三维复杂速度模型的交切法地震定位

地震定位是地震监测与减灾研究重要基础.基于均匀或横向均匀介质模型,利用震源轨迹确定震源位置的交切法具有稳健和效率高的优点,但定位精度较低,特别是震源深度.为提高震源定位精度,我们提出适用于三维复杂速度模型的地震定位交切法.将地壳速度模型由均匀或横向均匀介质模型扩展为三维复杂速度模型;均匀或横向均匀介质模型对应的原假设为球面或双曲面的震源轨迹通过最小走时树射线追踪技术予以确定.确定震源位置的震源轨迹以到时差作为约束条件;将震源定位于震源轨迹交汇最密集的点处,即总的到时差残差(RDT)最小的点处.定位结果的不确定性可通过RDT值较小节点的空间分布予以定性表示.考察了准确速度模型、扰动速度模型、扰动观测到时及地震在台网外等4种情况下改进方法的地震定位效果,结果表明改进的交切法可用于三维复杂速度模型的地震定位;综合利用P波与S波的到时差信息,可明显改善震源位置约束;使用多条震源轨迹进行定位,有助于减少由随机因素导致的定位误差.     

三维复杂速度模型中地震事件震源轨迹的计算

地球内部三维速度图像的广泛建立为进行高精度的地震定位提供了良好条件.使用震源轨迹确定震源位置不仅稳健而且直观,但三维复杂速度模型中的震源轨迹难以给出解析解.为此,本文提出了一种较准确地计算三维复杂速度模型中震源轨迹的数值方法.根据震源轨迹在残差场中的特点：（1）震源轨迹位于残差正负极性彼此不同的邻点之间;（2）绝对梯度在震源轨迹的法线方向最大;（3）在法线方向上越靠近震源轨迹残差绝对值越小,对于每个模型节点分别和残差正负极性与其不同的邻点组成的点对,将其中绝对梯度最大的点对作为震源轨迹法线点对,选取法线点对中残差绝对值较小的点（即震源轨迹所在模型单元的节点）作为震源轨迹代表点;在绝对残差场中数值较小的连通区域（可能有多个）内,利用最小走时树算法依次计算出每个连通区域中地震波从绝对残差最小点至同一连通区域内震源轨迹代表点的射线路径作为震源轨迹.算例表明：本文方法适用于三维复杂速度模型,对震源轨迹的稳定性及构成段数没有限制,计算的震源轨迹精细且较完整、可用于高精度的地震定位.     

利用DE算法反演地壳速度模型和地震定位

利用差异演化（Differential Evolution）非线性全局优化算法，设计了一种反演地壳速度模型和进行地震定位的方法，并给出了反演结果的具体分析.利用有限差分算法计算速度模型的走时场，可以节省大量的计算量，加快计算过程.反演得到的地壳速度模型和地震的震源参数可以直接用于地震层析成像研究，还可以利用地震层析成像得到的三维速度结构对地震重新定位，从而得到较为精确的震源参数.地壳速度模型的反演方法也可以用于三维速度结构的反演.     

复杂速度模型的地震交切定位方法

交切法是最基本的地震定位方法之一,该定位方法具有稳定性强、速度快的优点。由于速度模型假设为均匀或水平均匀介质,与实际介质有较大偏差,因此定位精度较低,特别是震源深度。为克服传统交切法的缺点,我们对其进行了改进。在改进的方法中,震源轨迹不再假设为圆形或双曲线形,而是利用最小走时树射线追踪方法进行精确计算。数值模型计算表明,本文提出的改进方法,可用于复杂速度模型的地震定位。     

一种基于插值技术高精度计算稀疏网格地震定位中震源轨迹的方法

区域和全球地震定位越来越多地基于更接近实际的横向非均匀速度模型.速度模型主要来自于地震体波层析成像结果,分辨率不是很高.这样,模型宜于以稀疏网格剖分以减少计算时间和计算机内存需求.当剖分的模型单元较大时,基于射线追踪技术计算复杂介质地震定位中震源轨迹的方法——选取震源轨迹所经过单元节点(位于单元中心,称为震源轨迹节点)为轨迹参考点,利用最小走时树射线追踪方法计算绝对残差场中连接轨迹参考点的射线路径作为震源轨迹——计算结果误差较大,难以满足精定位需要.针对该问题,本文对其进行了改进:不将震源轨迹节点作为轨迹参考点,而是基于插值技术计算每个轨迹节点其法线点对(即该节点与其周围残差正负极性不同的相邻节点组成的点对中梯度绝对值最大的那对)间残差为零的点作为震源轨迹参考点.算例表明:和原方法相比,改进方法计算的震源轨迹更为精细,计算精度提高数十(线性插值)至数百倍(非线性插值),而计算效率基本保持在同一数量级,使利用震源轨迹进行直观、快速和高精度的区域或全球地震事件定位成为可能;壳幔界面反射纵波(PmP)对震源的约束和直达纵波(Pg)相似;同一台站PmP-Pg波到时差约束的震源轨迹对震源深度有很好的约束.     

