古大陆斜坡方向的确定方法

寒武—奥陶纪时,在中国南方曾经存在一个十分壮观的大陆斜坡,现今主要沿着无锡—皖南—赣西北—鄂东南—湘西—黔东—广西呈—带状分布。据研究,该大陆斜坡发育完好,无论其规模还是矿藏拥有量均为国内外所罕见。因此,进一步加强斜坡的基础地质研究,对于阐明其形成机理和演化发展就显得尤为重要。本文在这里仅讨论九种确定古大陆斜坡方向(倾向)的方法,旨在为恢复岩相古地理以及解释斜坡沉积物的成因提供理论依据和基础地质资料。     

MOMENT DEFICITS ON THE MAJOR FAULTS AND EARTHQUAKE HAZARD ASSESSMENT IN THE EASTERN HIMALAYAN SYNTAXIS

The Eastern Himalayan Syntaxis(EHS)is a critical region for studying the tectonic evolution of Tibetan plateau, which was affected by the intense seismic activities. We use the theory of moment balance, GPS velocities and historical earthquake records to analyze the moment deficits in the EHS, assess the future seismicity and further to predict the recurrence interval of the 1950 Chayu M_S8.6 earthquake. We first collected multiple sets of GPS velocity fields and combined them to reduce the systematic bias. Then a micro-blocks model, constrained by GPS velocities, was built by TDEFNODE software to simultaneously invert the fault elastic strain parameters and rigid motion parameters based on the grid research and simulated annealing methods. The long-term slip rates on the faults were further estimated by the differential motions between the neighboring blocks. The results show that the nearly NS dextral strike-slip faults, Naga Fault and Sagaing Fault, slip with the average rates of ~10.6 and ~16.6mm/a, which are consistent with the lateral extrusion in the Tibetan plateau. However, the Main Frontal Thrust shows a distinguished sinistral strike-slip feature(6~10mm/a), possibly caused by the NNE pushing from the Indian plate to the Eurasian plate. On the other hand, because the EHS is located in frontal area of the collision between Indian and Eurasian plate, most faults show thrusting feature. The most obvious one is the Mishimi Fault, slipping with the rate of 23.3mm/a, implying that the convergence rate of the Indo-European plates is largely absorbed by this fault. The moment accumulation rate in the EHS is higher than the average rate in the Tibetan plateau and the total moment accumulation is(1.15±0.03)×10²² N·m in the last 200a. About 59.7% and 21.6% of the moment accumulation rate concentrate on the Main Frontal Thrust and Mishimi Fault. Second, we selected the earthquake records occurring on the upper crust since 1800AD to analyze the moment release in the EHS based on the data from the International Seismological Centre, United States Geological Survey, and catalogue of historical strong earthquakes in China and some other previous studies. In addition, the Global Centroid Moment Tensor Project and linear regression method were adopted to estimate the relationship between body wave magnitude(m_b), surface wave magnitude(M_S), local magnitude(M_L)and the moment(M₀). Then we further estimated the total fault moment release in the EHS, (5.50±2.54)×10²¹N·m, which is significantly lower than the total moment accumulation. About 79.2% of the moment release occurs on the Mishimi Fault, this is because the 1950 M_S8.6 Chayu earthquake is assumed to have ruptured on this fault. Finally, the present-day moment deficits on the faults in the EHS were calculated by the differences between the moment accumulation and release, which represent the possibility to produce earthquakes on the upper crust faults in the future. The largest moment deficit was found on the Main Frontal Thrust near Bhutan, which is able to rupture with M_W8.1+. Similarly, earthquakes with M_W7.5+ and M_W7.3+ have the potentials to occur on the Naga Fault and the Jiali Fault near Tongmai. However, the future earthquake scales may be less than M_W7.1 on the remaining faults. Moderate minor earthquakes are the main activity in the area near the Yarlung Zangbo Suture zone and the southern Sagaing Fault. Although the Chayu M_S8.6 earthquake occurred near the Mishimi Fault and the eastern MFT, the earthquake risk on those two faults cannot be ignored. Meanwhile, no matter which fault produced the Chayu earthquake, its recurrence will likely be 660a to 1 030a.     

