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| 1 | The seismicity and tectonic stress field characteristics of the Longmenshan fault zone before the Wenchuan M_S8.0 earthquake显示文摘The seismicity of Longmenshan fault zone and its vicinities before the 12 May 2008 Wenchuan MS8.0 earthquake is studied.Based on the digital seismic waveform data observed from regional seismic networks and mobile stations, the focal mechanism solutions are determined.Our analysis results show that the seismicities of Longmenshan fault zone before the 12 May 2008 Wenchuan earthquake were in stable state.No obvious phenomena of seismic activity intensifying appeared.According to focal mechanism solutions of some small earthquakes before the 12 May 2008 Wenchuan earthquake, the direction of principal compressive stress P-axis is WNW-ESE.The two hypocenter fault planes are NE-striking and NW-striking.The plane of NE direction is among N50o?70oE, the dip angles of fault planes are 60o?70o and it is very steep.The faultings of most earthquakes are dominantly characterized by dip-slip reverse and small part of faultings present strike-slip.The azimuths of principal compressive stress, the strikes of source fault planes and the dislocation types calculated from some small earthquakes before the 12 May 2008 Wenchuan earthquake are in accordance with that of the main shock.The average stress field of micro-rupture along the Longmenshan fault zone before the great earthquake is also consistent with that calculated from main shock.Zipingpu dam is located in the east side 20 km from the initial rupture area of the 12 May 2008 Wenchuan earthquake.The activity increment of small earthquakes in the Zipingpu dam is in the period of water discharging.The source parameter results of the small earthquakes which occurred near the initial rupture area of the 12 May 2008 Wenchuan earthquake indicate that the focal depths are 5 to 14 km and the source parameters are identical with that of earthquake. | Zhiwei Zhang Wanzheng Cheng Xiang Ruan Peng Wu | 2009 | Earthquake Science2009，22，2： | 3 |
| 2 | Spatio-temporal variation and focal mechanism of the Wenchuan M_S8.0 earthquake sequence显示文摘Based on abundant aftershock sequence data of the Wenchuan MS8.0 earthquake on May 12, 2008, we studied the spatio-temporal variation process and segmentation rupture characteristic.Dense aftershocks distribute along Longmenshan central fault zone of NE direction and form a narrow strip with the length of 325 km and the depth between several and 40 km.The depth profile(section of NW direction) vertical to the strike of aftershock zone(NE direction) shows anisomer-ous wedgy distribution characteristic of aftershock concentrated regions;it is related to the force form of the Longmenshan nappe tectonic belt.The stronger aftershocks could be divided into northern segment and southern segment apparently and the focal depths of strong aftershocks in the 50 km area between northern segment and southern segment are shallower.It seems like ‘to be going to rupture' segment.We also study focal mechanisms and segmentation of strong aftershocks.The principal compressive stress azimuth of aftershock area is WNW direction and the faulting types of aftershocks at southern and northern segment have the same proportion.Because aftershocks distribute on different secondary faults, their focal mechanisms present complex local tectonic stress field.The faulting of seven strong earthquakes on the Longmenshan cen-tral fault is mainly characterized by thrust with the component of right-lateral strike-slip.Meantime six strong aftershocks on the Longmenshan back-range fault and Qingchuan fault present strike-slip faulting.At last we discuss the complex segmen-tation rupture mechanism of the Wenchuan earthquake. | Wanzheng Cheng Zhiwei Zhang Xiang Ruan | 2009 | Earthquake Science2009，22，2： | 2 |
| 3 | Research on the Dynamic Change of Regional Stress Fields before the M_S8.0 Wenchuan Earthquake显示文摘Using the digital telemetric seismic waveform data of Chengdu and Kunming, this article studies the focal mechanism solutions and the apparent stress values of a large number of small earthquakes, and then analyzes the dynamic variation of regional stress fields and the spatio-temporal distribution of apparent stress values. The annual variation values of the azimuth of average principal stress field before the May 12, 2008 M_S8.0 Wenchuan earthquake in the Sichuan-Yunnan region were 58° from 2003 to 2004, 85° from 2003 to 2005,61° from 2006 to 2007 and 90° from 2006 to April 2008 respectively. In recent years, deflection or disturbances occurred in the azimuth of the average principal stress field in the Sichuan-Yunnan region. Analysis shows that this may be related to the change of stress field states of crustal blocks before and after the December 26, 2004 M_S9.0 Sumatra earthquake and the 2008 M_S8.0 Wenchuan earthquake. The ratio of thrust-type earthquakes in the Sichuan-Qinghai block was on the higher side in the period from 2006 to 2007, and the source faulting type of the regional moderate and small earthquakes had changed before the M_S8.0 Wenchuan earthquake. The change of state of the stress field is consistent with the changes