Table of Content

03 September 2003, Volume 25 Issue 3

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Brief Report

SLIP-RATE ON THE EASTERN ALTYN TAGH FAULT SINCE 20ka BP

WANG Feng, XU Xi-wei, ZHENG Rong-zhang

2003, 25(3):  349-358. 

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As northern boundary of the Tibet Plateau, the Altyn Tagh Fault attracts the attention of lots of geo scientists from different parts of the world. The offset and slip-rate of the Altyn Tagh Fault are a topic full of disputation. To make clear the slip-rate of the Altyn Tagh Fault is important for understanding the coupling relationship between the uplift and strike-slip on the northern Tibet Plateau. On the eastern Altyn Tagh Fault, to the west of the outlet of the Shulehe River, the late Quaternary terraces and alluvial fans are offset by the fault. There are some typical offset landforms in this region, and by investigating these geomorphological features, slip-rate of the Altyn Tagh Fault can be evaluated. To the West of Shulehe River, three sites, Erjiatai, Niuyagou gully and Bagexia were surveyed for obtaining the slip-rate on this part of the Altyn Tagh Fault. The range of fieldwork region for this study is about from 94°24′E to 94°37′E. At Erjiatai, offset of the lowest terrace T₁ cannot be identified because of its absence near the fault. The offsets of T₂, T₃ and T₄ terraces are 40m, 100m and 220m, respectively, and the TL ages of T₁, T₂ and T₃ terraces are 7.82ka BP, 22.53ka BP and 53.26ka BP. According to these data, we can calculate the average slip-rates of the fault in different periods since late Quaternary. The slip-rate of the fault at this site is about 4～4 5mm/yr since 50ka BP. At Niuyagou, the fault offset all the terraces but T₁. The offsets of T₂, T₃ and T₄ terraces are 6.4m, 40m and 120m, respectively, and the TL ages of T₁, T₂ and T₃ terraces are 6.97ka BP, 10.06ka BP and 28.81ka BP. So the slip-rate of the fault here is about 4～4.2mm/yr since 30ka BP. At Bagexia, the lowest terrace T₁ is not offset by the fault but the other terraces above T₁ are offset by the fault. Here the slip-rate of the fault is a little higher than the other two sites described above. Here the slip-rate of the fault since 20ka BP is about 4.7mm/yr. Based on the above mentioned data the following conclusion can be drawn: The slip-rate of the eastern Altyn Tagh Fault to the west of Shulehe River since 20ka BP is about 4～5mm/yr and the slip-rate become a little higher westward. Regarding the strike of the Altyn Tagh Fault in this region is about N73°E, we may estimate that the strike-slip on the eastern Altyn Tagh Fault assimilates the north-south shortening of the Tibetan Plateau at a rate of 1.5mm/yr at maximum.

HOLOCENE PALEO-SEISMIC ACTIVITIES ON THE KOUQUAN FAULT ZONE, DATONG BASIN, SHANXI PROVINCE

XIE Xin-sheng, JIANG Wa-li, WANG Rui, WANG Huan-zhen, FENG Xi-ying

2003, 25(3):  359-374. 

