青藏高原北部地区地震统计区划分及地震活动特征

地震统计区的划分是研究地震活动性的重要前提和基础,通过对青藏高原北部地区构造地质背景的分析,依据地球物理特征和强震活动特点,讨论和划分该区地震活动统计区,探讨研究区地震活动的复发特点、地震构造特征、潜在地震危险性及强度,分析研究区未来强震发生的强度和可能地点,结果表明,目前青藏高原北部地区处于第8个活跃期,仍存在发生强震的可能,且未来数年存在发生7级以上地震的可能,应密切东昆仑断裂带东段和祁连山中西段地区。     

祁连山构造带地震迁移活动特征分析

祁连山构造带地震迁移活动研究表明,存在不同震级层次的地震迁移现象:①大震顺构造带迁移由东而西震级衰减;②中强地震沿构造带东、中、西段交替发生;③横构造带地震跳迁会影响顺构造带地震的有序迁移;④祁连山中东段—东段存在发生破坏性地震的背景。     

甘肃东南地区构造活动与2013年岷县—漳县MS6.6级地震孕震机制

位于南北地震带中北段的甘东南地区,其构造变形和构造活动特征与青藏高原向北东方向的扩展密切相关,该地区复杂的构造几何形态主要受控于东昆仑断裂和西秦岭北缘断裂,区域新构造运动主要动力来源于青藏高原向北东的扩展.近年来,甘东南地区中强地震频发,本文主要通过对该地区构造活动特征、历史地震等资料的综合分析讨论,结合地球物理、地震学和野外调查等资料,认为青藏高原东北部东昆仑断裂的向北挤压和向东的运动是该地区构造应力集中的主要原因,也是该地区中强地震的主要孕震环境和机制,而西秦岭北缘断裂的走滑及向南北两侧逆冲“花状构造”是临潭—宕昌断裂带上中强地震频繁发生的一个重要动力因素.2013年7月22日发生在甘肃岷县—漳县的M_S6.6级地震正好位于临潭—宕昌断裂带中东段上,是该断裂分段不均匀活动的结果.     

昆仑山口西8．1级地震前兆异常时空特征初探

通过对青藏高原北部地区前兆观测手段共88个台点的资料系统分析、整理、汇总,认为在昆仑山口西8．1级地震中短期内前兆异常具有明显的特征,并认为8．1级地震的发生不仅仅是大构造断裂带活动的产物,更是在某种特殊的力源作用下,大区域构造应力发生显变化,并在几条边界构造大断裂的相互作用下发生的1次强震活动。     

2022年青海门源6.9级地震前祁连山地震带平静打破异常特征

结合2021年度地震活动性异常,系统梳理门源6.9级地震前祁连山地震带5级地震平静打破和祁连山中东段ML 3.6地震平静打破的时空异常特征,认为祁连山地震带5级地震平静对平静区及周边发生6级以上地震具有指示意义,祁连山中东段ML 3.6地震平静对祁连山中东段5级以上地震有良好的对应关系.平静区的打破使中强地震发震的时间...     

青藏高原北部地区地震窗口特征及其预报意义

利用青藏高原北部地区弱震活动资料筛选出3个具有中短期预报效能的地震窗口,分析了Ms≥5．0地震前各个窗口的异常特征,提取出半定量的中短期预报指标．即：在中强地震前80％的异常出现在地震前1～6个月,以中心异常出现时间向后推1～6个月作为预测的发震时间;异常幅度与未来震级之间没有明显的对应关系．对于7级以上大地震,古浪窗的显异常主要对应边界水平剪切走滑活动断裂上发生的地震;舟曲窗和乌海窗的显异常主要对应活动块体内部挤压逆冲断裂上发生的地震．而5～6级的中等地震没有明显的对应规律．     

