Table of Content

06 March 2002, Volume 24 Issue 1

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Brief Report

CHARACTERISTIC FEATURES OF THE SURFACE RUPTURES OF THE HOH SAI HU (KUNLUNSHAN) EARTHQUAKE (M_S8.1), NORTHERN TIBETAN PLATEAU, CHINA

XU Xi-wei, CHEN Wen-bin, YU Gui-hua, MA Wen-tao, DAI Hua-guang, ZHANG Zhi-jian, CHEN You-min, HE Wen-gui, WANG Zan-jun, DANG Guang-ming

2002, 24(1):  1-13. 

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On 14 November 2001, an extraordinarily large earthquake occurred on the Hoh Sai Hu segment of the Eastern Kunlun Fault, northern Tibetan Plateau. This event, named as Hoh Sai Hu (Kunlanshan) Earthquake, is the largest earthquake occurred in China continent for the past 50 years. The moment magnitude of this earthquake reaches 7.7 to 7.9 (USGS National Earthquake Information Center, 2001; Harvard CMT Catalog, 2001) and the surface wave magnitude reaches 8.1 (China Digital Seismic Network, 2001). Field investigation indicates that the surface rupture zone produced by this earthquake is striking N80°±10°W with a length of 350 km, which initiates from 91°E in the west nearby the east of Buka Daban Feng, a snow-capped summit with an altitude of 6 800m, extends eastwards along the fault traces of the Hoh Sai Hu segment, and terminates at the 94.8°E in the east. The surface ruptures of this earthquake consist of shear fractures, transtensional fractures, tension gashes and mole tracks arranged in en echelon. The shear fractures are N80°～90°W trending and dominated by left-lateral slip. Transtenssional fractures are several to tens meters long, the strike of which varies from N62°E to N65°E or from N70°E to N75°E, and are dominated by left-lateral slip with a component of tensile opening, the width of which decreases with depth. The shear and/or transtensional fractures are arranged in left-stepping or right-stepping to form releasing or restraining steps, on which tension gashes or mole tracks are developed. Tension gashes strike N45°～50°E and are developed at a releasing step to connect with the boundary shear or transtensional fractures which constrain the step in most cases. The tension gashes may also be arranged in en echelon pattern along the surface rupture zone, and especially at the termination of the surface ruptures. The mole tracks of 1.5 to 3 m height are trending 295°to 330°, which are well developed at the right-steps of the shear and/or transtensional fractures of different scales along the surface rupture zone. This surface rupture pattern appears to be purely strike-slip characterized by several meters of left-lateral offset. The maximum left-lateral offset we observed reaches 6 m at a site (93°05.384’E, 35°47.623’N), where a shallow channel bed was left-laterally offset by a single pure shear fracture. The macroscopic epicenter of the Hoh Sai Hu (Kunlunshan) earthquake is then inferred to be located at the piedmont area to the northeast of Hoh Sai Hu Lake, about 80 to 90km west of Kunlunshan Pass, in terms of the features of surface ruptures. It is postulated that this earthquake may trigger the occurrence of future large earthquake on the Dongdatan-Xidatan segment to the east of the Hoh Sai Hu segment of the Eastern Kunlunshan Fault,reflecting the eastward motion or flowing of the Tibetan Plateau along the fault.

LATE QUATERNARY SLIP RATE OF THE XIAOJIANG FAULT AND ITS IMPLICATION

HE Hong-lin, Yasutaka Ikeda, SONG Fang-min, DONG Xing-quan

2002, 24(1):  14-26. 

