A Preliminary Discussion on the Mechanism of Abnormal Intensity Zones of the M5.1 Wen'an Earthquake in 2006

An earthquake measuring 5.1 on the Richter scale struck Wen'an county,Hebei Province on July 4,2006. No casualties have been reported,with only slight damage. The disaster level of this event is the slightest on the M4.9～5.1 events record tally since the CCDSN (China Center of Digital Seismic Network) was built in 1999. The epicenter intensity of this earthquake was low,while in areas like Beijing,which is 100km far away,abnormal high intensity zones appeared. This article discusses the reasons behind this abnormal phenomenon,with the diagrams of intensity isolines drawn by the intense seismic stations and networks in the capital circle area as references,as well as the seismogenic mechanism in the source,the seismic histories,the geological structures and the ray pathways of seismic waves in areas within 150km around the epicenter. It was concluded that the special dynamic and geological situations were the main causes for the lower intensity degree and slight damage in the epicenter area,but higher intensity in the surrounding areas.

Temporal-spatial Variations of Dynamic Source Parameters in the Capital Circle Region and Its Surrounding Areas before the 2006 M_S5.1 Wenan Earthquake in Hebei Province

Source spectra,corner frequency and zero frequency amplitudes in near-source conditions were measured using waveform data from 989 earthquakes with magnitudes larger than ML2.0 observed by the Beijing Digital Telemetry Seismic Network in the Capital Circle Region of China and its surrounding areas from January 2002 to June 2006 by the Brune model.Relevant formulas that were used for the calculation of dynamic source parameters include rupture radius,seismic moment,seismic energy,stress drop,and apparent stress.Scaling relations and characteristics of temporal-spatial variations of these dynamic parameters before the MS5.1 Wenan earthquake in Hebei Province that occurred on July 20,2006 were analyzed.Results show that apparent stress,stress drop,and the ratio of seismic energy to the rupture radius had relatively high values in some areas before the Wenan earthquake.These high-value concentration areas were mainly distributed in the North China Plain seismic zone.As is seen from the time curves,parameters,such as apparent stress,stress drop,and ratio of seismic energy to rupture radius underwent significant ascending processes before the Wenan earthquake,but the variation in the corner frequency showed a descending trend.This result might be related to the enhancement of stress in the North China Plain seismic zone before the earthquake.

2020年唐山古冶5.1级地震前后岩石圈磁场异常变化分析

基于唐山及周边地区2019年9月-2021年4月连续4期流动地磁观测资料,获得唐山地区岩石圈磁场异常变化模型,以此分析2020年7月12日唐山古冶5.1级地震前后半年期岩石圈磁场的时空变化特征。结果表明:(1)岩石圈磁场变化的水平矢量在地震前出现明显的方向转向和幅值弱化的现象,地震前后经历了“由震中向周边分散→由周边向震中汇聚发震→方向反转”的时空演化过程;(2)震中附近各测点岩石圈磁场总强度值在地震前后具有规律性时序变化,均呈现“震前缓慢减小→同震时期急剧增大→震后急剧减小”的特征;(3)基于压磁效应和断层亚失稳模型分析,地震前后岩石圈磁场异常的时空变化特征或许是由震中附近主要断裂地应力的积累和释放所致。

文安和古冶2次M_(S)5.1地震活动特征对比

对2006年文安M_(S)5.1地震和2020年古冶M_(S)5.1地震前的异常平静图像、区域-时间-长度(RTL)曲线、地震图像信息(PI)热点进行了对比分析。研究结果表明:①2次M_(S)5.1地震前存在不同程度的时空平静异常图像;②RTL方法显示,文安地震前半年存在显著平静异常,古冶地震前半年内未出现平静异常;③PI方法显示,在3年预测窗长和0.1°×0.1°网格尺度下,2次地震的震中附近皆存在热点,文安地震的有效热点相比于古冶地震对地点指示意义更好,发震风险性更高。该研究对华北地区M_(S)5.0以上地震活动特征总结和地震危险性评估具有一定的参考意义。

