Lg amplitude ratios H/Z and station terms in the six eastern provinces of China

Using the data set of about 1 Hz Lg amplitudes from 80 stations in the six eastern provinces of China, the ratios of horizontal to vertical amplitude of Lg waves,H/Z are determined. The mean lg(H/Z)-value is equal to 0.13. For the station bases of soft deposit,granite and sedimentary rock, lg(H/Z)-values average 0.55, 0.04 and 0.10, respectively. For the three kinds of station bases, the station terms of horizontal amplitude,C _h average 0.65, −0.09 and 0.00; that of vertical amplitude,C _z average 0.32, −0.04 and 0.00, respectively. There are the relations betweenC _h ,C _z and lg(H/Z):C _h =0.01+2.10C _z andC _h =−0.15+1.46 lg(H/Z). The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 281–286, 1991. Project sponsored by the Chinese Joint Seismological Science Foundation.     

中国东部六省 Lg 波振幅比 H/Z 和台站项/

根据中国东部六省80个台的约1Hz Lg 波振幅资料得出,Lg 波水平与竖直分量之比1g H/Z 值平均为0.13.对于松散沉积、花岗岩和沉积岩类台基,1g H/Z 值分别平均为0.55,0.04和0.10对此三类台基,水平向振幅台站项 Ch,分别平均为0.65,-0.09和0.00;竖直向振幅台站项 C2分别为0.32-0.04和0.00.Ch,CZ 和1gH/Z 间存在关系 Ch=0.01+2.10Cz和 Ch=-0.15+1.46 1gH/Z.      

Lg magnitude scale in the mainland of China

The data of short-period seismograms had been collected widely in the mainland area of China not including Xinjiang and Tibet. The physical quantities of Lg wave are determined respectively in the five subregions. The group velocities of priminary arrival and maximum amplitude of Lg wave are equal to 3.54±0.02 km/s and 3.30±0.05 km/s, respectively. The periods of Lg waves are between 0.2s to 1.2s, averaging 0.7s. The γ-values of Lg waves in the five subregions are equal to 0.0034±0.0001 km⁻¹ for East, 0.0031±0.0004 km⁻¹ for Southwest, 0.0027±0.0004 km⁻¹ for Northeast, 0.0022±0.0001 km⁻¹ for South, and 0.0021±0.0002 km⁻¹ for Northwest subreqion, respectively. The average γ-value for the five subregions, γ=0.0027±0.0006 km⁻¹. The relations among the amplitude ratioH/Z, the station correctionD _z andD _h of amplitudes, and among them and station site condition are discussed. The subregional magnitude calibration functions ofm _Lg had been established according to each regional γ-value. From these, the unified magnitude calibration function of Chinese mainland not including Xinjiang and Tibet was given by

Electromagnetic induction in the British Isles region: analogue model and field station results

The results of a laboratory electromagnetic analogue model study, which employs a horizontal inducing field over a simple model of the British Isles region, delineate the location and frequency dependence of the major coast effect induction anomalies of the Scotland region. Contours of amplitudes, amplitude ratios, and in-phase and quadrature parts of the model field measurements are presented. The model vertical magnetic fields for two orthogonal source field polarizations and field station values for two hypothetical events for corresponding polarizations are compared.While major discrepancies occur between model and field H_z amplitudes, the H_z gradients across Scotland, which can be attributed to the coast-effect, are comparable in value, although sometimes reversed in sign. Superimposed on this coast-effect, the field data indicate the existence of current concentrations associated with the Great Glen and the Southern Uplands faults and possibly also of currents within the Scottish mainland near the east and west coasts.     