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The Xiaojiang fault is the southern portion of the Kangding fault zone, which is a noticeable active fault in southwest China and plays an important role in the left-lateral extrusion of the Tibetan Plateau. We determined average slip rates of the Xiaojiang fault, on the basis of offset landforms that were precisely mapped and dated by radiocarbon or thermoluminescence methods. The Xiaojiang fault consists of two subparallel fault strands in its middle segment; the average left-lateral slip rates on the west and east strands were determined to be 7.0～9.0mm/yr and 6.0～7.5mm/yr, respectively. Thus, the total slip rate on the Xiaojiang fault amounts to 13.0～16.5mm/yr. This value is approximately equal to that on the Xianshuihe fault, the northern part of the Kangding fault zone, and about twice as high as that on the Anninghe and Zemuhe faults, the middle part of the Kangding fault zone. Our results suggest that there must be other active faults that accommodate the missing slip in the middle of the Kangding fault zone; the most probable candidate may be the Puxionghe-Butou fault zone located several ten kilometers to the east of the middle segment.

PRELIMINARY RESULTS OF THE INVESTIGATION OF PALEO-EARTHQUAKES ALONG THE DALIANGSHAN FAULT ZONE, SICHUAN PROVINCE, CHINA

SONG Fang-min, LI Ru-cheng, XU Xi-wei

2002, 24(1):  27-34. 

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Four prospecting trenches were excavated along 4 subsidiary faults of the Daliangshan fault zone, and 9 paleo-seismic events were revealed by these trenches. West of Doukanzi village, the Tc-1 trench is located on the northern segment of the Shimian-Haitang-Yuexi subsidiary fault. The occurrence time of 2 paleo-earthquakes revealed by this trench is 35ka and 3.5ka, respectively. In the north of Keqilada village, the Tc-2 trench is located on the middle-northern segment of the Puxiong-Zhuhe subsidiary fault. The occurrence time of 2 paleo-seismic events revealed by this trench is 28～30ka and 24ka, respectively. The trench Tc-3 in the south of Tuodu village is located on the middle-northern segment of the Tuodu-Butuo subsidiary fault. The occurrence time of 3 paleo-earthquakes revealed by this trench is 25ka, 10ka and 7.5ka, respectively. The trench TC-4 at Cizijiao village is located on the northern segment of the Jifulada-Jiaojihe subsidiary fault, and the occurrence time of 2 paleo-seismic events revealed by this trench is 15ka and 4.5ka. Among the 9 events, 4 events occurred in Holocene time. The occurrence of the 9 events indicates that the Daliangshan fault zone has been active since late Pleistocene. The vertical displacements produced by these 9 earthquakes are 0.5～1.5m. In comparison to the displacement produced by the historical earthquakes along the Xianshuihe-Xiaojiang fault zone, it can be postulated that the maximum magnitude of these 9 events is M≥7. The study of fault segmentation has shown that a major fault zone can be divided into several segments with independent rupturing history. The segments are separated from each other by relatively large scale bending, turning, crossing and step-over, which may effectively inhibit the propagation of the rupture. The four subsidiary faults of the Daliangshan fault zone are 60～90km in length, and the width of the step-over zone between two subsidiary faults is 5～15km, which is wide enough to inhibit the propagation of rupture on the adjacent faults. That is why the subsidiary faults become an independent rupturing unit. It is concluded, therefore, that the paleo-seismic events revealed by the 4 trenches were independent rupturing events.

RESEARCH ON THE ACTIVE FAULTS AND PALEOEARTHQUAKES IN THE WESTERN JIUQUAN BASIN

MIN Wei, ZHANG Pei-zhen, HE Wen-gui, LI Chuan-you, MAO Feng-ying, ZHANG Shu-ping

2002, 24(1):  35-44. 