基于潮汐附加构造应力的2013年灯塔M_(S)5.1地震射出长波辐射变化分析

针对2013年1月23日辽宁灯塔M_(S)5.1地震,利用引潮力附加构造应力(Additional Tectonic Stress Caused By Tidal Force,ATSCTF)计算模型,计算得到震中位置(41.5°N,123.2°E)在地震前5周以及震后3周(2012年12月16日—2013年2月15日)的ATSCTF变化。地震发生时,ATSCTF垂直方向分量处于高相位点附近,显示引潮力对本次正断层走滑型地震具有诱发作用。以ATSCTF变化周期的各低相位点时间(2012年12月19日、2013年1月4日、2013年1月18日、2013年2月2日)数据分别为背景,各周期期后数据分别与背景逐日相减,计算研究区(36°N~46°N,118°E~128°E)范围内,National Oceanic and Atmospheric Administration(NOAA)卫星射出长波辐射数据(Outgoing Long Wave Radiation,OLR)在各ATSCTF周期时段分布及其变化。结果显示,无震的ATSCTF变化的A、B、D周期,震中附近OLR无变化;发震的ATSCTF变化的C周期,在空间上,该地区震前OLR仅震中及其南侧区域发生了显著连续升高变化过程,在时间上经历了初始微异常→异常加强→高峰→衰减→发震→平静的演化过程,与岩石应力加载—破裂经历:初始微动破裂→扩张破裂→应力闭锁→地震爆发→平静的力学演化过程中各阶段红外辐射特征一致;显示引潮力对处于临界状态的活动断层具有诱发作用,而OLR是地震构造应力应变过程辐射表征。

Using Airgun Source Signals to Study Regional Wave Velocity Changes before and after the Yunlong M_S5.0 and Yangbi M_S5.1 Earthquakes

On the basis of the airgun source signals recorded by the stations from January,2016 to June,2017,we use cross-correlation detection technology to obtain the characteristics of the stable phase travel time change of each station.We used the Yunlong MS5.0 and Yangbi MS5.1 earthquakes as samples.According to regional characteristics,13 stations with high signal-to-noise ratios and complete data were selected(including 3 fixed stations and 10 active source stations).They are divided into four regions,and on the basis of the GNSS baseline data,the characteristics of regional wave velocity changes before and after the earthquake are analyzed.The results show that the station phase travel time change and the regional stress characteristics represented by the GNSS baseline data have good correlation in the short-term.Due to different degrees of regional stress,there are differences in the travel time changes of different stations in the four regions.Before the Yunlong MS5.0 and Yangbi MS5.1 earthquakes,with regional stress adjustment,there is an upward trend in the travel time changes of related stations in the adjacent areas of up to 0.02 s.The difference is that there are differences in the time nodes and duration of the travel time anomalies,and there is a reverse descent process after the Yangbi MS5.1 earthquake.There are different degrees of travel time fluctuations in the relevant stations before and after the two earthquakes,but the fluctuation range before and after the earthquake was small.Compared with the water level change of the reservoir,the adjustment of the regional stress is more likely to have a substantial impact on the travel time changes of the relevant stations.

Temporal vP/vS variation characteristics in different zones of China's Capital Circle area before and after Wen'an earthquake

On July 4, 2006, an earthquake of MS5.1 took place in Wen’an, Hebei Province, just at the south center of China’s Capital Circle area digital seismograph network. It is the strongest event recorded ever since the network went into operation in 2002. We processed the vast amounts of phase data yielded by the 107 digital seismic stations between 2002~2007 using Wadati method. In order to improve the precision and stability of shear and compressional wave velocities (vP/vS) calculation, we impose a number of restrictions on the computation environment and condition, e.g., the earthquakes are densely concentrated, selected stations are limited in range, the number of stations in- volved in the computation is larger than 5 and linear fitting features high precision and small error. Under these restrictions, the study shows that vP/vS in and around Wen’an and Tangshan underwent a normal-low-normal proc- ess one year before Wen’an earthquake, vP/vS became obviously low and the low ratio lasted for about one year, meanwhile, little variation of vP/vS was seen in Xingtai, northwest of Beijing, southwest of Beijing, Beijing-Tianjin and Beijing; after the quake, the vP/vS returned normal in Wen’an and Tangshan. Error and stability analysis of the calculated result for vP/vS shows it is convincible that anomaly appeared in and around Wen’an and Tangshan be- fore Wen’an earthquake.

