Strain-geoelectric resistivity precursor and the virtual dislocation model to the Tangshan earthquake

The complete records of geoelectric resistivity before two earthquakes were analyzed, including 16 stations within 240 km around the Tangshan earthquake and 2 stations within 50–60 km from theM6.1 Datong earthquake. By eliminating various disturbances in the records and realizing the precursory anomalies to be reliable, the authors studied the distribution of the geoelectric precursor field, which proves to be physically related to the earthquake source stress field. Comparision of the sign distribution of coseismic resistivity changes with the solution of earthquake source mechanism indicates that, the coseismic resistivity changes are of opposite sign but similar spatial distribution with respect to the corresponding resistivity precursor changes. Therefore, from the resistivity observations we are of the opinion that the Tangshan earthquake is an elastic rebound process. A virtual dislocation model of geoelectric precursor for the Tangshan earthquake is proposed, in which the geoelectric precursors are supposed to be caused by the strain accumulation due to a virtual dislocation, which is opposite in sign to the actual slip taking place at the earthquake occurrence. Taking into account of the non-linear characteristics of the amplification factorK=(Δρ/ρ)/ɛ in a range of 10⁻⁷–10⁻⁵ strain changes, the theoretical distribution of geoelectric precursors for the Tangshan earthquake was calculated based on the theory of fracture mechanics and reasonably selected dislocation parameters. The results show that the semi-quantitative theoretical values are in good agreement with the observed, suggesting that the virtual dislocation model of the geoelectric precursor is appropriate to the Tangshan earthquake. Contribution No. 96A0023, Institute of Geophysics, SSB, China. This work was supported by the National Natural Science Foundation of China and the Science and Technology Department of SSB.     

Research on coda Q distribution in Beijing and its surrounding regions

Based on the single scattering model of seismic coda waves, we have calculated theQ-factor in Beijing and its surrounding regions by means of calculating the power density spectrum in frequency domain with a fixed time window. The digital seismic data of 69 earthquakes from Beijing Telemetered Seismographic Network are used. These earthquakes were recorded from January 1, 1989 to December 31, 1990 at 20 stations. This paper shows the variations of the codaQ-factors in the studied region with different sites, frequency and lapse time, and the temporal change of the codaQ-factors in these two years. The results indicate that codaQ-factor depends strongly on the lapse time and frequency. It is assumed that whenQ _C=Q ₀f^η, for the three time windows of 15–30s, 30–60s and 60–90s, the average values ofQ ₀ are 48, 115 and 217; and the average values ofη are 0. 89, 0.91 and 0.74, respectively. Contribution No. 95A0009, Institute of Geophysics, SSB, China. This work is a contract subject 85-04-01-02 of the State Seismological Bureau, China.     

On the hydro-spectrum of deep wells and the response ability of well-aquifer system

onclusions 1) A set of period components corresponding well to those of Earth tides and barometric pressure can be separated from the hydro-spectra of deep well. The well-aquifer system responds to these components in some degree, which shows that groundwater regime is influenced by Earth tides and barometric pressure. However, in different frequency bands a well-aquifer system possesses different transfer ability for various inputs. 2) The transfer function quantitatively describes the response ability of well-aquifer system to Earth tides and barometric pressure. It is a characteristic parameter that the system itself possesses. According to the transfer function value of well-aquifer system to a certain input factor, we can objectively understand the influence of the input on the groundwater regime in a deep well. This is of important significance for revealing the formation of groundwater regime in a deep well and for evaluating the seismic supervising well. The work was supported by National Natural Science Foundation of China (No. 49372149).     

Is a type of low frequency ULF magnetic disturbance a kind of earthquake precursors?