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The NNE-trending Kouquan Fault Zone is located on the western side of the Datong Basin, Shanxi Province. It is one of the three major boundary faults controlling Cenozoic sedimentation of the Datong Basin. The Huairen depression, where the greatest thickness of Cenozoic sediments may reach up to 1,800m, is located on the eastern side of the fault (see Fig. 1 in text). It is indicated by field investigation that a segment as long as 120km of the Kouquan Fault zone has been active in Holocene, and that three levels of morphologic surfaces formed in Holocene have been offset by this active segment. The highest one among the three surfaces is the back edge of a pluvial fan located on the western side of the Datong Basin. The upper layer of this pluvial fan consists of mid Holocene deposits, dated as 5ka BP. The other two lower surfaces are the 1st terrace and the 2nd terrace embedded into the pluvial fan. Among them, the 2nd terrace is a denudation terrace with a height of 8m; while the 1st terrace is accumulated with a height of 1.5～4m. The dating data show that the near surface layer of the 1st terrace is of 2.52ka BP in age (See Fig.2 & 3 in the text). Moreover, field investigation also shows that the Kouquan Fault has had obvious right lateral strike-slip component in Holocene as more than ten east west directed gullies were offset right laterally for 8～20m when crossing the fault trace. Two exploratory trenches were dug during our investigation on the northern and southern segments of the Kouquan Fault, respectively. Their locations were chosen right beneath the fault plane where the fault offsets the 1st terrace of the gully (see Fig. 4 & 6 in the text). The two trenches are of 40km apart. Their dimensions are 11m in length with a depth of 4～5.5m and 22m in length with a depth of 3.2～8.8m, respectively. The two trenches reveal a line of evidence of the Holocene activities of the Kouquan Fault. The Wudao trench has revealed 3 fault planes, and in the middle part of the profile a drag structure in dark brown clay layer has been observed, appearing as the ascending of the western side and the descending of the eastern side (Fig. 5). The Shanghuangzhuang trench reveals a fault bundle of 13m in width that consists of 5 groups of faults. A vertical displacement of 2.9m for the two sides of F₁～F₄ Faults can be determined from the vertical dragging of the dark gray clay layers on the trench profile. The F₅ Fault at the easternmost end of the profile consists of two vertical fault planes about 0.8m apart. No vertical displacement of clay layers on both sides of F₅ is observed, but the layers between the two fault planes have nearly horizontal layering and show stratum discontinuity on both sides of the fault, indicating a strike-slip activity of the fault. Moreover, the shanghuangzhuang trench log shows multiple yellow sand soil veins in the vicinity of the fault planes (Fig. 7). The time sequence of the paleo-seismic activities revealed by the Wudao and the Shanghuangzhuang trench logs might correspond to each other. That is to say that there have been 4 paleo-earthquake events since 13.7ka BP. Three of them occurred about 2.25, 5.68, and 13.73ka BP, respectively. The rest event occurred between 6.76ka and 10.82ka. The average recurrence time of these 4 events would be 3,737yr, and the interval between the latest and its previous one is 3,160yr. The average minimum co seismic vertical displacement of the 4 events reaches 1.8m. These new data are of great significance to earthquake safety assessment of the Datong area.

STRUCTURAL GEOMETRY AND ACTIVITIES OF THE FUKANG-JIMSAR FAULT ZONE, XINJIANG

YOU Hui-chuan, REN Li-sheng, ZHANG Yu-mei

2003, 25(3):  375-384. 

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Based on detailed field investigations, this paper describes geometrical characteristics and tectonic activities of the Fukang-Jimsar Fault zone in eastern Tianshan, Xinjiang, China. This fault zone strikes nearly in east-west, dipping to south, and extends over a length of 140km. It is a major geological/geomorphological margin controlling the neotectonic movement in this region. On the southern side of the Fukang-Jimsar Fault develop Mesozoic strata, forming low mountains and hills; the northern side of the fault corresponds to a large Quaternary depression plain. The Fukang-Jimsar Fault consists of 7 segments of secondary gently dipping reverse faults with lengths of 12～24km, i.e. the Ganquanpu, Sangonghe, Sigonghe, Ganhezi, Quanquanzi, Dalongkou and Dongfeng Muchang Faults. In seismic profiles these faults make up an anticline thrust structure in the range of 5～6km depth, having dip angles of 20°～30° at the lower part and 40°～50° at the upper part(sometimes nearly 60°～80° at the ground). All these fault segments display the evidence of late Quaternary activities such as fault scarps, offset terraces and young sedimentary deposits etc. The steep dipping distal segments of the fault experienced 7 times of activities in Holocene (with an occurrence interval of 1,500yr), and have relatively small vertical slip-rate (0.10～0.40mm/yr); while the gently dipping middle segments are characterized mainly by strong thrusting, and have relatively high vertical slip-rate in Holocene (0.40～1.43mm/yr).

APPLICATION OF MOHR CIRCLE ANALYSIS METHOD TO INTERPRETATION OF DATA FROM MAGNETOTELLURIC SOUNDING PROFILE IN WEST SICHUAN-EAST TIBET

SUN Jie, JIN Guang-wen, BAI Deng-hai, WANG Li-feng

2003, 25(3):  385-393. 

