Study on the variation characteristics of the geoelectric field preceding earthquakes

Earthquake events occurred in Nantong, Jiangsu Province on Nov. 3, and Dec. 25, 2001 in which the biggest mag-nitudes were ML=3.8 and ML=4.1, respectively. This paper firstly explains the principle of the eliminating noisemethod by the multi-dipole observation system of geoelectric field. Then based on the observation data of themulti-dipole observation system obtained by ZD9A telluric current monitors installed in Chongming and Nanjingstations, we study the abnormal variation of the geoelectric field preceding the earthquakes. The study shows that:a) Eliminating noise method of multi-dipole observation is an excellent method by which many kinds of geoelec-tric field noises can be eliminated successfully and the geoelectric precursor information can be recognized; b) Thegeoelectric precursor signals for the events were recorded on the NS and NE dipoles in Chongming station 42 days,20 days and 2 days before the earthquakes respectively, in which the station is near the epicenter, and the longesttime of persisting period was 9 days. The abnormal variation signals of geoelectric field observed in Nanjing sta-tion are all the noises but not the seismic electric signals, in which the station is not near the epicenter; c) Dipoledistribution method of common electrode is not good in the multi-dipole observation system of the geoelectricfield.

Detection of ULF electromagnetic emissions as a precursor to two earthquakes in China

The geomagnetic data recorded by Kashi and Jinghai observatories in China were analyzed with improved polarization method.We compared the result around 0.01 Hz which is thought to be useful to detect the ULF anomaly with the result around 0.1 Hz which was inferred from the earthquake depth according to the skin effect,and found that 0.1 Hz is more proper to detect the ULF anomaly for both earthquakes studied in this paper.

Seismotectonics of the 2008 and 2009 Qaidam Earthquakes and its Implication for Regional Tectonics

Three magnitude >6 earthquakes struck Qaidam, Qinghai province, China, in November 10th 2008, August 28th and 31st 2009 respectively. The Zongwulongshan fault has often been designated as the active seismogenic structure, although it is at odd with the data. Our continuous GPS station (CGPS), the Xiao Qaidam station, located in the north of the Qaidam basin, is less than 30 km to the southwest of the 2008 earthquake. This CGPS station recorded the near field co-seismic deformation. Here we analyzed the co-seismic dislocation based on the GPS time series and the rupture processes from focal mechanism for the three earthquakes. The aftershocks were relocated to constrain the spatial characteristics of the 2008 and 2009 Qaidam earthquakes. Field geological and geomorphological investigation and interpretation of satellite images show that the Xitieshan fault and Zongwulongshan fault were activated as left lateral thrust during the late Quaternary. Evidence of folding can also be identified. Integrated analyses based on our data and the regional tectonic environment show that the Xitieshan fault is the fault responsible for the 2008 Qaidam earthquake, which is a low dip angle thrust with left lateral strike slip. The Zongwulongshan fault is the seismogenic fault of the 2009 earthquakes, which is a south dipping back thrust of the northern marginal thrust system of the Qaidam basin. Folding takes a significant part of the deformation in the northern marginal thrust system of the Qaidam basin, dominating the contemporary structure style of the northern margin of the Qaidam basin and Qilianshan tectonic system. In this region, this fault and fold system dominates the earthquake activities with frequent small magnitude earthquakes.

Automatic recognition of damaged town buildings caused by earthquake using remote sensing information: Taking the 2001 Bhuj, India, earthquake and the 1976 Tangshan, China, earthquake as examples

In the high-resolution images, the undamaged buildings generally show a natural textural feature, while the damaged or semi-damaged buildings always exhibit some low-grayscale blocks because of their coarsely damaged sections. If we use a proper threshold to classify the grayscale of image, some independent holes will appear in the damaged regions. By using such statistical information as the number of holes in every region, or the ratio between the area of holes and that of the region, etc, the damaged buildings can be separated from the undamaged, thus automatic detection of damaged buildings can be realized. Based on these characteristics, a new method to automatically detect the damage buildings by using regional structure and statistical information of texture is presented in the paper. In order to test its validity, 1-m-resolution iKonos merged image of the 2001 Bhuj earthquake and grayscale aerial photos of the 1976 Tangshan earthquake are selected as two examples to automatically detect the damaged buildings. Satisfied results are obtained.