in block displacement fields revealed by GPS data and the crustal shortening velocity vertical to the Longmenshan fault zone. Based on the radiation energy calculated from all bands of the seismic waveform, the value of apparent stress σ_app is obtained. The fluctuation shape of the fitting trend of the apparent stress is related to the intensity of regional seismicity. It reveals that the micro-dynamic fluctuation process of the regional stress value is similar to the azimuth transition of the regional principal compressive stress field, which can be used to probe for pregnant physical processes. Areas with a higher value of apparent stress σ_app are possible areas of potential seismic risk. It can be seen from the spatial distribution of the medium and short-term apparent stress σ_app before the M_S8.0 Wenchuan earthquake, the Longmenshan fault zone is in a low stress distribution area, and the relatively high apparent stress is in the peripheral area. These images may show medium and short-term locking phenomena near the seismogenic tectonics of the M_S8.0 Wenchuan earthquake. For example, changes with time of the focal parameter consistency of the sub-blocks in Sichuan and Yunnan Provinces, continual increase of thrust-type earthquakes in the Sichuan-Qinghai block and the appearance of spatial distribution areas of high apparent σ_app stress. The work on this aspect was continued after the M_S8.0 Wenchuan earthquake, and the results seem to be shown a clearer relationship between these phenomena and future great earthquakes. | Cheng Wanzheng Ruan Xiang Qiao Huizhen Zhang Zhiwei Zhang Yongjiu | 2009 | Earthquake Research in China2009，23，3： | 2 |
| 4 | DEFORMATION RATE CHANGES OF TECTONIC BELTS ALONG BOUNDARIES OF YUNNAN SICHAUN BLOCK AND RELATION TO GROUPED STRONG EARTHQUAKES显示文摘Deformation measurements such as short range leveling, short baseline, continuous cross fault strain measurement are carried out at different intervals from 1982 to 2001.All these measurement sites are built across the boundary tectonic belts of Yunnan Sichuan block. On the basis of these deformation data, the annual deformation rates at all sites are calculated and their change curves with time are plotted respectively. With these calculated results, we analyze the vertical and horizontal movements of tectonic belts of Yunnan Sichuan block, and the relationship to grouped strong earthquakes occurred in the block are discussed as well. These results show that the tectonic activities in the western and southeastern Yunnan are intensive. Along some eastern boundary belts: from Qujiang, Xiaojiang fault belts in the south of Yunnan to Xianshuihe fault belts in northwestern Sichuan, present annual rate of horizontal deformation decreases.Along Xianshuihe, Xiaojiang and Longmenshan fault belts the rates of vertical deformation change are small,but the vertical deformation change rates along Anninghe and Zemuhe fault belts are comparatively large. The comprehensive analysis shows that grouped strong earthquakes will occur probably when the deformation rate changes sharply. Thus we think that sharp changes of deformation rates may be one of the seismic precursors. | Cheng Wanzheng and Yang Yongling (Earthquake Administration of Sichuan Province, Chengdu 610041, China) | 2003 | 大地测量与地球动力学2003，23，B12： | 1 |
| 5 | Discussion on 'Was the 2008 Wenchuan earthquake triggered by Zipingpu Reservoir?'显示文摘Some recent publications presented a result suggesting that Zipingpu reservoir hastened the occurrence of the 2008 MS8.0 Wenchuan earthquake by tens to hundreds of years.Their researches calculated the Coulomb stress change induced by Zipingpu reservoir on the rupturing fault of Wenchuan earthquake.Their results,however,are critically dependent upon the 3-D event location,reservoir location,and the fault plane orientation.We repeated Ge et al.'s work in this paper and found that an improper dip angle parameter of their 2-D fault model might lead to a wrong conclusion.Both the modeling results based on the 2-D model and 3-D model with proper fault parameters will show Coulomb stress changes alone were neither large enough nor had the correct orientation to affect the occurrence of Wenchuan earthquake,which supports our recent argument based on the local seismicity analysis and the induced Coulomb stress change calculation with a 3-D model. | Shiyong Zhou Kai Deng Cuiping Zhao Wanzheng Cheng | 2010 | Earthquake Science2010，23，6： | 1 |
| 6 | Feasibility Study on Determining Focal Mechanism Solutions of Small Earthquakes Using the Velocity Amplitude Ratio of -and -Waves显示文摘The focal mechanism parameters of small earthquakes are determined by the maximum velocity and displacement amplitude ratio of the direct -and -waves recorded by digital stations. The displacement is obtained from the velocity by emulation, and the two results are compared and analyzed. Results of theoretical analysis and practical measurement indicate that the two results of velocity and displacement are consistent, and it is feasible that the maximum displacement amplitude ratio be replaced by the maximum velocity amplitude ratio of the direct -and -waves recorded by regional seismic networks when determining focal mechanism solutions of small earthquakes. | Zhang Yongjiu Cheng Wanzheng | 2008 | Earthquake Research in China2008，22，3： | 0 |