用于地震预警的三维实时定位方法

为了尽快定位地震,在“着未着”定位算法基础上,尝试引入三维地壳模型,构建具有三维空间格点分布的走时表.根据已触发台站的到时和未触发台站的位置信息设计概率分布函数,通过八叉树搜索方法,快速给出震源在三维空间的可能位置.使用波前追踪算法,计算中国几个地区三维地壳模型的走时网格,利用中国地震台网资料,对区域内发生的地震进行定位分析.结果显示,在一定的台网密度条件下,三维实时定位方法能在震后数秒给出震源位置,可满足地震预警要求.     

复杂介质地震定位中震源轨迹的计算

在地震定位中常常需要求解震源轨迹,但由于复杂介质中的震源轨迹较为复杂,难以给出其解析解,因此震源轨迹的计算通常仅限于简单介质模型.本文基于最小走时树射线追踪技术,提出了一种计算复杂介质中震源轨迹的方法.为回避发震时间问题,以观测到时差作为震源轨迹的约束条件.首先从模型节点中选出少量理论到时差与观测到时差之绝对差,即双重时差较小的点作为震源轨迹的代表点,然后以其中双重时差最小的点为初始点,在双重时差场中利用最小走时树射线追踪方法计算出初始点到其他震源轨迹代表点的射线路径作为震源轨迹.当选的震源轨迹代表点较多时,得到的震源轨迹较为粗略,此时可去掉射线经过次数较少的代表点的射线路径使震源轨迹更为精细.为减少计算量,对最小走时树射线追踪方法的终止条件做了修正.以一个复杂介质模型中的地震为例,计算了包括速度扰动、到时扰动等不同情况下的震源轨迹,结果表明所提出的震源轨迹计算方法切实可行.     

利用NLLoc方法对新疆精河地震的定位研究

利用非线性定位方法NLLoc,对新疆精河地区2017年8月9日至9月30日期间M≥3.0级的22个地震进行重新定位,并与中国地震台网中心正式目录结果进行对比,讨论了NLLoc方法在速度模型复杂的新疆地区定位的可行性.研究结果表明,NLLoc方法重定位新疆精河地震事件中,在发震时刻、震中方面误差较小,满足定位精度要求,但深度值上相差较大.NLLoc方法定位结果相对较为集中,主要分布于14～21 km,与CAP反演的深度分布为12-21 km,平均深度17 km,主震的震源深度21 km结果相符合,且与新疆地区的平均震源深度21±10 km和北天山地震带的平均震源深度19 km的结论相符.结果显示:NLLoc方法可用于地壳速度结构变化大、分布复杂地区的新疆地震定位,且有助于更好地解决震源深度测定问题,提高地震定位精度.     

地震干涉技术的三维震源定位方法

快速、精确地进行震源定位是地震学的基础性研究课题之一.为了满足目前相关地震工作的需要,找到一种与实际情况相适应的有效震源定位方法具有切实意义.本文研究了干涉成像方法,根据地下介质构造以及对应传播速度,利用干涉的方法对震源位置进行成像.这种算法对地震记录的要求较低,不需要精确的初至时间,适应于多震源延时激发且信噪比较低的微震定位.基于二维干涉定位我们得到了三维干涉定位方法,确定了对应的成像理论.首先在二维水平层状介质模型中测试,定位结果趋近模拟位置.之后建立了三维均匀介质模型,进行三维弹性波场的数值模拟,对接收到的数据进行互相关运算,利用结果进行干涉成像,可以得到较为精确的水平位置,再结合二维模型中的方法确定深度,定位结果与模拟震源相符.最后根据三维模型下的微震数据同时定位出多个微震震源,证实了算法在微震定位中切实有效.     