祁连山地震带小震活动增强指标提取与预报效能分析

总结2015年以来、2010年以来和1980年以来祁连山地震带活动增强指标,得到祁连山地震带5年、10年以及40年尺度的小震频度预报效能。祁连山地震带M_L2.0以上地震月频度持续增强的异常出现时间在3个月以上,异常出现后1~3个月,祁连山地震带及边邻地区发生5级以上地震以及青藏高原东北缘发生6级以上地震的概率较高。作为短期预报指标,祁连山地震带的地震活动增强对甘肃及边邻地区的地震预报工作具有重要的指示意义。     

青藏高原北部地区强震活动特征与地震大形势

本文研究了青藏高原北部地区强震活动的特征,结果是:地震活跃期为230—250年,平静期为130—150年;喜马拉雅南缘、青藏高原南部地区强震与本区强震明显相关,喜马拉雅南缘与缅甸北部地区发生强震后本区发震的概率分别为87.5%和66.7%;本区7级地震分带交替发生,最先发生的两个6级地震对预测未来7级地震发震地点有一定意义;中强震在8级地震前不活跃,而在7级地震前比较活跃,且青海北部与西部发生6级地震后,甘肃东南部或甘青川交界有5级地震活动。据此判断本区未来10年内存在发生7级强震的危险。     

青藏高原北部地区地震基本活动状态定量评价

基于区域构造背景、应力场状态及动力学环境等,将青藏高原北部地区划分为4个相对独立的统计单元,即祁连山地震带、甘东南地区、柴达木一共和地块及库玛地震带.通过地震频度和b值拟合确定了各个地区不同下限震级的地震目录完整性起始时间;确定了各构造单元中强以上地震活跃与平静交替活动的特征;定量计算了在平均状态中强以上地震活动特征参数及平静阶段与活跃阶段地震活动特征参数;分别获得了各个区域在平均、活跃与平静状态下的小震活动状态参数.     

青藏高原地震活动特征及当前地震活动形势

青藏高原是我国现代构造活动和地震活动最强烈的地区,自有地震记录以来,在高原内记录到多达18次8级以上巨大地震和100余次7~7.9级地震,它们均发生在喜马拉雅板块边界构造带和板内断块区及其次级断块的边界活动构造带上.自1900年有地震仪器记录以来,青藏高原曾经历了3次地震活动丛集高潮,即1920-1937年,1947-1976年和1995-现在.在每次地震活动丛集期都形成以8级地震为核心的7级以上地震活动系列,它们分别是20世纪20-30年代的海原-古浪地震系列、50-70年代察隅-当雄地震系列和20世纪末期以来昆仑-汶川地震系列.每一个地震系列都有自己的主体活动区,最新的昆仑-汶川地震系列的主体活动区为巴颜喀喇断块.青藏高原地震活动高潮与全球M_w≥8.0巨大地震活动高潮紧密相关,昆仑-汶川地震系列与自2001年至今的全球最新地震活动高潮相对应,它们反映了两者的动力学联系.经过详细对比研究认为,它们至今均仍在延续之中,全球板块边界构造带8~9级地震和板内大陆断块区的7~8级地震都仍在连续发生.研究了全球和区域地震活动的相关关系及青藏高原地震活动的时空分布特征,指出了该区当前地震活动的总体形势,评价了其近期地震危险性,提出了加强地震监测的建议.     

