Application of Characteristics of Seismogenic Structures in the Determination of Parameter of Potential Seismic Source Areas

INTRODUCTIONThe method of probabilistic seismic risk analysis was proposed by Cornell in1968(Cornell,1968).After more than30years development,it has become the main method for seismic riskassessment of engineering sites and seismic zonation,and has been u…     

A Computer Model for Cyclic Activity of Strong Earthquakes in Continental Seismic Zones

Based on the study of activity of earthquakes with M**********>7.0 in mainland China,we have found a dynamic pattern,i.e.,the cyclic characteristics in time and migration from one seismic zone to another in space.In order to understand the physical mechanism of this pattern,we use a nonlinear dynamic model to simulate the seismic activity in fault zones under a unified tectonic stress field.The basic elements in our model consist of a Maxwell body and a rigid sliding block.Basic elements in a column represent a fault,and coupling elements connecting different columns simulate the interaction among faults and fault segments.The results provide insights to the cyclic activity of strong earthquakes and to the feature of mutual influence between strong earthquakes in groups in the climax of seismic activity.     

Thermal Process in the Crust and Upper Mantle in Xingtai-Shulu area of Hebei Province,China

Study of geothermal field data,terrestrial heat flow values,and other geophysical data from the Xingtai-Shulu area of Hebei Province made us more understanding of the distribution of geothermal fields and deep structures and their interrelation.The study illustrates that the geothermal field has an apparent lateral inhomogeneity and is evidently correlated by the structure of the crust and upper mantle in the area.The relation of the geothermal field distribution to the structure indicates that in comparison with the depression zone,the uplift zone has a higher heat flow value and a larger geothermal gradient.The analysis of the relation between distribution of earthquake epicenters and geothermal field and mathematical simulation of thermal stress in the area suggests that thermal stress plays an important role in the process of earthquake generation.     

Exploration and Study on Deep Crust Structural Characteristics in the Jiashi-Artux Seismic Region

The data from two deep seismic sounding profiles was processed and studied comprehensively. The results show that crnst-mantle structures in the investigated region obviously display layered characteristics and velocity structures and tectonic features have larger distinction in different geological structure blocks. The boundary interface C between the upper and lower crust and Moho fluctuate greatly. The shallowest depths of C (30.0km) and Moho (45.5km) under Jiashi deepen sharply from Jiashi to the western Kunlun mountain areas, where the depths of C and Moho are 44.0km and 70.0km, respectively. The higher velocity structures in the Tarim massif determine its relatively “stable“ characteristics in crust tectonics. The phenomenon in the Jiashi region, where the distribution of earthquake foci mostly range from 20kin to 40kin in depth, may infer that the local uplift of C and Moho interface, anomalonsly lower velocity bodies and deep large faults control earthquake occurrence and seismogenic processes in the Jiashi strong earthquake swarm.     

Corresponding Relation Between the Space-time Evolution of Seismic Apparent Strain and the Region of Strong Earthquakes in Yunnan

On the basis of our predecessors' research,we study the distribution and the space-time evolution characteristics of the seismic apparent strain field in Yunnan since the 1970's using the seismic data of Yunnan and its surrounding areas.The result shows that there is a rather strong corresponding relationship between the anomaly region of seismic apparent strain and strong earthquakes.In the nine earthquakes studied,anomaly areas of seismic apparent strain had appeared before eight earthquakes,including five occurring in the anomaly region and three on the edge.Finally,the investigative result is demonstrated primarily.     

The Volutionary Characteristics of Seismic Activity in the SichuanYunnan Region,China

Based on the earthquake activity characteristics of the diamond block in the Sichuan-Yunnan region and by using the method of the meso-scope damage dynamics and damage evolution,we studied the damage evolution process for moderately strong earthquakes along two seismicbelts.The original combination patterns of all the units which illuminate the changes from stable state to destroyed state are given.All these patterns can direct the earthquake prediction practice in this region     

