The Effects of Attenuation and Site on the Spectra of Microearthquakes in the Shillong Region of Northeast India

Microearthquake spectra from the Shillong region are analyzed to observe the effect of attenuation and site on these spectra. The spectral ratio method is utilized to estimate the Q values for both P- and S-waves in the subsurface layer, wherein the ratio of spectral amplitudes at lower and higher frequencies are taken into consideration for three stations at varying epicentral distances. Average estimates of Q _P and Q _S are 178 and 195. The ratio of Q _S to Q _P is estimated to be greater than 1 in major parts of the Shillong area, which can be related to the dry crust prevailing in the Shillong region. Typically, the variation in corner frequencies for these spectra is inferred to be characteristic of the site. Simultaneously, observations from spectral content of local earthquakes recorded at two different stations with respect to the reference site yield greater amplification of incoming seismic signals in the frequency range of 2–5 Hz, which is found to be well supported by the existing local lithology pertinent to that region.     

Seismotectonic zones demarcation in the Shillong Plateau using the microearthquakes and radon emanation rate

The Shillong Plateau signifies the intense tectonic processes that the region has experienced during the Tertiary Indo-Tibetan and Indo-Burman collisions. An attempt has been made to study the microearthquake and radon emanation rate to understand and identify the seismotectonic zones.     

Seismic source zonation for NE India on the basis of past EQs and spatial distribution of seismicity parameters

Journal of Seismology - The North East (NE) of India is an earthquake (EQ) prone zone and has experienced considerable damages in the past due to high intensity EQs. With rapidly growing...     

Estimation of Maximum Earthquakes in Northeast India

We attempt to estimate possible maximum earthquakes in the northeast Indian region for four seismic source zones, namely EHZ, MBZ, EBZ, and SHZ, which encapsulates the various seismogenic structures of the region and also for combined source zones taken as a single seismic source regime. The latter case exhibits a high maximum earthquake estimate of M_W 9.4 (±0.85) through Bayesian interpretation of frequency magnitude distribution with Gamma function implicating a moderate deviation from the standard Gutenberg Richter model at the higher magnitudes. However, tapering Gutenberg Richter models with corner magnitudes at M_W 8.01, 8.7 and 9.1, respectively indicated maximum values corresponding to M_W 8.4, 9.0, and 9.3. The former approach was applied to each of the source zones wherein the data are presented in parts according to the data completeness, thereof. EHZ, MBZ, EBZ and SHZ are seen with maximum earthquakes of M_W 8.35 (±0.59), 8.79 (±0.31), 8.20 (±0.50), and 8.73 (±0.70), respectively. The maximum possible earthquakes estimated for each individual zone are seen to be lower than that estimated for the single regime. However, the pertaining return periods estimated for the combined zone are far less than those estimated for the demarcated ones.     

Seismic recurrence parameters for India and adjoined regions

Journal of Seismology - This article focuses on estimating the seismic recurrence parameters of India and adjoining regions based on a comprehensive catalogue assimilated from various sources. The...     

Ground motion parameters of Shillong plateau: One of the most seismically active zones of northeastern India

Strong ground motion parameters for Shillong plateau of northeastern India are examined. Empirical relations are obtained for main parameters of ground motions as a function of earthquake magnitude, fault type, source depth, velocity characterization of medium and distance. Correlation between ground motion parameters and characteristics of seismogenic zones are established. A new attenuation relation for peak ground acceleration is developed, which predicts higher expected PGA in the region. Parameters of ...     

Calibration of the Tibetan Plateau Using Regional Seismic Waveforms

We use the recordings from 51 earthquakes produced by a PASSCAL deployment in Tibet to develop a two-layer crustal model for the region. Starting with their ISC locations, we iteratively fit the P-arrival times to relocate the earthquakes and estimate mantle and crustal seismic parameters. An average crustal P velocity of 6.2–6.3 km/s is obtained for a crustal thickness of 65 km while the P velocity of the uppermost mantle is 8.1 km/s. The upper layer of the model is further fine-tuned by obtaining the best synthetic SH waveform match to an observed waveform for a well-located event. Green's functions from this model are then used to estimate the source parameters for those events using a grid search procedure. Average event relocation relative to the ISC locations, excluding two poorly located earthquakes, is 16 km. All but one earthquake are determined by the waveform inversion to be at depths between 5 and 15 km. This is 15 km shallower, on average, than depths reported by the ISC. The shallow seismicity cut-off depth and low crustal velocities suggest high temperatures in the lower crust. Thrust faulting source mechanisms dominate at the margins of the plateau. Within the plateau, at locations with surface elevations less than 5 km, source mechanisms are a mixture of strike-slip and thrust. Most events occurring in the high plateau where elevations are above 5 km show normal faulting. This indicates that a large portion of the plateau is under EW extension.     