2022年7月2日皮山M_(S)5.1地震宏观异常调查与分析

以地震宏观异常现象的历史发展过程为出发点,具体描述和田地区皮山县震中周围动物震前、震后异常变化现象和特征。阐述了新疆和田地区皮山5.1级地震现场震中西南方向6~15 km村庄农户圈养的牛、羊等不同动物的反常现象,并针对在地震前后动物发生异常变化的18个调查点、98次异常进行详细统计。其中,皮山县乔达乡8个调查点7种动物宏观异常显著,异常数量总计46次;昆泉镇10个调查点4种动物宏观异常显著,异常数量总计32次。经统计,临近地震发生前24~36小时左右,狗、鸡、鸽、鸭反应异常显著,总宏观异常次数占比较高,动物主要表现为躁动不安、狂吠不止。由此分析得出:临震前,在不同时间段的11个调查点附近7种动物具有较明显的异常反应,从时间尺度上来看,离震中越近的地方,动物异常越早,异常动物数量、种类越多;反之越少。宏观异常现象具有向震中区域靠拢的趋势,空间分布较为集中。

Characteristics of Seismic Activity in a 3-Dimensional Space before the Wen'an Earthquake,Hebei Province by Relocation of Small Earthquakes in the China's Capital Area

Characteristics of seismic activity before the M5. 1 earthquake in Wen＇an, Hebei Province on July 4, 2006 are analyzed by relocation of small earthquakes in the China＇s capital area, and some results are obtained as follows：① The seismic activity of ML ≥ 3.0 in the middle part of the Hebei plain seismic belt displayed a feature of strengthening （ lasting 43 months） quiescence （17 months） five years before the Wen＇an earthquake. Simultaneously, the strain release curve showed a variation process of accelerating-flatting. ② A seismogenic gap in a three-dimensional space, located at the depth of 15km - 20km, with 70km long in latitude direction and 90km long in longitude orientation, was formed by M≥2.0 earthquakes four years prior to the Wen＇an earthquake. The initial rupture point of the Wen＇an earthquake is situated at the bottom of the gap. ③ The focal depths of earthquakes with ML ≥ 2. 0 in the middle part of the Hebei plain seismic belt gradually increased from 10km to 30kin during the period from April, 2003 to October, 2004. Meanwhile, the seismic activity obviously strengthened in the middle and lower crust （from 20km to 30km in depth）.

Dynamic Variations of the Gravity Field before and after the M5. 1 Wen’an Earthquake in Hebei Province

In order to explore the abnormal variations before and after the Wen＇an MS. 1 earthquake on July 4, 2006, the gravity data observed by the Wenba Gravity Observation Network before and after the earthquake was analyzed. The relationships between gravity change and vertical displacement and shallow groundwater level were discussed, and elevation correction of the gravity was preliminarily performed. The results show that there were abnormal gravity changes before the MS. ! earthquake, which appeared as gravity rising in the whole observation network, especially in the south part. A high gradient of gravity variation appeared around the epicenter before the MS. 1 earthquake, especially during the short period between October 2005 to April 2006. The boundary of the positive and negative gravity variations gradually deflected and began to recover from October 2007.

