Self-organized Fractal Seismicity and b Value of Aftershocks of the 2001 Bhuj Earthquake in Kutch (India)

— The devastating intraplate earthquake of M_w 7.7 of 26 January, 2001 took place along the south-dipping reverse fault in the lower crust ( 23 km) of Kutch, Gujarat, India, obliterating some 14,000 people. The aftershock activity has ensued for three years. We analyzed 997 aftershocks of M 3.0 to study the b value and fractal correlation dimensions in time and space. The b value is found to be 0.8 ± 0.03 from the Gutenberg-Richter relation and 0.76 ± 0.02 from the maximum-likelihood, suggesting a typical value for the intraplate region. The spatial correlation is 1.71 ± 0.02, indicating that events are approaching a two-dimensional region. Further, the temporal correlation dimension is estimated to be 0.78 ± 0.02, confirming the structure is mono-fractal in time domain. The depth section of b value shows a peak at 15–35 km depth range coinciding with the maximum occurrence of aftershocks ( 47%), which is inferred as a fluid-filled highly fractured rock matrix with fractures of high density. It will be important to note that tomographic results also suggest a low Vp, low Vs and a large Poissons ratio for the same depth range, further confirming this inference. Additionally, we have studied the variation of D₂^s and b value with time. During the first two months of aftershock activity the results show a marked negative correlation between spatial correlation dimension D₂ (large) and b value (low), indicating the predominance of large events associated with weak clustering. The negative correlation means the stress release along faults of a larger surface area. After two months the fractal dimension (D₂^s) and b value suggests a positive correlation implying more numerous smaller shocks with stress release along faults of a smaller surface area. This would indicate a reduced probability of large magnitude earthquakes due to fragmentation of the fault zone.Acknowledgement. The authors thank Dr. V.P. Dimri, Director, NGRI for his encouragement and kind permission to publish this work. The Department of Science and Technology, New Delhi supported this study.     

云南强震前地震活动因子A值异常动态演变特征

运用地震活动因子A值参数全面、深入地研究了云南强震前尤其是强震活跃期首发强震前地震活动在时间、空间的动态演变过程,研究结果显示:(1)强震前1～2年,在未来震中周围A值出现显著高值变化,异常阀值为A≥0.6;(2)强震前3～4年,A≥0.6的高值异常区主要分布在云南省外围或边界附近,省内A值异常较少。随着强震的临近,A值异常面积逐渐增大,并且向省内、未来震中迁移,震前1年或1～2年云南地区A值异常单位面积和省内外异常单位面积的比值出现明显转折回升。     

山西地震带中段M≥6地震活动特征初探

利用历史地震资料，分析研究了山西地震带中段（包括大同、忻定、太原、临汾和运城）的Ｍ≥６地震活动特征，并对这几个盆地中强地震活动时序进行了分析对比，发现它们具明显的节奏活动特点。并对今后百年内山西地震带中段的中强地震活动趋势作出了估计。     

Moment Tensor Solutions and Triggering Environment for Earthquakes in Koyna-Warna Water Reservoirs Region,India

We consider nine earthquakes in the Koyna-Warna reservoir region on the western side of the Peninsular India. The deviatoric moment tensors of these earthquakes have been evaluated by minimizing the least-squares misfit between observed and synthetic seismograms. We use broadband seismograms of observatories at KARD and PUNE which are at distances of nearly 50 and 150 km, respectively, from the epicenters. Both surface wave inversion and the difference between the arrival times of SH and SV show the presence of an anisotropic crust. However, we have obtained an equivalent isotropic structure by improving the published crustal structures of this area through inversion of surface wave group velocity data. The deviatoric moment tensors of the earthquakes are decomposed into two components: double-couple and compensated linear vector dipoles (CLVD). The double-couple components of all the nine earthquakes show normal faulting with minor strike slip; the T axis is consistently subhorizontal with an average azimuth of 260.6° and the P axis is nearly vertical. The fault planes of six events give average strike direction and dip, respectively as 194.0° and 51.8° and are associated with the main fault of the area. The other three events lie in the southern part of this area and have strike direction between SSE and SE which is parallel to the tectonic features in this part. The CLVD component is generally within 20 percent of the total moment tensor. Recent studies show that anisotropy can produce source mechanism with CLVD up to 30 percent and can also cause high pore fluid pressure leading to fault instability more rapidly compared to conventional mechanism in an isotropic medium. It appears that the anisotropic crust, noted in the present work, is generating the CLVD component and also gives the proper environment to trigger earthquakes by reservoirs through pore fluid pressure.     