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The Western Jiuquan (Jiuxi) Basin is located in the westernmost part of the Hexi Corridor. The basin is bounded by the Qilian Mountain fault on the south, by Alytn Taugh fault on the north, and by Jiayuguan fault on the east, respectively. The Hexi Corridor is one of the seismically active regions in western China. According to historical records, a large number of strong earthquakes had occurred in this area. Recently, we have discovered three Holocene active faults through the interpretation of aerial photos and field investigation in the Jiuxi basin. These three faults are called Xinminpu, Yinwashan and Yumen faults, respectively. The Xinminpu fault is a Holocene thrust fault, which is 17km in length, striking 315°and dipping southwest, located in the northern part of the basin. A fault scarp of 14m height was developed on the hanging wall of the fault, and it is superposed by the newly formed fault scarp with free surface. The rate of vertical motion along the fault is determined to be 0.24mm/yr. The Yinwashan fault is a Holocene thrust fault located on the alluvial fan at the eastern piedmont of the Yinwashan Mountain, striking 315°with a length of 17km and dipping southwest. The rate of vertical motion along the fault is determined to be 0.18mm/yr. Two Holocene paleoearthquake events have been identified through trenching on the fault. The first event occurred 10.64±0.83ka B.P., while the second event occurred between 4.09±0.31ka B.P. and 8.22±0.63ka B.P. The Yumen fault is also a Holocene thrust fault, which is nearly EW-striking and south dipping, located on the alluvial fan at the northern piedmont of the Qilianshan Mountain. A fault scarp of less than 2m height was developed along the fault. The fault scarp was perhaps produced by a historical earthquake. The rate of vertical motion along the fault is determined to be 0.25mm/yr. Two Holocene paleoearthquakes were revealed by trenching on the fault. The first earthquake occurred at 3.05±0.24 ～3.20±0.25ka B.P. The second occurred after 3.05±0.24ka B.P. As mentioned above, all the three Holocene faults belong to thrust fault, and thus no obvious horizontal displacement can be observed along the fault. This may indicate that this area is dominated by compressional deformation. According to historical records, the Jiuquan earthquake of 756 A.D. is the latest historical earthquake occurred in this area. It is postulated that the Xinminpu fault or Yumen fault would be ruptured during this earthquake, but currently we are unable to determine which fault was ruptured by this earthquake on the basis of available dating data. The ages of paleoearthquakes and the characters of surface ruptures along the three faults suggested that the three faults were activated independently or sometimes in cluster.

PROBABILITY OF LARGE EARTHQUAKE RECURRENCE ALONG THE JINGTAI-TIANZHU ACTIVE FAULT

GAN Wei-jun, LIU Bai-chi

2002, 24(1):  45-58. 

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Based on the results of large scale geological mapping (1/50000), we established a reasonable rupture-segmentation model for the Jingtai-Tianzhu active fault. The model comprises two large pull-apart basins on both ends of the fault and two relatively large discontinuous structures along the fault.The basins are considered as "stable barriers" constraining the maximum surface rupture of the Jingtai-Tianzhu active fault, while the two large discontinuons structures are considered as "unstable barriers". Thus, the Jingtai-Tianzhu active fault can be divided into three rupture units or segments the Jinqianghe segment, Maomaoshan segment and Laohushan segment with lengths of 34 km, 56 km and 62 km, respectively.Existing evidences show that each segment has its own characteristic earthquakes, and all the segments may rupture together because of the interaction between each other. Using the "real time probabilty model" and considering the interaction between the neighboring rupture segments, we estimated the recurrence probabilities of large earthquakes along the fault. The results suggest that in the next 100 years, the Laohushan segment has little possibility for an earthquake larger than M S7.2 to occar, whereas the other two segments have some potentialities. In the future, although the pessibility of rupture to occur on Maomaoshan and Jinqianghe segments is random it seems that the rupture on one segment will irritate the rupture on the other segment. That is, the next large earthquake on the Jingtai-Tianzhu fault is most proloably the resault of a combined rupture of the Maomaoshan and Jinqianghe segments. The magnitude is estimated at about M S7.5, and the preferred probabilities in the next 10, 20, 50, 100 years are 14%, 27%, 56% and 81%, respectively.

CHARACTERISTICS OF FOCAL RUPTURE OF THE JIASHI STRONG EARTHQUAKE SWARM

SHAN Xin-jian, HE Yu-mei, ZHU Yan, MA Bao-zhu, YANG Cheng-rong

2002, 24(1):  59-68. 