中国东北深源地震定位方法解析

黑龙江省有时会发生震源深度300-600 km的深源地震,且地震释放能量及有感范围均较大。该地区发生M > 6.0深源地震会产生较大社会影响,对于深源地震,快速准确地确定震源参数具有重要意义。利用黑龙江省测震台网自"十五"项目投入运行以来记录的31条深源地震数据,采用LocSAT和HypoSAT定位方法进行重新定位,将定位结果与中国地震台网中心统一编目给出的目录数据进行对比,寻求更加适用该区深震的定位方法。分析表明,对于黑龙江省测震台网记录的东北地区深源地震,使用HypoSAT方法进行定位结果优于LocSAT定位方法。     

皖东北地区多层砌体校舍抗震调查与震害预测

基于皖东北地区多层砌体校舍抗震性能调查,结合汶川地震的震害,对皖东北地区中小学多层砌体校舍的特点、抗震能力进行阐述,运用易损性概率分析法对所调查的校舍进行震害预测,并提出了相应的防震减灾对策,以期为该地区校舍安全工程提供参考.     

Recent seismicity in Northeast India and its adjoining region

Recent seismicity in the northeast India and its adjoining region exhibits different earthquake mechanisms – predominantly thrust faulting on the eastern boundary, normal faulting in the upper Himalaya, and strike slip in the remaining areas. A homogenized catalogue in moment magnitude, M _W, covering a period from 1906 to 2006 is derived from International Seismological Center (ISC) catalogue, and Global Centroid Moment Tensor (GCMT) database. Owing to significant and stable earthquake recordings as seen from 1964 onwards, the seismicity in the region is analyzed for the period with spatial distribution of magnitude of completeness m _t, b value, a value, and correlation fractal dimension D _C. The estimated value of m _t is found to vary between 4.0 and 4.8. The a value is seen to vary from 4.47 to 8.59 while b value ranges from 0.61 to 1.36. Thrust zones are seen to exhibit predominantly lower b value distribution while strike-slip and normal faulting regimes are associated with moderate to higher b value distribution. D _C is found to vary from 0.70 to 1.66. Although the correlation between spatial distribution of b value and D _C is seen predominantly negative, positive correlations can also be observed in some parts of this territory. A major observation is the strikingly negative correlation with low b value in the eastern boundary thrust region implying a possible case of extending asperity. Incidentally, application of box counting method on fault segments of the study region indicates comparatively higher fractal dimension, D, suggesting an inclination towards a planar geometrical coverage in the 2D spatial extent. Finally, four broad seismic source zones are demarcated based on the estimated spatial seismicity patterns in collaboration with the underlying active fault networks. The present work appraises the seismicity scenario in fulfillment of a basic groundwork for seismic hazard assessment in this earthquake province of the country.     

Seismic <Emphasis Type="Italic">b</Emphasis>-Value and the Assessment of Ambient Stress in Northeast India

Seismicity data of northeast India, recorded between 1986 and 1999 by a local network, are analysed for estimation of b-values. Based on the obtained values, viz. low (b ≤ 0.5), moderate (0.5 < b ≤ 0.7) and high (b > 0.7), the study area is classified into different seismic-domains. An assessment of stress level is also carried out in identifying seismic-domains. Seismic activities, though mostly confined in some sectors, are presumably triggered by mutual interaction of the Shillong Plateau, Mikir Hills, Indo-Burman Ranges and the easternmost part of the Himalayas, and the contributions from deep-seated fractures cannot be ignored. The results resemble the seismic character of a foreland setting adjacent to a convergent margin. The b-values estimated for 240 square grids of dimension 0.6° × 0.6° over five seismic domains indicate wide variation. An analysis of cumulative seismic moment release (M _O) in different layers also indicates an anomaly in reference to the total seismic-energy budget of the five zones. The lower b-value and higher M _O recorded at relatively lower depth (~30 km) towards the southwest of the study area might be associated with upward bulging of a strong lithosphere. The bulging is perhaps regionally compensated by the downward flexing of the descending Indian lithosphere beneath the Upper Assam area; features unequivocally observed in any foreland setup. Towards the north and east of the study area, random variations of in both b-value and M _O along the converging zone suggest a varied tectonic environment with active interaction between the tectonic elements in these areas.     