山西地震带地震活动趋势的基本预测

对山西地震带的中长地震趋势进行了探讨。并从华北第３、４地震活动期的对比、山西地震带地震活动的参与以及活动规律等方面进行了研究，确定出山西地震带目前所处的活动阶段，并对可能发生地震的３要素作出了初步预测。     

Interpretation of Low Seismicity in the Eastern Anatolian Collisional Zone Using Geophysical (Seismicity and Aeromagnetic) and Geological Data

The eastern Anatolia is a continental collisional zone between the Arabia-Eurasia plates and is currently being accompanied by the westward escape of the Anatolian crustal block to the west-southwest along two major strike-slip fault zones, the NATF and the EATF. Although these major fault zones have experienced historical earthquakes with moderate to high magnitude, notably there is a low seismicity zone on the active EATF between the Bitlis and Pötürge massifs. The low seismicity zone is characterized by thinner crustal structure relative to its environs, the shallow Curie Point Depth (SCPD, ca 12–14 km in between 39–40°E and 38.5–39°N in the easternmost part of the Anatolian plate) and moderate to high b values (more than 0.7). We consider that the shallow CPD and moderate to high b values in the low seismicity zone characterized by thinner crustal area are closely related with the higher thermal structure of the crust, which most probably resulted from crust-hot asthenospheric mantle interactions.     

Seismicity of the Koyna region and regional tectonomagmatism of the Western Margin (India)

Recent findings on the Meso-Cenozoic tectonomagmatism and deep-seated anomalous geophysical structures suggest a close linkage between the seismicity of the Koyna region, the Westernghat uplift (WG-U) and associated thermomechanical and fluid activities. The WG-U seems to be the result of late Cretaceous thermal mobilization, erosion of the Deccan trap cover and superposition of compressional stress. The association of seismicity with uplift seems to result from movement of deep-seated heat and fluids/volatiles along the edges (or boundary faults) of the uplift; because the force required for crustal deformation depends on the relief. Observed gradients in relief may be attributed to the differential erosion-rates and heat inputs, due to the time gap of 50 Ma in the break-ups and plume activities on the eastern and western sides and consequence magmatism. Further, the geology and tectonics strongly indicate that the western margin (WM) is a relic of a mobile arm (MA), that included Madagascar, and which formed a part of the Proterozoic mobile belt of greater India (fort>85 Ma). The mobile nature of the WM facilitates mantle upwellings and transient elevation of isotherms at depth, raising the possibility of intermittent metamorphism and greater deformation.Superposition of the ongoing compression and uplift-induced forces make local permeability and pore-fluid pressure vital in triggering the seismic slip over the Peninsular shield. Certain representative model calculations have been carried out to estimate change in the e.m. induction characteristics caused by an intermittent hydraulic connectivity. The results show a drop in the resistivity which could be a useful monitoring index. The close connection of uplift and fluid activity as discussed here seems applicable for other active parts of the South Indian Shield (SIS) also.     

喜马拉雅地震带地震活动性参数确定

GB 18306—2015《中国地震动参数区划图》发布距今已有十多年,期间,地震资料（尤其是青藏地震区地震资料）发生了较大变化,及时开展相关地震带地震活动性参数研究对重大建设工程抗震设防安全具有重要意义。本文研究补充了600余条地震目录,其中8.0～8.9级地震7次,7.0～7.9级地震35次,并分析了地震资料的完整性：由于历史、地理、人文的原因,喜马拉雅地震带地震遗漏较严重,7级地震直至1897年才基本完整,6级地震直至1950年才基本完整,5级地震直至1980年才基本完整;本文根据8级以上地震密集活动时段划分了地震活动周期,公元1803年以来可以划分出2个活动周期,并对未来地震活动进行了预测;采用多种方案拟合了喜马拉雅地震带的b值和4级以上地震年发生率：b=0.90,v4=162,并与前人的计算结果进行了简单对比。     

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。     

The Characteristics of Recent Geodetic Deformation and Seismicity in North-South Seismic Zone and East of the Qinghai-Xizang Block

In this paper,the corresponding relation of the evolutionary characteristics of geodetic deformation fields with seismic activity for more than 20 years in the North-South seismic zone and East of Qinghai-Xizang Mess has been investigated.Not only is geodetic deformation in non-homogeneity for the space-time distribution but also deformation fields are in macroscopic similarity for the identical time interval.The inherited tectonic movement is a total tendency of recent crustal movement,and the motion mode is in undulations.There are stages of accumulation and release-adjusting of strain energy in crustal movement processes,which may be the dynamic mechanism of relatively quiet and active seismicity.The analysis of the crustal movement tendency since 1991 is of some significance for judging the stress state and the large seismic situation in the area.     