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A series of strong earthquake swarm including 7 earthquakes with M_S≥6.0 occurred in Jiashi, Xinjiang during January-November 1997-an exceptional case for the intraplate seismicity. Using GDSN broadband P wave records and first motion direction of primary wave in short period records from regional seismic network of China, we have investigated the focal mechanism solutions of earthquakes occurred during January 23 to November 4, 1997 in Jiashi, as well as the rupture process of 5 strong earthquakes of Jiashi swarm, including subsequent and adjacent earthquakes. The focal mechanism solutions of the Jiashi strong earthquake swarm indicate that the earthquakes are characterized mainly by strike-slip and normal dip-slip rupturing. The axis of principal compressive stress is oriented mainly to NNE, and is nearly vertical. The axis of principal extensional stress is oriented to NW, and is nearly horizontal. The directions of principal stresses are different from those of the regional modern stress field, which is dominated by a NW-oriented compressive stress. It is clear that the Jiashi seismogenic zone has a distinct localized character. The processes of focal rupturing of these strong earthquakes are relatively simple, and characterized by relatively small area of rupture and shorter rise-time. They are a brittle rupturing that is generated from one point and propagates rapidly to all directions without a specific extensional direction, and differ significantly from the rupturing process of the Artux earthquake. The results of this study show that the Jiashi strong earthquake swarm bears a close relation with the vertical and lateral inhomogeneity of the crustal structure near the source region. In contrast, the Artux earthquake bears a close relation with modern regional tectonic movement. It is concluded, therefore, that the Jiashi strong earthquake swarm is the products of multiple rapid brittle rupturing along NNE direction in the source area due to the action of both vertical and horizontal stresses, and hence the focal mechanism of these earthquakes is dominated mainly by left-lateral strike-slip and normal dip-slip faulting.

TECTONIC STRESS FIELD AND SEISMIC TECTONICS CONTROLLED BY PLASTIC-FLOW NETWORK IN NORTH CHINA

WANG Sheng-zu, ZHANG Liu

2002, 24(1):  69-80. 

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The tectonic stress field and seismic tectonics in the North China region are investigated in terms of the "plastic-flow network and multi-layered tectonic deformation" model for continental dynamics. It is presented in the model that in consideration of the basic state of the transition of deformation regime from brittle to ductile with increase of the depth, the lithosphere is divided into several tectonic deformation layers, i.e. the ductile lower lithosphere (including the lower crust and lithospheric mantle) with netlike plastic-flow and the brittle upper lithosphere (including seismogenic layer and the shallow crust) with shear and tensile fractures, as well as the transitional layers (i.e. low-velocity / high-conductivity layers) discontinuously distributed between those mentioned above; the long-range transmission of driving force applying at continental plate boundary is carried out mainly by the netlike plastic-flow in the lower lithosphere, which controls intraplate tectonic stress field, tectonic deformation and seismic activities; the plastic-flow network, which is composed of two families of plastic-flow belts (i.e. ductile shear belts) intersecting each other, can be referred to as a "slip-line" network developed under large deformation condition. The previous study indicates that the plastic-flow network system developed under the compression at the Himalayan driving boundary spreads over a broad area in central-eastern Asia. The North China region discussed in this paper is located in the northeast part of this network system. The directions of the maximum compressive stresses in the lower lithosphere can be estimated from the lines bisecting conjugate angles of the network. Comparing these directions with those in the seismogenic layer inferred using focal-mechanism solutions, it is shown that both of them are identical with each other in general tendency, indicating that the stress field in the upper layer is controlled by that in the lower layer; meanwhile, the directions of the stress vectors in the seismogenic layer tend to deviate to some extent from the general tendency owing to the influences of local factors and estimation errors. It is emphasized that large-scale seismic belts are in fact a response to and manifestation of the plastic-flow belts in the lower lithosphere, while the large-scale seismotectonic zones in the seismogenic layer and shallow crust are controlled by the underlying plastic-flow belts. There are 6 left-handed and 6 right-handed plastic-flow belts involved in the region studied and, correspondingly, the seismotectonic zones upon them are progressively developing with different apparent maturity, Λ, i.e. the ratio of the length occupied by seismic faults to the general length of the corresponding segment of the zone. A seismotectonic zone is considered to be "mature" when the Λ-value is greater than or approximate to 0.8, such as the Yuncheng-Datong zone (i.e. Shanxi zone), Luoyang-Sanhe zone (or called Cixian-Xingtai-Sanhe zone) and Zhangjiakou-Penglai zone. On the contrary, the previously-active fault zones may transit to be quiescent since they depart from the plastic-flow belts, such as the Tancheng-Lujiang and Changdong fault zones. The analysis of 6 cases of deep-shallow combinations of seismotectonics indicates that the crossing angles between strikes of seismic faults and shallow faults in the region studied are not greater than 6.3°～8.1°, implying that the former can be inferred roughly from the latter; most of seismic faults cross the related plastic-flow belts with small angles, being the longitudinal-type faults, while a few of seismic faults with large angles, being the lateral-type; the horizontal-sliding angle of seismic faults (i.e. the angle included between the directions of the maximum compressive stress and the normal to fault strike) ranges from 44.2° to 65.7°, which are suitable for strike-slip.

