Correlation of earthquake occurrence rates in the Baikal region and mongolia: Episodes of synchronism

Variations in annual numbers of earthquakes (the earthquake occurrence rate) that hit the Baikal region and Mongolia during the period from 1964 through 2001 are studied in this work. Correlation analysis of the different-length series of annual numbers N of earthquakes of representative energy classes makes it possible to reveal the effects of synchronous changes in the earthquake occurrence rate in seven regions and eleven areas in the Mongolia-Baikal region, located far apart. The analysis of the shock occurrence rate revealed episodes of short-period synchronization of seismic processes in the Mongolia-Baikal region at the end of the 1960s, early in the 1980s, and in the middle of the 1990s. The episode of synchronization in the earthquake occurrence rate in the early 1980s is observed in all the territories under study, but the episode at the end of the 1960s is less distinctly seen in Mongolia and is revealed mainly in the data series with a length of three years. The synchronization in the seismicity in Mongolia and in the southern PreBaikal region in 1995 requires further investigations, involving the dynamic parameters of the earthquake sources. The observed synchronism in the annual number of earthquakes indicates that the seismic processes become active nearly simultaneously over the huge territory of the Mongolia-Baikal region and produce a short-term coherent change in the shock occurrence rate in the spatial-temporal distribution of the seismicity. The observed spatial and temporal correlation in the seismicity is a sign of the seismogenic link between the Baikal region and Mongolia.     

The Structure of Seismicity Field in the Baikal Rift Zone

Izvestiya, Physics of the Solid Earth - Abstract—A joint analysis of seismicity in the Baikal Rift Zone (BRZ) is carried out using modern instrumental data (Baikal Branch of the Federal...     

Variations in short-period shear-wave attenuation in the Baikal Rift Zone and their relationship to seismicity

This paper presents results from a study of variations in short-period shear-wave attenuation in the lithosphere of the Baikal Rift Zone (BRZ). We used earthquake records made at the Ulan-Bator station (ULN) at epicentral distances of ～400–1300 km. The ratios of maximum amplitudes in the Sn and Pn waves were considered. We show that these ratios are on the whole considerably higher than those in other areas of Central Asia. It was found that zones of low shear wave attenuation coincide with the rupture zones of large earthquakes that occurred during the 19th and 20th centuries. We identified zones of high attenuation where no large (M～ > 7.0) seismic events have occurred during at least 180 years. The hypothesis we propose is that precursory processes before future large earthquakes may be occurring in these zones. We discuss the question of whether wave attenuation characteristics may be related to seismicity.     

Space-time analysis of earthquake-generating structures in the Baikal Rift system

The epicenter field of earthquakes that occur in the Baikal Rift System (BRS) was analyzed to identify certain areas that have stable concentrations of seismic events; these are reflections (in time) of deformable earthquake-generating volumes in the tectonosphere, or earthquake-generating structures. The patterns that are identifiable in the interrelationships and interactions of earthquake-generating structures as found by analysis of the space-time characteristics of the seismic process characterize the present-day dynamics of the BRS. Time series of earthquake-generating structures were analyzed for the 1964?C2002 period using the following parameters: the number of earthquakes (N) and the logarithm of total energy released by the earthquakes (log E_sum). The tool was frequency-time analysis (FTAN), which revealed dominant periodic components in the seismic process for each earthquake-generating structure. We also used correlation analysis based on the parameter N to determine the order of activation for the earthquake-generating structures and to identify periods of synchronization in the seismic process.     

Fractal analysis of space-time variations in the epicentral field of earthquakes for several areas in the Baikal Rift Zone

Fractal analysis techniques were used to study space-time variations in the epicentral field of earthquakes for several areas in the Baikal Rift Zone (BRZ). In each area the epicentral field is found to be a system consisting of several subsystems in the shape of maxima of increased fractal dimension D _s , which occurred approximately at the same locations with constant or variable periodicity. We detected simultaneity and migration in the appearance of some maxima of D _s . Most local areas in the regions under study typically have an unformed type of epicentral field, to use A.V. Solonenko’s terminology, and this makes it difficult to predict the time of earthquake occurrence at a specified location using simple statistical methods. We used the variations in D _s in different regions to study the behavior of the present-day seismotectonic process in the BRZ as a whole.     