The Characteristics of Seismic Activity Before the Devastating Earthquake with M_W9.0 Off the West Coast of Northern Sumatra

The characteristics of seismic activity in different time-spatial domain before the M_W9.0 earthquake were studied. The results are as follows:The activity of the deep earthquakes in the north boundary zone of the Australian plate had been evidently strengthened since 1994, showing an increased frequency, magnitude and depth, especially in regards to the heterogeneous distribution of the earthquake depth (namely between 500km and 689km). Meanwhile the shallow earthquakes of M≥7.0 in the Sumatra island and its vicinity had been obviously strengthened too, and formed a strengthening area with a length of about 1000 km and width 300 km. ②The time distribution of global strong earthquakes with M≥7.0 shows that the character of anomalous seismic quiescence-activity one year before the M_W9.0 earthquake and during its active period, the strong earthquakes formed a seismic belt striking in NWW direction. At the same time, there is a seismic gap formed by earthquakes of M≥5.0 in the epicenter and its neighboring region. ③Two deep earthquakes of M≥7.0 occurred in the west and in the east of the north boundary zone of the Australian plate half year ago. It is notable that one of them occurred in the Sumatra island where no deep earthquake with M≥6.0 has occurred in the past thirty years. ④The space distribution of moderate shocks occurring three days ago exhibited a NWW-strike seismic belt along the north boundary zone of the Australian plate. ⑤The activity of volcanoes distributed in the north boundary zone of the Australian plate had been strengthened in the past 4 years, especially several months before the occurrence of the M_W9.0 earthquake.

3-D rheologic model of earthquake preparation (Ⅱ): Strain field and its applications

On the basis of the three-dimensional elastic inclusion model, the analytic expression of viscoelastic strain field is derived, i.e., the analytic expression of viscoelastic strain at an arbitrary point (x, y, z) in x-axis, y-axis and z-axis produced by three-dimension inclusion in the semi-infinite rheologic medium defined by the standard linear rheologic model, namely the normal strains εxx(r, t), εyy(r, t) and εzz(r, t), the shear strains εxy(r, t) and εyx(r, t), εyz(r, t)and εzy(r, t), εxz(r, t) and εzx(r, t), and the bulk-strain θ (r, t). By computing the spatial-temporal variation of bulk strain on the ground produced by a spherical rheologic inclusion in a semi-infinite rheologic medium, we obtained some significant results that the bulk-strain variation with time produced by a hard inclusion has three stages (α, β,γ) with different characteristics, which are similar to those of most geodetic deformation curves, but not the case for those by a soft inclusion. It is meaningful that these theoretical results have been applied to explain preliminarily the characteristics of stage variation of spatial-temporal evolution, the pattern and quadrant distribution of earthquake precursors, the changeability, spontaneity and complexity of short-term and imminent-term precursors. It offers a theoretical base to found the physical model of earthquake precursors and a reference to predict physically the earthquakes.

Characteristics of seismic activity before several large Sumatra, Indonesia, earthquakes

Two great earthquakes of MS8.5 and MS8.3 determined by the China Earthquake Networks Center (CENC) occurred successively on September 12 and 13, 2007 in the sea area to the south of Sumatra, which is another group of large earthquakes after MS8.7 event on December 26, 2004 and MS8.5 event on March 29, 2005.

3-D rheologic model of earthquake prepa-ration (III): Precursor field

On the basis of the theory of viscoelastic displacement and strain field for the three-dimensional rheologic model of earthquake preparation, this paper mainly studies the theoretical solution of precursor field for the three-dimensional rheologic model of earthquake preparation. We derive the viscoelastic analytical expressions of the ground tilt, underground water level, earth resistivity at an arbitrary point (x, y, z) in the rheologic medium, and analyzed the earth resistivity preliminarily, providing a certain theoretical basis for the precursor analysis of seis-mogenic process.