2020年古冶M_(S)5.1地震前后岩石圈磁场变化特征

总结归纳河北及邻近地区2014年以来4级以上中强地震震兆异常特征位置,结合2020年古冶M_(S)5.1地震发生前后岩石圈磁场H矢量、D、F总场3个要素地磁异常变化图像,分析发震前后岩石圈磁场震兆异常变化特征。结果表明:①河北及邻近地区7个M≥4地震均发生在岩石圈磁场各地磁要素0值线附近的高梯级带、0值线附近及矢量方向转向等特征部位;②古冶M_(S)5.1地震符合河北中强地震岩石圈磁场震兆异常空间位置特征,概率达75%;③丰南M_(S)4.5地震后唐山岩石圈磁场异常区异常特征出现增强,原地震危险区有再次发震的可能。

2022年6月24日新疆鄯善M_(S)5.1地震序列特征及震前异常分析

介绍2022年6月24日鄯善M_(S)5.1地震基本参数、地震构造背景、震源机制解、地震序列特征和震前地震活动性异常和地球物理观测异常。结果:①本次地震发生在天山地震带的东段,发震构造为博罗科努-阿奇克库都克断裂,震源错动方式为逆冲型。②此次地震序列类型为孤立型,序列余震不丰富,频度和强度衰减较快。③震前存在地震活动性异常5项,分别为天山中段M_(S)≥4.0地震平静、天山中段M_(S)≥4.0地震增强、天山中段M_(S)3.0-4.9地震增强区、天山中段M_(S)≥3.0增强、M_(S)≥3.0地震条带。④震前存在地球物理场异常2项,为新41井水位和库米什钻孔倾斜EW分量的速率变化异常。

2019年松原M_(S)5.1地震烈度评定及震害特征简析

2019年5月18日松原市发生M_(S)5.1地震,基于地震灾害现场调查、震源机制解、震区地震构造、强震动观测等,评定了地震烈度分布。此次地震极震区烈度为Ⅵ度,等震线长轴方向为北东向,面积为237km^(2)。调查结果表明,砖木结构受损比例最高,土木结构震害特征最重,简析震区土木结构、砖木结构、单层砖混结构和多层砖混结构房屋的震害特征,发现震区多次地震的积累,加大了本次地震中房屋的破坏程度。总结提出增强房屋抗震性能、加强防震减灾知识宣传教育等防震减灾建议。

Study on the focal mechanism of the M5. 1 Badong earthquake in Hubei

The focal mechanism solutions of the MS. 1 Badong earthquake and subsequent 34 aftershocks at ML 2.0 or more were calculated using the P-wave first motion method; the main earthquake was normal fault dip slip type, and the slip types of the seismogenic rupture surfaces of the subsequent aftershocks primarily include normal dip slip （14 times）, reverse dip slip （9 times）, normal strike slip （9 times） and reverse strike slip （2 times）. The MS. 1 Badong earthquake activities may be related to the stress adjustment caused by the rise of the groundwater level and the decrease of the frictional resistance between structural planes of rock forma- tions due to the effect of reservoir water penetration, and related to the joint activities of the NE-strike Gaoqiao fault and the near EW-strike Daping fault.

2022年3月17日甘肃肃南M_(S)5.1地震总结

系统总结2022年3月17日甘肃肃南M_(S)5.1地震发生前的地震活动和地球物理观测异常,其中:①地震活动异常:震前存在地震成组活动异常;②地球物理观测异常:震中200 km范围内存在6项地球物理场异常,其中5项异常为趋势异常,但均非此次肃南M_(S)5.1地震的相关异常,而高台地震台形变异常虽为短期异常,但异常转折变化与本次地震无关;此外,肃南M_(S)5.1地震发生前,我国西北地区存在地磁低点位移、加卸载响应比和逐日比3项异常,但因后续于3月26日发生青海德令哈M_(S)6.0地震,认为该地磁预测指标对应成组地震活动,不能作为单一地震的震前异常。肃南M_(S)5.1地震余震较少,其序列活动特征显示为主余型;主震震源机制显示断层破裂为走滑型,与距离最近的疏勒南山断裂构造性质相符。综合分析认为,此次地震对于祁连山地震带及其周边后续中强地震的发生具有一定预测意义。