A type of relatively low frequency ULF magnetic disturbance has been generally observed by the magnetic induction-meter since it’s operation at Baijiatuan observatory in Beijing. As the amplitudes and frequencies of such disturbance signals change with time and are different from the disturbances produced by some man-made magnetic sources, therefore they are usually taken as a kind of earthquake precursors for ULF electro-magnetic emission. However, a comparative analysis with magnetic storms on the magnetogram obtained by variometerat the same station indicates that such signals always occur in the period of magnetic storm occurrence and so far we have not found the cases where such signals occur with no magnetic storms, therefore this kind of ULF magnetic disturbances should not be taken as a ULF earthquake precursors of the electromagnetic emission at present. The main features of such signals are as follows: the signals occur discontinuously during storms and the duration of each time section with such signals and the occurrence rates of the sections and the disturbance amplitudes are usually related to the types and intensities of storms. The wave form characteristics of the ULF disturbance are also related to the types of storms. Generally, the amplitudes and durations for SC storms are stronger and longer respectively than those for GC stroms, and if a storm is with largerK index, then relatively large amplitudes and higher rates and longer durations as well as variable frequencies will be observed andvice versa. Most of the start time and the time section with strongest disturbance recorded by the ULF unit and by the variometer are not consistent with each other, and the same one is only about 44%. The periods and intensities of such disturbance signals are in the ranges of few to several seconds and 0.04–8 nT respectively. The predominant frequency is about 0.06 Hz and certain energies are also distributed on the harmonic frequencies. Contribution No. 95A0063, Institute of Geophysics, SSB, China.     

Visco-Elastic Waves in Rock-Like Composites

The subject of this paper is the treatment of rocks - and, especially, fluid-saturated and partially saturated reservoir rocks, as composite visco-elastic media. By this we mean to study and partially answer the question of how the effective material (frequency-dependent and complex-valued stiffness/density) parameters can be estimated from a knowledge of the constituents of the rocks, their volume fractions, the statistical distribution of sizes, shapes, orientations and positions of the individual particles (minerals of quartz, clay, etc.) and cavities (pores, cracks, etc.); in addition to parameters related to the fluid and its ability to flow, at the scale of the microstructure as well as that of the wavelength (assumed to be long compared to the scale-size of the microstructure).Our approach is to develop and combine a theory of stochastic waves with established results for the micromechanics of defects in solids, as well as state-of-the-art models of wave-induced fluid flow. Specifically, we first derive an exact formal expression for the effective material parameters in terms of a dynamic T-matrix for the material, which satisfies a single integral equation of the Lippmann-Schwinger type (known from quantum scattering theory), but formulated in an abstract vector space, associated with the combination of the strain and velocity fields into a more general state vector . Inclusions-based models are developed on the basis of standard many-body techniques, known from the static T-matrix approach as well as nuclear collision theory. The t-matrix of a low-aspect-ratio spheroidal crack is expressed in terms of the familiar displacement discontinuity parameters of Hudson, via the so-called K-tensor, which is of interest in itself, for example, when connecting cracks to pores (in the presence of multiple solid constituents) on the basis of an expression for the t-matrix of a communicating cavity.The present theory can in principle be used beyond the Rayleigh limit, but explicit estimates of the effective material parameters have so far been derived only under the assumption that (scattering attenuation can be ignored) the wavelength is large compared to the scale-size of a representative volume element. Starting with the dynamic equations of motion, we show that the behaviour of the mean wave in the Rayleigh limit is indeed determined by the effective stiffness tensor associated with a static theory of composites, in conjunction with the spatially averaged density for the heterogeneous material as a whole. Thus, we have provided justification to the procedure we used in a series of related papers, where we started out with the static equilibrium condition and employed the elastic/visco-elastic correspondence principle. Numerical examples (dealing with the effects of randomly oriented cracks on the isotropic velocity and attenuation spectra of a dual porosity model of clay-sand mixtures, and the effects of spatial distribution on the anisotropic attenuation spectra of fully aligned cracks that are partially saturated with two different fluids) will be provided in order to complement those in our earlier papers.     