Characteristics of seismic activity before the M_S8.0 Wenchuan earthquake

The characteristics of spatio-temporal seismicity evolution before the Wenchuan earthquake are studied.The results mainly involve in the trend abnormal features and its relation to the Wenchuan earthquake.The western Chinese mainland and its adjacent area has been in the seismically active period since 2001,while the seismic activity shows the obvious quiescence of M≥7.0,M≥6.0 and M≥5.0 earthquakes in Chinese mainland.A quiescence area with M≥7.0 has been formed in the middle of the North-South seismic zone since 1988,and the Wenchuan earthquake occurred just within this area.There are a background seismicity gap of M≥5.0 earthquakes and a seismogenic gap of ML≥4.0 earthquakes in the area of Longmenshan fault zone and its vicinity prior to the Wenchuan earthquake.The seismic activity obviously strengthened and a doughnut-shape pattern of M≥4.6 earthquakes is formed in the middle and southern part of the North-South seismic zone after the 2003 Dayao,Yunnan,earthquake.Sichuan and its vicinity in the middle of the doughnut-shape pattern show abnormal quiescence.At the same time,the seismicity of earthquake swarms is significant and shows heterogeneity in the temporal and spatial process.A swarm gap appears in the M4.6 seismically quiet area,and the Wenchuan earthquake occurred just on the margin of the gap.In addition,in the short term before the Wenchuan earthquake,the quiescence of earthquake with ML≥4.0 appears in Qinghai-Tibet block and a seismic belt of ML≥3.0 earthquakes,with NW striking and oblique with Longmenshan fault zone,is formed.

Study on the Precise Location of the Xinjiang Yutian M_S7.3 Earthquake

The seismic waveform of the Yutian MS7.3 earthquake,Xinjiang on February 12,2014 was recorded clearly and completely by the Digital Seismic Networks of Xinjiang,Qinghai,Tibet,and Xinjiang Hotan array,so the method of joint location by regional seismic network and seismic array can be used to accurately determine the earthquake source location.The following technologies were used in the process of location:(1) We selected seismic stations equally located around the epicenter of the MS7.3 earthquake with an average interval of about 15 degrees in the initial location.(2) The recording waveforms of Yutian seismic station were rotated to the radial and tangential directions to precisely obtain the arrival time of S-waves to determine the epicentral distance.(3) The velocity model was used in the determination of location of the epicenter,based on the historical records of earthquakes in the area within a radius of 1.0° from the source as the center,and the velocity model is obtained after re-fitting and calibration.(4) Based on the waveform records of the Hotan seismic array,the method of waveform beaming was used to determine the azimuths and perform the correction of the epicenter location with these azimuths.(5) The deterministic method was used to measure the source depth.Finally,it is concluded that the Yutian MS7.3 main shock hypocenter location is 36.197°N,82.467°E,focal depth 12 km and original time 17:19:48.2 p.m.February 12,2014.

Study on the Recurrence Probability of Strong Earthquakes of Faults

Based on the physical model of Brownian passage time,the probabilities of recurrence of strong earthquakes on the major active faults in China are calculated in different predictive time spans,based mainly on the analysis of the earthquake preparation process before a strong earthquake occurs. Furthermore,the seismic risks on active faults are studied. The results show that the earthquake probabilities on the Xianshuihe fault,the Altyn Tagh fault,the east Kunlun fault and Xiaojiang fault are significantly greater than other faults in the Chinese mainland,which indicates that the level of stress accumulation on these faults are higher than on other faults. Therefore,these faults may have a seismic risk for strong earthquake in future.

Research on the Result of Calibration for the Location of the Seismic Array in Shanghai

The paper proposes the calibration theory and methods for the Shanghai seismic array and analyzes the calibration results. As a result, the calibration results for the seismic array based on 2 typical earthquakes have been drawn; the difference of calibration results between Hokkaido and Honshu region in Japan is investigated. And calibration results of different directions, different epicenter distances and different magnitudes are probed into. The result shows that the location of earthquakes on the Shanghai seismic array is greatly improved.

Envelope Functions of Time Histories in Seismic Safety Evaluation and Scenario Earthquakes

It has been a period of time since the concept of scenario earthquake was proposed, but this concept has rarely been used in seismic safety evaluation in China since then. Meanwhile, because of the uncertainties of magnitudes-distances pairs, there is large arbitrariness while determining the envelope function of time histories in seismic hazard analysis. In this paper, we describe a method to control the envelope functions of the time histories by introducing the most-likely combinations of magnitude and distance of the scenario earthquakes based on a probabilistic method, revise the software of the ellipse model for seismic hazard analysis, and give a computation example.

Support vector machine method for fore-casting future strong earthquakes in Chinese mainland

Statistical learning theory is for small-sample statistics. And support vector machine is a new machine learning method based on the statistical learning theory. The support vector machine not only has solved certain problems in many learning methods, such as small sample, over fitting, high dimension and local minimum, but also has a higher generalization (forecasting) ability than that of artificial neural networks. The strong earthquakes in Chinese mainland are related to a certain extent to the intensive seismicity along the main plate boundaries in the world, however, the relation is nonlinear. In the paper, we have studied this unclear relation by the support vector machine method for the purpose of forecasting strong earthquakes in Chinese mainland.