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Mohr circle analysis is a qualitative method for interpretation of magnetotelluric (MT) data. It uses various rotational invariants of impedance tensors and their composite as its basic parameters, so as to visually and clearly displays the features, such as dimensions, 2D-deviation degree, anisotropy, strike orientation, and impedance of the main axis of electrical structures. The conspicuous feature of the method is that it can be used for determining the dimensions of structures. For 1D structure, the Mohr circle is degraded into a point on the abscissa. For 2D structure, the center of Mohr circle is on the abscissa, where the longer the radius of circle, the sharper the electrical anisotropy is. And for 3D structure, the center of Mohr circle deviates from the abscissa, and the farther the deviation of the center from the abscissa, the sharper the 3D feature is. The Mohr circle analysis method was used in analysis of 76 magnetotelluric sounding data from the whole sounded area in west Sichuan-east Tibet. The result obtained shows that 2D regional structures are the main features of the region, and the lateral anisotropy is less intense. The strike of regional structures is approximately sub-longitudinal and the maximum deviation does not exceed ?30?. The analysis result of data only from four observation sites is presented in detail here due to the limited space of the paper.

TEMPORAL AND SPATIAL DISTRIBUTIONS OF STRONG EARTHQUAKES IN CHINA'S CONTINENT IN THE PAST CENTURY

HONG Han-jing, LIU Pei-xun, YU Yong, TAO Wei, ZHENG Xiu-zhen

2003, 25(3):  394-402. 

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The temporal and spatial distributions of strong earthquakes in the past century in China's continent are analyzed in this paper. Both the temporal and spatial distributions of strong earthquakes display a clustering feature. The activity of strong earthquakes is undulate and clustered in the major region in certain period. Based on both temporal and spatial distributions of strong earthquakes in China's continent, a synthetic division of dynamic phases is carried out. The activities of strong earthquakes have approximately 13-year's periods in China's continent in the past century. In each period, most strong earthquakes cluster in a major region and a minor region, corresponding to a wave of energy release on the time axis. The major region and minor region are located on opposite sides of the Tibet Plateau. Quite a portion of strong earthquakes is developed in the original seismogenic area, but most events are developed outside the last major or minor regions. This may imply the fluctuation of energy accumulation in large-scale area. The division of active and quiescent periods of seismic activity is of benefit to understanding the undulation of energy release. However, both the length of active period and the intensity of energy vary with time, while within the active period the seismic activity is uneven. Comparatively, the division of dynamic phases is more advantageous to strong earthquake prediction.

WELL WATER TEMPERTURE BEHAVIORS IN CAPITAL CIRCLE AND THEIR FORMATION MECHANISM

CHE Yong-tai, LIU Xi-lan, YAO Bao-shu, YU Jin-zi, ZHANG Pei-ren, LIU Wu-zhou, YANG Ming-bo, ZHU Zi-qiang, CAO Xin-lai, HUANG Fu-qiong, GU Yuan-zhu, LIU Cheng-long, LI Hai-xiao, SONG Xiao-bing, FAN Xiu-li

2003, 25(3):  403-420. 

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On the basis of systematically checking up nearly ten years observation data of 11 water temperature observation wells in the Capital Circle, the behaviors of well water temperature are divided into several types, such as perennial tendency, annual, monthly and daily behaviors. The mechanisms and characteristics of these behaviors are then analyzed. The perennial tendency can be sub divided into smooth, ascending, descending, fluctuate and composite subtypes; the annual behaviors can be sub divided into smooth-stepwise, ascending, descending and fluctuate subtypes; the monthly behaviors can be sub divided into smooth, smooth-stepwise, smooth-fluctuate, descending-fluctuate, ascending fluctuate and composite subtypes; the daily behaviors are mostly fluctuate types, but 5 wells have tidal effects and most wells posses stepwise or pulsed changes. Up to now, the main factors that have been found to have influence on the behaviors of water temperature include the infiltration recharge of atmospheric water, lateral recharge of groundwater, groundwater exploration in adjacent area, and water turbulence in the observation wells, etc. In addition, the instability of observation instruments has great impact on water temperature behaviors. After the normal behaviors are recognized and all kinds of interference factors are eliminated, water temperatures are found to have good reflection on seismic event. Relatively distinct short imminent term anomalies have been found before some moderate-strong earthquakes, and remarkable co seismic anomalies can be recognized while distant strong earthquakes occur. Moreover, some pre seismic anomalies are also identified before the occurrence of a few distant strong earthquakes. Therefore, the further study on the behaviors of water temperature, and the elimination of all kinds of interference factors including stepwise or pulsed variation caused by the instability of observation instruments, will remarkably enhance the capacity and effectiveness of the reflection of water temperature behavior on seismic event. This will also help to improve the capability of short imminent term earthquake predication in our country.

