerical analysis

Abstract Stress measurement is performed to estimate the states of in situ rock stress at the Torigata open‐pit limestone mine in Japan using the compact conical‐ended borehole overcoring (CCBO) technique. A set of back and forward analyses are then carried out to evaluate the states of regional and local in situ rock stresses and the mine‐induced rock slope stability using a 3‐D finite element model. The maximum horizontal local in situ rock stress measured by the CCBO technique acts in the northeast–southwest direction. The horizontal regional tectonic stresses obtained by the back analysis are in good agreement with those of the horizontal local in situ rock stress measured by the CCBO technique. However, the horizontal regional tectonic stress is more compressive than the horizontal local in situ rock stress. This is because the horizontal regional stress due to gravity is not considered in the back‐analyzed horizontal regional tectonic stress, but it is included in the local in situ rock stress measured by the CCBO technique. The local stress obtained by the forward analysis, especially its horizontal components, is in good agreement with the horizontal local in situ rock stress measured by the CCBO technique, and the magnitude of the vertical normal stress increases more rapidly than those of the horizontal normal stresses with depth. As a result, the ratio of the horizontal normal stress to the vertical normal stress is largest at the nearest excavation level and decreases with depth. This means that the stress field within the mine‐induced rock slope is affected by the horizontal components of the local in situ rock stress.     

Site Response for the RSM Seismic Network and Source Parameters in the Central Apennines (Italy)

—?Thirty-three earthquakes which occurred in the Central Apennines (Italy) with Ml ranging from 2.4 to 3.7 have been spectrally analysed using digital recordings from twelve stations of the Rete Sismometrica Marchigiana (RSM) network. Data corrected for geometrical spreading and quality factor Q have been inverted by means of the Generalised Inversion Technique. Site responses have been compared with those obtained by H/V ratio. Site amplifications have been observed both at stations placed on Pleistocene sediments and at one station located at 1800?m altitude. Source parameters have been calculated by fitting the spectra with an automatic procedure adopting the ω² source model. The seismic moments range from 9.23?×?10¹⁹ to 4.28?×?10²¹ dyne-cm with an average M ₀ (S) to M ₀ (P) ratio of 1.13?±?0.38. The stress drops are generally low and they vary between 1.1 and 10.2?bar when estimated by using S source spectra, and between 0.5 and 7.1?bar when the P-source spectra are fitted. For the considered range of seismic moments we observe that the stress drop does not have significant dependence on event size.     

Seismic failure probability of concrete slab on CFR dams with welded and friction contacts by response surface method

In this study, failure probability of the concrete slab on concrete-faced rockfill (CFR) dams with welded and friction contact is investigated under earthquake effects by reliability analysis. For this purpose, Torul CFR dam is selected as an example and numerical solutions are performed by considering combination of reliability analysis–finite element method. 1992 Erzincan earthquake acceleration record is used in the finite element analysis considering deconvolved-base rock input model. In this model, the ground motion to be applied to the foundation base rock is obtained by deconvolution of the free-field surface record. In the materially nonlinear analysis, Drucker–Prager model is used for concrete slab and multi-linear kinematic hardening model is utilized for rockfill. Geometrically nonlinearity is also taken into account. Viscous boundary conditions are defined in the finite element model for both foundation soil and reservoir water. The hydrodynamic pressure of the reservoir water is considered using 2D fluid finite elements based on the Lagrangian approach. Both welded contact and friction contact based on the Coulomb’s friction law are defined in the structural connections. Improved Rackwitz–Fiessler method is used with response surface method in the reliability analysis. The tensile and compression strengths of the concrete slab are utilized in the implicit limit state functions considering various thicknesses. The probability of failure of the most critical points in the concrete slab is obtained. According to this study, the probabilities of failure obtained from the CFR dam including friction contact are lower. When the welded contact is considered in joints, the probability of failure of the concrete slab is 1 due to tensile stress limit state and compression stress limit state only if concrete slab is linear. The most critical probability of failure of the concrete slab appears in the case that the concrete slab is linear and rockfill is materially nonlinear. The probability of failure of the concrete slab decreases if the nonlinearity of the concrete is considered. Also, hydrodynamic pressure decreases the reliability of the concrete slab.     

