帕克菲尔德地震预报实验场:2004年6级地震及其对地震物理和地震预测研究的影响

介绍了美国中加州帕克菲尔德(Parkfield)地震预报实验场的建设、运行和试验场工作期间发生的2004年6级地震的情况.着重阐述了在实验场20多年的地震预测研究中遇到的一系列科学同题以及一些新的认识.这些科学认识包括:对活动构造破裂分段的研究是地震长期预测的基础;无震滑动是地震预测中的一大难点和障碍;地震复发模型具有的相当大的不确定性且在预测应用上有很大的局限性;地震前兆的复杂性、差异性及其对地震预测中常用原则--前兆异常重现性和相似性的挑战等.这些认识对地震预测研究与实验场建设有一定的启示意义.     

地震预测研究的几点建议

地震预测研究面临低水平的预测与政府和公众高期望值的矛盾,认为加强机理研究是根本出路之一。从震源物理和地震前兆研究的现状、特点、研究的方法等方面进行阐述,明确需要进一步研究和关注的问题,并提出几点建议。应找出新思路,发展适合我国大陆地质条件的震源物理理论和方法。     

地震震源的前兆系统和不均匀性及非线性对系统的影响

研究了地震震源前兆系统的几个特点。震源和前兆是在应力应变场不同分量作用下发育成的。不均匀性和非线性因素对震源和前兆有强烈的影响。讨论了介质和应力应变场不均匀性对地震震源形成和地震前兆影响的差别。在不同的应力场中,前兆的非线性关系首先在应变灵敏性上显示出来。应变灵敏性的不完备导致了“错误预测”和“漏报”这类预测错误。由地震前兆形成的条件可以得出复杂的前兆分布以及前兆相对震中转移规律的结论。研究了前兆信号曲线组成的几个特点,这些特点可以使地震短期前兆的区分更有根据。     

地震震游的前兆系统和不均匀性及非线性对系统的影响

研究了地震震源前兆系统的几个特点。震源和前兆是在应力应变场不同分量作用下发育成的。不均匀性和非线性因素对震源和前兆有强烈的影响。讨论了介质和应力应变场不均匀性对地震震源形成和地震前兆影响的差别。在不同的应力场中，前兆的非线性关系首先在应变灵敏性上显示出来。应变灵敏性的不完备导致了“错误预测”和“漏报”这类预测错误。由地震前兆形成的条件可以得出复杂的前兆分布以及前兆相对震中转移规律的结论。研究了前兆信号曲线组成的几个特点，这些特点可以使地震短期前兆的区分更有根据。     

地震短期预测方法物理机理的研究途径

通过实验室研究与野外观测实验分析，可以揭示典型短期前兆现象的动力机制和响应条件。基于特殊条件岩石力学实验结果，给出典型地震活动图像产生的条件及其物理机制。进行多种类型、不同尺度动力作用的野外试验，对流体前兆成因过程的机理进行揭示和解释。进行地电阻率、地电场的典型观测研究，建立异常识别的综合分析方法等。这些研究工作是探索地震短期预测方法物理机理的有效途径。     

《地震》征稿简则

<正>《地震》是由中国地震局地震预测研究所、中国地震学会地震预报专业委员会、中国地震学会地震流体专业委员会、中国地震学会地震电磁学专业委员会联合主办的学术性期刊。《地震》的办刊宗旨是交流地震观测、地震前兆和强震机理与预测等研究成果,推动地震预报探索研究及其成果在防震减灾中的应用。读者对象主要是从事地震观测分析、地震前兆探索、地震机理与预测研究的广大地震科技工作者和相关科     

地震短期预测的概率方法浅议

和天气预报相比,地震预测,特别是地震的短临预测更为困难。概率方法真实地反映了地震发生的随机性,真实地反映了人类对于地震认识的局限性,加快短期预测的概率方法研究与应用是十分必要的。概率预测是经验预测、统计预测和物理预测的综合,各类地震预测方法特别是物理预测方法的发展还很不充分,目前仍处于探索阶段。从目前我国地震预测的现状来看,应大力发展统计预测方法。地震目录可为我们提供对背景地震发生率的估计,而前兆的出现使我们可以得到地震发生率大大高于背景发生率的时段,其增益越强,时段越短,越接近于理想预测。前兆和地震之间的关系的统计检验对于概率预测起着十分重要的作用。该文提出,应按实用化的要求逐步规范各种地震预测方法。这对地震预测研究的健康发展是至关重要的。     