GEOCHEMISTRY OF GEOTHERMAL WATER AND NOBLE GASES IN YANHUAI BASIN, CHINA

WANG Guang-cai, ZHANG Zuo-chen, WANG Min, WANG Ji-hua, LIU Wu-zhou, YI Li-xin, SUN Ming-liang

2003, 25(3):  421-429. 

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In the past several years, a series of geologic, seismologic, geochemical and geophysical investigations have been conducted to understand the crustal structure, activity of active faults and seismic risk in the Yanhuai Basin. During the investigations, a large number of data about the pre-seismic, co-seismic and post-seismic behaviors of subsurface fluids have been obtained by the Seismic Precursory Monitoring Stations in the area. This paper deals with the geochemistry of groundwater and noble gases in this region. It is aimed at the understanding of the background of groundwater, which is sensitive indicator for both earthquake and active faults in the region. The study shows that geothermal water in this region originates mainly from meteoric rainfall, the circulation depth is 1 600～3 800m, the temperature of water at the depths is 57～112℃, and the residence time is 50～30 320yr. Groundwater in this region can be divided chemically into HCO₃-Na, HCO₃-Cl-Na, HCO₃-SO₄-Na-Mg and SO₄-Na types, the TDS and pH of which range from 527.6～938.0 ppm and 7.50～8.73, respectively, belonging to low TDS and neutral-alkaline thermal water. The results of water-rock equilibrium calculations by NETPATH 2.0 show that groundwater in this area is over-saturated with regard to quartz and in equilibrium with chalcedony. The noble gases of geothermal water originate from meteoric, crust and mantle sources, respectively. The anomalies of geothermal water and gases regime before the occurrence of Datong, Baotou and Zhangbei earthquakes were closely related to the faults and their activity, i.e., those anomalies occurred in the places where the circulation depth of the groundwater is great, or even the circulation depth is shallow but having an evidence of mantle gases releasing. The residence time of geothermal water and the discovery of mantle gases in this region are firstly reported in this paper. These results are of significance to further study of active faults, earthquake risk and groundwater utilization in this region.

WEATHERING FEATURES OF VOLCANIC EJECTA IN THE NORTHERN HAINAN ISLAND AND THEIR IMPLICATIONS

ZHANG Bing-liang, HONG Han-jing, SHI Lan-bin, LIN Chuan-yong, LI Jian-ping

2003, 25(3):  430-437. 

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It is well known that weathering process initiates as soon as the volcanic products reach the earth surface. As a result, various kinds of secondary clay minerals and their polytypes are formed in different periods of weathering process. Based on this fact, the time sequence and succession of volcanic eruption can be inferred. It may provide, therefore, a convenient dating method applicable to the prediction and assessment of volcanic hazards. In this paper, the weathering features of clay minerals from 3 profiles of volcanic ejecta in Leihuling-Ma'anshan area, northern Hainan Island are studied in detail by using various techniques including scanning electron microscopy, energy spectroscopy and x-ray diffraction analysis. The results of this study have led to some new insights about the clay minerals and their weathering features, as well as the time sequence of volcanic eruption in this area. (1) The spheroidal halloysite is predominant in the secondary clay minerals formed by the weathering of volcanic products in this area. The interlayer water within halloysite decreases with the development of weathering, and as a result halloysite changes its form in a sequence of 10Å halloysite→(7Å+10Å)halloysite→7Å halloysite. (2) The Fe³⁺ content in halloysite is within the range of 9.9%～28.8%, and the average SiO₂/Al₂O₃ ratio is 1.62, belonging to iron rich halloysite. (3) Based on final products of weathering, it is postulated that the succession of volcanic eruption in this area was southeast Leihuling→north Ruhongcun village→east Ruhongcun village. (4) Taken the study of weathering products of volcanic ejecta in Japan as a reference, the eruption time of volcano in this area can be inferred to be 6～12ka BP.