Estimating the energy of seismotectonic deformations in the lithosphere of the Baikal Rift Zone

Estimation and comparison of the energy of seismotectonic deformations in the lithosphere of the Baikal Rift Zone (BRZ) based on observations of large (M ≥ 6) earthquakes for the period of instrumental recording (1950–2002), for a historical period lasting 210 years (1740–1949), and inferred from palaeo-seismological materials for the past 2000 years, all indicate that the hypothesis of a stationary seismic process is appropriate for the region. The locations of maxima of the density of seismotectonic strain energy released during the time intervals under investigation show that most of the failures in the lithosphere occurred approximately in the same areas, which may be interpreted as stress concentrators. The isolines of increased density for the energy of seismotectonic deformations align themselves along the rift features from southwest to northeast in the Baikal region and this allows one to treat the BRZ lithosphere as an extended zone of enhanced, inhomogeneous, energy release of endogenous geotectonic processes. We assessed the power of the seismotectonic processes that reflect the release of endogenous energy through earthquakes. Identification of areas with deficits in the energy of seismotectonic deformations (“energy gaps”) is an important step toward long-term solution of seismic-safety problems for the Baikal region.     

蒙古-贝加尔裂谷区地壳应变场及其地球动力学涵义

蒙古-贝加尔地区是现今构造最活跃的大陆地区之一,其地壳构造运动及变形对我们理解大陆动力学问题具有重要的科学意义.基于融合的这一区域的GPS速度场,本文计算了该区应变率场和应变能变化率场.结果显示,蒙古褶皱带以南区域表现为NNE-SSW方向的压缩状态,主压应变率约为-2.0×10^-9/a,剪应变及面膨胀均较弱,表明蒙古褶皱带比较稳定.贝加尔裂谷整体处于拉张状态且伴有较强的剪应变和面膨胀,暗示可能有多种机制控制裂谷的张裂过程.蒙古高原西部有两条高应变率的构造带,结合深部存在地幔热柱等证据,我们认为这两条构造带及所围限的区域共同构成Amurian板块的西部边界—一条弥散变形的边界带.蒙古-贝加尔地区剪应变分布与0~40 km的地震活动性基本一致,表明该地区形变在地壳尺度耦合程度较高.地幔对流拖曳力场与主应变轴方向及应变率场的一致性表明,地幔对流可能是蒙古—贝加尔地区区域构造动力学过程主要控制因素之一.     

Research on determining the annual earthquake occurrence rates of potential seismic source areas

To actually reflect the seismic temporal-spatial inhomogeneity of intra continental strong earthquakes of North China in seismic hazard analysis, several seismological and geological characteristics have been selected and quantized to describe the seismicity features in time and space of every magnitude interval with the thought of dividing the interesting magnitude range into several intervals and using of spatial probability distribution function. A component analysis method with orthogonal transformation is introduced to avoid the repeated use of the same element and the subjective effects in determining the annual earthquake occurrence rates of earthquake. By passing synthetic fuzzy judgement on the nonintercorrelated new characteristics, the annual occurrence rates of every magnitude interval of each potential source area are obtained associated with the adjustments of earthquake reducing process after the occurrence ofM>7 quake. An intensity map of the Beijing-Tianjin-Zhangjiakou area is calculated as an example which shows a close coincidence with the seismic temporal-spatial inhomogeneity of North China. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 496–504, 1991.     

The “seismic quiescence” effect in the Baikal Rift Zone

This paper presents the results from long-term observations of parameters of the Earth’s natural impulsive electromagnetic field (ENIEMF) in the VLF range recorded in the Baikal Rift Zone. We consider the “seismic quiescence” effect and examine possible factors responsible for changes in the daily variations of the magnetic component before earthquakes. We demonstrate daily variations of the ENIEMF magnetic component and their correlation to local storm activity during a seismically quiet period.     

Source parameters of the earthquakes of the Baikal rift system

The dynamic parameters of the earthquake source—the seismic moment, the moment magnitude, the source radius, the stress drop, and the amplitude of displacement—are determined by the amplitude Fourier spectra of the body shear waves (S-waves) for 62 earthquakes of the Baikal rift system with the energy class of K _P = 9.1–15.7. In the calculations I used the classical Brune model. The seismic moment of the earthquakes being investigated changes from 3.65 × 10¹¹ N m to 1.35 × 10¹⁸ N m, and the radii of earthquake sources vary from 390 m to 1.84 km. The values of the drop in stress Δσ grow with an increase in the seismic moment up to 1.7 × 10⁸ Pa. For the group of weak earthquakes (M _w = 1.7–3.3), extremely low values of the drop in stress 10³–10⁴ Pa are observed. The maximum amplitude of displacement in the source amounts to 5.95 m. The empirical equations between the seismic moment and the other dynamic parameters of the source are determined. The regional dependence of the seismic moment and energy class is obtained: log M ₀ ± 0.60 = 1.03K _P + 3.17. The character of the relationship between the seismic moment and the corner frequency indicates that the classical scaling law of the seismic spectrum for the earthquakes in question is not fulfilled. The obtained estimates of the dynamic parameters are in satisfactory agreement with the published data concerning the analogous parameters of the other rift zones, which reflects the general regular patterns of the destruction of the lithosphere and the seismicity in the extension zones of the lithosphere.     

