On Shear Wave Splitting in the Zhangjiakou-Bohai Sea Seismic Belt

The Zhangjiakou-Bohai Sea seismic belt is an important seismic zone in North China. The direction of principal compressive stress is near EW in this region. According to digital seismic data from the Capital Region from September 2005 to September 2010,and using the SAM method,the spatial distribution of the crust anisotropy characteristics are studied and discussed in the middle-eastern part of the Zhangjiakou-Bohai Sea seismic belt. The principal polarization direction,which is near EW direction,is obvious in the middleeastern Zhangjiakou-Bohai Sea seismic belt. The spatial distribution of polarization direction crossing the Zhangjiakou-Bohai Sea seismic belt shows that there is little difference among the Yanshan uplift area,inside of the seismic zone and North China basin,and the principal polarization direction is near EW.

A DEEP SEISMIC REFLECTION PROFILE ACROSS ALTUN FAULT BELT

Altun fault is regarded as a large\|scale sinistral strike\|slip fault, it is composed of several faults with the different character, and there is a special geological structure in the fault belt, and they constitute the northwestern margin fault belt of the Qinghai\|Tibetan plateau. In order to investigate the deep crust structure in the Altun region, layers which Tarim lithosphere subducted beneath the Qinghai\|Tibetan plateau, the forward structure of the subduction plate and the scale of the plate subduction, a deep seismic reflection profile was designed. Data collection work of the deep seismic reflection profile across Altun fault was completed during 24/8/1999 to 25/9/1999. The profile locates in Qiemo county, Xinjiang Uygur Autonomous Region, the southern end of the profile stretches into Altun Mountains, the northern end locates in the Tarim desert margin. The profile is nearly SN trending and crosses the main Altun fault. The profile totally is 145km long, time record is 30 seconds, the smallest explosive amount is 72～100kg, the biggest explosive amount reaches 200～300kg, the explosive distance is 800m, and detectors are laid at a 50m distance.

Comparative Study of Global Seismicity on the Hot Engine Belt and the Cooling Seismic Belt—Improvement on Research Ideas of Earthquake Prediction

The study in this paper analyzes and compares the distribution on the global engine active seismic zone and cooling seismic belt basing on the ANSS earthquake catalog from Northern California Earthquake Data Center. An idea of the seismogenesis and earthquake prediction research is achieved by showing the stratigraphic structure in the hot engine belt. The results show that the main engine and its seismic cones are the global seismic activity area, as well as the subject of global geological disaster. Based on the conjecture of other stratum structure, the energy of crustal strong earthquake and volcano activities probably originates from the deep upper mantle. It is suggested that the research on earthquake and volcano prediction should focus on the monitor and analysis on the sub-crustal earthquake activities.

Crustal structure of the eastern Dabie orogenic belt from the seismic tomography and wide-angle reflection studies

The studies of seismic tomography and wide-angle reflection have been carried out to reveal the velocity structUrebeneath the eastern Dabie orogenic belt. The result from the seismic tomography shows the high velocity bodiesmight be positioned to a depth of only about 1 .5 km below sea level within the Dabie ultra-high pressure metamorphic (UHPM) belt; the fan-profile shows the Shuihou-Wuhe fault, the demarcation between the South Dabieand the North Dabie, slopes to the south-west at a dip angle of about 45° in the bottom of upper crust. The wideangle reflection shows the middle crustal boundaries and the complex features from the lower crust.

Twin lintel belt in steel for seismic strengthening of brick masonry buildings

A single-room,single-storey full-scale brick masonry building with precast RC roofing system was tested thrice under displacement controlled lateral cyclic loading,to assess the effectiveness of the basic repair and seismic strengthening techniques.Initially,the virgin building specimen was loaded laterally to f^tilure.In the second stage,the damaged building was repaired by stitching across the cracks,and tested under the same lateral loading.In the third stage,the twice-damaged structure was repaired once more by stitching and strengthened by twin lintel belt in steel and vertical comer reinforcement, and re-tested.The building strengthened by twin lintel belt in steel showed about 28% higher strength under lateral loading than the virgin building.

