One feature of the activated southern Ordos block:the Ziwuling small earthquake cluster

Small earthquakes （Ms〉2.0） have been recorded from 1970 to the present day and reveal a significant diffrence in seismicity between the stable Ordos block and its active surrounding area. The southern Ordos block is a conspicuous small earthquake belt clustered and isolated along the NNW direction and extends to the inner stable Ordos block; no active fault can match this small earthquake cluster. In this paper, we analyze the dynamic mechanism of this small earthquake cluster based on the GPS velocity field （from 1999 to 2007） , which are mainly from Crustal Movement Observation Network of China （CMONOC） with respect to the north and south China blocks. The principal direction of strain rate field, the expansion ratefield, the maximum shear strain rate, and the rotation rate were constrained using the GPS velocity field. The results show that the velocity field, which is bounded by the small earthquake cluster from Tongchuan to Weinan, differs from the strain rate field, and the crustal deformation is left-lateral shear. This left-lateral shear belt not only spatially coincides with the Neo-tectonic belt in the Weihe Basin but also with the NNW small earthquake cluster （the Ziwuling small earthquake cluster）. Based on these studies, we speculate that the NNW small earthquake cluster is caused by left-lateral shear slip, which is prone to strain accumulation. When the strain releases along the weak zone of structure, small earthquakes diffuse within its upper crust. The maximum principal compression stress direction changed from NE-SW to NEE-SWW, and the former reverse faults in the southwestern margin of the Ordos block became a left-lateral strike slip due to readjustment of the tectonic stress field after the middle Pleistocene. The NNW Neo-tectonie belt in the Weihe Basin, the different movement character of the inner Weihe Basin （which was demonstrated through GPS measurements） anti the small earthquake cluster belt reflect the activated southern margin of the Ordos block, which was generated through readjustment of the tectonic stress field after the middle Pleistocene.

Variation Characteristics of Focal Mechanisms of Small Earthquakes before Four Strong Earthquakes in Xinjiang

With the point source dislocation model and the velocity structure of a layered medium,focal mechanisms of small earthquakes are calculated using the maximum amplitude of the direct P- and S-waves in the vertical component. By system clustering,and using the vector synthesis method,the average focal mechanism solution is obtained. Using the above method,this paper analyzes the variation characteristics of the source ruptures and the P-axis azimuths of small earthquakes around the seismic zones before four strong earthquakes occurring since 2003 in the western part of north Tianshan and the middle part of Tianshan. The result shows that 2 ~ 3 years before the strong earthquakes,the focal mechanism types of small earthquakes are distributed randomly, and obvious dominant distributions are observed one year before the strong earthquakes. There are obvious changes in the P-axis azimuth.

Analysis on Completeness of Small Earthquake Data in the Northeast Seismic Zone and Its Influence on Seismicity Parameters

Taking the northeast seismic zone as an example and based on the magnitude-frequency distribution principles of the G-R relationship and the study on the temporal and spatial distribution characteristics of the minimum magnitude of completeness M C in each time interval,this paper makes a statistical analysis on the seismicity parameters of the research area and explores the influence of the completeness of small earthquake data on the seismicity parameters and seismic hazard analysis.It shows that the completeness analysis of the regional small earthquakes data provides a possible means for obtaining more accurate seismicity parameters that can better represent the actual regional seismicity level for areas of low seismicity.The research methods of this article and its conclusion can be used as a reference for the completeness analysis of earthquake data and seismic activity research.

Static and Dynamic Scaling Relationships for Moderate and Small Earthquakes in the Yunnan Region

Source spectra for moderate and small earthquakes are obtained after removing the path effect, site effect, and instrument response, etc. in the observed S-wave spectra. Based on the Brune source model and by means of genetic algorithm, the source parameters including seismic moment, stress drop, source dimension, etc. are determined, the radiated seismic energy for small-to-moderate earthquakes is measured with consideration of underestimation and compensation brought forth by limited bandwidth of the instrument, and the scaling relationships of static and dynamic parameters for earthquakes

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）.

Study on the Anomaly of Small Earthquake Activity Before Moderate-strong Earthquake in the Northeast Margin of the Qinghai-Xizang (Tibet) Block

26 earthquakes with MS ≥5. 0 have been recorded in the northeast margin of the Qinghai- Xizang (Tibet) block since 1980,22 of which were relatively independent of other moderate- strong earthquakes. Research on the increase of small earthquake activity before the 22 moderate-strong earthquakes has indicated that small earthquake activity was enhanced before 17 of the moderate-strong earthquakes. Though the increased seismicity is a common phenomenon in the northeast margin of the Qinghai-Xizang ( Tibet ) block,we have difficulty in predicting the moderate-strong earthquakes by this phenomenon. In order to predict the moderate-strong earthquakes through the increased seismicity of small earthquakes,this paper attempts to propose a new method, which calculates small earthquake frequency through the change of distribution pattern of small earthquakes, based on the characteristics of small earthquake activity in the northeastern Qinghai-Xizang (Tibet) block,and then make primary applications. The result shows that we are able to obtain obvious anomalies in the frequency of small earthquakes before moderate strong earthquakes through the new method,with little spatial range effect on the amplitude of this small earthquake frequency anomaly. We can obtain mid to short-term anomaly indices for moderate-strong earthquakes in the northeast margin of the Qinghai-Xizang (Tibet) block.

“Small earthquake modulation window” and its applications in medium－term prediction

Based on the observations of many years, it has been found that “small earthquake modulation windows” exist inthe situation of some special geological structures, which respond sensitively to the variations of regional stressfields and the activities of earthquake swarms greater than moderate strong magnitude, and can supply some precursory information. More than two “small earthquake modulation windows” can also provide a general orientation of the first main earthquake of a earthquake cluster. Compared with “seismic window” based on frequency itis no doubt that the “modulation-window” has an unique characteristic of applicational significance to mediumterm earthquake prediction with a time scale of two or three years.

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.

Application of fuzzy clustering method to determining sub-fault planes of earthquake from aftershocks sequence

Earthquake rupture process generally involves several faults activities instead of a single fault A new method using both fuzzy clustering and principal component analysis makes it possible to reconstruct three dimensional structure of involved faults in earthquake if the aftershocks around the active fault planes distribute uniformly. When seismic events are given, the optimal faults structures can be determined by our new method. Each of sub-fault planes is fully characterized by its central location, length, width, strike and dip. The resolution determines the number of fault segments needed to describe the earthquake catalog. The higher the resolution, the finer the structure of the reconstructed fault segments. The new method successfully reconstructs the fault segments using synthetic earthquake catalogs. By taking the 28 June 1992 Landers earthquake oceured in southern California as an example, the reconstructed fault segments are consistent with the faults already known on geological maps or blind faults that appeared quite frequently in longer-term catalogs.

小浪底水库诱发地震监测及研究

利用2009年小浪底地震台网数字化改造后记录到的2009—2011年水库周围20km范围内发生的地震，分析其时空分布特征以及其中61次地震的震源机制解。结果表明，小浪底水库存在诱发地震活动，以弱震和微震为主，最大ML2．8。地震集中分布在2个丛集区，分别与库区的石井河断层、塔底断层有关。库区地震的张应力轴与构造背景最大主压应力轴方向基本一致，倾角30。左右，表明这些地震是在受到一定的相对直立压应力作用下发生的，可能与水库蓄水后库水载荷的长期作用有关。