Estimation of site amplification,attenuation and source spectra of S-waves in the East-Central Iran

Generalized inversion of the S-wave amplitude spectra from the strong-motion network data in the East-Central Iran has been used to estimate simultaneously source parameters, site response and the S-wave attenuation (Q_s). In this regard, 190 three-component records were used corresponded to 40 earthquakes with the magnitudes M3.5–M7.3. These earthquakes were recorded at 42 stations in the hypocentral distance range from 9 to 200 km. The inverse problem was solved in 20 logarithmically equally spaced points in the frequency band from 0.4 to 15 Hz. The frequency-dependent site amplification was imposed, as a constraint, on two reference site responses in order to remove the undetermined degree of freedom in the inversion and obtain a unique inverse solution. Also, a geometrical spreading factor was assumed for removing the trade-off between geometrical spreading and anelastic attenuation. Different source parameters, such as seismic moment (M₀), seismic energy (E_s), corner frequency (f_c) and Brune stress drop (Δσ), were estimated for each event by fitting an ω² model to the spectra obtained from the inversion. The stress drop values of earthquakes, obtained in this research, are in good agreement with those of other studies. Also average site response values were correlated to the average shear wave velocities in the uppermost 30 m, in high and low frequency bands. The peak frequencies of site amplifications, estimated by the generalized inversion method, where in good agreement with those of horizontal to vertical (H/V) spectral ratios for the S-wave portion of records. However, no perfect matching in amplitude was obtained due to the deficiencies of the H/V ratio technique. By supposing a free shape for Q factor, a frequency dependent function was found, the logarithm of which could be approximated by a linear function, Q(f)=151f^0.75. The uncertainties of model parameters have been evaluated by covariance matrix of least-square fit. The residuals were also analyzed in order to assess the validity of the model. The analysis of residuals with respect to magnitude and distance indicates that they are distributed normally with approximately zero mean. The robustness of the results has been studied concerning their sensitivities to the omission of different datasets, selected randomly from original database. The results obtained here can be used in predicting ground-motion parameters applying stochastic methods.     

Effect of overconsolidation ratio on dynamic properties of binary mixtures of silica particles

Overconsolidated soils are ubiquitous in nature due to multiple mechanisms; however, the stress-history-based studies of small strain stiffness on binary mixtures, such as silty sand, are limited even though natural sand deposits are commonly mixtures of sand particles with varying amounts of fines. Consequently, this study quantified the stress-history-based dynamic properties of binary mixtures, such as sand-sand mixtures with different size small particles, and silty sand mixtures with small amounts of non-plastic fines, up to the critical fines content. By performing bender element tests on those mixtures according to fines content, size ratio, and overconsolidation ratio, the stress-history-based G_max of binary mixtures was evaluated. For the relevant data analysis, the OCR (overconsolidation ratio) exponent in the G_max formulation was expressed in terms of stress exponents during loading and unloading. It was found that the effect of OCR on the estimation of G_max increased with a decrease in size ratio (or increase in size difference), since the stress exponents during loading increased more significantly with a decrease in size ratio due to the pronounced change in interparticle coordination between large grains. However, the variation of stress exponents during unloading of different mixtures was relatively small due to the prevalent elastic deformation. It was demonstrated that the maximum stress history effect of tested mixed soils was observed at a fines content of approximately 5%, which was smaller than critical fines content of silty sand. This behavior was attributable to the delay in critical fines content observed during unloading, when compared to that observed during loading.     

Seismic sources and attenuation properties at the Campi Flegrei volcanic area

Microearthquake digital data collected at Campi Flegrei during the recent (1982–1985) ground uplift episode have been analyzed in order to infer source and medium seismic properties. The main results obtained from these analyses are:

1. Hypocenter distribution and the size of the seismic zone do not change with time and do not depend on the ground uplift rate. Events occurred clustered in time with no simple causal relations between the cluster occurrences and their energy.
2. Anelastic attenuation does not depend strongly on frequency, showing a constant pattern at high frequencies. The observed values of low and high frequency attenuation, due to the short source receiver distances, do not seriously affect the spectral content of signals radiated by the sources.
3. A constant Brune stress drop pattern (～4–5 bars) as a function of seismic moment is observed. This indicates that the manner of fracturing is almost independent on magnitude of earthquakes (hypothesis of self-similarity (Aki, 1967)). Seismic processes in a prefractured medium can explain the observed small stress drop values.
4. Focal mechanisms from moment tensor estimates show that radiation patterns are mostly well interpreted in terms of double couple source models.
5. The scaling of peak ground motion parameters (A _max andV _max vs seismic moment) can be explained by an ω² source model (constant stress drop) multiplied by an exponential function with a small decay parameter, which takes into account the measured attenuation.