On the topographic coupling at the core-mantle interface

The mean tangential stresses at a corrugated interface between a solid, electrically insulating mantle and a liquid core of magnetic diffusivity λ are calculated for uniform rotation of both mantle and core at an angular velocity Ω in the presence of a corotating magnetic field B. The core and mantle are assumed to extend indefinitely in the horizontal plane. The interface has the form z = η(x, y), where z is the upward vertical distance and x, y are the zonal and latitudinal distances respectively. The function η(x, y) has a planetary horizontal length scale (i.e. of the order of the radius of the Earth) and small amplitude and vertical gradient. The liquid core flows with uniform mean zonal velocity U₀ relative to the mantle. Ω and B possess vertical and horizontal components.The vertical (poloidal) component B_p is uniform and has a value of 5 G while the horizontal (toroidal) field B_T = B_pαz, where α is a constant. When |α| ? 1, the mean horizontal stresses are found to have the same order of magnitude (10^?2 N m^?2) as those inferred from variations in the decade fluctuations in the length of the day, although the exact numerical values depend on the orientation of Ω as well as on the wavenumbers in the zonal and latitudinal directions.The influence of the steepness (as measured by α) of the toroidal field on the stresses is investigated to examine whether the constraint that the mean horizontal stresses at the core-mantle interface be of the order of 10^?2 N m^?2 might provide a selection mechanism for the behaviour of the toroidal field in the upper reaches of the outer core of the Earth. The results indicate that the restriction imposed on α is related to the value assigned to the toroidal field deep into the core. For example, if |α| ? 1 then the tangential stresses are of the right order of magnitude only if the toroidal field is comparable with the poloidal field deep in the core.     

中国东部六省的 Lg 震级(上)——测定方法和结果

本文系统地分析了根据 Lg 波埃里相和非埃里相衰减系数确定单台和台网震级公式、震级标准差以及台站校正值的方法.根据中国东部苏、鲁、豫、皖、冀、浙六省地区约1秒 Lg 波衰减系数的测定结果,导出六省地区的 Lg 震级公式:mLg(z,h)=lgdLg(z,h)+5/6lg+0.0019+2.09 mmx(z,h)=lgdmx(x,h)+lg+0.0013+1.27 计算了该区域内19个地震的各震级值,以及80个台站的台校值.单台震级标准差为0.15-016震级单位,台网震级标准差为0.03-0.04震级单位.此外,还测定了111个地震的 mmxz和 mmxh值,验证或补充了19个地震得出的某些关系和结果.      

Vertical mixing in Überlinger See,western part of Lake Constance

Depth variable vertical eddy diffusion coefficients for heat (K _z) were calculated from continuously measured temperature profiles in Überlinger See (western part of Lake Constance). The temperatures were averaged over vertical intervals of 10 m yielding 14 discrete values (maximum depth of Überlinger See: 147 m). A linear fit from 10 June to 29 September 1987 was used to smooth the significant temperature fluctuations caused by internal seiches of Lake Constance.Assuming horizontal homogeneity for the smoothed data the Gradient-Flux-Method was applied to compute vertical diffusion coefficientsK _z at different depths using the depth variable volumes and surfaces of the 14 layers. The resulting mean diffusion coefficients for the period from June to September are 0.04 cm²/s near the thermocline and up to 0.8 cm²/s in deeper strata (accuracy: ± 50%). It is shown that horizontal mixing between Überlinger See and Obersee (main lake) alters the computation ofK _z by less than 50%.A relationship betweenK _z and stability (Brunt-Väisälä) frequencyN is found which corresponds well to the theory of internal wave induced turbulence.Combining the diffusion coefficients with measured phosphorus profiles, a phosphorus flux from the hypolimnion to the epilimnion of (0.7 ± 0.4) mg P m^–2 d^–1 was calculated, corresponding to about 20% of the average external loading per area of Lake Constance in 1986.     

Relation between the atmospheric characteristics in the antarctic regions and the space weather factors

Considerable variations in the cloud cover level and air temperature, related to the variations in GCRs and IMF, have been revealed based on an analysis of the meteorological and aerological data obtained at Vostok station from 1974 to 1994. It has been found out that the cloud cover at Vostok decreased, on average, by 35% a day after powerful Forbush decreases in GCRs following a considerable increase in the southward IMF component. In the years of solar activity minimum, when the variations in SCRs and GCRs are insignificant, the cloudiness and surface temperature increase on a day of B _z minimum and decrease on a day of maximum as compared to the average level. On days of B _z minimum, the air temperature rises at altitudes of h = 3.5–7 km, remains almost unchanged at an altitude of h = 8 km, and slightly decreases at higher altitudes. An increase in cloudiness at altitudes below 8 km causes warming, probably due to the greenhouse effect, because the temperature of the Earth’s surface decreases.     