Intensive Observation of Reservoirinduced Seismicity and Preliminary Analysis on the Causes of Earthquakes in the Three Gorges Reservoir Area,Yangtze River

In accordance with the requirements of the National Key Technology R&D Program of the 11th "Five-year Plan",a densified seismic network consisting of 26 seismic stations was established at the Three Gorges Reservoir area in the section of Hubei Province in March 2009(21 short-period seismographs,5 broadband digital seismographs).From March to December,2009,a total of 2,995 ML-0.8~2.9 earthquakes were detected during the trial impounding of the Three Gorges Reservoir(water level rose from 145 m to 172.8m).Using the double difference earthquake location algorithm,2,837 earthquakes were precisely re-located.The results show that the pattern of small local earthquake swarms in the Three Gorges Reservoir area took on a linear distribution or mass-like cluster distribution,the mass-like clusters of events were generally within a distance of 5 km from waterfront,and the linear distribution of the earthquakes could be extended to a distance of 16 km away from the waterfront.In the Hubei section of the Three Gorges Reservoir,earthquakes were mainly concentrated in the northern end of the Xiannvshan and Jiuwanxi faults near the Xiangxihe River,and along the banks of the Yangtze River at the west of Xietan township and the Shenlongxi area on the northern bank in the Badong region,with focal depths less than 10km,and 4km in average.Earthquake frequency in the reservoir region had a positive correlation with reservoir water level fluctuations,indicating that the seismicity belongs to reservoir induced earthquakes.Along the Shenlong River in the reservoir area,earthquakes showed three linear distributions in the northern Badong county,and distributed according to Karst distribution.There are underground rivers in the carbonate strata.When the reservoir was impounded,water permeated into the underground rivers,thus inducing earthquakes.Earthquakes in the areas on the cross-river segment of Xiannvshan fault,the Jiuwanxi fault and at the areas west of Xietan,Shazhen and Xizhen,may be related to the softening of discontinuities,such as the Nukou fault,the Xiannvshan fault,or the bedding joints,which would lead to failure of rock masses,thus,inducing earthquakes.However,convincing conclusions about the triggering mechanism still need further study.Additionally,near the areas south of Wenhua and Yanglin of Zigui county and at Rangkou town east of Badong county,mining-induced earthquakes occurred at the mines nearby,and on the shores of the reservoir are some collapse earthquakes.     

印尼8.7、8.5级远震对云南地区小震的激发和地下流体异常响应初探

2004年12月26日和2005年3月29日,印度尼西亚苏门答腊西北海域相继发生8.7和8.5级地震。通过对这两次地震与云南地震活动关系和地下流体记录异常变化的分析,认为:印尼两次8级地震的发生可能会使云南提前进入新一轮强震活跃期;远震对云南地区小震的激发和导致地下流体记录出现异常变化的因素虽然都与大地震面波的强弱、地质构造环境等有关,但机理不同。     

2015年7月3日新疆皮山MS6.5地震序列活动性及重新定位研究

2015年7月3日新疆皮山M_S6.5地震发生在塔里木块体与西昆仑交会地区, 该地区近年来中强地震较为活跃。 本文对2015年新疆皮山M_S6.5地震序列及其附近地区中强震序列b值等参数进行了研究。 另外, 还利用新疆台网的震相报告, 采用双差定位方法对余震进行了重新定位, 对皮山余震序列的主震破裂区地震活动时空变化特征进行了分析和讨论, 为该地区地震危险性评估提供参考。     

中强地震活动区地震年平均发生率确定方法的比较

Two kinds of methods for determining seismic parameters are presented,that is,the potential seismic source zoning method and grid-spatially smoothing method. The Gaussian smoothing method and the modified Gaussian smoothing method are described in detail, and a comprehensive analysis of the advantages and disadvantages of these methods is made. Then,we take central China as the study region,and use the Gaussian smoothing method and potential seismic source zoning method to build seismic models to calculate the mean annual seismic rate. Seismic hazard is calculated using the probabilistic seismic hazard analysis method to construct the ground motion acceleration zoning maps. The differences between the maps and these models are discussed and the causes are investigated. The results show that the spatial smoothing method is suitable for estimating the seismic hazard over the moderate and low seismicity regions or the hazard caused by background seismicity; while the potential seismic source zoning method is suitable for estimating the seismic hazard in well-defined seismotectonics. Combining the spatial smoothing method and the potential seismic source zoning method with an integrated account of the seismicity and known seismotectonics is a feasible approach to estimate the seismic hazard in moderate and low seismicity regions.     