Regional characteristics of stress field in the southern part of the north－south seismic belt in China and its relation with plate movement

The regional characteristics of stress field in the southern part of the northusouth seismic belt (NSB) have beenanalyzed in detail based on the mechanism solutions of 134 medium and large earthquakes from 1933 to 1991.The results show that the southern part of the NSB is a shallow earthquake zone where most earthquakes arecaused by the strike-slip faulting. There is a systematic distribution of the directions of P-and T-axes in thewestern and the eastern regions of the southern part of the NSB. P-and T-axes in the western region are in theNE-SW direction and in the NW-SE direction. P-and T-axes in the eastern region are oriented in NW-SE andNE-SW, respectively. The directions of p-axes in the western and the eastern regions show a pattern of a reversal 'V' as a whole. The boundary between the eastern and western regions coincides with that between the Tibetan Plateau and the Yangtze crustal block. Based on a lot of mechanism solutions, the result indicates that thedirection of P-axes roughly shows the consistent distributions from the Himalayan collision zone to the easternregion and from the eastern coast collision zone in Taiwan to the eastern region of southern part of the NSB, respectively. It is suggested that the tectonic force due to relative movement between the indo-Australian and theEurasian plates is transmitted from the Himalayan collision zone to the western region of the southern part ofNSB, simultaneously, the tectonic force due to the relative movement between the Philippine Sea and theEurasian plates is transmitted from the eastern region coast in Taiwan to the eastern region of the southern partof NSB, and control the stress field there, respectively.

Global shear stress pattern in main seismic active belts

Based on the relationship between body wave magnitued m b and seismic moment M 0 presented by PEI SHAN CHEN (1981), by using the moment tensor and focal mechanism solution in the earthquake reports of EDR, a global shear stress pattern including shear stress values and directions of P and T principal axis was obatained. The distribution of ambient shear stress values in the globe is: the highest in intraplate followed by subduction zone, and the lowest in oceanic ridge. The horizontal directions of the maximum principal stresses we got are coincident very well with the result of Zoback (1989). The detail analysis of the stress state in Tonga region shows that: The subduction slab bends down in shallow by press and bends up in deep because of the resistance from deeper part. Between them, the slab is in an equilibrum state. After analysizing the global stress distribution, we got the result that: the plate is driven by a drag force from under its bottom, the plate motion results in its extensional state in oceanic ridge and compressive state in subduction zone.

Study on Evolution of Gravity Fieldand Earthquake Prediction in theNorth-south Seismic Belt and theEastern Qinghai-Xizang Block

The relation between the dynamic evolution feature of gravity field and strong seismicity is studied. The result shows that the regional gravity field variation enjoys inhomogeneity of spatial and temporal distribution and gravity change in different regions. It may be resulted from active faults and seismogenic process, and may be due to microdynamic activity of regional strain energy, which might be accumulated or released in different stages, and there exists transformation process of stress.

Analysis of Spatial and Temporal Variation Characteristics of the bvalue in the Hetao Seismic Belt

Focusing on the b-value as the research target and under the theoretical framework that the b-value is determined by stress state and medium properties, the variation characteristics of the b-value in the Hetao seismic belt are analyzed. Earthquakes with ML≥1. 5,which have occurred in the Hetao seismic belt since 1970 are selected to conduct the quantitative detection of the non-uniform temporal change of Mcusing the EMR method. Based on the actual situation of seismic activity,the lower limit magnitude is set as ML2. 0 to calculate the b-value. The temporal variation of the b-value is calculated and scanned using the least square method. The results show that there is a good corresponding relationship between the temporal variation of the b-value,strong earthquake activity,network distribution and aftershock deletion. We also calculate and scan the spatial variation of the b-value by using maximum likelihood. The results show that the spatial difference is possibly caused by stress state and crustal medium properties. The tectonic dependence of the b-value is obvious. In addition,the sufficient earthquakes samples in each magnitude interval are still a key step to improve the calculation accuracy of the b-value.

Ancient terrane boundaries as probable seismic hazards: A case study from the northern boundary of the Eastern Ghats Belt, India

In the eastern part of the Indian shield, late Paleozoic-Mesozoic sedimentary rocks of the Talchir Basin lie precisely along a contact of Neoproterozoic age between granulites of the Eastern Ghats Mobile Belt （EGMB） and amphibolite facies rocks of the Rengali Province. At present, the northern part of the basin experiences periodic seismicity by reactivation of faults located both within the basin, and in the Rengali Province to the north. Detailed gravity data collected across the basin show that Bouguer anomalies decrease from the EGMB （~＋15 mGal）, through the basin （^-10 mGal）, into the Rengali Province （^-15 mGal）. The data are consistent with the reportedly uncompensated nature of the EGMB, and indicate that the crust below the Rengali Province has a cratonic gravity signature. The contact between the two domains with distinct sub-surface structure, inferred from gravity data, coincides with the North Orissa Boundary Fault （NOBF） that defines the northern boundary of the Talchir Basin. Post-Gondwana faults are also localized along the northern margin of the basin, and present-day seismic tremors also have epicenters close to the NOBF. This indicates that the NOBF was formed by reactivation of a Neo- proterozoic terrane boundary, and continues to be susceptible to seismic activity even at the present-day.