Low CodaQcin the Epicentral Region of the 2001 Bhuj Earthquake ofMw7.7

On 26 January, 2001 (03:46:55,UT) a devastating intraplate earthquake of M_w 7.7 occurred in a region about 5 km NW of Bhachau, Gujarat (23.42°N, 70.23°E). The epicentral distribution of aftershocks defines a marked concentration along an E-W trending and southerly dipping (45°) zone covering an area of (60 × 40) km². The presence of high seismicity including two earthquakes of magnitudes exceeding 7.7 in the 200 years is presumed to have caused a higher level of shallow crustal heterogeneity in the Kutch area; a site lying in the seismic zone V (zone of the highest seismicity for potentially M8 earthquakes) on the seismic zoning map of India. Attenuation property of the medium around the epicentral area of the Bhuj earthquake covering a circular area of 61,500 km² with a radius of 140 km is studied by estimating the coda-Q_c from 200 local earthquakes of magnitudes varying from 3.0–4.6. The estimated Q₀ values at locations in the aftershock zone (high seismicity) are found to be low in comparison to areas at a distance from it. This can be attributed to the fact that seismic waves are highly scattered for paths through the seismically active and fractured zone but they are well behaved outside the aftershock zone. Distribution of Q₀ values suggests that the local variation in Q₀ values is probably controlled by local geology. The estimated Q₀ values at different stations suggest a low value of Q=(102 ± 0.80)^*f^{(0.98 ± 0.02)} indicating an attenuative crust beneath the entire region. The frequency-dependent relation indicates a relatively low Q_c at lower frequencies (1–3 Hz) that can be attributed to the loss of energy due to scattering attenuation associated with heterogeneities and/or intrinsic attenuation due to fluid movement in the fault zone and fluid-filled cracks. The large Q_c at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneity is expected. Based on the attenuation curve estimated for Q₀=102, the ground acceleration at 240 km distance is 13% of 1 g i.e., 0.13 g agreeing well with the ground acceleration recorded by an accelerograph at Ahmedabad (0.11 g). Hence, it is inferred that the Q₀ value obtained from this study seems to be apt for prediction of ground motion for the region.     

九寨沟MS 7.0地震前单台波速比变化特征

收集、整理四川省地震台网2013年5月至2017年9月产出的震相观测报告数据,采用单台多震和达法,重点分析2017年8月8日九寨沟M_S 7.0强震前龙门山断裂带中北段地区波速比变化特征。结果表明：龙门山断裂带中段西侧的茂县地震台,在发震前2年左右,波速比出现一次持续时间约一年半的下降异常;发震前半年左右,波速比出现明显回返特征,波速比回返至均值上下时即发生九寨沟M_S 7.0地震;震源区周边其他6个地震台波速比在发震前变化较平稳。茂县地震台波速比低值异常对判定九寨沟M_S 7.0强震前龙门山断裂带中北段可能存在孕震体具有重要参考价值。     

Vp and Vp/Vs structures in the crust and upper mantle of the Taiwan region, China

A tomographic study of the V _p and V _p/V _s structures in the crust and upper mantle beneath the Taiwan region of China is conducted by simultaneous inversion of P and S arrival times. Compared with the previous tomographic results, the spherical finite difference technique is suitable for the strong heterogeneous velocity structure, and may improve the accuracy in the travel time and three-dimensional ray tracing calculations. The V _p and V _p/V _s structures derived from joint inversion and the relocated earthquakes can provide better constraints for analyzing the lateral heterogeneity and deep tectonic characters in the crust and upper mantle. Our tomographic results reveal significant relations between the seismic wavespeed structure and the tectonic characters. In the shallow depth, sedimentary basins and orogen show distinct wavespeed anomalies, with low V _p, high V _p/V _s in basins and high V _p, low V _p/V _s in orogen. As the suture zone of Eurasian Plate and Philippine Sea Plate, Longitudinal Valley is characterized by a significant high V _p/V _s anomaly extending to the middle-lower crust and upper mantle, which reflects the impact of rock cracking, partial melting, and the presence of fluids. In the northeast Taiwan, the V _p, V _p/V _s anomalies and relocated earthquakes depict the subducting Philippine Sea Plate under the Eurasian Plate. The high V _p of oceanic plate and the low V _p, high V _p/V _s atop the subducted oceanic plate extend to 80 km depth. Along the east-west profiles, the thickness of crust reaches 60 km at the east of Central Range with eastward dipping trend, which reveals the eastward subduction of the thickened and deformed crust of the Eurasian continental plate. Supported by Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KZCX3-SW-234-2), National Basic Research Program of China (Grant No. 2007CB411701), National High Technology Research and Development Program of China (Grant No. 2006AA09A101-0201) and National Natural Science Foundation of China (Grant Nos. 40804016, 40704013)     

