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2012年6月24日宁蒗5．7级地震发生前,地震活动性存在中期异常,地下流体存在中短临异常。地震活动性方面存在滇西地区5级以上地震东西迁移规律性活动、滇西地区4级以上地震成条带分布和程海断裂发生4级窗口地震等中期震兆异常。水氡、固体二氧化碳、水温和流量存在中短临前兆异常。地下流体前兆中期、短期和短临阶段都出现同步性异常。 相似文献
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在地震短临跟踪工作过程中,我们发现,2008年3月21日盈江5．3级地震前,在地震活动性方面,长中短临不同时段内呈现出各种形态的异常,小震活动呈现条带、围空和平静－活跃的过程。地下流体前兆异常水氡趋势性中期异常呈现同步性转折,水氡突变性短临异常呈现同步性突跳,近源区和邻区的水汞呈现同步性高值异常;而且,地下流体前兆异常形成区域与中强震发生地点有一定的构造关联性。因此,对2008年3月21日盈江5．3级地震前地震活动性长中短临各阶段呈现的异常．中短临地下流体前兆异常,进行了系统回顾和总结,理清预报思路,以便更好地指导今后的工作。 相似文献
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汶川地震与松潘地震前异常现象分析比较 总被引：1，自引：0，他引：1
分析了1976年松潘7.2级地震和2008年汶川8.0级地震前异常现象。汶川8.0级地震为地震活跃期的首发大地震,震前区域性中小地震相对平静,区域地震活动b值为0.77;松潘7.2级地震是活跃期间发生的地震,震中附近地区地震活动密集,曾发生松潘黄龙6.5级地震,震前区域地震活动b值为0.62;汶川地震前区域主压应力方位与错动类型总体上与8.0级地震给出的解大体一致,这与松潘7.2级地震前的情况有显著差异,震前地震与余震震型有差异。根据观测数据的分析和异常判定结果认为,松潘地震与汶川地震前的观测异常没有一个台项是重复出现的,短临异常数前者显著多于后者。松潘地震与汶川地震前的宏观异常现象差异极大,更未出现火球、地光临震异常现象。这说明,已有的短临前兆观测异常的出现仅有个案特征,而无普适意义。 相似文献
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:1 998年 1 1月 1 9日云南宁蒗发生了 6.2级地震 ,根据震前震源区及周围地区中小地震活动异常图像 ,分析研究了在区域应力场作用下 ,震源区介质由稳定状态进入非稳定状态过程地震活动异常动态演变特征。同时 ,根据序列参数 h、b值的动态变化 ,给出序列异常变化特征。主要特征为 :1 .地震孕育中期阶段 ,3级以上地震活动主要分布在震源外围地区 ,震源区高度平静 ;地震孕育发展至短期阶段 ,震源外围地震活动消失 ,震源周围地震活动迅速增强 ,且随着地震的逼近 ,地震活动逐渐向震源区收缩 ;至短临阶段 ,孕震区出现显著的小震群活动。 2 .宁蒗 6.2级地震在其 5.3级前震前 ,震群发育 ,震群参数 h值小于 1 .0 ,且随着地震的逼近 ,震群参数 h值逐渐减少、b值逐渐增大。 3 .5.2级前震和6.2级主震前 ,序列参数 h值均出现明显快速减小的动态异常过程。此外 ,5.3、5.2级前震后 ,其 2级以上余震衰减迅速 ,且余震又少又小。 相似文献
7.
2003年云南大姚6.2级、6.1级地震中短期地震学异常演化特征 总被引：1，自引：0，他引：1
通过对2003年云南大姚6．2级、6．1级地震中短期地震学特征的分析和初步研究，认为大姚地震孕育过程中的短期阶段，震中附近的地震学异常以中小地震活动增强为主，表现为震区附近中小地震频度增加，4级震群活动、A值、GL值出现异常，同时震源深度加深等；在由中期向短期过渡的过程中，震源区异常消失相对平静，使多种地震学异常收缩至震源区附近；地震孕育进入短临阶段，震中附近地区再次出现了3～4级小震活动增强的特征，云南省地震活动出现向震中附近迁移的趋势。整个孕震过程，地震活动体现了活动-平静-活动的动态演化特征。 相似文献
8.