四川九寨沟7.0级地震中长期预测的回顾认识

回顾了2013年郭增建提出的基于地震活动性的"静中动"和后期与作者共同发展的"准静中动"方法,以1987年1月8日在甘肃南部迭部县发生的5.9级地震作为标志性的"准静中动"地震,结合国家地震局在1989年综合划定的1990—2000年南北地震带10年地震危险区,用中长期预测的时间尺度,指出了南北地震带北段三个未来可能发生6~7级地震的地区,其中一个就是四川省九寨沟地区。回顾分析中对"静中动"方法预测效能、存在的问题及一些中长期预测的问题进行讨论,得到两点认识:一是通过这次地震的再次验证,证明"静中动"方法是一种可行的预测地震的独立指标方法,在验证的基础上对该方法进行了优化认识;二是基于"静中动"方法与南北地震带北段10年地震危险区对应地震较为准确的启发,初步认为前兆可以划分为平静期的前兆与活跃期的前兆,而前者可能对于地震的预测意义更大,未来可以沿这一思路开展进一步的论证和研究。     

苏联地震预报研究及其实验场的发展概述

苏联是一个多地震的国家,也是开展地震预报探索研究较早的国家之一。近20年来取得了一些明显的进展,并已在本国的地震学领域中占有一定的地位。苏联对地震预测预报的探索工作可归纳为二十年代、五十年代、六十年代至今的三个阶段,每个阶段都把测震学和地震前兆的观测作为重要内容。从第二阶段探索工作起,苏联便采取了实验场的方式,在中亚塔吉克共和国的加尔姆地区建立了第一个实验场,并开展了较大规模的综合观测。特别是近年来固体物理(首先是断裂物理和力学)取得的成就以及电子技术在地球物理实验中的应用,为地震预报研究奠定了基础。本文仅根据所收集到的资料,对苏联地震活动区域分布和地震预报的历史,苏联地震预报体系与实验场,特别是中亚地区的实验场情况,以及地震预报研究实验场的主要成果和经验以及预报实例等方面,进行了概述。     

本刊已加入期刊网、数据库的声明

《地震》是由中国地震局地震预测研究所、中国地震学会地震预报专业委员会、中国地震学会地震流体专业委员会、中国地震学会地震电磁学专业委员会联合主办的学术性期刊。《地震》刊物的办刊宗旨是以交流地震观测、地震前兆和强震机理与预测等研究成果,推动地震预报探索研究及其成果在防震减灾中的应用。读者对象主要以从事地震观测分析、地震前兆探索、地震机理与预测研究的广大地震科技工作者和相关科技领域的科技人员。     

A Monte Carlo study of rainfall forecasting with a stochastic model

A procedure for short-term rainfall forecasting in real-time is developed and a study of the role of sampling on forecast ability is conducted. Ground level rainfall fields are forecasted using a stochastic space-time rainfall model in state-space form. Updating of the rainfall field in real-time is accomplished using a distributed parameter Kalman filter to optimally combine measurement information and forecast model estimates. The influence of sampling density on forecast accuracy is evaluated using a series of a simulated rainfall events generated with the same stochastic rainfall model. Sampling was conducted at five different network spatial densities. The results quantify the influence of sampling network density on real-time rainfall field forecasting. Statistical analyses of the rainfall field residuals illustrate improvement in one hour lead time forecasts at higher measurement densities.     

Earthquake Engineering and Engineering Vibration Aims And Scope

正This journal is established by the Institute of Engineering Mechanics(IEM),China Earthquake Administration,to promote scientific exchange between Chinese and foreign scientists and engineers so as to improve the theory and practice of earthquake hazards mitigation,preparedness,and recovery.To accomplish this purpose,the journal aims to attract a balanced number of papers between Chinese and     

