Activation of seismicity in Central and South Asia after the Makran earthquakes: Possible acceleration of preparation of large seismic events in the Tien Shan region

Two zones of seismicity (ten events with M _w = 7.0–7.7) stretching from Makran and the Eastern Himalaya to the Central and EasternTien Shan, respectively, formed over 11 years after the great Makran earthquake of 1945 (M _w = 8.1). Two large earthquakes (M _w = 7.7) hit theMakran area in 2013. In addition, two zones of seismicity (M ≥ 5.0) occurred 1–2 years after theMakran earthquake in September 24, 2013, stretching in the north-northeastern and north-northwestern directions. Two large Nepal earthquakes struck the southern extremity of the “eastern” zone (April 25, 2015, M _w = 7.8 and May 12, 2015, M _w = 7.3), and the Pamir earthquake (December 7, 2015, M _w = 7.2) occurred near Sarez Lake eastw of the “western” zone. The available data indicate an increase in subhorizontal stresses in the region under study, which should accelerate the possible preparation of a series of large earthquakes, primarily in the area of the Central Tien Shan, between 70° and 79° E, where no large earthquakes (M _w ≥ 7.0) have occurred since 1992.     

Stress drop in the sources of intermediate-magnitude earthquakes in northern Tien Shan

The paper is devoted to estimating the dynamical parameters of 14 earthquakes with intermediate magnitudes (energy class 11 to 14), which occurred in the Northern Tien Shan. For obtaining the estimates of these parameters, including the stress drop, which could be then applied in crustal stress reconstruction by the technique suggested by Yu.L. Rebetsky (Schmidt Institute of Physics of the Earth, Russian Academy of Sciences), we have improved the algorithms and programs for calculating the spectra of the seismograms. The updated products allow for the site responses and spectral transformations during the propagation of seismic waves through the medium (the effect of finite Q-factor). By applying the new approach to the analysis of seismograms recorded by the seismic KNET network, we calculated the radii of the sources (Brune radius), scalar seismic moment, and stress drop (release) for the studied 14 earthquakes. The analysis revealed a scatter in the source radii and stress drop even among the earthquakes that have almost identical energy classes. The stress drop by different earthquakes ranges from one to 75 bar. We have also determined the focal mechanisms and stress regime of the Earth’s crust. It is worth noting that during the considered period, strong seismic events with energy class above 14 were absent within the segment covered by the KNET stations.     

中天山地区的Pn波速度结构与各向异性

利用宽频带流动地震台阵GHENGIS和吉尔吉斯地震台网KNET记录的地震波走时数据,反演了中天山地区的Pn波速度结构和各向异性.结果表明,中天山上地幔顶部平均速度偏低,具有构造活动地区的特点和明显的横向非均匀性;中天山南部地幔上涌区的Pn波速度非常低,表明存在较高的热流活动.Pn波速度的变化与地震分布有着密切的对应关系：地震大都发生在中天山北部Pn波高速区上方,而南部的Pn波低速区上方几乎没有地震.这一现象说明地幔上涌引起高温极大地降低了岩石层地幔的强度,并以热传导的方式进入地壳使其失去地震破裂强度而发生韧性变形.中天山北部和南部的各向异性也存在一定的差异,南部各向异性的快波方向为近南北方向,与SKS波的各向异性特征基本一致,反映了地幔物质的迁移方向;北部各向异性的快波方向呈向南凸出的旋转趋势,估计与哈萨克地台南缘楚河盆地地壳块体向天山挤入造成应力场的改变和岩石层变形有关.     

Temporal and spatial distributions of precursory seismicity rate changes in the Thailand-Laos-Myanmar border region: implication for upcoming hazardous earthquakes

To study the prospective areas of upcoming strong-to-major earthquakes, i.e., M _w  ≥ 6.0, a catalog of seismicity in the vicinity of the Thailand-Laos-Myanmar border region was generated and then investigated statistically. Based on the successful investigations of previous works, the seismicity rate change (Z value) technique was applied in this study. According to the completeness earthquake dataset, eight available case studies of strong-to-major earthquakes were investigated retrospectively. After iterative tests of the characteristic parameters concerning the number of earthquakes (N) and time window (T _w ), the values of 50 and 1.2 years, respectively, were found to reveal an anomalous high Z-value peak (seismic quiescence) prior to the occurrence of six out of the eight major earthquake events studied. In addition, the location of the Z-value anomalies conformed fairly well to the epicenters of those earthquakes. Based on the investigation of correlation coefficient and the stochastic test of the Z values, the parameters used here (N = 50 events and T _w  = 1.2 years) were suitable to determine the precursory Z value and not random phenomena. The Z values of this study and the frequency-magnitude distribution b values of a previous work both highlighted the same prospective areas that might generate an upcoming major earthquake: (i) some areas in the northern part of Laos and (ii) the eastern part of Myanmar.     