鄂尔多斯盆地南部黄土塬区地震资料处理与解释 第二部分:地震资料解释

鄂尔多斯盆地南部的黄土高原,由于长期 风化、侵蚀,形成了树枝状水系与沟、塬、梁、峁、 坡并存的独特黄土地貌。针对这种复杂地表条件长 庆物探已建立了沿沟弯线为主、辅以黄土塬多线和 网状三维的采集方法;确立了高精度静校正、适度 去噪、共反射面元优化及已知井反射系数序列控制 的高保真处理方法、本文着重介绍处理、解释二大 环节的关键方法及应用效果。     

青藏高原北部地区地震空区初步研究

研究了青藏高原北部地区地震空区的时空演化特征，结果表明：大部分Ms≥5地震在孕育发展过程中会形成空区，从孕震空区形成到发震，最短时间1个月，最长时间2a，优势发震时段为4～15个月。孕震空区可作为青藏高原北部地区Ms≥5地震1年尺度的中期预报指标；逼近地震的出现可作为孕震空区形成之后半年内发震的短期指标。     

俯冲带中的洋壳在中国东北上地幔底部分离的地震学证据

中国东北西太平洋俯冲带位于欧亚板块与西太平洋板块的交界部位,西太平洋板块的俯冲运动从日本海沟开始一直延伸到了中国东北地区.层析成像(Gudmundsson & Sambridge,1998;Huang & zhao,2006)结果表明,西太平洋板块和该区域'660'的相互作用,导致了该区域上地幔结构的复杂性.     

Probabilistic Assessment of Earthquake Recurrence in Northeast India and Adjoining Regions

Northeast India and adjoining regions (20°–32° N and 87°–100° E) are highly vulnerable to earthquake hazard in the Indian sub-continent, which fall under seismic zones V, IV and III in the seismic zoning map of India with magnitudes M exceeding 8, 7 and 6, respectively. It has experienced two devastating earthquakes, namely, the Shillong Plateau earthquake of June 12, 1897 (M _w 8.1) and the Assam earthquake of August 15, 1950 (M _w 8.5) that caused huge loss of lives and property in the Indian sub-continent. In the present study, the probabilities of the occurrences of earthquakes with magnitude M ≥ 7.0 during a specified interval of time has been estimated on the basis of three probabilistic models, namely, Weibull, Gamma and Lognormal, with the help of the earthquake catalogue spanning the period 1846 to 1995. The method of maximum likelihood has been used to estimate the earthquake hazard parameters. The logarithmic probability of likelihood function (ln L) is estimated and used to compare the suitability of models and it was found that the Gamma model fits best with the actual data. The sample mean interval of occurrence of such earthquakes is estimated as 7.82 years in the northeast India region and the expected mean values for Weibull, Gamma and Lognormal distributions are estimated as 7.837, 7.820 and 8.269 years, respectively. The estimated cumulative probability for an earthquake M ≥ 7.0 reaches 0.8 after about 15–16 (2010–2011) years and 0.9 after about 18–20 (2013–2015) years from the occurrence of the last earthquake (1995) in the region. The estimated conditional probability also reaches 0.8 to 0.9 after about 13–17 (2008–2012) years in the considered region for an earthquake M ≥ 7.0 when the elapsed time is zero years. However, the conditional probability reaches 0.8 to 0.9 after about 9–13 (2018–2022) years for earthquake M ≥ 7.0 when the elapsed time is 14 years (i.e. 2009).     

中印地震和滑坡灾害研讨会在印度举行

介绍了2002年1月21－23日在印度国家科学院举行的“中印地震和滑坡灾害研讨会”概况及印度有关科研机构的研究计划和进展情况。     

青藏高原东北缘背景噪声特征分析

采用2009年青藏高原东北缘55个数字化宽频带地震台站的垂直分量波形数据,计算了台站间噪声互相关函数,得到了Rayleigh经验格林函数。利用信噪比和归一化平均能量流的方法分析了青藏高原东北缘背景噪声源的方位分布和季节性变化特征。结果表明,5~10 s周期噪声源方位分布上较为稳定,不随季节变化,噪声源能量集中在105°~165°方向上,噪声源主要来自于太平洋。10~20 s周期噪声源季节性变化特征明显,夏季的噪声能量主要集中在165°~210°方向上,噪声源来自于印度洋海洋活动;冬季主要集中在300°~350°和165°~210°方向上,噪声能量主要来源于太平洋和北大西洋。