RUPTURE CHARACTERISTICS OF THE ZHANGBEI EARTHQUAKE SEQUENCE ANALYZED BY EXACT FOCAL LOCATION

GAO Jing-chun, DIAO Gui-ling, ZHANG Si-chang, CAI Hua-chang, ZHANG Hong-zhi, LAI Xiao-ling, LI Qin-zu, WANG Qin-cai, LI Song-lin, ZHANG Yan-qing, ZHU Zhen-xing

2002, 24(1):  81-90. 

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Based upon the far-field and near-field digital seismic records, the hypocenter locations of M_L≥3 earthquakes of the Zhangbei earthquake sequence occurred during January, 1998-March, 1999 have been re-determined by using relative locating method. The results show that the process of the earthquake sequence can be divided into three stages. The first stage occurred in January 1988, which is the stage of the occurrence and adjustment of the main shocks. The seismic activity in this stage occurred mostly along NWW-SEE direction. The second stage occurred during February ～August 1998, which is the active period of moderate aftershocks occurred mostly along NNE direction. The third stage was represented by the occurrence of M 5.6 earthquake on March 11, 1999, the hypocenter of which was aligned also along NNE direction, but was out of the hypocenter area of the main shock. It seems that the two hypocenters were not connected with each other. The rupture characteristics of the Zhangbei earthquake sequence is given on the basis of the relocation of the hypocenters and the revised focal mechanisms, as well as macroscopic intensity distribution data of this earthquake sequence. It is suggested that the earthquake sequence consists of one main left-lateral strike-slip rupture plane, and 2 right-lateral strike slip secondary rupture planes. The main rupture plane is NWW-trending, NNE-dipping at an angle of 44°, and 11.5km in length. The two secondary rupture planes are NNE-trending with a high dip angle. The main rupture plane is conjugate with the NNE-trending secondary rupture planes while the two NNE-trending rupture planes are aligned in right-step en ecelon. The three rupture planes occurred sequentially, the depth of the which is 1.4～7.6 km at shallow part of the crust. The present study shows that it is a doable approach to research the focal rupture of a strong earthquake sequence from three dimensional space by using exact hypocenter 1ocation, focal mechanism and macro-intensity distribution data in "tectonically stable area" where no active fault is found.

PROBABILITY MODEL FOR STRONG EARTHQUAKE RECURRENCE AND QUANTITATIVE ESTIMATES OFSEISMIC POTENTIAL IN THE BASINN-AND-RANGE PROVINCE,NORTHWEST OF BEIJING

CHEN Li-chun, RAN Yong-kang

2002, 24(1):  91-100. 

