MULTI-SCALE DECOMPOSITION OF BOUGUER GRAVITY ANOMALY AND SEISMIC ACTIVITY IN NORTH CHINA

1　IntroductionThecrustalstructureofNorthChinahasbeenstudiedalotwithgravitymethod[1 ,2 ] .Theresultsareundoubtedlyofgreatsignificancetoabetterun derstandingofthecrustalstructureofthisregion .Butsofar,neithervertical/lateraldistributionscalesnorcharacteristicsofanomalousdensitybodiesofrockmediaintheregionhavebeenrevealedsystem aticallyintermsofrockdensity .Assurfacegravityanomalyreflectstheoveralleffectofgravityanoma liesofsubsurfaceanomalousgeologicalbodiesatdif ferentdepths,ondifferentscales…     

Multiresolution wavelets and natural time analysis before the January–February 2014 Cephalonia (Mw6.1 & 6.0) sequence of strong earthquake events

On January 26 and February 3, 2014, Cephalonia Island (Ionian Sea, Greece) was struck by two strong, shallow earthquakes (moment magnitudes Mw6.1 and Mw6.0, respectively) that ruptured two sub-parallel, strike-slip faults, with right-lateral kinematics. The scope of the present work is to investigate the complex correlations of the earthquake activity that preceded the Mw6.1 event in the broader area of the Cephalonia Island and identify possible indications of critical stages in the evolution of the earthquake generation process. We apply the recently introduced methods of Multiresolution Wavelet Analysis (MRWA) and Natural Time (NT) analysis and for the first time we combine their results in a joint approach that may lead to universal principles in describing the evolution of the earthquake activity as it approaches a major event. In particular, the initial application of MRWA on the inter-event time series indicates a time marker 12 days prior to the major event. By using this time as the initiation point of the NT analysis, the critical stage of seismicity, where the κ₁ parameter reaches the critical value of κ₁ = 0.070, is approached few days before the occurrence of the Mw6.1 earthquake.     

A wavelet thresholding technique for local geoid and deflection of the vertical determination using a planar approximation

A computational scheme using the wavelet transform is employed for local geoid determination, where wavelet multiresolution analysis (MRA) is introduced as an alternative approach to the well-established fast Fourier transform (FFT). The Stokes and Vening Meinesz integrals are approximated in finite MRA subspaces. The algorithm is built using an orthogonal wavelet base function. The characteristics of the base function and its effect on the final result are investigated. Hard and soft thresholding are tested in the compression of the kernel as well as global thresholding is compared to level-wise thresholding to optimize the compression level with an acceptable accuracy. Both global and level-wise thresholding are combined in order to achieve the maximum compression level, with acceptable geoid accuracy. The compression rate depends on the degree of singularity of the kernel. In the case of Stokes, a 94 per cent compression level is achieved with 1 cm (rms) accuracy in comparison to FFT and numerical integration approaches. Due to its stronger singularity, in the case of the Vening Meinesz kernel, 97 per cent compression rate is achieved with a 0.07 arc-second (rms) accuracy. The compression percentages achieved in this study are higher than those reported in pervious studies, which shows that this algorithm is very suitable for use in local geoid determination.     

Localization of gravity and topography: constraints on the tectonics and mantle dynamics of Venus

We develop a method for spatio-spectral localization of harmonic data on a sphere and use it to interpret recent high-resolution global estimates of the gravity and topography of Venus in the context of geodynamical models. Our approach applies equally to the simple spatial windowing of harmonic data and to variable-length-scale analyses, which are analogous to a wavelet transform in the Cartesian domain. Using the variable-length-scale approach, we calculate the localized RMS amplitudes of gravity and topography, as well as the spectral admittance between the two fields, as functions of position and wavelength. The observed admittances over 10 per cent of the surface of Venus (highland plateaus and tessera regions) are consistent with isostatic compensation of topography by variations in crustal thickness, while admittances over the remaining 90 per cent of the surface (rises, plains and lowlands) indicate that long-wavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere. The global average crustal thickness is less than 30 km, but can reach values as large as 40 km beneath tesserae and highland plateaus. We also note that an Earth-like radial viscosity structure cannot be rejected by the gravity and topography data and that, without a mechanical model of the lithosphere, admittance values cannot constrain the thickness of the thermal boundary layer of Venus. Modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae, the effective elastic plate thickness, T_e , was less than 20 km. Estimates of T_e at highland rises are consistently less than 30 km. Our inability to find regions with T_e > 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models.     