On the correlation between seismicity characteristics and S-wave attenuation in the ring structures that appear before large earthquakes

We discuss seismicity characteristics in the source zones of two great earthquakes: the December 26, 2004 Sumatra (M_w = 9.0) and the November 14, 2001 Kunlun (M_w = 7.8) events. Ring structures of low magnitude seismicity have been forming prior to these earthquakes for several decades. We studied the short period shear-wave attenuation field in the area of these ring structures. The method we used is based on the analysis of the rate of attenuation for the early Sn and Lg codas to detect attenuation inhomogeneities in the uppermost mantle. We show that the ring structures have comparatively high attenuation of shear waves compared with the crustal volumes inside the rings. The fact that there is no recent volcanism in the area of the seismicity rings shows that this effect is due to a high content of free fluids in the uppermost mantle. Proceeding by analogy with our results, we identified a zone in northern Tien Shan that is anomalous for these parameters; the zone may be related to the precursory process of a large earthquake. We discuss the geodynamic mechanisms that may be responsible for fluid concentration in the seismicity rings.     

地震活动性预测并划分未来地震发生的潜在区域方法研究

采用网格地震方法,选取面积百分比(33%),确定圈闭区域的半径,统计后期地震发生在圈闭区域内的概率。在我国4个不同构造环境和地震活动水平的地区,分别统计了后期地震发生在圈闭区域内的概率。结果表明,4个地区后期地震发生在圈闭区域的概率远远大于面积百分比。说明未来地震是高度地聚集在过去地震发生的区域内,采用地震活动性预测并划分未来地震发生的潜在区域的方法是可行的。     

Archaeoseismological studies and structural position of the medieval earthquakes in the South of the Issyk-Kul depression (Tien Shan)

We carried out archaeoseismological studies in the Southern Issyk-Kul region (Kyrgyz Tien Shan) and obtained radiocarbon datings of the collected samples. These data suggest that the sources of strong earthquakes have occurred in this territory in the 11th and (probably) 16th centuries. These earthquakes had magnitude M ≥ 7 and seismic intensity of at least I ≥ 9. The sources of these earthquakes were associated with the local adyr (piedmont) faults—components of the Pre-Terskei border fault. Our results demonstrate considerable underestimation of the seismic hazard for the South Issyk-Kul region in the latest Seismic Zoning Map of Kyrgyz Republic (2012), which should be taken into account in the construction of the new seismic zoning map for Kyrgyzstan.     

Heterogeneities in the field of short period seismic wave attenuation in the lithosphere of central Tien Shan

Records of deep-focus Hindu Kush earthquakes in the depth ranges 70–110 and 190–230 km made by 45 digital and analogue seismic stations were analyzed to study the attenuation field of short period seismic waves in the lithosphere of central Tien Shan. The dynamic characteristics studied include the ratio of peak amplitudes in S and P waves (S/P) and the ratio of the S-wave maximum to the coda level in the range t = 400 ± 5 s, where t is the lapse time (S/c400) for 1.25 Hz. Comparatively high values of S/P are shown to prevail in most of the area, corresponding to lower S-wave attenuation. Upon this background is a band of high and intermediate attenuation in the west of the area extending along the Talas-Fergana fault in the south and afterwards turning north-northeast. The rupture areas of the two largest (M ≥ 7.0) earthquakes which have occurred in Tien Shan during the last 25 years are confined to this band. Abnormally high values of S/c400 were obtained for stations situated in the rupture zone of the August 19, 1992, magnitude 7.3 Suusamyr earthquake and around it. For two of the stations we found considerable time variations in the coda envelope before the earthquake. The effective Q was derived from compressional and shear wave data for the entire area, as well as for the band of high attenuation. Comparison with previous data shows that the attenuation field in the area has changed appreciably during 20–25 years, which can only be due to a rearrangement of the fluid field in the crust and uppermost mantle. It is hypothesized that a large earthquake is very likely to occur in the northern part of the attenuating band.     