东昆仑断裂带玛曲—玛沁段大震危险性分析和探讨

在巴彦喀拉块体边界地震活动和构造分析的基础上,从地震活动空间图像、时间序列演化两方面对巴彦喀拉块体北边界东昆仑断裂带玛曲—玛沁空区的大震危险性进行探讨。与以前震例进行对比发现当前甘青川交界区域已形成与共和大震前相似的平静区,活动性参数计算表明东昆仑断裂带玛曲—玛沁段有逼近的大震危险性。     

漫湾水库蓄水前后库区地震活动性与构造应力场分析

对漫湾水库蓄水诱发地震作了分析，结果表明，蓄水后库区小震频次明显增加，近距离地震明显增多，在蓄水首次达到高水位和由高水位突降至低水位时，诱发了最强地震。表明蓄水对库区构造应力场产生了影响，使得局部应力场较区域构造应力场作用方向出现明显偏转，作用方式发生改变，呈现较大的垂直作用效应，节面错动出现较大正倾滑动分量。     

Study on Uncertainties of Seismicity Parameters b and v4 in Seismic Statistical Zones

For several seismic statistical zones in North China,the key factors causing uncertainties in the important seismicity parameters b and ν_4 and the features of their uncertainties are discussed in this paper.The magnitude of uncertainty is also analyzed.It can be seen that the key influencing factors are statistical period,methods of processing statistical samples,lower limit magnitude and the annual average occurrence ratio of large earthquakes.The variation ranges of b and ν_4 in the Tancheng-Lujiang zone are as high as 0.2 and 1.4 respectively,which are similar to those in the Fenwei zone.They are much smaller however in the Hebei zone because of its sufficient statistical samples.     

Multifractal Analysis of Earthquakes in the Southeastern Iran-Bam Region

Earthquakes in Iran and neighbouring regions are closely connected to their position within the geologically active Alpine-Himalayan belt. Modern tectonic activity is forced by the convergent movements between two plates: The Arabian plate, including Saudi Arabia, the Persian Gulf and the Zagros Ranges of Iran, and the Eurasian plate. The intensive seismic activity in this region is recorded with shallow focal depth and magnitude rising as high as M_w = 7.8. The study region can be attributed to a highly complex geodynamic process and therefore is well suited for multifractal seismicity analysis. Multifractal analysis of earthquakes (m_b ≥ 3) occurring during 1973 – 2006 led to the detection of a clustering pattern in the narrow time span prior to all the large earthquakes: M_w = 7.8 on 16.9.1978; M_w = 6.8 on 26.12.2003; M_w = 7.7 on 10.5.97. Based on the spatio-temporal clustering pattern of events, the potential for future large events can be assessed. Spatio-temporal clustering of events apparently indicates a highly stressed region, an asperity or weak zone from which the rupture propagation eventually nucleates, causing large earthquakes. This clustering pattern analysis done on a well-constrained catalogue for most of the fault systems of known seismicity may eventually aid in the preparedness and earthquake disaster mitigation.     

汾渭地震带的b值和年平均发生率的研究

为弄清当前汾渭地震带的 b值和年发生率的情况 ,用 Mapinfo的图象分析和检索功能对汾渭地震带的 b值进行了统计计算 ,推导出 4 .0级以上地震的年平均发生率 ,并对计算结果进行了分析 ,得出近 10 a由于带内强震数量很少 ,b值和年平均发生率变化不大的结论     

云南“三江”流域地震活动性研究

利用云南区域台网1966年至2006年的地震目录,系统研究了云南“三江”（怒江,澜沧江,金沙江）流域内的地震活动性,给出了该区域、以及以各河流形成的3个次区域的地震活动时空分布特点和b值结果,并进行了讨论,根据研究结果,认为拟建、在建的水电工程可能造成水库诱发地震是一个值得高度重视的问题。