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Based on the results of quantitative research on active faults,and especially the detailed investigation and systematic summary on paleoearthquakes in the basin-and-range province,northwest of Beijng by Ran Yongkang et al., a catalog of strong earthquakes since 25ka B.P. (including historical earthquakes) has been compiled for the 15 individual segments of 7 active faults in this area. Furthermore,this catalog is used to estimate the strong earthquake recurrence interval (T),average strong earthquake recurrence interval (T_a) and value of normalized function (T/T_a) for all specific fault segments in the light of the thought of the NB model.In these estimations,the recurrence intervals of strong earthquakes on 2 specific segments are estimated separately for longer-interval and shorter-interval,and correspondingly their T_a and T_a/T values are estimated in the same manner.The reason is that we found that the recurrence of characteristic earthquakes follows a fractal-law,i.e. the recurrence of characteristic earthquake follows a quasi-periodic model with longer quiet and active periods,while within the active period it follows a quasi-periodic model with shorter interval.Moreover,the recurrence of small earthquakes within the relatively short active period may follow a clustering model. As the 15 fault segments have a similar tectonic environment,it is considered that they have a similar recurrence model of strong earthquakes.Therefore, the obtained 35 T/T_a values are treated as an equal basis for the statistical analysis.The result of statistic analysis shows that the distribution of normalized strong earthquake recurrence internal follows a normal distribution with mean (μ) equal to 1.000 3 and standard deviation (σ) of 0.246 4,and we call it the Northwest Beijing model.Using this model and also comparing with the NB model,we have estimated the probabilities of strong earthquake occurrence on 25 segments of 15 faults in the basin-and-range province,northwest of Beijing.The results indicate that the potential fault segments for the occurrence of future strong earthquake should be the North marginal fault of the Tiznzhen-Yanggao basin,the South marginal fault of the Yangyuan basin,the Xingwei segment of the North marginal fault of the Yanqing basin,the Yanshui segment of the North marginal fault of the Fanshan basin,and the South marginal fault of the Xuanhua basin.

CHRONOLOGICAL STUDY OF FAULTING EVENTS OF GAOLIYING FAULT, BEIJING

YIN Gong-ming, LU Yan-chou, WEI Lan-ying, ZHANG Jing-zhao

2002, 24(1):  101-110. 

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Dating the age of faulting is critical to the studies of active tectonics, paleoearthquake and neotectonics, but is sometimes difficult of access. At present, two methods are commonly used to date the age of the last faulting. The one is to date the direct products of faulting, such as fault gouge and colluvial wedge, while the other is to date the youngest sediment that was offset by faulting or the oldest sediment that was not affected by faulting. In the region from Tuoli to Yongdinghe River, western Beijing, three types of faulting can be identified along the Gaoliying fault. The first type is that the fault displaces the older loess layer, but is covered by the younger loess layer, such as the cases at Lujing and Xiaoyouying. The second type is observed at Xinkaikou, where the fault offsets the pre-Quaternary bedrocks, but does not affect the Quaternary covering layer (loess). The third type is identified at Xinzhuang village, where the fault dissects the pre-Quaternary bedrocks, resulting in fault gouge, but no Quaternary sediment covering the faults. According to the types of faulting and the characteristics of sediments, two dating methods were used to date the ages of faulting events on the Gaoliying fault in the region from Tuoli to Yongdinghe River, Beijing. Thermoluminescence dating method is suitable to dating eolian deposits, such as loess, and thus is used to date the loess samples affected by faulting or deposited after faulting. Electron Spin Resonance (ESR) dating method is currently the most reliable method to date fault gouge, and thus is used to date the ages of fault gouges collected from Xinzhuang and Dayuanshang villages, respectively. Based on the ages of faulting, it is coucluded that at least 3 faulting events had occurred on the Gaoliying fault at 270～360ka B.P., 130～140ka B.P and 1.8～4.2ka B.P, respectively.

CRUSTAL MOVEMENT IN SHENHUWAN BAY AREA, FUJIAN PROVINCE SINCE THE MIDDLE STAGE OF LATE PLEISTOCENE

XU Qi-hao, FENG Yan-ji, SHI Jian-sheng

2002, 24(1):  111-123. 