Focal Depths of Earthquakes in the Crimea–Black Sea Region

The problem of determining focal depths of earthquakes in the Crimea–Black Sea region is considered. Based on the results of interannual studies, it is found that the focal depths of Crimean earthquakes are mainly crustal, with maximum values of up to 60 km. Some recent publications, however, have described deep-focus earthquakes with depths of up to 300 km which were “revealed” in the Crimean region. In this respect, there arose the need to study such a large difference in estimated focal depths. Convincing examples show that the sensational “revelation” of deep earthquakes in Crimea was caused by incorrect processing of the experimental data, in particular, due to (1) a sharp distortion in the recorded arrival times of body waves, (2) exclusion of data from stations nearest to a source, (3) unreasonable arbitrary selection of data from seismic stations, and (4) dropping of data from the worldwide seismological network, including those on deep seismic phases. Thus, the conclusions about the presence of deep mantle earthquakes in Crimea are erroneous. We have redetermined the parameters of hypocenters and verified that the focal depths of earthquakes in the Crimea–Black Sea region are no more than 60 km. Based on these data, we analyze the features of the spatial distribution of focal depths to show that earthquake sources are grouped along conduits that dip southeastward, from the continental part of Crimea toward the Black Sea Basin, in the case of grouping of sources in the Alushta–Yalta and Sevastopol areas. The seismic focal layer of the Kerch–Anapa area dips northeastward, from the Black Sea beneath the North Caucasus.     

On the Problem of Deep Earthquakes in Crimea–Black Sea Region

It is a common opinion that only crustal earthquakes can occur in the Crimea–Black Sea region. Since the existence of deep earthquakes in the Crimea–Black Sea region is extremely important for the construction of a geodynamic model for this region, an attempt is made to verify the validity of this widespread view. To do this, the coordinates of all earthquakes recorded by the stations of the Crimean seismological network are reinterpreted with an algorithm developed by one of the authors. The data published in the seismological catalogs and bulletins of the Crimea–Black Sea region for 1970–2012 are used for the analysis. To refine the coordinates of hypocenters of earthquakes in the Crimea–Black Sea region, in addition to the data from stations of the Crimean seismological network, information from seismic stations located around the Black Sea coast are used. In total, the data from 61 seismic stations were used to determine the hypocenter coordinates. The used earthquake catalogs for 1970–2012 contain information on ~2140 events with magnitudes from–1.5 to 5.5. The bulletins provide information on the arrival times of P- and S-waves at seismic stations for 1988 events recorded by three or more stations. The principal innovation of this study is the use of the original author’s hypocenter determination algorithm, which minimizes the functional of distances between the points (X, Y, H) and (x, y, h) corresponding to the theoretical and observed seismic wave travel times from the earthquake source to the recording stations. The determination of the coordinates of earthquake hypocenters is much more stable in this case than the usual minimization of the residual functional for the arrival time of an earthquake wave at a station (the difference between the theoretical and observed values). Since determination of the hypocenter coordinates can be influenced by the chosen velocity column beneath each station, special attention is focused on collecting information on velocity profiles. To evaluate the influence of the upper mantle on the results of calculating the velocity model, two different low-velocity and high-velocity models are used; the results are compared with each other. Both velocity models are set to a depth of 640 km, which is fundamentally important in determining hypocenters for deep earthquakes. Studies of the Crimea–Black Sea region have revealed more than 70 earthquakes with a source depth of more than 60 km. The adequacy of the obtained depth values is confirmed by the results of comparing the initial experimental data from the bulletins with the theoretical travel-time curves for earthquake sources with depths of 50 and 200 km. The sources of deep earthquakes found in the Crimea–Black Sea region significantly change our understanding of the structure and geotectonics of this region.     

The K Method for Estimating Earthquake Activity Parameters and Effect of the Boundary Uncertainty of the Source Region:Discussion on the Seismic Zoning Method

Two aspects of a new method,which can be used for seismic zoning,are introduced in this paper.On the one hand,the approach to estimate b value and annual activity rate proposed by Kijko and Sellevoll needs to use the earthquake catalogue.The existing earthquake catalogue contains both historical and recent instrumental data sets and it is inadequate to use only one part.Combining the large number of historical events with recent complete records and taking the magnitude uncertainty into account,Kijko's method gives the maximum likelihood estimation of b value and annual activity rate,which might be more realistic.On the other hand,this method considers the source zone boundary uncertainty in seismic hazard analysis,which means the earthquake activity rate across a boundary of a source zone changes smoothly instead of abruptly and avoids too large a gradient in the calculated results.     

