Multifractal measures of earthquakes in west Taiwan

The generalized fractal dimension for epicentral distribution of earthquakes in west Taiwan is measured. The entire area is first divided into two zones, i.e., north and south zones, after which the two zones are further separated into three subzones for the former and two for the latter. The logC _q (r) versus logr function, whereC _q (r) is the generalized correlation integral andr is the distance between two epicenters, shows that a linear relation between logC _q and logr exists in the range ofr smaller thanr _c . The value ofr _c is 25 km for the north zone, 40 km for the south and 12 km for the three north subzones. The valuesr _c =25 and 40 km are almost the smallest ones of the width of epicentral distributions of the north and south zones, respectively. The value ofr _c =12 km for the three north subzones is approximately the smallest size of the cluster of epicenters. For the plots of two south subzones, the pattern of data points does not bend in the range ofr in consideration, and, thus, there is not such a critical radius. TheD _q –q relations forq=0, 1, 2,..., 15 are constructed for the two zones and five subzones. Results show significant multifractality and a spatial variation in multifractality for epicentral distributions of earthquakes in west Taiwan.     

适用于全周期结构的速度脉冲型地震动强度表征参数研究

鉴于传统的地震动强度表征参数（IM）不能有效地反映速度脉冲型地震动的破坏特征,需要研究能够在全周期段内表征速度脉冲型地震动强度的参数。本文首先从NGA-West2强震数据库中选取了236条速度脉冲记录,分析了42种地震动参数之间的相似性和相关性,初步给出了速度脉冲型地震动强度的表征参数;其次,利用单自由度体系动力时程分析方法,研究了不同延性μ条件下速度脉冲型地震动强度表征参数与最大非线性位移u_m之间的相关性系数随周期T的变化特征,并通过线性回归和离差分析方法确定了初步给出的速度脉冲型地震动强度表征参数的有效性及其应用范围;最后,将速度脉冲对加速度反应谱的放大系数A_f作为速度脉冲型地震动强度表征参数,并验证其有效性。结果表明:① 当0＜T＜1 s时,S_a（T）作为速度脉冲型地震动强度表征参数的有效性较好;② 当1＜T＜3 s时,S_a,avg,S_v,avg,PPV,I_f和PGV作为速度脉冲型地震动强度表征参数的有效性较好;③ 当3＜T＜6 s时,S_a,avg,S_v,avg,PPV,I_f,PGV和S_a（T）不宜作为速度脉冲型地震动强度表征参数;④ 当0＜T＜6 s时,A_f作为速度脉冲型地震动强度表征参数的有效性较好,而且,当结构自振周期小于速度脉冲周期时,A_f作为速度脉冲型地震动强度表征参数时u_m的离散性最小。      

The synodic periods of planets in the solar system and synchronization of the timing of large earthquakes

We report the results from a search for statistically significant (at a significance level of <0.05) periodicities that synchronize the occurrence of large shallow (h ≤ 100 km) earthquakes in the eight world regions with the highest level of seismicity. The periodicities in question include the synodic periods of planets of the solar system (Mercury, Venus, Mars, and Jupiter), as well as the precession period of the lunar orbit. In five of the eight regions we have found statistically significant results for the period T ≈ 780 days, corresponding to the synodic period of Mars. The lunar cycle and the period of Mercury were significant in two regions. These results cannot be accounted for by invoking the disturbing influence of gravitational and tidal forces due to planets other than the Earth. The significant periodicities were used to develop a forecast (for the period 2004–2009) of the most probable time intervals for the occurrence of large earthquakes in four regions: Kamchatka, the South Kurils, northeastern Japan, and the Nankai trough.     