天山构造带及邻区地壳各向异性

天山构造带位于中国大陆西北部,是典型的岩石圈陆内缩短造山带.本文利用新疆区域数字地震台网2009年1月至2014年12月的近场小地震波形资料,采用剪切波分裂分析对天山构造带及邻区的地壳各向异性特征进行研究,获得了研究区域内39个台站的快剪切波偏振方向和慢剪切波时间延迟.剪切波分裂参数的空间特征显示,研究区上地壳各向异性具有分区性,各向异性特征与局部构造、地壳介质变形和应力分布有关.天山构造带的快剪切波偏振呈现出两个优势方向的特点,第一优势方向大致平行于台站附近断裂和天山构造带的走向,与断裂构造和应力的综合影响有关,另一个优势方向反映了主压应力的直接作用.北天山山前断裂带东段的断裂弯折部位和南天山局部地区的剪切波分裂参数与东、西两侧不同,与准噶尔盆地、塔里木盆地的南北向挤压作用密切相关.快剪切波偏振优势方向的剧烈变化揭示,在准噶尔盆地和塔里木盆地双向挤压隆起的过程中,天山构造带产生了强烈的局部不均匀变形.塔里木盆地西侧快剪切波偏振具有两个优势方向,一个为NNE方向,与帕米尔高原受到印度-欧亚板块碰撞产生的北向挤压作用有关,另一个为NW方向,指出了塔里木盆地区域主压应力方向.准噶尔盆地北部也存在NE和NW两个快波偏振优势方向,主要与断裂的影响有关.天山构造带区域内的慢剪切波时间延迟总体上低于塔里木盆地西侧和准噶尔盆地北部,同时慢剪切波时间延迟的结果也进一步证实了天山构造带的局部强烈变形.

     

A probabilistic model of seismicity: Kamchatka earthquakes

The catalog of Kamchatka earthquakes is represented as a probability space of three objects {Ω, $ \tilde F $ \tilde F P}. Each earthquake is treated as an outcome ω _i in the space of elementary events Ω whose cardinality for the period under consideration is given by the number of events. In turn, ω _i is characterized by a system of random variables, viz., energy class k_i, latitude φ _i , longitude λ _i , and depth h _i . The time of an outcome has been eliminated from this system in this study. The random variables make up subsets in the set $ \tilde F $ \tilde F and are defined by multivariate distributions, either by the distribution function $ \tilde F $ \tilde F (φ, λ, h, k) or by the probability density f(φ, λ, h, k) based on the earthquake catalog in hand. The probabilities P are treated in the frequency interpretation. Taking the example of a recurrence relation (RR) written down in the form of a power law for probability density f(k), where the initial value of the distribution function f(k ₀) is the basic data [Bogdanov, 2006] rather than the seismic activity A ₀, we proceed to show that for different intervals of coordinates and time the distribution f _elim(k) of an earthquake catalog with the aftershocks eliminated is identical to the distribution f _full(k), which corresponds to the full catalog. It follows from our calculations that f ₀(k) takes on nearly identical numeral values for different initial values of energy class k ₀ (8 ≤ k ₀ ≤ 12) f(k ₀). The difference decreases with an increasing number of events. We put forward the hypothesis that the values of f(k ₀) tend to cluster around the value 2/3 as the number of events increases. The Kolmogorov test is used to test the hypothesis that statistical recurrence laws are consistent with the analytical form of the probabilistic RR based on a distribution function with the initial value f(k ₀) = 2/3. We discuss statistical distributions of earthquake hypocenters over depth and the epicenters over various areas for several periods     

Seismotectonic deformations and recent tectonics of the Tien Shan

Seismotectonic strains (STSs) are calculated on the basis of the catalog of focal mechanisms of earthquakes including more than 5000 events compiled at the Institute of Seismology, National Academy of Sciences of the Kyrgyz Republic, in order to investigate deformation processes in the Tien Shan (38°–44°N, 68°–80°E). A modern approach to STS classification based on the identification of 11 deformation settings (including four main, two extreme, and five transitional settings) is applied for constructing STS maps. Areal distributions of the Lode-Nadai coefficient, vertical component, and angle of the stress-state type are obtained. The results of the calculations are verified by comparing them with STS calculations using focal mechanisms of 116 strong earthquakes from the centroid moment tensor catalog of Harvard University that occurred in the studied region in 1976–2003.     