GEO-SLICER—A NEW TECHNIQUE FOR DETECTING ACTIVE FAULT

HE Hong-lin, Eikichi TSUKUD, Kouichi SHIMOKAWA, Keita TAKADA, Tsuyoshi HARAGUCHI, Takeshi NAKADA, Kunihiko SHIMAZAKI

2003, 25(3):  438-449. 

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In order to mitigate the disaster in urban area caused by an earthquake under the city, it is very important to detect and locate the distribution of active faults in urban area, and to keep away from these active faults in future construction. Many techniques and methods for detecting active fault have been developed in the past century. Nevertheless, some methods are not suitable to being applied in urban area. It is necessary, therefore, to develop and apply some other new methods for detecting active fault in urban area. This paper introduces Geo slicer method, a newly developed technique in Japan for detecting active fault. A Geo-slicer is made of steel and has a simple structure composed of a sampling box and its shutter, and very easy to be used in the field. The Geo-slice can be taken to a laboratory for close examination or can be displayed at a meeting or even stored for future re-examinations. Moreover, this technique enables us to carry on high-resolution analyses not only in active fault research but also in many other fields with less expenditure of time, labor and money, such as the researches of liquefaction, landslide and fluviolacustrine or littoral sediments. Three examples show the application of this technique to the studies of active fault, liquefaction caused by earthquake, and littoral sediment. The first example is the Geo-slicer investigation on the Hayward Fault of San Andreas Fault system east of the San Francisco Bay. The object of the investigation is to detect fault trace 3m under ground surface, where investigation by trenching is difficult. As a result, four geo slices revealed fault traces and a firstly discovered paleo earthquake under a 3m deep trench. The second is the geo-slicer investigation of liquefaction caused by the 2000 M 6.6 Totori earthquake in Japan. The geo slices taken in Oone Island of Shimane Prefecture, Japan showed the structure of liquefaction in 3.5m deep infilled sandy soil behind a mole. The third is the geo-slicer investigation of grouted sediment near the mouth of the Columbia River, the United States. Those geo-slices revealed liquefaction relics not only caused by 1700 Cascadia earthquake but also by other older events. Finally, the advantages and shortcomings of this method have been summarized. The advantages include the following five aspects: it may decrease the earthwork to 1/100 of a trench and reserve effectively the geologic information; can be applied to weak strata where trenching is difficult; can be applied to detect active fault in urban area; multi geo-slicer and 3D-geo-slicer can be taken in a narrow area; can be conserved for future re-examinations. The shortcomings are also summarized as several aspects, such as difficult to apply to the gravel composed of big boulders and highly viscous clay sediment, and higher expenditure in developing countries.

SEISMOTECTONIC SETTING OF THE 1949 KUQA, XINJIANG M7.4/EARTHQUAKE

XU Jie, ZHOU Ben-gang, YANG Xiao-ping, WANG Xiang-yu, ZHOU Qing, CHEN Guo-guang, RAN Hong-liu, LIU Zhan-po

2003, 25(3):  450-459. 

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Previous studies suggested that the Kuqa, Xinjiang M7.4 earthquake of 1949 is associated with the NEE trending Qiulitage Fault of the active tectonic zone at the southern foot of the Tianshan Mountain. Recently we have made a more detailed analysis of the seismotectonic setting of this event based on new data and results of crustal seismic sounding, petroleum geology, and research of active faults. Our work indicates that the underthrusting of the Tarim block beneath the Tianshan, as well as faulted block uplift and lateral overthrusting and spreading of the upper crust of Tianshan have given rise to the development of the Kuqa basin and thrust-fold system along the southern margin of the Tianshan Mountain. All these are thin-skinned structures developed in the sedimentary cover. The NE-trending Kuqa-Yixi No.1 Well Fault is an important transfer fault in the Kuqa basin, which is located in the NE-trending Urumqi-Kuqa-Keping tectonic zone. The results of seismological research on the M7.4 earthquake indicate that the earthquake bears no relation to the Qiulitage Fault, but is related to the Kuqa-Yixi No.1 Well Fault.