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Crustal movement in Shenhuwan Bay area during the early and middle stages of late Pleistocene was dominated by subsidence, resulting in the deposition of alluvial and fluvial gravel-bearing sandy clays. Since the middle stage of late Pleistocene, crustal movement here has been characterized by uplift followed by subsidence. In the past 1 500 years, the crust was relatively stable. During the time period of 25.58± 2.49ka～15.46ka B.P., the crustal movement in Shenhuwan Bay was characterized by uplift simultaneous with the eustatic change of sea-level. The amplitude of uplift reached up to 8m, and the rate of uplift was about 0.78mm/yr. During that time, the coastal environment was relatively stable, and oysters were growing in Shenhuwan Bay. In 15.46～11.00ka B.P., however, the crust in Shenhuwan Bay was rapidly uplifted. The amplitude of uplift reached up to 10m or more, while the rate of uplift reached up to 2mm/yr. In that period, the Shenhuwan Bay had possibly experienced sudden uplift caused by strong earthquakes, resulting in the rapid escape of the ancient oyster bank from the tidal zone to avoid wave erosion. At that time, several low-lying depressions filled with late Pleistocene gravel-bearing sandy clays were formed in the Shenhuwan Bay, while the ancient oyster bank had become a tableland. Afterward, the crust tended to be stable, and forest was flourishing in the low-lying land of the Shenhuwan Bay. At 7 000yr B.P. a sudden crustal subsidence had occurred in the Shenhuwan Bay due to a strong earthquake, which caused the transformation of low-lying land into the lagoon environment, and hence the ancient oyster bank became closer to the sea level. Since 2,800yr B.P. the Shenhuwan Bay had been slightly uplifted again, and the lagoon had become land. A strong earthquake occurred at 2 000yr B.P. had given rise to the sudden crustal subsidence in Shenhuwan Bay, causing the submergence of the ancient forest into a deeper sea bottom, and the return of the ancient oyster bank to the tidal zone or offshore. It is concluded that in the past 45,000 years, the Shenhuwan Bay had experienced a complicated process including subsidence-uplift-sudden uplift-stable-sudden subsidence-stable-slight uplift-sudden subsidence-stable, and at least 3 strong paleoearthquakes. It seems that the Shenhuwan Bay had experienced 3 times of land-forming and 3 times of sea-forming processes.

THE RELATIONSHIP BETWEEN LOW VELOCITY LAYERS AND RHEOLOGY OF THE CRUST IN NORTH CHINA AND ITS EFFECT ON STRONG EARTHQUAKE

ZHOU Yong-sheng, HE Chang-rong

2002, 24(1):  124-132. 

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Seismic tomographic data show that crustal low-velocity layers and mantle uplift do exist in a vast area along Beijing-Tianjin-Tangshan-Zhangjiakou, where a lot of large earthquakes had occurred. However, such low-velocity layer and mantle uplift can not be found in Ordos region, where no strong earthquake had occurred. Geological study shows that the compositions of the crust are similar in these two areas. The main difference of the two areas is that rifting had occurred in Beijing-Tianjin-Tangshan-Zhangjiakou area at Cenozoic. We suggest, therefore, that the crustal low velocity layers in North China were controlled by the Cenozoic rifting and mantle uplifting, which caused the rise of temperature and hence giving rise to plastic deformation of lower part of the middle crust and lower crust. The lattice preferred orientation of minerals formed by plastic deformation can cause wave anisotropy, while subgrain substructure produced by dislocation creep may reduce the elastic wave velocity of rocks. The low velocity layers in lower part of the middle crust and lower crust, therefore, should be the result of plastic deformation of rocks. The low velocity layers in the upper part of the middle crust, however, might represent the low angle detachment or ductile shear zones formed during rifting, while low velocity might be resulted from anisotropy of rocks and possibly the effect of fluid. The crustal structure of Ordos region is similar to "ice-water" structure, and that of Beijing-Tianjin-Tangshan-Zhangjiakou area is like "sandwich". A large number of studies show that the crustal weak layers (lower velocity and plastic deformation layers) enhance the decoupling of stronger layers, and this may play an important role in the generation of large earthquake.