区域地壳速度模型对准确测定地震参数的重要性研究

研究区域地壳速度模型是地震学研究中的经典问题,而震中位置和震源深度的准确确定与地壳速度模型密切相关。论述了中国各区域测震台网速度模型的现状,并分别以福建和内蒙古地壳速度模型的地震定位结果为例,说明合适的地壳速度模型对于准确测定地震参数的重要性。     

Earthquake location in transversely isotropic media with a tilted symmetry axis

The conventional intersection method for earthquake location in isotropic media is developed in the case of transversely isotropic media with a tilted symmetry axis (TTI media). The hypocenter is determined using its loci, which are calculated through a minimum travel time tree algorithm for ray tracing in TTI media. There are no restrictions on the structural complexity of the model or on the anisotropy strength of the medium. The location method is validated by its application to determine the hypocenter and origin time of an event in a complex TTI structure, in accordance with four hypotheses or study cases: (a) accurate model and arrival times, (b) perturbed model with randomly variable elastic parameter, (c) noisy arrival time data, and (d) incomplete set of observations from the seismic stations. Furthermore, several numerical tests demonstrate that the orientation of the symmetry axis has a significant effect on the hypocenter location when the seismic anisotropy is not very weak. Moreover, if the hypocentral determination is based on an isotropic reference model while the real medium is anisotropic, the resultant location errors can be considerable even though the anisotropy strength does not exceed 6.10%.     

Fast and accurate earthquake location within complex medium using a hybrid globallocal inversion approach

A novel hybrid approach for earthquake location is proposed which uses a combined coarse global search and fine local inversion with a minimum search routine, plus an examination of the root mean squares (RMS) error distribution. The method exploits the advantages of network ray tracing and robust formulation of the Fréchet derivatives to simultaneously update all possible initial source parameters around most local minima (including the global minimum) in the solution space, and finally to determine the likely global solution. Several synthetic examples involving a 3-D complex velocity model and a challenging source-receiver layout are used to demonstrate the capability of the newly-developed method. This new global-local hybrid solution technique not only incorporates the significant benefits of our recently published hypocenter determination procedure for multiple earthquake parameters, but also offers the attractive features of global optimal searching in the RMS travel time error distribution. Unlike the traditional global search method, for example, the Monte Carlo approach, where millions of tests have to be done to find the final global solution, the new method only conducts a matrix inversion type local search but does it multiple times simultaneously throughout the model volume to seek a global solution. The search is aided by inspection of the RMS error distribution. Benchmark tests against two popular approaches, the direct grid search method and the oct-tree important sampling method, indicate that the hybrid global-local inversion yields comparable location accuracy and is not sensitive to modest level of noise data, but more importantly it offers two-order of magnitude speed-up in computational effort. Such an improvement, combined with high accuracy, make it a promising hypocenter determination scheme in earthquake early warning, tsunami early warning, rapid hazard assessment and emergency response after strong earthquake occurrence.     

Numerical Green’s function method:Application to quantifying ground motion variations of M7 earthquakes

The concept of "numerical Green’s functions" (NGF or Green’s function database) is developed. The basic idea is: a large seismic fault is divided into subfaults of appropriate size, for which synthetic Green’s functions at the surface (NGF) are calculated and stored. Consequently, ground motions from arbitrary kinematic sources can be simulated, rapidly, for the whole fault or parts of it by superposition. The target fault is a simplified, vertical model of the Newport-Inglewood fault in the Los Angeles basin. This approach and its functionality are illustrated by investigating the variations of ground motions (e.g. peak ground velocity and synthetic seismograms) due to the source complexity. The source complexities are considered with two respects: hypocenter location and slip history. The results show a complex behavior, with dependence of absolute peak ground velocity and their variation on source process directionality, hypocenter location, local structure, and static slip asperity location. We concluded that combining effect due to 3-D structure and finite-source is necessary to quan- tify ground motion characteristics and their variations. Our results will facilitate the earthquake hazard assessment projects.     

基于Pn/Pg相对定位方法研究2017年8月8日九寨沟M7.0地震起始破裂深度

本文利用Pn/Pg相对定位方法,测定了2017年8月8日四川九寨沟M7.0主震及部分余震的起始破裂深度.地震的起始破裂深度是理解地震孕震机理的重要参数,而九寨沟地区台网稀疏,地壳速度结构复杂,基于传统的到时定位方法测定地震起始破裂深度误差较大.Pn/Pg相对定位方法首先基于流动观测近台记录对2017年8月10日M4.1和11月7日M4.5余震震源位置进行测定,选择其为参考事件,再利用Pg校正主震水平位置,Pn约束震源起始深度,基于参考事件可以有效降低速度模型对震源位置测定的影响.结果显示：九寨沟主震起始破裂深度约9 km,早期余震的震源深度分布在7~13 km,主要集中在主震起始破裂深度附近.     