Seismic anisotropy in a hydrocarbon field estimated from microseismic data

The study of seismic anisotropy in exploration seismology is gaining interest as it provides valuable information about reservoir properties and stress directions. In this study we estimate anisotropy in a petroleum field in Oman using observations of shear‐wave splitting from microseismic data. The data set was recorded by arrays of borehole geophones deployed in five wells. We analyse nearly 3400 microearthquakes, yielding around 8500 shear‐wave splitting measurements. Stringent quality control reduces the number of reliable measurements to 325. Shear‐wave splitting modelling in a range of rock models is then used to guide the interpretation. The difference between the fast and slow shear‐wave velocities along the raypath in the field ranges between 0–10% and it is controlled both by lithology and proximity to the NE‐SW trending graben fault system that cuts the field formations. The anisotropy is interpreted in terms of aligned fractures or cracks superimposed on an intrinsic vertical transversely isotropic (VTI) rock fabric. The highest magnitudes of anisotropy are within the highly fractured uppermost unit of the Natih carbonate reservoir. Anisotropy decreases with depth, with the lowest magnitudes found in the deep part of the Natih carbonate formation. Moderate amounts of anisotropy are found in the shale cap rock. Anisotropy also varies laterally with the highest anisotropy occurring either side of the south‐eastern graben fault. The predominant fracture strikes, inferred from the fast shear‐wave polarizations, are consistent with the trends of the main faults (NE‐SW and NW‐SE). The majority of observations indicate subvertical fracture dip (>70° ). Cumulatively, these observations show how studies of shear‐wave splitting using microseismic data can be used to characterize fractures, important information for the exploitation of many reservoirs.     

Some results of microseismic monitoring in the Elbrus Region

The results of microseismic monitoring in the adit of the neutrino laboratory (Institute of Nuclear Research, Russian Academy of Sciences) in the Elbrus Region are presented. Three-component computer recording was carried out in the frequency band 10–120 Hz with 5-second averaging of the amplitudes of vibrations. We consider the results of processing the data yielded by the monitoring in the adit during nine months since March 2008. The seismic field at the observation point is composed of three main contributions: the local earthquakes associated with two main fluid-magmatic sources, which are located nearby the adit; a slowly varying wave field, which is probably related to the same sources; and the industrial noise, namely, the vibrations caused by the traffic of carriages and a weak stationary background noise.     

Multifractal of spatial distribution of seismicity in Liaoning area

Making use of multifractal theory and corresponding computational method and according to the feature of evolution of spatial distribution with respect to seismicity by earthquake data in Liaoning area, earthquake activity of the area has been studied in detail. The results show that the evolution of increase in seismicity and distributive process in space are a multifractal structure. Whole characteristic of evolution in fractal increasing process of seismicity is described by obvious variation in regard toτ(q)-q curve,f(α) spectrum and other parameters before and after moderate and strong earthquakes.     

Slow hydromagnetic oscillations in a rotating spheroidal shell

Abstract

The ray method is used to study slow hydromagnetic waves in an incompressible, inviscid, perfectly conducting fluid of constant density in the presence of a constant toroidal magnetic field. The fluid is bounded below by a rigid sphere and above by a rigid spheroidal surface, and the mean fluid layer thickness is assumed to be small. Both the general time-dependent and time-harmonic (free oscillation) problems are studied and dispersion relations and conservation laws are derived. These results are applied to free oscillations with constant azimuthal wave number in a spherical shell and then compared to those of previous authors. Such oscillations propagate to the east and are trapped between circles of constant latitude. Wave propagation in axisymmetric shells is then studied with emphasis on the relationship between shell shape and direction of propagation, and it is found that such shells can sustain westward propagating modes wherever the shell thickness decreases sufficiently rapidly from a maximum at the poles to zero at the equator; no shells exist which can sustain westward propagation at the equator.     