The rocking response of large flexible structures to earthquakes

The rocking response of structures subjected to strong ground motions is a problem of ‘several scales’. While small structures are sensitive to acceleration pulses acting successively, large structures are more significantly affected by coherent low frequency components of ground motion. As a result, the rocking response of large structures is more stable and orderly, allowing effective isolation from the ground without imminent danger of overturning. This paper aims to characterize and predict the maximum rocking response of large and flexible structures to earthquakes using an idealized structural model. To achieve this, the maximum rocking demand caused by different earthquake records was evaluated using several ground motion intensity measures. Pulse-type records which typically have high peak ground velocity and lower frequency content caused large rocking amplitudes, whereas non-pulse type records caused random rocking motion confined to small rocking amplitudes. Coherent velocity pulses were therefore identified as the primary cause of significant rocking motion. Using a suite of pulse-type ground motions, it was observed that idealized wavelets fitted to velocity pulses can adequately describe the rocking response of large structures. Further, a parametric analysis demonstrates that pulse shape parameters affect the maximum rocking response significantly. Based on these two findings, a probabilistic analysis method is proposed for estimating the maximum rocking demand to pulse-type earthquakes. The dimensionless demand maps, produced using these methods, have predictive power in the near-field provided that pulse period and amplitude can be estimated a priori. Use of this method within a probabilistic seismic demand analysis framework is briefly discussed.     

Geodynamic processes in seismically active areas of the Tien Shan from monitoring data with the use of nuclear explosions

Recent geodynamic processes in the Tien Shan region are studied by the analysis of time series of effective velocities and traveltime delays relative to the IASPEI-91 traveltime curve of the weakly refracted wave P _n from nuclear explosions at the Semipalatinsk test site over the period of 1968–1989. The time series were constructed for 10 seismic stations located at distances of 800–1200 km from the test site in the regions of the Northern, Central, and Southern Tien Shan. The twenty-year period of observations at stations in the North Tien Shan showed a significant decrease in traveltime delays by 0.20–0.76 s, which corresponds to a 0.2–0.7% increase in seismic velocities. An opposite pattern is observed at stations of the Central and Southern Tien Shan: traveltime delays increased by 0.2–0.5 s and, accordingly, seismic velocities dropped by 0.2–0.5%. These results suggest the predominance of compression processes in the crust and upper mantle during the period of observations in Northern Tien Shan and extension processes in the Central and Southern Tien Shan. The series of velocities and traveltime residuals are characterized by the presence of rhythmic oscillations of various amplitudes and periods against a linear trend. A correlation between variations in kinematic parameters and yearly numbers of earthquakes is observed at all stations. Diagrams of the spectral time analysis reveal rhythms with periods of 2–3 and 5–7 yr. The data obtained in this study are consistent with results of studying the stress-strain state of the Tien Shan crust from focal mechanisms of earthquakes and the velocities of recent crustal movements from GPS data. It is found that the amplitude of variations in kinematic parameters of the P wave at stations located in seismically active regions (the Tien Shan, Kopet Dagh, the Caucasus, Altai, and Sayany) is two to five times higher compared to aseismic regions (the Russian and Kazakh plains).     

中天山及邻区S波分裂研究及其动力学意义

本文利用天山及其邻区布设的37个宽频带地震台站记录到的远震波形数据,分别采用最小能量法和旋转相关法对SKS和SKKS波震相进行了偏振分析,计算出了台站下方介质的S波分裂参数：快波的偏振方向（φ）和慢波延迟时间（δt）.本文研究结果表明：中天山内部大多数台站的各向异性快波方向呈NEE-SWW向,与天山走向平行,慢波时间延迟为0.4～1.7 s,这是塔里木、哈萨克斯坦的南北双向俯冲及其导致的天山地区岩石圈地幔南北向缩短变形的直接反映.本文研究发现中天山北部部分台站下方地震各向异性快波方向与慢波延时随方位角呈现规律性的变化,可能暗示该区上地幔各向异性不能仅用单层水平各向异性这一简单模式来解释.75°E以西的天山地区台站下方S波快波方向和延时具有强烈的横向变化,可能与研究区下方存在的小规模对流有关.中天山不同地段地震台站下方各向异性明显不同,进一步证实了天山地区构造变形的复杂性.     

Modelling of depth phases and source processes of some central Aleutian earthquakes

Short-period synthetic seismograms are computed to determine the relative amplitudes and arrival times of P, pP, pwP (water surface reflection), and sP phases. Except along nodal planes of upgoing p, pwP is of greater amplitude than sP. For central Aleutian earthquakes, pwP dominates over sP and pP at North American stations for thrust mechanisms and modelled crustal structures. The pwP phase is clearly identified in three Aleutian events and can be used to constrain focal depths. Complex fracturing processes are identified in two of the events. In simple events, smaller phases which are consistent from station to station can be identified as sub-surface reflections and used for modelling the structure of the forearc.     