These results support the hypothesis of earthquakes generated by simple shear fractures along prefractured structures as a response to changes in the stress field due to the ground deformation.     

Seismicity and strain accumulation around Karliova Triple Junction (Turkey)

GPS studies in Turkey date back to the early 1990s, but were mostly focused on the seismically active North Anatolian Fault System (NAFS), or on the more populated Western Anatolia. Relatively few studies were made of the seismically less-active East Anatolian Fault System (EAFS), although it has the potential to produce large earthquakes. In this study, we present the results of a combination of geodetic and seismological data around the Karliova Triple Junction (KTJ), which lies at the intersection of the North- and East Anatolian Fault Systems. In particular, the geodetic slip rates obtained through block modeling of GPS velocities were compared with b-values to assess seismicity in the region. Yedisu segment, one of the best-known seismic gaps in Turkey, was specifically analyzed. The relatively low b-values across Yedisu segment verify the accumulation of seismic energy in this segment, and the GPS-derived geodetic slip rates suggest that it has the potential to produce an earthquake of Mw 7.5 across an 80-km rupture zone.Additionally, analysis of earthquake data reveals that the study area has a ductile or rigid–ductile behavior with respect to its surroundings, characterized by varying b-values. Although, seismic events of moderate- to high magnitudes are confined along the major fault zones, there are also low-seismicity zones along the eastern part of the Bitlis Suture Zone and around Yedisu. Since the high seismicity areas within the region may not accumulate sufficient stress for a large earthquake to occur, it is considered that the deformation in such areas occurs in a ductile manner. On the other hand, the areas characterized by low b-values may have the capacity of stress accumulation, which could lead to brittle deformation.     

Source Parameters and Scaling Relations for Reservoir Induced Seismicity in the Longtan Reservoir Area

We investigate the relationship between the impoundment and seismicity in the Longtan reservoir, southwestern China and find evidence that the seismicity was reservoir induced. After the reservoir impoundment, a pronounced increase in seismicity was observed in five clusters mainly concentrated in the areas where few earthquakes had occurred before the first filling. The observed induced seismicity shows a strong correlation with the filling cycles. The activity levels in the five clusters are different due to differences in the structures and permeabilities of the faults. Source parameters for 1,616 earthquakes with M _L 0.1–4.2 recorded by 24 fixed and temporary stations deployed around the reservoir were calculated after applying corrections for geometrical spreading, frequency-dependent Q, and site effects. The static stress drop and apparent stress in this area both appear to increase with increasing seismic moment over the entire magnitude range. Our results show that reservoir induced earthquakes have ten times lower average stress drop than natural tectonic earthquakes. These results may indicate that the reservoir induced seismicity can occur with a lower tectonic stress due to the high pore pressures of the underground medium, and that the effect of the water decreases the coefficient of friction.     

Numerical evaluation of the dynamic response of pipelines to vibrations induced by the operation of a pavement breaker