Meridional distribution of226Ra in the eastern Pacific along GEOSECS cruise tracks

We present the distribution of²²⁶Ra in eight vertical profiles from the eastern Pacific. The profiles are located along a meridional trend near 125°W, from 43°S to 29°N. Surface²²⁶Ra concentrations are about 7 dpm/100 kg, except for the two stations south of 30°S where the higher values are due to the Antarctic influence. Deep waters show a distinctive south-to-north increase in the²²⁶Ra content, from about 26 to 41 dpm/100 kg near the bottom. Unlike in the Atlantic and Antarctic Oceans, the effect of²²⁶Ra injection from bottom sediments is clearly discernible in the area. The presence of this primary²²⁶Ra can be traced up to at least 1–1.5 km above the ocean floor, making this part of the sea bed among the strongest source regions for the oceanic²²⁶Ra. Numerical solutions of a two-dimensional vertical advection-diffusion model applied to the deep (1.2–4 km)²²⁶Ra data give the following set of best fits: upwelling velocity(V_z) = 3.5m/yr, vertical eddy diffusivity(K_z) = 0.6cm²/s, horizontal (north-south) eddy diffusivity(K_y) = 1 × 10⁷cm²/s, and water-column regeneration flux of²²⁶Ra(J) = 3.3 × 10^?5dpmkg^?1yr^?1 as an upper limit. These parametric values are in general agreement with one-dimensional (vertical) model fits for the Ra-Ba system. However, consideration of²²⁶Ra balance leads us to suspect the appropriateness of describing the vertical exchange processes in the eastern Pacific with constantV_z and K_z. If future modeling is attempted, it may be preferable to treat the area as a diffusion-dominant mixing regime with depth-dependent diffusivities.     

Lateral Variations of Lg Coda Q in Southern Mexico

—Broad band digital three-component data recorded at UNM, a GEOSCOPE station, were used to estimate Lg coda Q for 34 medium size (3.9 ≤m _b ≤ 6.3) earthquakes with travel paths laying in different geological provinces of southern Mexico in an effort to establish the possible existence of geological structures acting as wave guides and/or travel paths of low attenuation between the Pacific coast and the Valley of Mexico. The stacked spectral ratio method proposed by XIE and NUTTLI (1988) was chosen for computing the coda Q. The variation range of Q ₀ (Q at 1?Hz) and the frequency dependence parameter η estimates averaged on the frequency interval of 0.5 to 2?Hz for the regions and the three components considered are: i) Guerrero region 173 ≤Q¯ ₀≤ 182 and 0.6 ≤Q¯ ₀ ≤ 0.7, ii) Oaxaca region 183 ≤Q¯ ₀ ≤ 198 and 0.6 ≤Q¯ ₀ ≤ 0.8, iii) Michoacan-Jalisco region 187 ≤Q¯ ₀ ≤ 204 and 0.7 ≤Q¯ ₀ ≤ 0.8 and iv) eastern portion of the Transmexican Volcanic Belt (TMVB) 313 ≤Q ₀≤ 335 and η = 0.9. ¶The results show a very high coda Q for the TMVB as compared to other regions of southern Mexico. This unexpected result is difficult to reconcile with the geophysical characteristics of the TMVB, e.g., low seismicity, high volcanic activity and high heat flow typical of a highly attenuating (low Q) region. Visual inspection of seismograms indicates that for earthquakes with seismic waves traveling along the TMVB, the amplitude decay of Lg coda is anomalously slow as compared to other earthquakes in southern Mexico. Thus, it seems that the high Q value found does not entirely reflect the attenuation characteristics of the TMVB but it is probably contaminated by a wave-guide effect. This phenomenon produces an enhancement in the time duration of the Lg wave trains travelling along this geological structure. This result is important to establish the role played by the transmission medium in the extremely long duration of ground motion observed during the September 19, 1985 Michoacan earthquake. ¶The overall spatial distribution of coda Q values indicates that events with focus in the Michoacan-Jalisco and Oaxaca regions yield slightly higher values than those from Guerrero. This feature is more pronounced for the horizontal component of coda Q. A slight dependence of average coda Q ^?1 on earthquake focal depth is observed in the frequency range of 0.2 to 1.0?Hz approximately on the horizontal component. Deeper (h > 50?km) events yield lower values of Q ^?1 than shallower events. For frequencies higher than 1.0?Hz no clear dependence of Q ^?1 on focal depth is observed. However, due to the estimates uncertainties this result is not clearly established.     