龙门山断裂带大地震孕震环境的岩石磁学证据

地震断裂带的孕震环境对于研究地震的发生至关重要.本文以汶川地震断裂带科学钻探2号（WFSD-2）钻孔岩心中的假玄武玻璃、碎裂岩及其围岩为研究对象,通过岩石磁学测试,并结合显微结构观察探讨龙门山断裂带大地震的孕震环境.WFSD-2岩心碎裂岩中的假玄武玻璃是龙门山断裂带曾经发生过大地震活动的岩石学证据,假玄武玻璃具有高磁化率特征,而碎裂岩的磁化率值与围岩相似,假玄武玻璃中的新生磁铁矿是其高磁化率值异常的重要原因之一.假玄武玻璃中较少量的新生磁铁矿暗示了假玄武玻璃的生成环境为含氧量较低的高温还原环境.大地震断裂的摩擦热是改变龙门山断裂带中假玄武玻璃岩石磁学特征的主导因素,流体作用较弱.无高磁化率异常的碎裂岩经历的温度小于300℃,推测假玄武玻璃的生成深度较深.WFSD-2岩心中20余层假玄武玻璃脉体证明映秀—北川断裂带是一条长期活跃的断裂带,龙门山断裂带上曾经发生了多次大地震断裂活动,这些大地震可能发生在深度较大、流体作用较弱的还原性孕震环境中.

     

广义地震活动窗及其对震前异常过程的跟踪应用

提出了广州地震窗的概念,并以此作为研究范围以突出局部构造特点。用多年地震频次等值线划分广义地震窗的方法,在中国东部得到４５个广义地震窗,它们具有明显的构造意义,应用原有地震活动参数提出一个相对指标ｆ,用以消除局部的地域特点和地震活动涨落的时段特点,提高原指标的异常信息提取能力。用地震活动指数Ａ（ｂ）值的相对指标ｆ（Ａ（ｂ）,提出了用多窗异常组合图综合平面的时间的三维信息以跟踪异常过程,结果显示主震     

The pattern of the paleo- and present-day stresses of Northern Tien Shan

The study of neotectonic stresses of the North Tien Shan is carried out within the northern slopes of the Kyrgyz Ridge bordering the Chyua submontane depression. The purpose of the study was to reconstruct the neotectonic stresses from the geological indicators and to compare the obtained results with the data on the present-day stress state of this region. The work is based on the analysis of the field in situ data acquired in the expeditions in 2009 and 2011. For the first time for the Northern Tien Shan region, the general (averaged) neotectonic stresses, which are distinct between the uplifts and depressions, are reconstructed. The deformation of the positive topographic features at the recent tectonic stage takes place in the conditions of thrust faulting with the meriodional horizontal compression axis and subvertical tension axis, while deformation of the negative landforms is dominated by normal faulting with the vertical compression axis and northnortheast trending subhorizontal tension axis. Based on the field data, separate sites with different types of local stress tensors, which are determined by the Lode-Nadai parameter, are revealed. The neotectonic geodynamical regime within the depressions and uplifts and the variations in the type of the stress tensor agree with the characteristics of the present-day stress state reconstructed from seismological data.     

Physical models of earthquake instability and precursory processes

This paper selectively reviews physical models of earthquake instability. In these models, instability arises as a result of interaction of a fault constitutive relation with deformation of the surrounding material that occurs in response to remote tectonic loading. In contrast to kinematic models in which the fault slip is imposed, it is calculated in physical models and, consequently, these models are essential for understanding precursory processes. Some kind of weakening behavior for the fault constitutive relation is required to produce an instability analogous to an earthquake. Two commonly employed idealizations discussed here are rate-independent slip weakening and rate/state-dependent friction. When these constitutive models are employed on surfaces embedded in elastic half-spaces or layers, possibly coupled to a viscoelastic substrate, the results are capable of simulating realistically some aspects of earthquake occurrence. Common to all models is the prediction that earthquake instability is preceded by precursory slip which produces a departure of surface strain-rate from the background level. Near the epicenter of a moderate to large earthquake, the magnitude of this departure appears to be well within the range of current geodetic measurement accuracy, and its duration is of the order of months to years. However, details depend on a variety of factors, including the modelling of the constitutive relation near peak stress, coupling of elastic crust to the asthenosphere, and coupling of deformation with pore fluid diffusion.     

Global seismicity of 1982: centroid-moment tensor solutions for 308 earthquakes

The centroid-moment tensor analysis is applied to digital recordings of 308 moderate and large earthquakes that occurred during 1982 and were recorded by the Global Digital Seismograph Network (GDSN) and International Deployment of Accelerometers (IDA) networks. The tables contain numerical results for all events. A graphical representation of the source geometry is shown for the full moment tensor solution, as well as for the “best double couple”.