宁蒗6．2级地震活动异常动态演变图象与序列主要特征 总被引：1，自引：0，他引：1
1998年11月19日云南宁蒗发生了6．2级地震,根据震前震源区及周围地区中小地震活动异常图像,分析研究了区域应力场作用下,震源区介质由稳定状态进行入稳定状态过程地震活动异常动态演变特征。同时,根据序列参数h、b值的动态变化,给出序列异常变化特征。主要特征为：1．地震孕育中期阶段,3级以上地震活动主要分布在震源外围地区,震源区高度平静;地震孕育发展至短期阶段,震源外围地震活动消失,震源周围地震活动迅速增强,且随着地震的逼近,地震活动逐渐向震源区收缩;至短临阶段,孕震区出现显的小震群活动。2．宁蒗6．2级地震在其5．3级地震前,震群发育,震群参数h值小于1．0,且随着地震的逼近,震群参数h值逐渐减少、b值逐渐增大。3．5．2级前震和6．2级震前,序列参数h值均出现明显快速减小的动态异常过程。此外,5．3、5．2级前震后,其2级以上余震衰减迅速,且余震又少又小。 相似文献
9.
通过普洱6．3、宁蒗6．2级和姚安6．5级3个在省内M≥5级地震平静了10余个月,打破5级平静后发生的M≥6级强震前水汞的异常分析,并着重短临异常研究,结果表明这3次地震前水汞均有中期和短、临异常,中、短、临异常台站数为两头少,中间多。中期异常表现为11％。22％的水汞观测台站震前出现2—10个月的中期异常;短期异常表现为震前2—3个月,平均44％的台站观测到短期异常,最大异常幅度与异常判定线比值在1．3—3．2倍之间：强震前都只有1个台站出现临震异常。短期异常有差异,宁蒗6．2级主震前,短期异常在2次前震前出现,普洱6．3、姚安6．5级地震前,水汞短期异常在打破5级平静的地震前出现,水汞短期异常有活跃、平静的现象。最早出现中期异常的台站相对靠近震中。 相似文献
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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MS 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MS 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MS 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。 相似文献
12.
P. Mandal Jainendra S. Joshi Sudesh Kumar Rajendra Bhunia B. K. Rastogi 《Pure and Applied Geophysics》2004,161(8):1635-1654
On 26 January, 2001 (03:46:55,UT) a devastating intraplate earthquake of Mw 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-Qc 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 Qc 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 Qc 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. 相似文献
13.
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) 相似文献
14.
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 vP/vS 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 vP/vS ratio in the north of study zone is higher than that in the south. The highest vP/vS 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. 相似文献
15.
In this paper we study the variation of Vp/Vs 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 Vp/Vs 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 Vp/Vs 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 Vp/Vs changes, such as geologic and structural differences, thermal effects, partial melting, and hydrothermal activity, we conclude that the most plausible explanation for the observed Vp/Vs reduction is the presence of hot-water at temperatures and pore-pressures near the water steam transition in the caldera geothermal reservoirs. 相似文献
16.
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. 相似文献
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18.
Abstract Petrogenetic grids for ultrahigh-pressure (UHP) metamorphism were calculated at different Xco2 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 Xco2= 0.01 and 0.5 are described. In the calculated P-T grid at Xco2= 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 Xco2= 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 Xco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three at Xco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low Xco2 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 Xco2 < 0.02 and their high-P limit is 31.7 kb (749°C) at Xco2= 0.01. Dolomite-magnesite-silica assemblages have large P-T stability fields even if Xco2 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 Xco2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low Xco2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of Xco2 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. 相似文献
19.
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. 相似文献
20.
Ultrasonic compressional wave velocity Vp and quality factor Qp 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 Vp and Qp 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. 相似文献