| 7 | Research on Seismicity and Source Parameters of Small Earthquakes in the Zipingpu Reservoir before the Wenchuan M_S 8. 0 Earthquake显示文摘Based on the digital seismic waveform data observed from regional and reservoir seismic networks,the seismicity and source parameters in the Zipingpu reservoir area from January 1,2000 to May 11,2008 before the Wenchuan MS8.0 earthquake are studied. The analysis shows a continuous activity of small earthquakes before the Wenchuan MS8. 0 earthquake. The level of seismic activity was from ML2.0 to ML3.0 in recent years. The impoundment of Zipingpu reservoir started in September 2005 and the obvious increment of seismic activities occurred in February 2008,but according to the analysis of the longer time series,the frequency and intensity of seismic activities do not exceed its range,which means it is still a fluctuation of regional seismic activities or tectonic activity. At the same time,we also studied the spatio-temporal distribution and the change of apparent stress difference in the source region before the great earthquake,and the results show that the apparent stress values of small earthquakes are generally higher than fitted values of apparent stress in the reservoir area in the nearly three months before the Wenchuan MS8.0 earthquake. It can be seen from the spatial distribution of apparent stress difference,that the epicenter of the earthquake is located at a low stress distribution area,and the relatively high apparent stress difference is at the east of the epicenter. Apparent stress increment occurred in local areas. | Zhang Zhiwei Cheng Wanzheng Zhang Yongjiu Xie Ronghua Fu Ying | 2010 | Earthquake Research in China2010，24，3： | 0 |
| 8 | Damage Tensor Analysis on Regional Seismic Status显示文摘In this paper, we researched the regional seismic status by using theories of the Damage Mechanics. The macroscopic damage status of the earth crust block, which is caused by earthquake fracture, is described with several concepts—the damage degree, the damage rate and the strain rate. In the earthquake process, the average strain rate of the studied block is equal to the sum of all seismic moment tensors of the earthquakes taking place in unit time and physical volume. To describe the anisotropy of microdamage of the crust block, we use the damage tensor that is expressed in the fissure density. By means of the transformation from the focal coordinate system to the observation system, we obtained the external normal vector of the focal fault plane expressed in its observation system and obtained the macrodamage degree of the researched block, which is calculated in dyadic. This provides a new analysis method for recognizing the underground damage status and the stress status. | Zhong Jimao Cheng Wanzheng | 2006 | Earthquake Research in China2006，20，3： | 0 |
| 9 | Research of the Activity of Earthquakes Induced by Water Injection of Salt Mining in Changning County,Sichuan Province显示文摘According to information from on-the-spot investigations, the tectonics of salt mining areas and digital seismic records, we studied the activity of earthquakes induced by water pumping in and out of the salt mines in Shuanghe town, Changning county, Sichuan Province. The study found that the rates of water injection and extraction in the Shuanghe salt mining region were evenly matched before April 2006 and earthquake activity was stable. On the other hand, shallow small and moderate earthquake numbers increased sharply after the water injection rate became much larger than the water extraction rate. Large injection over a long time may causes the permeation of water through pre-existing small fractures and micro-cracks in the Changning anticlinal and accelerate the rupture of micro tectonic formations in nearby regions, inducing small earthquake swarm activity. The Q value calculated by using digital earthquake records indicates a relatively inhomogeneous medium in this area. The results of the accurate location of small earthquakes show that sources are relatively shallow and are concentrated at a depth of 2km to 3km. Focal mechanism solutions reveal a normal dip-slip character of shallow earthquakes. All of these show certain characteristics of earthquake activity induced by water injection. | Ruan Xiang Cheng Wanzheng Zhang Yongjiu Li Jun Chen Yin | 2009 | Earthquake Research in China2009，23，2： | 0 |
| 10 | Study on the Source Parameters of the Barkam Earthquake Sequence显示文摘Using digital wave data recorded by the Barkam station, source parameters for 93 earthquakes with ML2.5~5.0 in the Barkam area are determined by spectral analysis. Preliminary analysis of the source parameters show that there is a high correlation between seismic moment M0 and local scale ML, which is lg 10 M0=0.92ML+10.24,correlation coefficient R is 0.86;Investigation of the relationship between stress drop and local scale ML indicates that the stress drop seems to be independent of ML. | Zhang Yongjiu Peng Liguo Cheng Wanzheng | 2006 | Earthquake Research in China2006，20，4： | 0 |
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