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 M L ≥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 M L ≥2.0 in the middle part of the Hebei plain seismic belt gradually increased from 10km to 30km 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).     

Multi-dipole observation system and study on the abnormal variation of the geoelectric field observed at Capital Circle area before the Wen'an M_S5.1 earthquake

On July 4,2006,a MS5.1 earthquake occurred in Wen'an county of Hebei Province of which the epicenter is near the Beijing city.The six geoelectric field monitoring stations have been in operation for several years around the Beijing area to examine the relationship between electric field changes and earthquake.This paper firstly explains the principle of the eliminating noise method by using multi-dipole observation system of the geoelectric field.Then the data observed at the stations are studied and a lot ...     

2006年文安地震孕育、发生的应力场动态图像

利用数字地震记录反演了河北地区ML≥2.0地震的震源机制解,通过对震源机制解各参数的统计分析,发现应力轴的优势取向和华北构造应力场完全吻合,反映它们受到大区域动力因素的制约。进行震源机制解一致性参数计算,并对一致性参数进行空间平面插值,得到了2006年文安地震前震源机制解一致性参数空间动态的变化特征。文安地震前在震源区附近出现了大范围的一致性参数低值区域,并且低值区域面积随时间增长逐渐扩大,这可能是中强震前的一种具有动力学意义的前兆现象。     

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 M5. 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 M5. 1 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 M5. 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.     

文安5.1级地震前的一些前兆异常

2006年7月4日文安5.1级地震前,保定流动测量队很多前兆监测手段都有前兆异常显示.该文对这些异常进行总结,同时分析其形态特征.     

河北文安MS5.1地震前首都圈及邻近地区震源动力学参数时空变化特征

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 Wenan 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 Wenan 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 Wenan 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.     

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 M5. 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 M5. 1 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 M5. 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.     

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.     

2006年河北文安M_W 5.1地震震源机制

使用河北省测震台网7个宽频带地震台记录,采用CAP方法反演2006年7月4日文安M_W 5.1地震震源机制解,并结合区域地质背景讨论发震构造。结果显示,文安5.1级地震的最佳双力偶解为:节面Ⅰ走向215°,倾角87°,滑动角-115°;节面Ⅱ走向118°,倾角25°,滑动角-8°,震源深度为15 km,联合地质构造确定地震断层面为隐伏的NE向断裂带,为正断层兼走滑断层,与其他研究结果基本一致。     

2006年7月4日文安5.1级地震

分析了震动京津的文安地震的特点:余震微小,最大仅1.5级,属于孤立型地震;震区烈度偏低、有感范围大;宏观震中与微观震中相差10余公里;震源较深;具有较大的正断层错动分量。对此作出初步解释,并认为可能是岩石圈减薄的新证据