The heterogeneous characteristics of crust-mantle structures and the seismic activities in the northwest Beijing region

Introduction Zhangjiakou-Bohai seismic zone is a major active seismic zone that passes through the north-ern part of North China. Zhangjiakou-Beijing area, the northwest part of North China, is located at the intersection position of Yanshan, Taihangshan uplift, and Shanxi down-faulted zone, and the geological structures in this area are extremely complicated. Many researchers in the geoscience circle always pay close attention to this region because Yanqing-Huailai region is located in th…     

The complexity feature of crust-mantle boundary in Zhangbei seismic region and its tectonic implication

The ProP waveform data obtained from a deep seismic sounding profile, which ran through Zhangbei seismic region, were processed by means of both seismic wave complexity coefficient and frequency spectrum analysis methods, and the complexity characteristics of crest-mantle boundary beneath the studied area and its adjacent region were determined. The results show that the place below epicenter can be taken as boundary, the northern side of which is Inner Mongolia axis with small complexity coefficient and the southern side of which is Huai＇an basin with large complexity coefficient. The different spectrum patterns at the two sides of the epicenter were inferred from spectrum analysis. In the epicentral area, there have been multi-period magmatic eruptions since Meso-Cenozoic and craters exist at the surface. From the velocity imaging of middle and upper crust in Zhangbei seismic region it can be found that there are crustal low velocity bodies around the craters and also there are low velocity zones, which went into deep crust. It is suggested that the distinct zones of crust-mantle boundary complexity may be the margin, where the magma had intruded due to magma activity in Meso-Cenozoic. The southern side with large complexity coefficient is deep magmatic activity area and the northern side with small complexity coefficient is stable crust-mantle tectonics. The difference of crust-mantle complexity provides deep background for the development of strong earthquake.     

The regional characteristics of seismic activity in China

ＴｈｅｒｅｇｉｏｎａｌｃｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆｓｅｉｓｍｉｃａｃｔｉｖｉｔｙｉｎＣｈｉｎａＺｈｅｎ－ＬｉａｎｇＳＨＩ，ＪｉａｎＷＡＮＧａｎｄＸｉａｏ－ＤｏｎｇＺＨＡＮＧ（时振梁，王健，张晓东）（ＩｎｓｔｉｔｕｔｅｏｆＧｅｏｐｈｙｓｉｃｓ，Ｓ...     

Study on the relationship between geothermal exploitation and seismic activity in Xi''''an region

IntroductionGeothermal energy is a new and clear energy resource. It is very useful in heating and generating electricity and medical treating. The Assembly of International Environmental Protection in 1992 predicted that the usage and development of geothermal energy would exceed the traditional energy such as oil and coal, and its developmental prospect will be very bright.There are great geothermal resources in Weihe basin. The average value of heat flow in Weihe basin is 7.88(10(2 W/m2…     

Study on distribution characteristics of strong earthquakes in Sichuan-Yunnan area and their geological tectonic background

Introduction Both Sichuan and Yunnan are provinces with more earthquakes. Based on catalogue of strong earthquakes in China compiled by the Prediction Department of China Earthquake Administration, there are 639 M5.0 earthquakes during 26 B.C.~A.D. 2001. Among them, 475 are M=5.0~5.9 events, 124 are M=6.0~6.9 events, 39 are M=7.0~7.9 events, and one is M=8 event occurred in Sichuan and Yunnan area. Here is one of the areas where seismic activities are most active in China. Sichuan-Yun…     

The abnormal mantle and deep tectonic process in the southern region of North China Plain

ＴｈｅａｂｎｏｒｍａｌｍａｎｔｌｅａｎｄｄｅｅｐｔｅｃｔｏｎｉｃｐｒｏｃｅｓｓｉｎｔｈｅｓｏｕｔｈｅｒｎｒｅｇｉｏｎｏｆＮｏｒｔｈＣｈｉｎａＰｌａｉｎＳｈｉ－ＹｕＧＡＯ（高世玉），Ｈｏｎｇ－ＸｉａｎｇＨＵ（胡鸿翔）ａｎｄＳｈａｎＤＩＮＧ（丁山）（Ｉｎ...     

The effect of wind on the gravity wave propagation in the Earth’s ionosphere

The effect of ionospheric wind on the gravity wave propagation is studied. These waves arise in the ionosphere due to intensification of their sources near the Earth’s surface during enhanced seismic activity. The influence of the wind on these waves is connected with the Ampere’s force that produces the ion-drag force acting on the atmosphere. This results in the occurrence of the discrete wave spectrum the maximum of which increases in proportion to the numbers of the natural scale. Furthermore, these waves are amplified during propagation from the source region in the direction perpendicular to the wind direction. These peculiarities of the gravity waves can be used for monitoring of seismic activity based on the ionosphere sounding.     