SOME PROBLEMS IN THE EXPLORATION AND RESEARCH OF BURIED ACTIVE FAULT

XIANG Hong-fa

2003, 25(3):  460-466. 

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The review of the research on buried active faults all over the world has indicated that the exploration and research of buried active faults have experienced 3 main development stages: the development of exploration technique (the early stage), the use of single exploration technique (the middle stage), and the use of multiple exploration techniques (the modern stage). In the 1990s, comprehensive exploration of buried active faults through shallow seismic prospecting and drill logs has been carried out in China, Japan and the United States, for the purpose of major engineering construction and seismic hazard reduction. Good progress has been made in locating buried active faults and timing of fault activity. According to the present situation of the research of buried active faults and its heavy responsibilities, great attention should be paid to the following problems: (1) how to enhance the coordination of various exploration techniques, especially the coordination of a series of drill logs with the corresponding age dating and shallow seismic prospecting; (2) attention should be paid to the improvement of the locating precision of buried faults and the determination of displacement amount; (3) how to focus our research on the meticulous quantitative determination of the age of fault activi～ty and the displacement amount. It is suggested, therefore, that the precise location of buried active faults and the quantitative research of their Quaternary activity, the relationship between the Meso Cenozoic active faults with the late Quaternary active faults, the 3-dimensional correlation between the seismogenic fault and the shallow level fault, as well as the the establishment and application of the 3-dimensional geologic structural model of buried active fault in plain area are the main key problems and difficulties in the research and exploration of buried active faults at present. According to the present development of the research of buried active faults and the demands of national economic construction, the research of buried active faults should be focused on the areas of the occurrence of magnitude above 7 earthquakes, large cities and coastal areas with high potential of seismic risk. Moreover, great effort should be made to the basic research of the classification, precise location and the latest activity of the buried faults. Therefore, to develop new exploration techniques by employing advanced technology such as aerial, surficial and subsurface prospecting is recommended as the research priority of the buried active faults.

HISTORY AND PRESENT SITUATION OF X-RAY COMPUTERIZED TOMOGRAPHY (CT) OF ROCKS

DING Wei-hua, WU Yan-qing, PU Yi-bin, CUI Zhong-xing, CAO Guang-zhu

2003, 25(3):  467-476. 

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Since the late 1980's, X-ray Computerized Tomography (CT) technique has become the main approach to detect the internal structure of rocks and the evolution of cracks within the rocks, owing to its high resolution, high scanning velocity and nondestructive measurement ability. The apparatus of X-ray CT of rocks is composed of two parts: CT equipment and loading system. The implication of X-ray CT of rocks is to get the information of the evolution of internal structures of rocks through the CT images of the rocks under various experimental conditions. The basic method of CT of rocks is to compare the CT images of the same scanned section of the rock under different stress level. Although it has only a ten years history, the CT of rock has made significant progress. In its initial stage, the CT technique was used for detecting the internal structures of rocks, and now it has been used to observe the evolution law of cracks and its application. The capacity of CT lies on the function of CT apparatus and loading system, as well as the processing capacity of the CT images. The CT of rocks is based on the analysis of the CT images of the cross-section of rock sample under different conditions. At present, it can be used for real-time scanning under uniaxial loading, triaxial loading, unloading and cyclic loading conditions. It discovers that the crack nucleates near the surface of rock sample, and then propagates toward the inner part of the sample. The evolution of the crack is step-by-step, i.e. the widening and propagation of the crack occur alternatively. The deformation and failure of rocks are intrinsically the evolution of crack. The width of the crack can be determined quantitatively in two ways: actual measurement and theoretical calculation. There are two methods for reconstruction of the 3-dimensional image of crack in rock: the stereo display of 3 dimensional sampling and the spatial connection of crack at CT image scale. By regarding the deformation process of rock as the damage process of material, the analysis of damage evolution and damage variables can be carried out on the basis of CT number. The applicability of CT results depends on the similarity of the prototype of the rockmass and the experimental condition. The main problem faced by CT technique is how to get precise CT images under various experimental conditions as well as the application of its results. The progress of CT technique for rock testing depends on its theory and technology, and on its applicability to the relevant fields.

AN ACOUSTIC EMISSION ACQUIRING SYSTEM WITH ULTRA-HIGH SPEED PARALLEL NET

LIU Li-qiang, LEI Xing-lin

2003, 25(3):  477-479,420. 