Multifractal Analysis of Earthquakes in the Southeastern Iran-Bam Region

Earthquakes in Iran and neighbouring regions are closely connected to their position within the geologically active Alpine-Himalayan belt. Modern tectonic activity is forced by the convergent movements between two plates: The Arabian plate, including Saudi Arabia, the Persian Gulf and the Zagros Ranges of Iran, and the Eurasian plate. The intensive seismic activity in this region is recorded with shallow focal depth and magnitude rising as high as M_w = 7.8. The study region can be attributed to a highly complex geodynamic process and therefore is well suited for multifractal seismicity analysis. Multifractal analysis of earthquakes (m_b ≥ 3) occurring during 1973 – 2006 led to the detection of a clustering pattern in the narrow time span prior to all the large earthquakes: M_w = 7.8 on 16.9.1978; M_w = 6.8 on 26.12.2003; M_w = 7.7 on 10.5.97. Based on the spatio-temporal clustering pattern of events, the potential for future large events can be assessed. Spatio-temporal clustering of events apparently indicates a highly stressed region, an asperity or weak zone from which the rupture propagation eventually nucleates, causing large earthquakes. This clustering pattern analysis done on a well-constrained catalogue for most of the fault systems of known seismicity may eventually aid in the preparedness and earthquake disaster mitigation.     

Subharmonic oscillations of a forced hydromagnetic cavity

Abstract

We study the propagation of nonlinear MHD waves in a highly magnetized plasma cavity. The cavity's moving boundaries generate Alfvén waves, which in turn drive and interact with slow magnetosonic waves. The interacting wave system is analyzed by a Galerkin and multiple-scale analyses leading to simple dynamical equations. When the frequency of the forcing provided by the moving boundaries and that of the fundamental Alfvén eigenmode are close, the cavity behaves like a Duffing oscillator. Application of the Melnikov function theory shows that the Alfvén wave's amplitude undergoes both flip and saddle-node bifurcations as the amplitude and the phase of the boundary forcing vary. Direct numerical integration confirms these results and provides an estimate of the amount of energy dissipated in the bifurcations.     

中国大陆地震活动时空分布多重分形特征

利用多重分形分析方法,考察了中国大陆各构造区地震活动广义应变释放时间和空间分布的多重分形特征。结果表明,中国大陆各构造区地震活动广义应变释放时空分布的多重分形特征差别明显。     

煤矿冲击地压的微地震监测研究

为了研究煤矿冲击地压与岩层在三维空间破裂之间的关系,进而探索依据岩层破裂规律预测和预报冲击地压的可能性,文中采用自行研制的防爆型微地震定位监测（MS）系统,基于定位原理,监测了山东华丰煤矿冲击地压煤层（四层煤）及其解放层（六层煤）开采过程中的岩层破裂过程和二次应力场分布变化的过程,得到了如下结论：冲击地压的发生与岩层破裂密切相关,四层煤下顺槽处于六层煤顶板破裂区的外边缘时,正处于高应力区内,在此处掘进容易引发冲击地压,必须将六层煤下顺槽位置向实体煤侧移动20 m以上,或将四层煤下顺槽位置内移20 m以上,才能消除四层煤的冲击地压;六层煤和四层煤开采时,工作面前方断层活化的距离分别为250 m和350 m左右,根据这一距离,及时对断层带进行卸压处理,可以消除由断层带引发的冲击地压;监测显示了工作面周围岩层的三维破裂形态和范围,为矿井确定防水煤柱的高度提供了可靠的依据;监测证明了厚层砾岩的破裂、断层活化、采场附近关键层的破裂是引起冲击地压的主要原因,证明了所研制的硬件和定位软件具有较高的精度和实用性,可以在煤矿和边坡、隧道等领域应用.     

Some mechanical analogies of magnetodynamic oscillations in a homogeneous medium

Zusammenfassung Es wird auf die Analogien hingewiesen zwischen dem einfachsten Fall der Resonanz einer ebenen MD-Welle im Modell homogener Schicht der Erdmagnetosph?re und horizontaler Schwingung einer homogenen elastischen S?ule mit einem befestigten Ende. Es wird qualitativ die Analogie zwischen dem Anprall eines materiellen K?rpers an eine elastische S?ule mit einem befestigten Ende und dem Stoss der Front von Sonnenwind auf die Erdmagnetosph?re angedeutet. Diese Ann?herung erm?glicht die Illustration einiger Züge von Pulsationen, welche pl?tzliche Ver?nderungen in magnetischem Felde der Erde (Impulse si, ssc u. ?.) begleiten und deren Beschreibung auf Grund pr?ziser Theorie der Verbreitung von MD-Wellen sehr schwierig ist.