Lateral variations of coda Q and attenuation of seismic waves in Southwest Anatolia

The seismic quality factor (Q _c) and the attenuation coefficient (δ) in the earth’s crust in southwest (SW) Anatolia are estimated by using the coda wave method based on the decrease of coda wave amplitude by time on the seismogram. The quality factor Q _o, the value of Q _c at 1 Hz, and its frequency dependency η are determined from this method depending on the attenuation properties of scattered coda waves. δ is determined from the observations of amplitude variations of seismic waves. In applying the coda wave method, firstly, a type curve representing the average pattern of the individual coda decay curves for 0.75, 1.5, 3.0, 6.0, 12.0, and 24.0 Hz values was estimated. Secondly, lateral variation of coda Q and the attenuation coefficients for three main tectonic patterns are estimated. The shape of the type curve is controlled by the scattering and attenuation in the crustal volume sampled by the coda waves. The Q _o and η values vary from 30 to 180 and from 0.55 to 1.25, respectively for SW Anatolia. In SW Anatolia, coda Q–f relation is described by and δ = 0.008 km⁻¹. These results are expected to help in understanding the degree of tectonic complexity of the crust in SW Anatolia.     

La descente et la montée étendues: the spatially <Emphasis Type="Bold">d</Emphasis>-anisotropic and the spatio-temporal case

There is a great demand for statistical modeling of phenomena that evolve in both space and time, and thus, there is a growing literature on correlation function models for spatio-temporal processes. In particular, various properties of these correlation functions have been studied only for the merely spatial or temporal case, fact that constitutes a strong motivation for our work. The goal of this paper is to inspect some properties, obtained with respect to partial differentiation and integration, of stationary spatio-temporal correlation functions for which anisotropy is obtained through isotropy between components as in Fernández-Casal et al. (Stat Comput 13(2):127–136, 2003). We show that through partial differentiation and integration it is possible to obtain permissible spatio-temporal correlation functions in the space–time domain. Other new results regard specific classes of space–time correlations introduced in recent literature. A curious result arises by differentiating scale mixtures of Euclid’s hat. Work partially funded by grant MTM2004-06231 from the Spanish Ministery of Science and Education.     

Advanced insights into sources of vertical velocity in the ocean

Estimating vertical velocity in the oceanic upper layers is a key issue for understanding ocean dynamics and the transport of biogeochemical elements. This paper aims to identify the physical sources of vertical velocity associated with sub-mesoscale dynamics (fronts, eddies) and mixed-layer depth (MLD) structures, using (a) an ocean adaptation of the generalized Q-vector form of the ω-equation deduced from a primitive equation system which takes into account the turbulent buoyancy and momentum fluxes and (b) an application of this diagnostic method for an ocean simulation of the Programme Océan Multidisciplinaire Méso Echelle (POMME) field experiment in the North-Eastern Atlantic. The approach indicates that w-sources can play a significant role in the ocean dynamics and strongly depend on the dynamical structure (anticyclonic eddy, front, MLD, etc.). Our results stress the important contribution of the ageostrophic forcing, even under quasi-geostrophic conditions. The turbulent w-forcing was split into two components associated with the spatial variability of (a) the buoyancy and momentum (Ekman pumping) surface fluxes and (b) the MLD. Process (b) represents the trapping of the buoyancy and momentum surface energy into the MLD structure and is identified as an atmosphere/oceanic mixed-layer coupling. The momentum-trapping process is 10 to 100 times stronger than the Ekman pumping and is at least 1,000 times stronger than the buoyancy w-sources. When this decomposition is applied to a filamentary mixed-layer structure simulated during the POMME experiment, we find that the associated vertical velocity is created by trapping the surface wind-stress energy into this structure and not by Ekman pumping.     

Use of mode-converted waves in marine seismic data to investigate the lithology of the sub-bottom sediments in Isfjorden,Svalbard

The compression wavefield is efficiently converted to shear-wave energy at post-critical angles in areas of high impedance contrast at the sea floor. We have analysed mode-converted shear waves in a data set acquired with a hybrid marine/land geometry in Isfjorden, Svalbard. Through a kinematic 2D ray-tracing modellingV _p/V_s ratios for part of the uppermost 5km of the crust are obtained. Low values (V _p /V _s =1.65) are tentatively associated with the section of Devonian sandstones which appears to attain a minimum thickness of 1.5km below 3 km depth about 10km west of Kapp Thorden.     