Space-time model for migration of weak earthquakes along the northern boundary of the Amurian microplate

In this paper, we aimed to investigate the statistical distributions of shallow earthquakes with 2 ≤ М ≤ 4, located in 13 rectangular areas (clusters) bounded by 120°E and 144°E along the northern boundary of the Amurian microplate. As a result of our study, the displacement of seismicity maxima has been determined and three recurrent spatial cycles have been observed. The clusters with similar distribution of earthquakes are suggested to alternate being equally spaced at 7.26° (360–420 km). A comparison of investigation results on the structure of seismicity in various segments of the Amurian microplate reveals the identity between the alternation pattern observed for meridional zones of large earthquakes and a distinguished spatial period. The displacement vector for seismicity in the annual cycles is determined, and the correspondence between its E-W direction and the displacement of the fronts of large earthquakes is established. The elaborated model of seismic and deformation processes is considered, in which subsequent activation of clusters of weak earthquakes (2 ≤ М ≤ 4), tending to extend from the Japanese-Sakhalin island arc to the eastern closure of the Baikal rift zone, is initiated by the displacement of the strain wave front.     

中国大陆与邻区强震分区活动性及其随时间演化

根据主要构造分布、震源机制解分布及地震活动状况，将中国大陆及邻区分成20个单元，6个区。利用中国地震局地球物理研究所提供的地震目录，计算近百年来各单元和各区的6级以上强震释放应变能，做出时间滑动后的应变能—时间曲线图，分析认为：(a)中国大陆及邻区百年来的强震活动是一个复杂的时空动态过程，每个区、带有其高、低潮，然而各地区没有统一的地震释放应变能高潮与低潮；(b)强震的活动与平静只是相对而言的，没有绝对的活动或平静；(c)微动态期划分较好的体现了强震活动特征，每个微动态期内至少有一个高潮期，且强震集中在一个主体地区内发生，各搬动态期的能量可以有起伏。通过进一步的讨论和分析，提出中国大陆地壳变形主要受三方面因素的控制：(1)印度板块、太平洋板块和菲律宾板块的活动及其变化控制中国强震活动图像的总体格局；(2)大陆地壳结构的非均匀性及其变化影响了主体地区的形态结构；(3)地壳形变的继承性影响强震活动主体地区形成与变迁的过程。     

Fault density,earthquakes, and the topography of crustal stratification interfaces: Central Magadan Oblast

This study is concerned with quantitative estimation of the relationship between earthquakes and tectonic crustal fragmentation based on a correlation analysis of fault density with seismicity parameters (the number and energy of earthquakes per unit area) for the Sredne-Yamskoi seismic junction and adjacent area. The highest level of seismic activity and the highest probability of earthquake occurrence with energy classes K ≥ 12 within areas that have a continental crust with a well-pronounced granite layer occur in those areas with the mean fault density. Within areas with a thinner granite layer in the crust, the most likely seismic events are K ≥ 12 earthquakes that occur in areas with lower fault density. We estimated the relationship between the degree of crustal fragmentation and the topography of stratification interfaces in the crust as identified by new interpretative gravimetry. Zones with the lowest degree of fragmentation tend to be areas where the top of the crystalline basement lies deeper.     

On the seismicity of central Kamchatka before the 1997, M = 7.9 Kronotskii earthquake

We report results from a detailed study of seismicity in central Kamchatka for the period from 1960 to 1997 using a modified traditional approach. The basic elements of this approach include (a) segmentation of the seismic region concerned (the Kronotskii and Shipunskii geoblocks, the continental slope and offshore blocks), (b) studying the variation in the rate of M = 4.5–7.0 earthquakes and in the amount of seismic energy release over time, (c) studying the seismicity variations, (d) separate estimates of earthquake recurrence for depths of 0–50 and 50–100 km. As a result, besides corroborating the fact that a quiescence occurred before the December 5, 1997, M = 7.9 Kronotskii earthquake, we also found a relationship between the start of the quiescence and the position of the seismic zone with respect to the rupture initiation. The earliest date of the quiescence (decreasing seismicity rate and seismic energy release) was due to the M = 4.5–7.0 earthquakes at depths of 0–100 km in the Kronotskii geoblock (8–9 years prior to the earthquake). The intermediate start of the quiescence was due to distant seismic zones of the Shipunskii geoblock and the circular zone using the RTL method, combining the Shipunskii and Kronotskii geoblocks (6 years). Based on the low magnitude seismicity (M≥2.6) at depths of 0–70 km in the southwestern part of the epicentral zone (50–100 km from the mainshock epicenter), the quiescence was inferred to have occurred a little over 3 years (40 months) before the mainshock time and a little over 2 years (25 months) in the immediate vicinity of the epicenter (0–50 km). These results enable a more reliable identification of other types of geophysical precursors during seismic quiescences before disastrous earthquakes.     