This paper studies the response of pipelines to vibrations induced by the operation of a pavement breaker during the rehabilitation of concrete pavements. An efficient two-and-a-half-dimensional (2.5D) formulation is employed, where the geometry of the structure and the soil is assumed to be invariant in the longitudinal direction, allowing for a Fourier transform of the longitudinal coordinate y along the structure to the wavenumber k_y. The dynamic soil–structure interaction problem is solved by means of a 2.5D coupled finite element–boundary element (FE–BE) method using a subdomain formulation. The numerical model is verified by means of results available in the literature for a buried pipeline subjected to incident P- and SV-waves with an arbitrary angle of incidence. The presented methodology is capable to incorporate any type of incident wave field induced by earthquakes, construction activities, traffic, explosions or industrial activities. The risk of damage to a high pressure steel natural gas pipeline and a concrete sewer pipe due to the operation of a pavement breaker is assessed by means of the 2.5D coupled FE–BE methodology. It is observed that the stresses in the steel pipeline due to the operation of the pavement breaker are much lower than those induced by the operating internal pressure. The steel pipeline behaves in the linear elastic range under the combined effect of the loadings, indicating that damage to steel pipelines close to the road due to the operation of a pavement breaker is unlikely. The maximum principal stress in the concrete pipe, on the other hand, remains only slightly lower than the specified tensile strength. The decision to use a pavement breaker should hence be taken with care, as its operation may induce tensile stresses in concrete sewer pipes which are of the same order of magnitude as the tensile strength of the concrete. Assessing the risk of damage by means of vibration guidelines based on the peak particle velocity (PPV) gives, for the particular cases considered, qualitatively similar results.     

Behavior of parameters of the ionospheric F2 layer at the turn of the Centuries: 1. critical frequency

The available massifs of experimental data on the critical frequency of the ionospheric F2 layer, foF2, covering the first decade of the new century, are considered. On the basis of studying these massifs, a conclusion is drawn that the scatter of foF2 values (measured by the standard deviation (SD)) relative to the dependence on solar activity has grown substantially over recent decades as compared to the period 1958–1979. The possible causes of the SD increase are considered. It is shown that the foF2 values for the period 1998–2010 decreased as compared to the period 1958–1979 by an average of 0.6 MHz which gives an estimate of the foF2 trend of ～-0.03 MHz per year. Linear trends in foF2 for some ionospheric stations are analyzed. It is obtained that, in spite of the scatter in the data, it is possible to obtain statistically significant trends for each considered situation (day and postsunset period in summer and winter). At the same time, the winter negative trends (～-0.052 MHz per year) are approximately a factor of 2 higher than the summer ones (～-0.024 MHz per year). Comparisons with the trends obtained for earlier periods show that the negative trend in foF2 increased substantially towards the first decade of our century.     

Time Correlation Analysis of the Microseismicity of the Low Eastern Flank of Mt. Etna Volcano (Italy)

—An analysis in terms of time correlation functions has been applied to the time distribution of microseismicity. We considered the single station detections (M < 2) recorded by each one of the four working stations of the network of the Osservatorio Sismologico di Protezione Civile, Acireale (Catania), located on the active faults systems (Timpe) of the low eastern flank of Mt. Etna. Information obtained on the time regularity of seismic crisis and on the time correlation between the activities recorded by different stations allowed a better understanding of the role of the two main structures. The Timpa di Santa Tecla is the most seismogenic structure in the area and probably played a role in the magma transfer process for the 1989 eruption. Modifications of the local stress field by actions of this feature may have produced gravitational instability along a second structural system (the Timpa di S. Leonardello) with a time delay of about 100 days. By contrast, magma penetration associated with the larger eruption of 1991–1993 had a deeper rise system and had no effect on local stress conditions, consequently there was no correlation between volcanism and seismicity.     

Evaluation of static and dynamic properties of sand–fines mixtures through the state and equivalent state parameters

The results of an experimental investigation on sands with low plastic fines content are presented. Specimens with a low plastic fines content of 0%, 15%, 30%, 40%, 50% and 60% by weight were tested in drained and undrained triaxial compression tests. The soil specimens were tested under three different categories: (1) at a constant void ratio index; (2) at the same peak deviator stress in a triaxial test; and (3) at a constant relative density. By a combination with our published experimental data in recent years, the critical state line and various state parameters have been proposed and discussed for a further understanding the behavior of sand–fines mixtures. Results indicated that a unique critical line was obtained from drained and undrained triaxial compression tests for each fines content. The effects of fines content on critical state line (CSL) were recognized and discussed. In addition, the results revealed that normalized peak undrained shear stress, cyclic resistance ratio, and compression index were found to be a good correlation with state parameter Ψ as well as equivalent state parameter Ψ*. An increasing state parameter decreased the normalized peak undrained shear stress, and cyclic resistance ratio; however, the compression index increased with an increase in state parameter. Finally, there were no correlations such as the coefficient of consolidation–state parameter and maximum shear modulus–state parameter due to the different testing condition.