On the sensitivity of tidal residuals off the west coast of Britain to mesh resolution

An irregular mesh model of the west coast of Britain is used to examine the sensitivity of tidal residuals to mesh resolution in the region. Computed residuals are compared with earlier published results determined with a high resolution (1 km grid) finite difference model of the eastern Irish Sea. Initial calculations show that tidal residuals are largest in nearshore regions particularly in the vicinity of headlands. Local refinement of the mesh in these regions leads to a more detailed picture of the flow field, particularly adjacent to the coast. Although large scale offshore features of the flow can be resolved using the high resolution finite difference model, such an approach leads to a “stair case” representation of the coastal boundary with an adjacent near coastal region of spurious tidal residuals. By using an irregular mesh that follows the coast, this effect is removed. In the Mersey river region the tidal residual is resolved with a mesh resolution of 120 m, although calculations show that its distribution is particularly sensitive to small scale features of the topography. A variable mesh that can accurately represent the lateral variations in river width and details of topography in both the nearshore and estuarine environment appears essential in modelling the coastal spread of freshwater plumes from rivers and pollutants discharged into the near coastal environment.     

Estimating the size of an earthquake using short-period seismograms of the first three seconds: A simulated experiment using the 1999 Chi-Chi earthquake sequence

Most of the present earthquake early warning systems are based on broadband or strong motion recordings. How-ever, the short-period instruments are still deployed. It is well-known that short-period recordings have saturation problems for large earthquakes when estimating the size of an earthquake. Thus, it is necessary to make clear the magnitude at which saturation starts to occur for the commonly used τc and Pd measurements, respectively. To investigate the possibility of using short-period seismic recordings for earthquake early warning, we conducted a simulated experiment using the strong motion data of the 1999 Chi-Chi earthquake sequence including its main shock and 31 aftershocks, with magnitude span from 4 to 7.6. The strong motion acceleration recordings were convolved with the instrument response of short-period seismographs in northern China to simulate short-period seismograms. Parameters τc and Pd from the first-three-second seismograms were calculated for the simulated short-period recordings and compared with that obtained by the original strong ground motion recordings. The result showed that to some extent, short-period recordings can be used for threshold earthquake early warning, while the magnitude saturation of Pd estimation can be up to 6.5, better than τc estimation.     

Ar/Ar and C geochronology of the Albano maar deposits: Implications for defining the age and eruptive style of the most recent explosive activity at Colli Albani Volcanic District,Central Italy

New ⁴⁰Ar/³⁹Ar and ¹⁴C ages have been found for the Albano multiple maar pyroclastic units and underlying paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near Rome, Italy, consisting of seven eruptions (Albano 1 = oldest). Both dating methodologies have been applied on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The ⁴⁰Ar/³⁹Ar ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, which are indistinguishable from the previously determined ⁴⁰Ar/³⁹Ar ages of the proximal Albano units 1, 2, 5 and 7, thus confirming their stratigraphic correspondence.     

First Ar/Ar age of the Ceprano man (central Italy)

The Ceprano calvarium, found in 1994 in Italy and attributed to Homo cepranensis, is one of the most celebrated hominin remains of Europe. It was considered at least 700 ka-old until a recent investigation incorporating magnetostratigraphy and K-Ar ages from the literature assigned to the calvarium an age of ∼450 (+50, −100) ka. Here we pin down the age of the Ceprano calvarium to 353 ± 4 ka (±1σ external) by means of new ⁴⁰Ar/³⁹Ar dating on K-feldspars retrieved from the sediments that hosted the skull. In absence of evidence of reworking, this refined age sinks the conviction that H. cepranensis belonged to human evolution at the Brunhes–Matuyama boundary (c.a. 781 ka). Our refined age indicates that H. cepranensis lived in central Italy probably during the cold period of marine isotope stage (MIS) 10, and that despite his archaic morphology and lack of Neanderthal traits, he was contemporaneous with more advanced species such as H. heidelbergensis.     