2022年4月6日四川兴文M_(S)5.1地震震害特征及成因分析

针对四川兴文M_(S)5.1地震,地震现场工作队采用抽样调查、单项调查、现场核实等方法对震中区房屋及其他工程进行了实地勘察,获取了此次地震的烈度分布及大量调查资料,本文在此基础上对本次地震震害特征及其成因进行了总结梳理,最后从地震灾害风险排查、房屋建筑抗震设防、地震科学基础研究等方面对震区防震减灾工作提出建议,为震后恢复重建和当地抗震设防能力提升提供参考依据。

河北文安MS5.1地震前首都圈及邻区震源动力学参数时空变化特征

本文根据近震源条件下,由北京数字遥测地震台网观测到的波形资料,利用Brune模式,得到了2002年1月至2006年6月期间首都圈及邻近地区989次ML≥2.0地震的震源谱,测定了它们的拐角频率和零频幅值。通过相关计算公式,得到了破裂半径、地震矩、地震能量、应力降、视应力等震源动力学参数。分析了这些动力学参数的标度关系以及在2006年7月4日河北文安MS5.1地震前的时空变化特征。结果表明,文安MS5.1地震前,视应力、应力降、地震能量与破裂半径比等出现了相对高值集中区,这些高值集中区主要分布在华北平原带上;从时间变化曲线可以看出,文安MS5.1地震前华北平原带上视应力、应力降、地震能量与破裂半径比等参数都表现出了明显的升高变化过程,而拐角频率则显示出趋势性下降变化特征。这可能与文安MS5.1地震前华北平原带上应力的增强过程有关。

2020年河北唐山M_(S)5.1地震b值时空变化特征

为了考察b值在2020年唐山M_(S)5.1地震前的时空变化特征,分别从大尺度区域及小尺度区域对2009年1月至地震发生前2个研究范围的b值变化进行时空扫描计算。结果显示:唐山M_(S)5.1地震震中位置b值被小尺度区域很好地表现出来,而大尺度区域b值空间扫描未表现出明显异常;从大尺度区域可以看出,河北怀安—北京、唐山地震余震区东北及西南段、渤海等区域处于低b值区,未来存在发生中强地震的可能。因此,在做b值研究时要选择适当的研究范围,防止研究范围过大b值异常区被遗漏。

2020年河北唐山5.1级地震前震源参数变化

利用谱分析方法对河北省测震台网2016年1月—2020年7月唐山老震区(117.5°~119.5°E,39°~41°N)85次M_(L)≥2.5地震进行震源参数分析,发现该时段唐山老震区应力降范围在0.1~10.3MPa,主要集中在0.1~1.0MPa,其中应力降小于2MPa的地震占81%。当2.5≤M_(L)≤3.5时,应力降、拐角频率与震级之间无明显的对应关系;M_(L)≥4.0地震较少,但应力降有随震级增大而增大的趋势,拐角频率有随震级增大而减小的趋势。2019年12月5日河北丰南M 4.5地震和2020年7月12日唐山M 5.1地震之间发生了7次M_(L)≥2.5地震,震级范围为2.5≤M_(L)≤2.8,提取该震级范围的震源参数进行时空分析,发现唐山M 5.1地震前有2次明显的高应力降事件,2次高应力降事件呈现出应力降数值高、拐角频率高、震源尺度小的特点,表明单位体积所释放能量较多。2次高应力降事件均发生在唐山M 5.1地震震中17km范围内,可能是唐山M 5.1地震前震源区构造应力较高的一个标志。

新疆皮山M_(S)5.1、叶城M_(S)5.0地震震源机制解分析

利用CAP方法反演2021年9月4日皮山M_(S)5.1地震和2021年9月5日叶城M_(S)5.0地震。反演得到皮山M_(S)5.1地震节面I:走向303°,倾角64°,滑动角74°;节面Ⅱ:走向156°,倾角29°,滑动角119°,P轴方位44°;矩震级为4.8,矩心深度为19 km。叶城M_(S)5.0地震节面I:走向287°,倾角50°,滑动角108°;节面Ⅱ:走向80°,倾角43°,滑动角70°,P轴方位4°;矩震级为4.76,矩心深度为19 km。两次地震的震源机制解基本一致,均为逆冲型破裂事件,与泽普断裂的断错性质一致。