Crustal thickness and vP/vS ratio in Shanxi Graben,China

Shanxi Graben is in the middle part of the North China Craton, from south to north. With the teleseismic data recorded by Regional Seismograph Networks and the temporary ZBnet-W Seismic Array around east part of Shanxi Graben, we measured the crustal thickness and v_P/v_S ratio beneath each station using the H-κ stack of receiver functions. The observed crustal thickness shows obvious lateral variation, increasing gradually from east to west in the Shanxi Graben. Beneath the Shanxi Graben the crust is relatively thicker than both sides of the south and the north. In addition, the v_P/v_S ratio in the north of study zone is higher than that in the south. The highest v_P/v_S ratio exists in the crust of the Xinding basin and the Datong basin. Our study also suggests that high velocity ratio might result from the strong activities of the magmation and volcanism.     

Vp/Vs ratios in the Yellowstone National Park region, Wyoming

In this paper we study the variation of V_p/V_s and Poisson's ratio (δ) in the Yellowstone National Park region, using earthquakes which were well recorded by a local seismic network. We find that the average V_p/V_s value within the geothermally active Yellowstone caldera is about 7% lower than in the area outside the caldera. Within the caldera itself there may be a further 2–7% reduction of V_p/V_s in the hydrothermally active Norris Geyser Basin, the Upper and Lower Geyser Basins, and the Yellowstone Lake and Mud Volcano regions. After considering various possible causes for V_p/V_s changes, such as geologic and structural differences, thermal effects, partial melting, and hydrothermal activity, we conclude that the most plausible explanation for the observed V_p/V_s reduction is the presence of hot-water at temperatures and pore-pressures near the water steam transition in the caldera geothermal reservoirs.     

兴蒙造山带诺敏河火山群地壳厚度与波速比研究

利用布设于兴蒙造山带诺敏河火山群地区的宽频带流动地震台站资料,基于接收函数方法,获取了该地区的地壳厚度与波速比值.研究结果显示,该地区的地壳厚度介于32~38 km,莫霍面深度在空间上分布特征与五大连池为中心的火山带分布具有较好的一致性：沿着火山带延展方向地壳较薄.该地区的波速比介于1.74~1.84,波速比在空间上与地壳厚度变化具有一致性：高波速比主要集中于靠近五大连池火山带地区,向诺敏河火山和小古里河火山延展.研究认为：诺敏河火山与五大连池火山带可能具有相同的岩浆来源,可能与富钾岩石圈地幔拆沉作用造成的地幔热物质上涌有关.研究区地壳厚度与波速比呈现负相关关系,暗示该地区可能发生过岩浆底侵作用.     

Petrogenetic grid for ultrahigh-pressure metamorphism in the model system CaO-MgO-SiO2-CO2-H2O

Abstract Petrogenetic grids for ultrahigh-pressure (UHP) metamorphism were calculated at different X_co2 conditions in the model system CaO-MgO-SiO₂-CO₂-H₂O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P-T grids at X_co2= 0.01 and 0.5 are described. In the calculated P-T grid at X_co2= 0.01, four out of 10 stable invariant points, Co-En-Ms-Tc, Co-Di-En-Tc-Tr, Co-Di-Ms-Tc-Tr and Di-En-Ms-Tc-Tr lie within the stability field of coesite. If the fluid phase has X_co2= 0.5, no invariant point is stable under UHP conditions. Some magnesite-bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO₂, Ms + Tr = Do + Fo + CO₂+ H₂O and Ms + Tc = Fo+ CO₂+ H₂O at X_co2= 0.01 and by reaction Ms + Tc = Fo + CO₂+ H₂O together with these three at X_co2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low X_co2 conditions include the following: Co-Do-Ms, Co-Di-Ms, Co-Di-Tc, Di-Ms-Tc, Di-En-Tc-, Di-En-Ms, Co-Di-En, Di-En-Fo, Di-Fo-Ms, Di-Do-Fo. Among them, talc-bearing assemblages are restricted to X_co2 < 0.02 and their high-P limit is 31.7 kb (749°C) at X_co2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if X_co2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite-dolomite assemblage is restricted only to a calc-silicate rock interlayered with marble where X_co2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low X_co2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of X_co2 on the stability of coesite-dolomite-magnesite, diopside-enstatite-magnesite, diopside-talc assemblages is examined and the occurrence of coesite-dolomite, magnesite-bearing and talc-bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P-T grids.     