Discussion about the relationship between seismic belt and seismic statistical zone

This paper makes a summary of status of delimitation of seismic zones and belts of China firstly in aspects of studying history, purpose, usage, delimiting principles, various presenting forms and main specialties. Then the viewpoints are emphasized, making geographical divisions by seismicity is just the most important purpose of delimiting seismic belts and the concept of seismic belt is also quite different from that of seismic statistical zone used in CPSHA method. The concept of seismic statistical zone and its history of evolvement are introduced too. Large differences between these rwo concepts exist separately in their statistical property, actual meaning, gradation, required scale, and property of refusing to overlap each other, aim and usage of delimitation. But in current engineering practice, these two concepts are confused. On the one hand, it causes no fit theory for delimiting seismic statistical zone in PSHA to be set up; on the other hand, researches about delimitation of seismic belts with purposes of seismicity zoning and studying on structural environment, mechanism of earthquake generating also pause to go ahead. Major conclusions are given in the end of this paper, that seismic statistical zone bases on the result of seismic belt delimiting, it only arises in and can be used in the especial PSHA method of China with considering spatially and temporally inhomogeneous seismic activities, and its concept should be clearly differentiated from the concept of seismic belt.

Long－ and intermediate－termseismic poten－tial of Fen－Wei seismic belt：active fault data application

Recurrence model for strong earthquakes on Fen Wei seismic belt is proposed on the basis of the collection and analysis of fault slip rate, paleoearthquake sequence, maximum displacement of each event etc. on 21 fault segments of the belt, which are active since late Late Pleistocene. And the long and intermediate term seismic potential of the belt has been evaluated through four approaches.

Characteristics of satellite-gravity variations in the North-South Seismic Belt before the 2013 Lushan earthquake

To study the characteristics of gravity variations in and near the North-South Seismic Belt before the 2013 Lushan earthquake,we used the geopotential-field models based on monthly data of the RI.~5 GRACE satellite to calculate the gravity changes. Here we present the patterns of annually cumulative variation, differentiatial variation and secular trend, as well as the continuous time-series at 4 characteristic sites during 2004 -2012. The result shows that the anomalous positive-to-negative transition zone, in which the epicenter of the 2008 Wenchuan earthquake was located, did not show any new gravity change before the Lushan earthquake, though located in the same zone.

Structural features of crust-mantle assemblage in the lithosphere along the Longitudinal Seismic Belt of China and their tectonic effect

The 1°×1° distribution map of crustmantle structural ratio R for the lithosphere along the Longitudinal Seismic Belt of China has been compiled using computer based on the results of geophysical prospecting by previous researchers, and the latest results by the present authors. Based on this map, an insight into the structural features of the crustmantle assemblage along the Longitudinal Seismic Belt has been gained, while their relation to seismic activity and the distributions of geothermal flux and intracrustal high conductivitylow velocity layers, as well as their tectonic effect to seismicity have been discussed.

Seismic Inventory Simulation and Conformance Test Based on the Monte Carlo Method——A Case Study of the Fenhe-Weihe Belt

Artificial earthquake catalogue simulation is one of the ways to effectively improve the incompleteness of the existing earthquake catalogue,the scarcity of large earthquake records and the improvement of seismological research. Based on the Poisson distribution model of seismic activity and the Gutenberg-Richter magnitude-frequency relationship,the Monte Carlo method which can describe the characteristics of the stochastic nature and the physical experiment process is used. This paper simulates the future seismic catalogues of the Fenhe-Weihe seismic belt of different durations and conducts statistical tests on them.The analysis shows that the simulation catalogue meets the set seismic activity parameters and meets the Poisson distribution hypothesis,which can obtain a better simulated earthquake catalogues that meets the seismic activity characteristics. According to the simulated earthquake catalogues,future earthquake trends in this region are analyzed to provide reference for seismic hazard analysis.

A fast automatic identification method for seismic belts based on distance correlation and its earthquake case

Earthquake prediction practice and a large number of earthquake cases show that anomalous images of small earthquake belts may appear near the epicenter before strong earthquakes.Through the research of earthquake cases,researchers have a relatively consistent method to determine the clarity of an identified seismic belt,but there is still a lack of method on seismic belt identification from the distribution of scattered points.Due to the complexity of exhaustive algorithm,the rapid automatic identification technique of seismic belts has been progressing slowly.Visual recognition is still the basic method of seismic belt identification.Based on the algorithm of distance correlation,this paper presents a fast automatic identification method of seismic belts.The effectiveness of this method was proved by 100 random earthquakes and an example of seismic belts of magnitude 4.0 before the 2005 Jiujiang M5.7 earthquake.The results show that:①the automatic identification of seismic belts should first identify the"relational earthquake",then identify the"suspected seismic belt",and finally use the criterion of seismic belt clarity to determine;②random earthquakes and real earthquakes identification results show that the distance correlation method can realize the fast automatic identification of seismic belts by computer.