Censoring and edge effects in areal and line transect sampling of rock joint traces

This paper reviews techniques for correcting for bias incurred when incomplete observations form part of a data set. The similarities and differences between biases due to censoring and edge effects are emphasized, the latter being of importance when the distribution of joint traces is estimated from an exposure. A new technique for analyzing line transect data is presented, and a new analysis of published data on joint trace lengths given.     

青藏高原西北地区与南北地震带地震活动性研究

通过研究青藏高原地区地震的发震时间和空间分布规律，发现青藏高原西北地区70%的6级以上地震发生在青藏高原地震活动高潮时期，以青藏高原西北地区地震的发震时间为基准，以一年的时间窗口去检测南北地震带发生的地震，发现青藏高原西北地区与南北地震带中强震发震时间接近，具有很强的关联。青藏高原西北地区发生的地震与南北地震带南、北、中段的地震活动相关性各不相同，地震活动频次上呈现出与南北地震带北段相关性最弱，与南段相关性最强，但在震级上表现出与南北地震带北段和中段强震活动关系密切，与滇缅构造转换区的中震联系紧密，图像信息方法为两个地区地震活动相关性提供了证据。研究同时发现以海原地震为起始地震时南北地震带的强震具有由北向南往复迁移的特征，南北地震带中段和滇缅构造转换区的地震迁移次数更多，表明两个地区地震活动确实联系紧密。这项研究对于南北地震带的地震危险性评价和"源线模式"地震预测方法具有重要的意义。     

Comparative study on Co<Subscript>2</Subscript> emissions from different types of alpine meadows during grass exuberance period

Potentilla fruticosa scrub,Kobresia humilis meadow andKobresia tibetica meadow are widely distributed on the Qinghai-Tibet Plateau. During the grass exuberance period from 3 July to 4 September, based on close chamber-GC method, a study on CO₂ emissions from different treatments was conducted in these meadows at Haibei research station, CAS. Results indicated that mean CO₂ emission rates from various treatments were 672.09±152.37 mgm^-2h^-1 for FC (grass treatment); 425.41± 191.99 mgm^-2h^-1 for FJ (grass exclusion treatment); 280.36±174.83 mgm^-2h^-1 for FL (grass and roots exclusion treatment); 838.95±237.02 mgm^-2h^-1 for GG (scrub+grass treatment); 528.48±205.67 mgm^-2h^-1 for GC (grass treatment); 268.97±99.72 mgm^-2h^-1 for GL (grass and roots exclusion treatment); and 659.20±94.83 mgm^-2h^-1 for LC (grass treatment), respectively (FC, FJ, FL, GG, GC, GL, LC were the Chinese abbreviation for various treatments). Furthermore,Kobresia humilis meadow,Potentilla fruticosa scrub meadow andKobresia tibetica meadow differed greatly in average CO₂ emission rate of soil-plant system, in the order of GG>FC>LC>GC. Moreover, inKobresia humilis meadow, heterotrophic and autotrophic respiration accounted for 42% and 58% of the total respiration of soil-plant system respectively, whereas, inPotentilla fruticosa scrub meadow, heterotrophic and autotrophic respiration accounted for 32% and 68% of total system respiration from GG; 49% and 51% from GC. In addition, root respiration fromKobresia humilis meadow approximated 145 mgCO₂m^-2h^-1, contributed 34% to soil respiration. During the experiment period,Kobresia humilis meadow andPotentilla fruticosa scrub meadow had a net carbon fixation of 111.11 gm^-2 and 243.89 gm^-2, respectively. Results also showed that soil temperature was the main factor which influenced CO₂ emission from alpine meadow ecosystem, significant correlations were found between soil temperature at 5 cm depth and emission from GG, GC, FC and FJ treatments. In addition, soil moisture may be the inhibitory factor of CO₂ emission fromKobresia tibetica meadow, and more detailed analyses should be done in further research.     