The revisedm Lg scale in six eastern provinces of China

The influence of the four different methods of measuring Lg amplitude, and the selection of different geometrical attenuation coefficient ζ-values (=5/6 or 1) on the determination ofγ value of Lg wave are discussed.γ=0.0034±0.0001km^?1 (when ζ=5/6) for six eastern provinces is redetermined. The revised magnitude calibration function ofm _Lg (mxh),q _E (Δ)=(5/6)logΔ+0.00147Δ+1.81 is deduced.     

Quantification relations between the sourc eparameters

The various useful source-parameter relations between seismic moment and common use magnitude lg(M ₀) andM _s,M _L,m _b; between magnitudesMs andM _L,M _s andm _b,M _L andm _b; and between magnitudeM _s and lg(L) (fault length), lg (W) (fault width), lg(S) (fault area), lg(D) (average dislocation);M _L and lg(f _c) (corner frequency) have been derived from the scaling law which is based on an “average” two-dimensional faulting model of a rectangular fault. A set of source-parameters can be estimated from only one magnitude by using these relations. The average rupture velocity of the faultV _r=2.65 km/s, the total time of ruptureT(s)=0.35L (km) and the average dislocation slip rateD=11.4 m/s are also obtained. There are four strong points to measure earthquake size with the seismic moment magnitudeM _w.

1. The seismic moment magnitude shows the strain and rupture size. It is the best scale for the measurement of earthquake size.
2. It is a quantity of absolute mechanics, and has clear physical meaning. Any size of earthquake can be measured. There is no saturation. It can be used to quantify both shallow and deep earthquakes on the basis of the waves radiated.
3. It can link up the previous magnitude scales.
4. It is a uniform scale of measurement of earthquake size. It is suitable for statistics covering a broad range of magnitudes. So the seismic moment magnitude is a promising magnitude and worth popularization.

     

A quantitative study of the energy release in the aftershocks of the Bhuj earthquake, 2001, India, using Lg phase

The devastating earthquake on 26 January 2001 at Bhuj, India, resulted in large-scale death and destruction of properties of several million US dollars. The moment magnitude of the earthquake was 7.7 and its maximum focal intensity exceeded X in MM scale. The rate of aftershocks of this earthquake, recorded at Gauribidanur seismic array station (GBA), shows a monotonic decay with time superposed with oscillations. For the Indian continent the Lg phase is a prominent arrival at regional distances. The estimate of Lg amplitude is obtained by optimally fitting the Lg wave train to a exponential decay curve. The logarithm of these amplitudes and logarithm of root mean square (rms) value of actual amplitudes of the Lg are calibrated with USGS m_b to create a local m_bLg magnitude scale. The energy released from these aftershocks is calculated from the rms value of Lg phase. The plot of cumulative energy release with time follows the power law of the form t^p, superposed with oscillations. The exponent of the power law, p, is estimated both by a time-window scanning method and by an interpolation method. The value of p is 0.434 for time-window scanning method and 0.432 for the interpolation method. The predominant periods found in the oscillatory part of the cumulative energy, obtained by differencing the observed from the power law fit, are 10.6, 7.9, 5.4, 4.6 and 3.5 h for time-window scanning method. The corresponding periods for interpolation method are 13.4, 11.5, 7.4, 4.2, 3.5, 2.6 and 2.4 h.     

THE VERTICAL MAGNETIC FIELD OF AN ALTERNATING CURRENT DIPOLE FOR HORIZONTALLY STRATIFIED MEDIA *

For the computation of the vertical component H_z of the magnetic field of a horizontal A.C. dipole lying on the earth's surface, a recurrence formula is presented for a horizontally stratified half space, to obtain the (n+ 1)-layer case from the w-layer case. By means of several computed diagrams for the two-layer case, H_z can be determined for different ratios of conductivity of the subsoil and that of the overburden. Thereby the distance from the dipole as well as the layer thickness h are expressed in terms of the wave length A of a plain wave in the overburden. Assuming a sufficiently large conductivity difference, the results show that evidence about the subsurface conditions can be obtained if the distance between the measuring coil and the dipole is of the order of A/3, and if the thickness h of the layer varies within the range A/100 < h < A/6. As an example for the 3-layer case, a nonconducting intermediate layer is assumed.     