The Key Technical Points and Main Characteristics of the New National Standard of Magnitude

The New Magnitude National Standard of General Rules for Earthquake Magnitude( GB17740-2017) is the state mandatory standard. It was released on May 12,2017,by the General Administration of Quality Supervision,Inspection and Quarantine of the Peoples Republic of China and the Standardization Administration of the Peoples Republic of China. This paper introduces the necessity of revising the national standard of magnitude,and the main contents,technical points and primary features of the new national standard of magnitude,so that it can be applied better in practice.     

Attenuation of Coda Waves at the Changbaishan Tianchi Volcanic Area in Northeast China

21 earthquakes recorded by a temporary seismic network in the Changbaishan Tianchi volcanic area in Northeast China operated during the summer of 2002 and 2003 were analyzed to estimate the S coda attenuation. The attenuation quality factor Q_c was estimated using the single scattering attenuation model of Sato (1977) in the frequency band from 4 to 24 Hz. All the events studied in this paper occurred at depths from 2 to 6 km with ML of 1.4–2.8. The epicentral distances are less than 25 km. For all events which occurred near the Tianchi Lake (caldera), the Q_c patterns obtained at the stations near the lake are similar, and the Q_c values are relatively small. At the stations located about 15 km east of the Tianchi Lake, however, the average Q_c is significantly higher. For an event which occurred 25km from the lake to the west, Q_c patterns derived at the stations near the lake are quite similar to the above mentioned Q_c for stations located in the east. Further study shows that Q_c value in the north and central areas of the volcano is relatively lower than that in the surrounding area. Compared to other volcanic areas in the world, the average Q_c of the Changbaishan Tianchi volcanic area is obviously lower. The deep seismic sounding and teleseismic receiver function studies indicated more than one lower velocity layer in the crust. The MT studies suggested the presence of high conductive bodies beneath the area. We interpret the strong attenuation of coda waves near the Changbaishan Tianchi volcano as being possibly related to high temperature medium caused by shallow magma chambers.     

A large excess of 210Po in the overlying water of the Zhubi Coral Reef flat, in the South China Sea

The temporal variability of ²¹⁰Po and ²¹⁰Pb was examined in the overlying water of the Zhubi Coral Reef flat to detect nutrient-like behavior of ²¹⁰Po. Different mechanisms influencing their geochemical behaviors were observed. Excess ²¹⁰Po relative to ²¹⁰Pb revealed an additional input of ²¹⁰Po other than in situ production from ²¹⁰Pb. The ²¹⁰Po input comes from the reef flat sediment through diffusion. The diffusion contributes 62% of the total ²¹⁰Po. This diffusion of ²¹⁰Po directly highlights its nutrient-like behavior. No input, but the slight removal, of ²¹⁰Pb was observed. Fractionation factors indicate that particulate matter prefers to adsorb ²¹⁰Po rather than ²¹⁰Pb. In combination with particulate composition, ²¹⁰Po diffusion was closely related to organic matter. These results reveal that ²¹⁰Po might be a potential tracer for quantifying nutrient recycling in the Coral Reef system.     

Am improved source mechanism for the 1935 Timiskaming,Quebec earthquake from regional waveforms

The Timiskaming earthquake, which occurred near the Quebec-Ontario border at the northwest end of the Western Quebec seismic zone in 1935, is one of the five largest instrumentally recorded southeastern Canadian earthquakes. Previous studies of this earthquake concentrated on modeling teismograms recorded at regional distances, a better constrained focal mechanism is obtained. The waveforms indicate thrust faulting on a moderately dipping northwest striking plane at a depth of 10 km. TheM _w of 6.1 determined in this study is in good agreement with previous magnitude estimates (m _b 6.1,M _s 6.0, andm _bLg 6.2–6.3). The focal mechanism is similar to those of many recent small to moderate earthquakes in the region, and the inferred (from theP axis) acting stress of northeast compression is consistent with the overall eastern North American stress field. The Lake Timiskaming Rift Valley in which the earthquake occurred, comprises several northwest striking faults consistent with the strike of the 1935 event. Thus, the 1935 earthquake appears to be a result of faulting on the reactivated Timiskaming graben.