Solute concentrations in the lower niger river and the source rock contribution

Water quality analyses for the Niger River for the 1980/81 hydrological year are presented. The samples were collected from the main river at Lokoja, and from two main tributaries, the Kaduna and the Benue Rivers. Different water types were distinguished by the concentrations of major ions. The type Ca > Na > Mg > K - HCO₃ > SO₄ > Cl was represented at all stations during at least part of the year. Chloride was found to dominate the sulphate ion in the Kaduna and Niger, while the Benue maintained a higher concentration of sulphate relative to chloride all year round. Distinct patterns of seasonal variation in the ion concentrations were observed, particularly for the samples collected at Lokoja. Low ion concentrations were prominent during periods of high discharge, while low flow periods coincided with high dissolved ion concentrations. The contribution of rainwater to the total dissolved solids in the river waters was assessed indirectly using rainwater chemistry data from the Gulf of Guinea. The estimated rainwater contribution to the Lower Niger amounts to 5.15 mg 1^?1. Geochemical weathering calculations involving reactions of the four major minerals of granitic rocks - anorthite, biotite, albite, and K-feldspar - with carbon dioxide and water, can account for the average water composition of the Lower Niger. The proportion of the ionic components was also related to the occurrence of the respective element in the minerals.     

Scenario predictions for potential near-field and far-field earthquakes affecting Hong Kong

Historical earthquakes noted in the written records of the South China region, including Hong Kong, are not well delineated along identified prominent fault sources. Despite the lack of any definitive, localised trend in the spatial distribution of seismic activity in the region, there does appear to be some major disparity in the seismic activity rates (especially for large magnitude earthquakes) between the near-field and the far-field regions of Hong Kong. Despite this observation, previous studies of the regional seismic activity and seismic ground motion hazard (the latter using a probabilistic seismic hazard assessment, PSHA) have considered very broad source zone regions, in which uniform levels of seismic activity have been assumed. The present paper further scrutinises this broad source zone (BSZ) approach by adopting a novel expanding circular disc (ECD) method to determine the rates of earthquake recurrence. Such a method is intended to counter-check previously developed models by determining earthquake scenario events in terms of magnitude–distance (M–R) pairs or combinations, having defined values of average return period. Unlike the BSZ approach, the ECD method specifically accounts for the supposed variations in the seismic activity rates between events in the near-field and the far-field of Hong Kong.The form of the developed method is particularly suited to the determination of design-level earthquake ground motions for bedrock sites, since it assumes a directionally-independent attenuation model as described in the companion paper. It is found that, whilst the BSZ approach may indicate the overall average levels of hazard that are representative of the South China region as a whole, it does not capture the large disparity in seismic activity rates between near-field and far-field events. This important feature is expected to have a significant impact upon engineering assessments of the seismic safety of structures in Hong Kong and elsewhere in the South China region. For example, it is found that for events with M≥6, the seismic activity rate (normalised by time and area) in the very far-field is around 3.5 times larger than in the near-field and medium-field of Hong Kong. The resulting design M-R combinations, covering a range of return periods from 70 to 2500 years, are limited, for very long return periods and for distant events, by the maximum credible earthquake (MCE) magnitude. Intensive research to determine this seismic hazard parameter is recommended, in order to refine further the results of the ECD analysis, which presently conservatively assumes the MCE to range between M=6 in the near-field of Hong Kong to M=8 in the very far-field, at distances greater than 280 km from Hong Kong.     

Depositional ages and characteristics of Middle–Upper Jurassic and Lower Cretaceous lacustrine deposits in southeastern Mongolia

Lower Cretaceous lacustrine oil shales are widely distributed in southeastern Mongolia. Due to the high organic carbon content of oil shale, many geochemical studies and petroleum exploration have been conducted. Although most of the oil shales are considered to be Early Cretaceous in age, a recent study reveals that some were deposited in the Middle Jurassic. The present study aims at establishing depositional ages and characteristics of the Jurassic and Cretaceous lacustrine deposits in Mongolia. The Lower Cretaceous Shinekhudag Formation is about 250 m thick and composed of alternating beds of shale and dolomite. The Middle Jurassic Eedemt Formation is about 150 m thick and composed of alternating beds of shale, dolomitic marl, and siltstone. The alternations of shale and dolomite in both formations were formed by lake level changes, reflecting precipitation changes. Shales were deposited in the center of a deep lake during highstand, while dolomites were formed by primary precipitation during lowstand. Based on the radiometric age dating, the Shinekhudag Formation was deposited between 123.8 ±2.0 Ma and 118.5 ±0.9 Ma of the early Aptian. The Eedemt Formation was deposited at around 165–158 Ma of Callovian–Oxfordian. The calculated sedimentation rate of the Shinekhudag Formation is between 4.7 ±2.6 cm/ky and 10.0 ±7.6 cm/ky. Shales in the Shinekhudag Formation show micrometer‐scale lamination, consisting of algal organic matter and detrital clay mineral couplets. Given the average thickness of micro‐laminae and calculated sedimentation rate, the micro‐lamination is most likely of varve origin. Both Middle–Upper Jurassic and Lower Cretaceous lacustrine oil shales were deposited in intracontinental basins in the paleo‐Asian continent. Tectonic processes and basin evolution basically controlled the deposition of these oil shales. In addition, enhanced precipitation under humid climate during the early Aptian and the Callovian–Oxfordian was another key factor inducing the widespread oil shale deposition in Mongolia.     