Seismic analysis of nailed vertical excavation using pseudo-dynamic approach

An attempt has been made to study the behavior of nailed vertical excavations in medium dense to dense cohesionless soil under seismic conditions using a pseudo-dynamic approach. The effect of several parameters such as angle of internal friction of soil(Φ), horizontal(k_h) and vertical(k_v) earthquake acceleration coefficients, amplification factor(f_a), length of nails(L), angle of nail inclination(α) and vertical spacing of nails(S_v) on the stability of nailed vertical excavations has been explored. The limit equilibrium method along with a planar failure surface is used to derive the formulation involved with the pseudo-dynamic approach, considering axial pullout of the installed nails. A comparison of the pseudo-static and pseudo-dynamic approaches has been established in order to explore the effectiveness of the pseudo-dynamic approach over pseudo-static analysis, since most of the seismic stability studies on nailed vertical excavations are based on the latter. The results are expressed in terms of the global factor of safety(FOS). Seismic stability, i.e., the FOS of nailed vertical excavations is found to decrease with increase in the horizontal and vertical earthquake forces. The present values of FOS are compared with those available in the literature.     

MEM Analysis of Geomagnetic Field Variations Over Narssarssuaq

—Maximum entropy spectral analysis (MESA) has been applied to 24 series of hourly daily data and only one daily mean series for the horizontal (H) and vertical (Z) components of the geomagnetic field for the year 1983 as observed at Narssarssuaq, Greenland (71.2°N, 36.7°E) (gm coordinate). The method has isolated some prominent medium frequency signal components. The maximum peaks for H are at 06 hr (0.174 cycles per day (cpd), 3.2 × 10⁴ db) and 08 hr (0.09 cpd, 3.5 × 10⁴ db). Similarly, the maximum peak in Z is observed at 04 hr (0.114 cpd, 5.7 × 10⁴ db). The spectral results for the daily-mean data indicate periods are greater than two days, with 178.5 days (nearly semiannual) being common to both H and Z. Other harmonics have been found for all the series of H and Z components which are mainly caused by the "Effective Period", i.e., the period produced by the combined effect of the sunspot numbers and the sun’s rotation period. Such frequencies correspond very well with those found in the geomagnetic indices A _p ?, C _p and AE. This suggests that the disturbance transient variations are caused by viscous interaction of the solar energies emanating from sunspot regions with the outermost magnetospheric boundary which, in turn, influences the magnetosphere-iono sphere coupling and produces the medium intensity long-duration continuous auroral activities (MILD CAAs) over high latitude regions. Thus, the higher latitude geomagnetic activities are nothing but the "effective period driven MILDCAAs" having a recurrence tendency of 27/n, where n is an integer.     

Lower crustal attenuation in northeastern Tibetan Plateau from ML amplitude

This study investigated the crustal attenuation structures of Sg and Lg waves of the northeastern Tibetan Plateau. We collected M_L amplitude data recorded at 168 permanent stations between 1985 and 2016 and 11 temporary broadband stations between 2014 and 2016. Detailed Q₀ variation maps of Sg and Lg waves were obtained by applying M_L amplitude tomography. The average Q₀ values of the Sg and Lg wave were 440 and 220, respectively. Relatively high attenuation anomalies of both waves appeared in the central and eastern regions of the Bayan Har Block and the east edge of the Qiangtang Block, which may be related to partial melting, high geotemperature, and strong tectonic processes. High attenuation anomalies were also found in the Qilian Orogenic Belt and Hetao Graben, which may be related to their active tectonic behavior and densely distributed faults. The relatively low attenuation anomalies of both waves were revealed in the Alax and Ordos blocks, Qaidam, Tarim, Qinghai Lake, and Gonghe basins, which can be explained by the tectonically stable properties and ancient composition of geological elements. These results indicate that the path between the highly attenuated lower crust of the Bayan Har Block and the Qilian Orogenic Belt is obstructed by three adjacent low attenuated areas (i.e., the Qilian, Qinghai Lake, and Gonghe basins); thus, it appears unlikely that a crustal flow channel from the interior of the Tibetan Plateau to the Qilian Orogenic Belt will form.     