Duration Magnitude Scale and Site Residuals for Northern Morocco

— The first empirical duration magnitude (M_D) formula is developed and tested for the Northern Morocco Seismic Network (NMSNET). This relationship is obtained by relating the IGN (Instituto Geografico National, Madrid) body-waves mbLg^IGN to the duration (), and the epicentral distance (), at 25 analogue stations of the NMSNET for 479 earthquakes with 2.5 mb 5.4, from March 1992 to February 2001. M_D estimates are significantly more precise while introducing a correction term for each of these stations, cSta_j. The magnitude for the i^th event (M_D)_i is the mean value of individual M_Dij=–0.14+1.63log₁₀(_ij)+0.031(_ij)+cSta_j. The cSta_j corrections reduce considerably the local site effects which influence the recorded durations and cause stations to either overestimate, or underestimate M_D up to 0.5 magnitude units. Average station M_D residuals (–cSta_j) are found to be independent of the distance from the epicenter to at least 10 degrees. It seems evident that regional geological features in the immediate behavior of stations have a systematic effect on the corresponding obtained residuals: older well-consolidated Precambrian crystalline rocks produce high negative residuals (shorter durations), younger unconsolidated sediments produce high positive residuals (longer durations), whereas, intermediate M_D site residuals appear to be the result of the effect of various factors, principally age and state of consolidation of the bedrock, combined with the local tectonic.     

滇西金满脉状铜矿床成矿年龄讨论

对滇西兰坪盆地内金满脉状铜矿床成矿主阶段的石英样品进行了40Ar 39Ar快中子活化测定,谱线特征为马鞍型,其坪年龄为(58 05±0 54)Ma,最小视年龄为(56 76±0 81)Ma,等时线年龄为(54 30±0 15)Ma,三者均十分接近,说明所测石英样品的年龄可靠。而石英的流体包裹体Rb Sr同位素分析数据因线性关系不太好,导致等时线年龄(68±76)Ma误差大,因而仅具参考意义。砂岩及矿化砂页岩中磷灰石的裂变径迹年龄为58 7～61 7Ma,反映了兰坪盆地内一期构造热演化事件的发生时间,其与40Ar 39Ar分析的年龄基本一致,说明金满脉状铜矿床的形成时代相当于喜马拉雅早期。结合滇西大区域内铜及其他矿种的成矿主要集中在喜马拉雅早期这一现象,认为滇西在此时期必定存在着诱发某种大规模成矿作用的关键因素,并在区域内统一制约着矿床的形成。     

Period variations of the Chandler wobble

Variations in the period of the Chandler wobble have been discussed since its discovery by Chandler in 1892. Various authors engaged in the investigation of polar motion time series suggest both a variable and an invariable period. It cannot be resolved by the analysis of time series whether the Chandler period is variable. By studying the influence of mass redistributions on the Chandler period it has been found that it is in fact variable, but the magnitude of such variation is much smaller than that found by polar motion time series analysis. For the currently available time series of polar motion, it is sufficient to assume an invariable Chandler period. AcknowledgmentsUseful discussions with Dr. F. Barthelmes and Dr. K. Fleming are gratefully appreciated.     

1303年山西洪洞8级大地震的时间有序性特征

1303年山西洪洞地震是我国历史记录中被确认的第一个8级大地震。引入有序系列和有序组合的含义,对1303年山西洪洞地震和1556年陕西华县地震分别形成的2个有序系列进行了研究,认为2个8级地震的发震时间有一个时间差(τ)。2个τ值之差用△τ来表示。若2个τ值相差不大,则4个地震(有时可以是3个地震)组成一个有序组合。2个有序系列可有7个有序组合。7个有序组合的τ值在100a～450a范围变化,但它们的△τ值仅在10d～2400d范围内变化,表示了2个有序序列之间的密切联系。震例表明,1303年洪洞大地震与其他8级地震之间的间隔的形成机制可能与沙罗周期有关。     

祁连山中东段地区三维速度结构研究

应用中——法数字化地震台网自1996年以来所记录到的微震S-P资料，对祁连山中东段地区三维速度结构与震源参数进行了联合反演，获得了该区的三维速度成像．速度结果反映出深部构造的一些重要信息，为进一步研究该区的活动构造及地震参数计算，提供了科学依据．