The emplacement of in situ greenstones in the northern Hidaka belt: The tectonic relationship between subduction of the Izanagi–Pacific ridge and Hidaka magmatic activity

Greenstone bodies emplaced upon or into clastic sediments crop out ubiquitously in the Hidaka belt (early Paleogene accretionary and collisional complexes exposed in the central part of northern Hokkaido, NE Japan), but the timing and setting of their emplacement has remained poorly constrained. Here, we report new zircon U–Pb ages for the sedimentary complexes surrounding these greenstones. The Hidaka Supergroup in the northern Hidaka belt is divided into four zones from west to east: zones S, U, and R, which contain in situ greenstones; and zone Y, which does not. Detrital zircons in zones S, U, and R have early Eocene U–Pb ages (55–47 Ma) and these strata are intruded by early Eocene granites (46–45 Ma), indicating that they were deposited between 55 and 46 Ma. Therefore, in situ greenstones in the northern Hidaka belt can only be explained by the subduction of the Izanagi–Pacific Ridge during 55–47 Ma. In contrast, the deposition of zone Y (the Yubetsu Group, younging to the west) began by 73–71 Ma, indicating that the accretionary prism in front of the paleo-Kuril arc formed at the same time as that in the Idonnappu zone and grew continuously until 48 Ma. The plutonic rocks that intruded the Hidaka belt are roughly divided into three stages: (1) early Eocene granites intruded the northern Hidaka belt at 46–45 Ma, during subduction of the Izanagi–Pacific Ridge; (2) the upper sequence of the Hidaka metamorphic zone was metamorphosed by magmatism at 40–37 Ma associated with the collision of the paleo-Kuril arc and NE Asia; and (3) younger granites intruded the entire Hidaka belt at 20–17 Ma in association with asthenospheric upwelling caused by back-arc expansion.     

On correlation between seismic activities and focal mechanisms of subsequent strong earthquakes

Introduction Earthquakes are direct results from tectonic deformation and crustal movements, which usu- ally contain abundant information of crustal stress status and medium property in the deep lithosphere. Seismic activities might reflect accumulation and concentration of crustal stress in the seismogenic process. And different mechanisms of rock ruptures lead to different precursory phe- nomena prior to earthquakes (ZHANG, et al, 2001). Therefore, seismic activities before a strong earthq…     

Features of isostatic gravity anomaly and seismic activity in the Central Asian region

Ｆｅａｔｕｒｅｓｏｆｉｓｏｓｔａｔｉｃｇｒａｖｉｔｙａｎｏｍａｌｙａｎｄｓｅｉｓ┐ｍｉｃａｃｔｉｖｉｔｙｉｎｔｈｅＣｅｎｔｒａｌＡｓｉａｎｒｅｇｉｏｎＳＨＥＮＧ－ＭＩＮＧＦＡＮＧ１）（方盛明）ＲＵＩＦＥＮＧ２）（冯锐）ＣＨＡＮＧ－ＺＨＥＮＧＴＩＡＮ...     

Triggered and tectonic driven earthquakes in the Koyna–Warna region,western India

Two strong M?>?5.0 earthquakes within a span of six months occurred in a triggered seismicity environment in the Koyna–Warna region in western India in 2000. The region is experiencing continued seismicity since the last five decades indicating that this region is close to critical stresses and minor perturbations in the stresses due to reservoir loading and unloading can trigger earthquakes. In the present study we applied the technique developed for identification of prognostic anomalies for tectonic earthquakes to the Koyna–Warna catalogue prior to these two earthquakes with an aim to study the process of source preparation for triggered earthquakes. In case of tectonic earthquakes, unstable conditions in a source zone develop gradually leading to a metastable zone which shows variations in certain seismicity parameters known as prognostic anomalies. Our results indicate that the variations in seismicity parameters before the two strong earthquakes in the Koyna region have a pattern of prognostic anomalies typical of tectonic earthquakes. We conclude that initiation of failure in a metastable zone can be caused both, by external impacts, reservoir loading and unloading in our case, and internal processes of avalanche-like failure development.     

Interrelation between seismicity parameters and delimiting potential seismic sources in a seismic statistical region and its influence on seismic risk estimation

In the paper, we have discovered the abnormal area distribution features of maximum variation values of ground motion parameter uncertainty with different probabilities of exceedance in 50 years within the range of 100°～120°E,29°～42°N for the purpose to solve the problem that abnormal areas of maximum variation values of ground motion parameter uncertainties emerge in a certain cities and towns caused by seismicity parameter uncertainty in a seismic statistical region in an inhomogeneous distribution model that considers tempo-spatial nonuniformity of seismic activity. And we have also approached the interrelation between the risk estimation uncertainty of a site caused by seismicity parameter uncertainty in a seismic statistical region and the delimitation of potential sources, as well as the reasons for forming abnormal areas. The results from the research indicate that the seismicity parameter uncertainty has unequal influence on the uncertainty of risk estimation at each site in a statistical region in the inhomogeneous distribution model, which relates to the scheme for delimiting potential sources. Abnormal areas of maximum variation values of ground motion parameter uncertainty often emerge in the potential sources of Mu≥8 (Mu is upper limit of a potential source) and their vicinity. However, this kind of influence is equal in the homogeneous distribution model. The uncertainty of risk estimation of each site depends on its seat. Generally speaking, the sites located in the middle part of a statistical region are only related to the seismicity parameter uncertainty of the region, while the sites situated in or near the juncture of two or three statistical regions might be subject to the synthetic influences of seismicity parameter uncertainties of several statistical regions.     