Regional minimum 1-D P-wave velocity model for a new seismicity catalogue with precise and consistent earthquake locations in southern Iran

For faster and more robust ray tracing in 1-D velocity models and also due to the lack of reliable 3-D models, most seismological centers use 1-D models for routine earthquake locations. In this study, as solution to the coupled hypocenter-velocity problem, we compute a regional P-wave velocity model for southern Iran that can be used for routine earthquake location and also a reference initial model for 3-D seismic tomography. The inversion process was based on travel time data from local earthquakes paired reports obtained by merging the catalogues of Iranian Seismic Center (IRSC, 6422 events) and by the Broadband Iranian National Seismic Network (BIN, 4333 events) for southern Iran in the period 2006 through July 2017. After cleaning the data set from large individual reading errors and by identifying event reports from both networks belonging to same earthquake (a process called event pairing), we obtained a data set of 1115 well-locatable events with a total number of 24,606 P-wave observations. This data set was used to calculate a regional minimum 1-D model for southern Iran as result of an extensive model search by trial-and-error process including several dozens of inversions. Significantly different from previous models, we find a smoothly increasing P-velocity by depth with velocities of 5.8 km/s at shallow and velocities of 6.4 km/s at deepest crustal levels. For well-locatable events, location uncertainties are estimated in the order of ±?3 km for epicenter and double this uncertainty for hypocentral depth. The use of the minimum 1-D model with appropriate station delays in routine hypocenter location processing will yield a high-quality seismic catalogue with consistent uncertainty estimates across the region and it will also allow detection of outlier observations. Based on the two catalogues by IRSC and BIN and using the minimum 1-D model and station delays for all stations in the region, we established a new combined earthquake catalogue for southern Iran. While the general distribution of the seismicity corresponds well with that of the two individual catalogues by IRSC and BIN, the new catalogue significantly enhances the correlation of seismicity with the regional fault systems within and between the major crustal blocks that as an assembly build this continental region. Furthermore, the unified seismic catalogue and the minimum 1-D model resulting from this study provide important ingredients for seismic hazard studies.     

云南腾冲地区三维地震定位

本文利用中国数字测震台网和流动台站的地震资料,基于参数优化的AICD自动拾取算法和质量评估方案得到了高质量的震相到时,并在此基础上使用一维、三维定位方法对腾冲地区的799次地震事件进行了重新定位。定位结果显示:水平方向上,一维、三维重定位结果相差较小;深度方向上,三维定位的震源成丛分布比一维定位结果更加密集,地震主要位于地壳内低速层之上。分别利用一维、三维定位方法对典型地震、人工震源进行定位,结果表明,三维定位的精度明显优于一维定位,其在水平、深度方向上的平均绝对定位误差分别为0.7 km和1.3 km。      

Accurate Location of Synthetic Acoustic Emissions and Location Sensitivity to Relocation Methods, Velocity Perturbations, and Seismic Anisotropy

Acoustic emission (AE) monitoring is a non-invasive method of monitoring fracturing both in situ, and in experimental rock deformation studies. Until recently, the major impediment for imaging brittle failure within a rock mass is the accuracy at which the hypocenters may be located. However, recent advances in the location of regional scale earthquakes have successfully reduced hypocentral uncertainties by an order of magnitude. The least-squares Geiger, master event relocation, and double difference methods have been considered in a series of synthetic experiments which investigate their ability to resolve AE hypocentral locations. The effect of AE hypocenter location accuracy due to seismic velocity perturbations, uncertainty in the first arrival pick, array geometry and the inversion of a seismically anisotropic structure with an isotropic velocity model were tested. Hypocenters determined using the Geiger procedure for a homogeneous, isotropic sample with a known velocity model gave a RMS error for the hypocenter locations of 2.6 mm; in contrast the double difference method is capable of reducing the location error of these hypocenters by an order of magnitude. We test uncertainties in velocity model of up to ±10% and show that the double difference method can attain the same RMS error as using the standard Geiger procedure with a known velocity model. The double difference method is also capable of precise locations even in a 40% anisotropic velocity structure using an isotropic model for location and attains a RMS mislocation error of 2.6 mm that is comparable to a RMS mislocation error produced with an isotropic known velocity model using the Geiger approach. We test the effect of sensor geometry on location accuracy and find that, even when sensors are missing, the double difference method is capable of a 1.43 mm total RMS mislocation compared to 4.58 mm for the Geiger method. The accuracy of automatic picking algorithms used for AE studies is ±0.5 μs (1 time sample when the sampling rate is 0.2 μs). We investigate how AE locations are effected by the accuracy of first arrival picking by randomly delaying the actual first arrival by up to 5 time samples. We find that even when noise levels are set to 5 time samples the double difference method successfully relocates the synthetic AE.