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NEW SOFTWARE FOR FORECASTING THE RISK OF VOLCANIC FALLING TEPHRA

ZHAO Yi, MA Bao-jun, SHI Xing-jue

2003, 25(3):  480-490. 

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This paper expatiates on the hazards of volcanic falling tephra, and demonstrates the pratical importance of making program for forecasting the risk of volcanic falling tephra. It describes in detail the main scheme of the program, basic idea of the design, and some considerations about the program. Based on the Visual Basic 6.0 platform, some new programs for various purposes are developed, such as for the calculation of tephra diameter parameters in the light of Sequential Fragmentation Theory by Wohletz;the calculation of magma viscosity by using Magma 3.2 software developed by Wohletz;the calculation of crystallization pressure;the calculation of eruption pressure;the calculation of wind parameters in particular period and region;the calculation of magma dynamic parameters;the calculation of erupted material quantity;the distribution of ballistic trajectories for blocks ejected during volcanic eruptions;the distribution of ballistic trajectories for blocks ejected during plume collapse and the distribution of falling tephra during diffusion. Moreover, the main scheme and functions of each program on the basis of Mapinfor 6.0 platform are introduced in detail, while the basic theory and scientific basis for these programs are provided as well. The St. Helens volcano eruption on May 18, 1980 is taken as an example. The distribution of tephra fallout of this eruption is calculated by using the program introduced in this paper. The obtained result is in good agreement with the observed data, indicating the applicability of this program. Finally, detailed analysis of the shortcomings of the software and the method for further improvement are provided as well.

OBSERVATION RECORDS OF THE ERUPTION OF WUDALIANCHI VOLCANOES IN 1720—1721AD

CHEN Hong-zhou, WU Xue-juan

2003, 25(3):  491-500. 

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Recently, the Man language archives about the eruption of Wudalianchi Volcanoes originally preserved in Heilongjiang General Yamen (Office) has been discovered and translated into Chinese language by Wu Xuejuan. The archives recorded in detail the whole process of the eruption of Wudalianchi Volcanoes from January 14, 1720 to March 18, 1721 that formed the Laoheishan crater, and the eruption from April 26, 1721 to May 28, 1721 that resulted in the formation of the Huo～shaoshan crater. These Man language archives record the time, place, and state of eruption, the formation of dammed lake, and the personnel who made the observation on the two volcanoes. It is the only organized observation on volcanic eruption in Chinese history. These Man language archives not only provide unique firsthand materials for volcanic observation and study, but also indicate clearly that China is one of the countries where volcanic observation was carried out early. The eruption of the Laoheishan and Huoshaoshan volcanoes was previously considered to occur in 1719—1721, but it is clear now that it occurred in 1720—1721.

CRYSTALLINE BASEMENT STRUCTURE OF CHANGBAISHAN TIANCHI VOLCANIC AREA

DUAN Yong-hong, ZHANG Xian-kang, YANG Zhuo-xin, ZHANG Cheng-ke, ZHAO Jin-ren, PAN Ji-shun

2003, 25(3):  501-508. 

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The crystalline basement velocity structure of Changbaishan Tianchi volcanic area is established by using Three-Dimensional Finite-Difference method and Pg data from four wide-angle reflection/refraction profiles and one three-dimensional array which was deployed in Changbaishan Mountain area. This paper presents the characteristics of the velocity structure of crystalline basement and the interface with 5.9～6.0km/s P-wave velocity in the crystalline basement. Inversion results reveal that the velocity of surface layer in the southern part is higher than that in the northern part of the studied area. In general, the depth of crystalline basement of the studied area is within the range of 2.0～3.0km, with the deepest of about 4.0km and the shallowest of about 1 5km near Songjiang. There are two low velocity areas in this region, one is beneath the Protection Station of Tianchi volcano, and the other is located beneath Erdaobaihe and Chixi Protection Station. In these two areas the depths of crystalline basement are about 4.0km. We found that the intense lateral variation of velocity and depth of crystalline basement in the seismic profile coincides well with the location of fault. Therefore, it is suggested that the intense lateral variation of velocity and the abrupt change of the depths of crystalline basement can be taken as an indication of the occurrence of fault.