The S to P convert wave from the bottom of sediment basin in the near-field seismic records

Introduction On October 18 and 19, 1989, a sequence of earthquakes with magnitude MS=5.5, 5.8 and 5.5 occurred in the southeast part of Datong basin, Shanxi Province, China. One year and five months later, on March 26, 1991, another strong earthquake with magnitude MS=5.8 strikes the Datong basin once more. After both October 1989 and March 1991 earthquakes, the Institute of Geophysics, China Seismological Bureau (IGCSB) made near-field observations for aftershocks. 5 DCS-302 digital…     

A study on characteristics of tectonic block motion and tectonic setting of strong earthquakes in northern part of the Shanxi fault depression zone

Introduction Shanxi fault depression zone (SFDZ) is one of important Cenozoic fault basin zones and strong earthquake belts in Chinese mainland. Its northern part has aroused wide research interests due to the complicated tectonics and high activity of strong earthquakes there. Early researches on this depression zone were carried out since 60s of last century (DENG, et al, 1973; DENG, YOU, 1985; LU, DING, 1985; XU, 1990; XU, et al, 1996, 2002). In 90s of last century, the geologica…     

Preliminary analysis on the tectonic stress level in the source region of Tangshan earthquake

Introduction Analyzing tectonic stress field based on focal mechanism data is an important way to the study tectonic evolvement of lithosphere and associated dynamic process. Such studies growrapidly in China and abroad (Zoback, 1992; Plenefisch, Bonjer, 1997; XU, 1985; CUI, XIE, 1999). At present most of the studies focus on the inversion of focal mechanism data for the direction and relative magnitude of stress tensor, and few on absolute stress. Using focal mechanism and fault scratch,…     

The characteristics of tectonic stress field about strike slip earthquake－generating structure in the Chinese mainland

（环文林，汪素云，宋昭仪）Ｔｈｅｃｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆｔｅｃｔｏｎｉｃｓｔｒｅｓｓｆｉｅｌｄａｂｏｕｔｓｔｒｉｋｅｓｌｉｐｅａｒｔｈｑｕａｋｅ－ｇｅｎｅｒａｔｉｎｇｓｔｒｕｃｔｕｒｅｉｎｔｈｅＣｈｉｎｅｓｅｍａｉｎｌａｎｄ￥Ｗｅｎ－Ｌｉｎ...     

The seismicity of the Kamchatka region: 1962–2011

This paper reviews the Kamchatka seismicity for a 50-year period of observation. These data were used to carry out a regionalization of Kamchatka’s seismic volume and adjacent areas. In all, ten zones were identified with differing activities and origins of seismicity. A comparative analysis was carried out for the seismicity in the more active zones. We found significant differences between the structures of the southern and the northern segment in the Kamchatka part of the Kuril-Kamchatka subduction zone. Seismological data corroborated a relationship between the subduction zone and the underthrusting of the Pacific plate under the Eurasian plate. These data from the 50-year period of observation helped identify a new Koryak seismic belt that encompasses the northwestern coast of the Bering Sea. We provide a brief review of macroseismic effects due to the most significant earthquakes for the 1962–2010 period.     

Temporal variation of b value associated with M ��4 earthquakes in the reservoir-triggered seismic environment of the Koyna�CWarna region, Western India

It is generally found that the b values associated with reservoir-triggered seismicity (RTS) are higher than the regional b values in the frequency magnitude relation of earthquakes. In the present study, temporal and spatial variation of b value is investigated using a catalog of 3,000 earthquakes from August 2005 through December 2010 for the Koyna?CWarna region in Western India, which is a classical site of RTS globally. It is an isolated (30?×?20?km²) zone of seismicity where earthquakes of up to M ??5 are found to occur during phases of loading and unloading of the Koyna and Warna reservoirs situated 25?km apart. For the Warna region, it is found that low b values of 0.6?C0.9 are associated with earthquakes of M ??4 during the loading phase. The percentage correlation of the occurrence of an M????4 earthquake with a low b value outside the 1?? or 2?? level is as high as 78?%. A drastic drop in the b value of about 50?% being reported for an RTS site may be an important precursory parameter for short-term earthquake forecast in the future.