小波神经网络的大地电磁数据静态效应处理

静态效应在频率域电法勘探中对电磁场信号产生很大的影响,对深部电性结构处理结果影响重大,对静态效应的良好处理直接影响分析结果。这里采用了小波神经网络算法建立模型来对静态效应进行处理,通过对模型实例仿真发现,小波神经网络算法对静态效应的处理有很好的效果。     

1961-2011年甘肃雷暴气候分布特征及变化趋势

利用1961-2011年甘肃省63个测站的雷暴观测资料,采用气候倾向率趋势系数和小波分析方法,研究了甘肃省雷暴气候变化特征。结果表明:年平均雷暴日数呈东北-西南走向,东北少、西南多,全省平均为24 d,有3个雷暴高发中心,地形是影响甘肃雷暴空间分布的主要因子。近51年中,1961-1990年为多雷暴期,1991-2011年为少雷暴期,雷暴日数总体呈减少趋势,其中甘南高原减少速率最快,每10年约减少4 d,这可能与对流有效位能和700 hPa相对湿度的下降有关。在春、夏、秋三季中,夏季雷暴日数减少的趋势最为明显,每10年减少3.4 d,尤以6月最甚。初、终雷暴日的地区差异较大,甘南高原初雷暴日出现最早,终雷暴日结束最晚,河西走廊初雷暴日出现最晚,终雷暴日结束最早,因此甘南高原是甘肃雷暴期最长的地区,达204 d,河西走廊则最短,为105 d。近51年来甘肃雷暴期的缩短主要是初雷暴日的显著推后和终雷暴日的提前所致。小波分析表明甘肃雷暴日数存在24 a和8 a的周期震荡,当前雷暴的发生正处在一个偏少的周期内。     

多尺度数字高程模型

以岩土工程边坡数值模拟仿真可视化为研究对象,分析、研究了数字高程模型(DEM)和样条函数。将非均匀B样条曲面拟合和准均匀B样条小波多尺度分解结合起来,实现多尺度DEM地形建模方法,设计了相应的算法。实验结果表明,其建模造型效果逼真,多尺度分辨率明显,是一种高效实用的建模方法。     

构造高阶B-样条正交小波的一个递推算法

Ｂ－样条正交小波在信号分析方面有着重要的应用。由于Ｂ－样条正交小波随着阶次的升高将导致频域内支集缩小，因而分辨信号不同频率成分的能力将大大增强。因此，研究高阶Ｂ－样条正交小波的简便构造方法很有实际意义。本文独立地给出了一个构造高阶Ｂ－样条正交小波的适于计算机数值计算的递推算法，并给出了构造１１阶以下Ｂ－样条正交小波所需的系数，以供读者利用。同时还给出了一个在频域内计算脉冲响应系数的简便方法。     

Regional gravity modeling in terms of spherical base functions

This article provides a survey on modern methods of regional gravity field modeling on the sphere. Starting with the classical theory of spherical harmonics, we outline the transition towards space-localizing methods such as spherical splines and wavelets. Special emphasis is given to the relations among these methods, which all involve radial base functions. Moreover, we provide extensive applications of these methods and numerical results from real space-borne data of recent satellite gravity missions, namely the Challenging Minisatellite Payload (CHAMP) and the Gravity Recovery and Climate Experiment (GRACE). We also derive high-resolution gravity field models by effectively combining space-borne and surface measurements using a new weighted level-combination concept. In addition, we outline and apply a strategy for constructing spatio-temporal fields from regional data sets spanning different observation periods.