Anomalies of Precursory Group and Grouped Strong Earthquakes in the Sichuan-Yunnan Region

Three methods of extracting the information of anomalies of a precursory group are put forward, i.e., the mathematical analyses of the synthetic information of earthquake precursors(S), the inhomogeneous degree of precursory groups ( ID ) and the values of short-term and impending anomaly in near-source area (NS). Using these methods, we calculate the observational data of deformation, underground fluid and hydrochemical constituents obtained from different seismic stations in the Sichuan-Yunnan region and conclude that the synthetic precursory anomalies of a single strong earthquake with Ms6.0 differ greatly from those of the grouped strong earthquakes, for the anomalous information of precursory groups are more abundant. The three methods of extracting the synthetic precursory anomaly and the related numerical results can be applied into the practice of prediction to the grouped strong earthquakes in the Sichuan-Yunnan region. Inhomogeneons degree (ID) of synthetic precursory anomaly can be identified automatically because it takes the threshold of distributive characteristics of the anomalies of precursory group as its criterion for anomaly.     

The Seismic Risk Assessment and Seismic Zoning of the Hong Kong Region

The Hong Kong-Kowloon Block,which has been uplifted continually and is stable since the late Cretaceous-Paleogene period,is identified and determined as a potential source with a maximum credible magnitude of M=5.5,while the late Cretaceous-Paleogene fault basins located around the Hong Kong-Kowloon Block are determinated as a potential source with a maximum credible magnitude of M≥6.0 based on the characteristics of seismo-tectonic activities in Hong Kong and its adjacent regions.The attenuation law of peak ground acceleration(PGA)and standard response spectra of acceleration(β)with damping 5% underlain by basement were determined by comparing the Chinese continent with the western U.S.A.in the similarity of tectonic environment of continental earthquakes,potential source with credible seismic magnitude of 5.0-7.5,and considering ground motion saturation near the source.The near source attenuation model of acceleration and response spectrum also reflect the field of Intensity distribution in East Chi     

The State of the Art in Studying the Deep Structure of the Earth’s Crust and Upper Mantle beneath the Baikal Rift from Seismological Data

Izvestiya, Physics of the Solid Earth - Abstract—This review discusses the major milestone results yielded by the regional seismological studies of the deep structure of the Earth’s...     

Geomagnetic Field Variations Caused by the Processes Occurring at Different Depths in the Earth’s Crust and Upper Mantle

Izvestiya, Physics of the Solid Earth - Abstract—The results of geomagnetic studies in the Tashkent and Fergana geodynamic test sites, Uzbekistan, are presented. Detail analysis of anomalous...     

Study on Integrated Recurrence Behaviors of Strong Earthquakes Along Entire Active Fault Zones in the Sichuan-Yunnan Region, China

Based on historical earthquake data, we use statistical methods to study integrated recurrence behaviors of strong earthquakes along 7 selected active fault zones in the Sichuan-Yunnan region. The results show that recurrences of strong earthquakes in the 7 fault zones display near-random, random and clustering behaviors. The recurrence processes are never quasiperiodic, and are neither strength-time nor time-strength dependent. The more independent segments for strong earthquake rupturing a fault zone has, the more complicated the corresponding recurrence process is. And relatively active periods and quiescent periods for earthquake activity occur alternatively. Within the active periods, the distribution of recurrence time intervals between earthquakes has relatively large discretion, and can be modelled well by a Weibull distribution. The time distribution of the quiescent periods has relatively small discretion, and can be approximately described by some distributions as the normal. Both the durations of the active periods and the numbers of strong earthquakes within the active periods vary obviously cycle by cycle, leading to the relatively active periods having never repeated quasi-periodically. Therefore, the probabilistic assessment for middle- and longterm seismic hazard for entireties of active fault zones based on data of historical strong earthquakes on the fault zones still faces difficulty.     