The empirical relationships between M s, m b and M L for China and vicinity

Data from 753 earthquakes are used to determine a relationship between surface-wave magnitude (M _s) and bodywave magnitude (m _b), and from 541 earthquakes to determine a relationship between surface-wave magnitude (M _s) and local magnitude (M _L) for China and vicinity: M _s=0.9883 m _b-0.0420, M _s=0.9919 M _L-0.1773. The relationship of M _s versus m _b is obtained for 292 events occurred in the Chinese mainland in the time period from 1964 to 1996, 291 events occurred in Taiwan in the time period from 1964 to 1995 and 170 events occurred in the surrounding area. Standard deviation of the fitting is 0.445. Relationship of M _s versus M _L is obtained for 36 events occurred in the Chinese mainland, 293 events occurred in Taiwan, China and 212 events occurred in the surrounding area. The total amount is 541 events. Standard deviation of the fitting is 0.4673. The uncertainties of the converted M _s in different magnitude intervals can be estimated using complementary cumulative distribution function (CCDF). In the relationship of M _s versus m _b, taking ±0.25 as a range of uncertainties, in magnitude interval m _b 4.0–4.9, the probabilities for the converted M _s taken value less than (M _s-0.25) and more than (M _s+0.25) are 17% and 27% respectively. Similarly, we have probabilities for m _b 5.0–5.9 are 34% and 20% and that for m _b 6.0–6.9 are 11% and 47%. In the relationship of M _s versus M _L, if the range of uncertainties is still taken as ±0.25, the corresponding probabilities for magnitude interval M _L 4.0–4.9 are 22% and 38%, for M _L 5.0–5.9 are 20% and 15% and for magnitude interval M _L 6.0–6.9, are 15% and 29%, respectively. The relationships developed in this paper can be used for the conversion of one magnitude scale into another magnitude scales conveniently. The estimation of uncertainties described in this paper is more accurate and more objective than the usual estimation expressed by deviation. The estimations described in this paper indicate various dispersions in different magnitude intervals of original data. The estimations of uncertainties described by probabilities can be well connected with the total estimations of uncertainties in seismic hazard assessment.     

Ultrasonic measurements of Vp and Qp: relaxation spectrum of complex modulus on basalt melts

Ultrasonic compressional wave velocity V_p and quality factor Q_p have been measured in alkali basalt, olivine basalt and basic andesite melts in the frequency range of 3.4–22 MHz and in the temperature range of 1100–1400°C. Velocity and attenuation of the melts depend on frequency and temperature, showing that there are relaxation mechanisms in the melts. Complex moduli are calculated from the ultrasonic data. The results fit well a complex modulus of Arrhenius temperature dependence with log-normal Gaussian distribution in relaxation times of attenuation. The analysis yields average relaxation time, its activation energy, relaxed modulus, unrelaxed modulus and width of Gaussian distribution in relaxation times. Relaxed modulus is smaller (17.5 GPa) for basic andesite melt of high silica and high alumina contents than for the other two basalt melts (18.1–18.4 GPa). The most probable relaxation times decrease from ～ 3 × 10^?10 s for basic andesite to ～ 10^?11 s for alkali basalt at 1400°C. Activation energies of attenuation, ranging from 270 to 340 kJ mol^?1 in the three melts, are highest in basic andesite. Longitudinal viscosity values and their temperature dependences are also calculated from V_p and Q_p data. The volume viscosity values are estimated from the data using the shear viscosity values. Longitudinal, volume and shear viscosities and their activation energies are highest in the basic andesite melt of the most polymerized structure.