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Address: Boční II, Praha 4-Spořilov.     

Spectral analysis of oscillations with a time-variable frequency

Summary The paper presents a method of evaluating oscillations with a time-variable frequency using a computer. The given function is first interpolated at points which are not distributed equidistantly in time, but the digitizing step varies with time according to a known optional regularity. The spectrum of the obtained function is computed for various of these interpolations and tests are run to determine when the interpolation best compensates the time variation of the frequency. The initial and terminal frequency in the given sample is then determined. The usability of this method with respect to various types of oscillations with a variable frequency and its accuracy in comparison to sonagrams are discussed.     

Improved microseismic event location by inclusion of a priori dip particle motion: a case study from Ekofisk

Microseismic monitoring in petroleum settings provides insights into induced and naturally occurring stress changes. Such data are commonly acquired using an array of sensors in a borehole, providing measures of arrival times and polarizations. Events are located using 1D velocity models, P‐ and S‐wave arrival times and the azimuths of P‐wave particle motions. However in the case of all the sensors being deployed in a vertical or near‐vertical borehole, such analysis leads to an inherent 180° ambiguity in the source location. Here we present a location procedure that removes this ambiguity by using the dip of the particle motion as an a priori information to constrain the initial source location. The new procedure is demonstrated with a dataset acquired during hydraulic fracture stimulation, where we know which side of the monitoring well the events are located. Using a 5‐step location procedure, we then reinvestigate a microseismic data set acquired in April 1997 at the Ekofisk oilfield in the North Sea. Traveltimes for 2683 candidate events are manually picked. A noise‐weighted analytic‐signal polarization analysis is used to estimate the dip and azimuth of P‐wave particle motions. A modified t‐test is used to statistically assess the reliability of event location. As a result, 1462 events are located but 627 are deemed to be statistically reliable. The application of a hierarchal cluster analysis highlights coherent structures that cluster around wells and inferred faults. Most events cluster at a depth of roughly 3km in the Ekofisk chalk formation but very little seismicity is observed from the underlying Tor chalk formation, which is separated from the Ekofisk formation by an impermeable layer. We see no evidence for seismicity in the overburden but such events may be too distant to detect. The resulting picture of microseismicity at Ekofisk is very different from those presented in previous studies.     

Constrained tomography of realistic velocity models in microseismic monitoring using calibration shots

The knowledge of the velocity model in microseismic jobs is critical to achieving statistically reliable microseismic event locations. The design of microseismic networks and the limited sources for calibration do not allow for a full tomographic inversion. We propose optimizing a priori velocity models using a few active shots and a non‐linear inversion, suitable to poorly constrained systems. The considered models can be described by several layers with different P‐ and S‐wave velocities. The velocities may be constant or have 3D gradients; the layer interfaces may be simple dipping planes or more complex 3D surfaces. In this process the P‐ and S‐ wave arrival times and polarizations measured on the seismograms constitute the observed data set. They are used to estimate two misfit functions: i) one based on the measurement residuals and ii) one based on the inaccuracy of the source relocation. These two functions are minimized thanks to a simulated annealing scheme, which decreases the risk of converging to a local solution within the velocity model. The case study used to illustrate this methodology highlights the ability of this technique to constrain a velocity model with dipping layers. This was performed by jointly using sixteen perforation shots recorded during a multi‐stage fracturing operation from a single string of 3C‐receivers. This decreased the location inaccuracies and the residuals by a factor of six. In addition, the retrieved layer dip was consistent with the pseudo‐horizontal trajectories of the wells and the background information provided by the customer. Finally, the theoretical position of each calibration shot was contained in the uncertainty domain of the relocation of each shot. In contrast, single‐stage inversions provided different velocity models that were neither consistent between each other nor with the well trajectories. This example showed that it is essential to perform a multi‐stage inversion to derive a better updated velocity model.