THEORETICAL APPROACH TO THE SOLUTION OF THE INFILTRATION PROBLEM

ABSTRACT

The one-dimensional transient downward entry of water in unsaturated soils is investigated theoretically. The mathematical equation describing the infiltration process is derived by combining Darcy's dynamic equation of motion with the continuity and thermodynamic state equations adjusted for the unsaturated flow conditions. The resulting equation together with the corresponding initial and boundary conditions constitues a mathematical initial boundary value problem requiring the solution of a nonlinear partial differential equation of the parabolic type. The volumetric water content is taken as the dependent variable and the time and the position along the vertical direction are taken as the independent variables. The governing equation is of such nature that a solution exists for t > 0 and is uniquely determined if two relationships are defined, together with the specified state of the system, at the initial time t = 0 and at the two boundaries. The two required relations are those of pressure versus permeability and pressure versus volumetric water content.

Since the partial differential equation has strong non-linear terms, a discrete solution is obtained by approximating the derivatives with finite-differences at discrete mesh points in the solution domain and integrated for the corresponding initial and boundary conditions. The use of an implicit difference scheme is employed in order to generate a system of simultaneous non-linear equations that has to be solved for each time increment. For n mesh points the two boundary conditions provide two equations and the repetition of the recurrence formula provides n—2 equations, the total being n equations for each time increment. The solution of the system is obtained by matrix inversion and particularly with a back-substitution technique. The FORTRAN statements used for obtaining the solution with an electronic digital computer (IBM 704) are presented together with the input data.

Analysis of the errors involved in the numerical solution is made and the stability and convergence of the solution of the approximate difference equation to that of the differential equation is investigated. The method applied is that of making a Fourier series expansion of a whole line of errors and then following the progress of the general term of the series expansion and also the behavior of each constituent harmonic. The errors (forming a continuous function of points in an abstract Banach space) are represented by vectors with the Fourier coefficients constituting a second Banach space. The amplification factor of the difference equation is shown to be always less than unity which guarantees the stability of the employed implicit recurrence scheme.

Experiments conducted on a vertical column packed uniformly with very fine sand, show a satisfactory agreement between the theoretically and experimentally obtained values. Many experimental results are shown in an attempt to explain the infiltration phenomenon with emphasis on the shape and movement of the wet front, and the effects of the degree of compaction, initial water content and deaired water on the infiltration rate.     

Late Permian post‐ophiolitic trondhjemites from Central Iran: a mark of subduction role in growth of Paleozoic continental crust

Late Permian trondhjemites in the Anarak area occur as stocks and dykes, which cross cut the Anarak ophiolite and its overlying metasedimentary rocks, and are exposed along the northern Anarak east–west main faults. These leucocratic intrusive bodies have enclaves of all ophiolitic units and metamorphic rocks. They are composed of amphibole, plagioclase (oligoclase), quartz, zircon and muscovite. Secondary minerals are chlorite (pycnochlorite), epidote, albite, magnetite and calcite. Whole‐rock major‐ and trace‐element analyses reveal that they are characterized by high SiO ₂ (67.8–71.0 wt%), Al ₂ O ₃ (14.9–17.1 wt%) and Na ₂ O (5.3–8.6 wt%), low K ₂ O (0.1–1.5 wt%; average: 0.8 wt%), low Rb/Sr ratio (0.01–0.40; average: 0.09), low Y (3–6 ppm), negative Ti, Nb and Ta anomalies, slightly negative or positive Eu anomaly, LREE enrichment and fractionated HREE. These rocks present 2 to 40 times enrichment in inclined chondrite‐normalized REE patterns. Geochemical characteristics of the Anarak trondhjemites all reflect melting of a mafic protolith at more than 10 kbar. The field evidence and whole‐rock chemistry reveal that these rocks have been crystallized from magmas derived from melting of subducted Anarak oceanic crust. This study reveals that melting of garnet amphibolite was an important element of continent formation in the study area.