Inversion of breakout data from inclined bore-holes for stress state of the upper crust in Jizhong depression

The forward and inverse problems of studying crustal stress state from breakout data of inclined boreholes are concisely stated. Direction of the maximum horizontal principal stress (compressive) and relative magnitudes of the horizontal stresses to the vertical stress in the upper crust in two regions of the Jizhong depression, the North of China, are obtained by analyzing the breakout data of 6 inclined wells. To get stable results in the analysis we searched for the unknown parameters both forwardly and inversely. The results show that the maximum azimuths of the horizontal principal compressive stresses in the central and southern part of the Jizhong depression are N86°E and N77°E, respectively, while the relative magnitudes of the three principal stresses in the upper crust (about 1000–4000 m) of the depression are variable. In the central part of the Jizhong depression we have foundS _H :S _V :S _h =1.38:1.00:0.57, whereS _H ,S _V andS _h are the maximum horizontal, vertical and minimum horizontal stress, respectively. This indicates that the present stress regime in this area is of strike-slip faulting type. In the southern part of the depression we have obtainedS _H :S _V :S _h =0.80:1.00:0.62, indicating a normal faulting stress regime in the shallow part of the crust.     

Formation of cracks due to moving source over a thick crust

Summary In this paper the problem of a point source of stress moving over the surface of a thick aelotropic plate resting of a rigid foundation has been considered. Following the method ofAleksandrov & Vorovich (1960) the stress componentsZ _x andZ _z have been expanded in series of ascending powers of 1/h when the source velocity is less than (c ₄₄/)^1/2. When the velocity exceeds (c ₄₄/)^1/2 it has been shown that two cracks are produced in different directions and their successive reflections at the upper and lower surface are also obtained.     

An analogue model study of electromagnetic induction in the Tasmania region

Laboratory analogue model magnetic measurements are carried out for a model of the region including Tasmania, Bass Strait with its highly conductive deep sedimentary basins, and the south coast of mainland Australia. The model source frequencies used simulate naturally occurring geomagnetic variations of periods 5–120 min. In-phase and quadrature magnetic H_x, H_y and H_z field measurements for the modelled region are presented for an approximately uniform overhead horizontal source field for E-polarization (electric field of the source in the N-S direction) and for H-polarization (electric field of the source in the E-W direction). Large anomalous in-phase and quadrature model magnetic fields are observed over Bass Strait and the coastal regions at short periods for both E- and H-polarization, but with increasing period, the field anomalies decrease more rapidly for E-polarization, than for H-polarization. The difference in response with polarization for the Bass Strait region is attributed to current induced in the deep ocean, for all periods, being channelled through Bass Strait for H-polarization but not for E-polarization. The persistent large coastal field anomalies elsewhere, for H-polarization, can be accounted for by the coastal current concentrations due to currents induced in the deep ocean for all periods deflected to the south and to the north by the shelving sea-floor and channelled through Bass Strait and around the southern coast of Tasmania. The phenomena of current deflection and channelling for H-polarization for the geometry of the southern Australia coastline and associated ocean bathymetry is particularly effective in producing field anomalies for a large period range.The coastal horizontal H_x and H_y field anomalies, present for E-polarization at short periods and for H-polarization at all periods, do not extend far inland, and thus, for inland station sites somewhat removed from the coast, should not present serious problems for magnetic soundings in field work. The sharp vertical field (H_z) gradient over Tasmania at short periods, which is predominantly in the E-W direction for E-polarization and the N-S direction for H-polarization, is strongly frequency dependent, becoming almost undetectable at 60 min. The behaviour of the H_z field gradients, however, are very similar from traverse to traverse over inland Tasmania, and thus, the effects of the ocean should not present too serious a problem in the interpretation of field station studies. The discrepancies between model and field station results should be useful in mapping geological boundaries in the region.