Large, moderate, small earthquakes and seismic fortification criterion

This paper discusses the relation between two-step seismic design and the standard of probability of exceedance, and the relation of three-levels seismic ground motion parameters given by probability method and comprehensive probability method. The relative size relations of the ground motions with 2%, 10%, 63% probability of exceedance in 50 years, namely "large earthquake","moderate earthquake", and "small earthquake", are discussed through a practical example of seismic hazard analysis. The methods to determine seismic fortification criterion are discussed.

The characteristics of Quaternary activity of faults in the sea area near the Yangtze River mouth

By shallow seismic prospecting, it is showed that the faults in the sea area near the Yangtze River mouth are mainly the NE and NW-trending faults. The main activity time of fault is Pliocene to Early Pleistocene, and the latest activity is up to Middle Pleistocene. The maximum of fault is generally several tens meters with the throw decreased upward. The dislocation near the bottom of Middle Pleistocene is 12～13 m. The average vertical displacement rate is on a level of 10-3 mm/a.

Primary component analysis method and reduction of seismicity parameters

In the paper, the primary component analysis is made using 8 seismicity parameters of earthquake frequency N (ML≥3.0), b-value, η-value, A(b)-value, Mf-value, Ac-value, C-value and D-value that reflect the characteristics of magnitude, time and space distribution of seismicity from different respects. By using the primary component analysis method, the synthesis parameter W reflecting the anomalous features of earthquake magnitude, time and space distribution can be gained. Generally, there is some relativity among the 8 parameters, but their variations are different in different periods. The earthquake prediction based on these parameters is not very well. However, the synthesis parameter W showed obvious anomalies before 13 earthquakes (MS≥5.8) occurred in North China, which indicates that the synthesis parameter W can reflect the anomalous characteristics of magnitude, time and space distribution of seismicity better. Other problems related to the conclusions drawn by the primary component analysis method are also discussed.

Performance Metrics and Delay Thresholds for Multi-Link Cluster Monitoring in Seismic Profession

Seismic cluster nodes can be monitored by monitoring system,but thresholds for link failure alarm in monitoring systems are not determined presently,especially in different types of cluster links.Communication link types are discussed in seismic profession.By analyzing the characteristics of various links,the main performance metric,network latency,was proposed,which influenced states of communication links and gave the monitoring results deviation formula for judging the cluster monitoring system at different delay thresholds settings based on multiple-link delay error ratio analysis algorithm we offered.From the final experimental data of the monitoring system,fault alarm thresholds settings were posed under five different communication links,which had the instruction significance to the cluster monitoring in seismic profession.

Study on the Crustal Velocity Model of Xinjiang and Its Subareas

In this paper,we firstly analyze the "3,400 travel time table "used for a long time in Xinjiang Seismological Network to obtain the velocity structure models in accord with the table by fitting. Then we fit the velocity of all seismic phases recorded in Xinjiang region in January 2009 ~ December 2013. Simulation analysis is done on the reliability and stability of the velocities,and a concept is proposed for building subarea crustal velocity models according to partitioning of seismic cluster regions. The crustal velocity model suitable for the Yutian area is fitted with the data of all phases of seismic events within a radius of 1°around the 2014 Yutian M_S7.3 earthquake since January 2009,and the model is applied to the relocation of the Yutian M_S7.3 earthquake and determination of focal depths of the earthquake sequence.

Levels of Probability of Exceedance for Current Code Spectra in China

Long-period acceleration spectra determined by seismic safety evaluation for project sites are generally lower than that given by relative code spectra.In this paper,we discuss the recurrence periods corresponding to the code spectra of different periods,by using ground motion attenuation laws for sites of types Ⅰ and Ⅲ.We show the results that the longer the periods of code spectra are commonly the more conservative the seismic level is.As for the project examples in this paper,when the periods are longer than 3.7 or 5.2 seconds for sites of types Ⅰ or Ⅲ,the recurrence periods corresponding to the code spectra are longer than 5000 years.We suggest that some problems need to be further discussed,including the reliability of the present attenuation laws,the performance of those project structures that suffer long period seismic waves and have been designed according to the conservative codes,and the effects of ground motion parameters such as velocity and displacement on seismic design.