The NE Directed Seismicity Belt in Tibet after the M_S8.1 Nepal Earthquake and Its Predictive Significance

After the 2015 M_S8. 1 Nepal earthquake,a strong and moderate seismicity belt has formed in Tibet gradually spreading along the northeast direction. In this paper,we attempt to summarize the features and investigate the primary mechanism of this behavior of seismic activity,using a 2-D finite element numerical model with tectonic dynamic settings and GPS horizontal displacements as the constraints. In addition,compared with the NEtrending seismicity belt triggered by the 1996 Xiatongmoin earthquake,we discuss the future earthquake hazard in and around Tibet. Our results show that: the NE-directed seismicity belt is the response of enhanced loading on the anisotropic Qinghai-Tibetan plateau from the Indian plate and earthquake thrusting. Also,this possibly implies that a forthcoming strong earthquake may fill in the gaps in the NE-directed seismicity belt or enhance the seismic hazard in the eastern( the north-south seismic zone) and western( Tianshan tectonic region) parts near the NE-directed belt.

Relationship between crustal magnetic anomalies and strong earthquake activity in the south segment of the China North-South Seismic Belt

The south segment of the China North-South Seismic Belt is located in the southeast margin of the Qinghai Tibet Plateau.This region is characterized by the frequent seismic activity in Chinese mainland.In this paper,the geomagnetic field model NGDC-720 and the data of terrestrial heat flow are used to investigate the distribution of crustal magnetic anomalies,the depth of Curie surface,and the characteristics of the crustal thermal structure in the south segment of the North-South Seismic Belt.The distribution characteristics of the vertical component AZ and the magnetic declination AD in the area with earthquakes over a magnitude of 6 and their aftershocks since 1970 are focused on.The results show that the earthquakes are mainly observed in the area of negative magnetic anomaly or the strong and weak transition zone.It especially shows in the AD.The Curie surface in the study area varies significantly,ranging from 20.8 to 31 km.The uplift area of the Curie surface is consistent with the high-value area of terrestrial heat flow.The high geothermal area corresponds to the strong earthquake activity area.The focal depth of most strong earthquakes is shallower than the depth of the Curie surface.The strong earthquakes mainly occur in the deep-shallow transition zones of the Curie surface.The results can be used as a reference for strong earthquake prediction in this area.

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 NorthSouth 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.

Crustal velocity, density structure, and seismogenic environment in the southern segment of the North- South Seismic Belt, China

The southern segment of the North-South Seismic Belt in China is a critical region for earthquake preparedness and risk reduction efforts.However,limited by the low density of seismic stations and the use of single-parameter physical structural models,the deep tectonic features and seismogenic environment in this area remain controversial.Thus,a comprehensive analysis based on high-resolution crustal structures and multiple physical parameters is required.In this study,we applied the ambient noise tomography method to obtain the three-dimensional(3D)crustal S-wave velocity structure using continuous waveform data from 112 permanent stations and 350 densely distributed temporary stations in the southern segment of the North-South Seismic Belt.Then,we obtained the high-resolution 3D density structure through wavenumber-domain 3D gravity imaging constrained by the velocity structure.The low-velocity and low-density anomalies in the upper crust of the study area were mainly distributed in the Sichuan Basin and around Dali and Simao,while the high-velocity and high-density anomalies were primarily distributed in the Panxi region,corresponding to the surface geological features.Two prominent low-velocity and low-density anomalies were observed in the middle and lower crust:one to the west of the Songpan-Garzêblock and Sichuan-Yunnan diamond-shaped block,and the other near the Anninghe-Xiaojiang fault.Combined with the spatial distribution of seismic events in the study area,we found that previous earthquakes predominantly occurred in the transition zones between high and low anomaly regions and in the low-velocity and low-density zones in the upper crust.In contrast,moderate-to-strong earthquakes mainly occurred within the transition zones between high and low anomaly regions and close to the high-velocity and high-density regions,often with low-velocity and low-density layers below their hypocenters.Fluids play a critical role in the seismogenic process by reducing fault strength and destabilizing the stress state,which may be a triggering factor for earthquakes in the study area.Additionally,the upwelling of molten materials from the mantle may lead to energy accumulation and stress conce-ntration,providing an important seismogenic background for moderate-to-strong earthquakes in this area.