Spatial and Spectral Analysis of Refined Gravity Data for Modelling the Crust–Mantle Interface and Mantle-Lithosphere Structure

We analyse spatial and spectral characteristics of various refined gravity data used for modelling and gravimetric interpretation of the crust–mantle interface and the mantle-lithosphere structure. Depending on the purpose of the study, refined gravity data have either a strong or weak correlation with the Moho depths (Moho geometry). The compilation of the refined gravity data is purely based on available information on the crustal density structure obtained from seismic surveys without adopting any isostatic hypothesis. We demonstrate that the crust-stripped relative-to-mantle gravity data have a weak correlation with the CRUST2.0 Moho depths of about 0.02. Since gravitational signals due to the crustal density structure and the Moho geometry are subtracted from gravity field, these refined gravity data comprise mainly the information on the mantle lithosphere and sub-lithospheric mantle. On the other hand, the consolidated crust-stripped gravity data, obtained from the gravity field after applying the crust density contrast stripping corrections, comprise mainly the gravitational signal of the Moho geometry, although they also contain the gravitational signal due to anomalous mass density structures within the mantle. In the absence of global models of the mantle structure, the best possible option of computing refined gravity data, suitable for the recovery/refinement of the Moho interface, is to subtract the complete crust-corrected gravity data from the consolidated crust-stripped gravity data. These refined gravity data, that is, the homogenous crust gravity data, have a strong absolute correlation of about 0.99 with the CRUST2.0 Moho depths due to removing a gravitational signal of inhomogeneous density structures within the crust and mantle. Results of the spectral signal decomposition and the subsequent correlation analysis reveal that the correlation of the homogenous crust gravity data with the Moho depths is larger than 0.9 over the investigated harmonic spectrum up to harmonic degree 90. The crust-stripped relative-to-mantle gravity data correlate substantially with the Moho depths above harmonic degree 50 where the correlation exceeds 0.5.     

Characteristics of Geoelectric and Geomagnetic Data before Strong Earthquakes in the North China Region

The earth resistivity and geomagnetic observation data of the Z component from many stations in near-epicentral areas of strong earthquakes are combined and calculated by using a spatial linearity method and a new characteristic value, the spatial linearity a, describing the precursory field in near-epicentral area of strong earthquake, is obtained. The analysis and calculation results of geoelectric and geomagnetic observation data prior to 8 earthquakes with M≥5.5 occurring in the North China region show that a value near the epicentral area tendsto decrease significantly half a year before strong earthquakes.     

Elementary Research on Activity Behavior of the Crust in Funan-Huoqiu Area, Southern Border of the North China Active Block Region

1.ESSENTIAL GEOLOGIC GEOMORPHIC CHARACTERIN THE STUDIDED REGIONThe studied region of the paper,the Funan-Huoqiu area,extends across the Jianghuai hills andHuaibei plain of Anhui Province,located in the south border area of the North China active blockregi…     

Analysis of Seismic Activity in the Middle Part of the North-South Seismic Belt—Joint Study on Deep Seismic Sounding Profile and Seismicity Parameters

Using the rich deep seismic sounding data recorded in the middle part of the North-South Seismic Belt in China, the horizontal and vertical profiles are constructed to obtain the seismic velocity structure, analyze the seismic distribution and calculate the seismic energy and the thickness of the seismogenic layer at the same time. On this basis, the seismicity parameters are calculated using the earthquake catalogue of the study area for the past 40 years, and the relationship between the b-value distribution and the velocity structure is analyzed. The results show an uneven b-value distribution in the study area and a segmented feature along the Longmenshan fault zone. Most of the earthquakes occur in the transition zone anomalies from the positive to the negative. In addition, the thickness of the crust drops from ~60 km to ~48 km from the Southeastern to the Northeastern Qinghai-Tibetan Plateau, but the thickness of the seismogenic layer increases gradually. It is speculated that the crustal composition of the Northeastern margin contains more felsic materials and has relatively stronger seismic activities than the Southeastern Qinghai-Tibetan Plateau, possibly associated with the subduction and compression of the Indian Ocean Plate.