Contemporary horizontal movements and seismicity of the south Baikal Basin (Baikal rift system)

The contemporary horizontal movements and deformations in the central and southern parts of the Baikal depression are analyzed, and their relationship with contemporary seismicity is studied. Based on the long-term measurements by the Baikal geodynamical GPS monitoring network, the refined estimate is obtained for the velocity of the divergence of the Siberian and Transbaikalian blocks, which is found to occur in the southeastward direction (130°) at 3.4 ± 0.7 mm per annum. This agrees with the parameters of the long-term extension component estimated from the geological data and with the direction of extension determined from the seismic data. The distribution of the displacement velocity across the strike of the rift, which gradually increases from one block to another, suggests a nonrigid behavior of the continental lithospheric plates at the divergent boundary. About 30% (1.0–1.5 mm per annum) of the total increase in the velocity is accommodated by the Baikal Basin. The strain rate within the trough reaches 3.1 × 10^?8 yr^?1 and decreases on either side across the structure. The character of distribution of the horizontal displacement velocities on the Baikal divergent boundary between the Eurasian and Amurian plates favors the model of passive rifting. The zones of highly contrasting topography and increased seismicity are localized within the area of contemporary deformations, and the seismic moment release rate directly depends on the strain rate. Here, the rate of the seismic moment release rate makes up a few percent of the geodetic moment accumulation rate calculated by the approach suggested by Anderson (1979). Based on the coherence between the graphs of the rates of geodetic moment accumulation and seismic moment release rate by the earthquakes with M ≥ 5.0 during the historical and instrumental observation periods, the contemporary seismic hazard for the South Baikal Basin could be assessed at a level of seismic event with M = 7.5–7.6.     

On the relation of variations in the travel times of seismic waves with the changes in the speed of the Earth’s rotation

The results of long-term sounding of the lithosphere by seismic waves from the deep-focus Hindu Kush earthquakes are presented. The travel time series of the first longitudinal wave on a fixed base are constructed for six seismic observation stations (SS) located on the Russian Platform (the Obninsk SS), on the Siberian Platform (the Eltsovka SS), on the Cis-Ural Trough (the Arti SS), in the Central-Ural Megazone (the Sverdlovsk SS), in the Transbaikalia (the Bodaibo SS), and in the Northern Tien Shan (the Przhevalsk SS). The time series duration in years for these stations ranged from 1964–1970 to 2007. The travel time series of seismic waves for the stations indicated are characterized by multi-slope negative linear trends caused by changes in the stress-strain state of rocks. From the comparison of the trend slopes at different stations it follows that the changes in the stress conditions within the lithosphere are relatively weak in the aseismic regions of the Russian and Siberian Platforms and in the Ural Megazone, whereas in the seismically active regions of Tien Shan, Transbaikalia and the Cis-Ural Trough they are more pronounced. The correlation has been observed between the time series trends of the average annual travel times of seismic waves and the time series of the Earth’s rotation speed. The strongest correlation between the series can be seen for the stations, located on the platforms with weak manifestations of both seismicity and active geodynamic processes. Within the long-term periods of deceleration and acceleration of the Earth’s rotation, travel times of seismic waves are decreased and increased, respectively.     

Searching for an Earthquake Precursor—A Case Study of Precursory Swarm as a Real Seismic Pattern Before Major Shocks

A long-range correlation between earthquakes is indicated by some phenomena precursory to strong earthquakes. Most of the major earthquakes show prior seismic activity that in hindsight seems anomalous. The features include changes in regional activity rate and changes in the pattern of small earthquakes, including alignments on unmapped linear features near the (future) main shock. It has long been suggested that large earthquakes are preceded by observable variations in regional seismicity. Studies on seismic precursors preceding large to great earthquakes with M ≥ 7.5 were carried out in the northeast India region bounded by the area 20°–32°N and 88°–100°E using the earthquake database from 1853 to 1988. It is observed that all earthquakes of M ≥ 7.5, including the two great earthquakes of 1897 and 1950, were preceded by abnormally low anomalous seismicity phases some 11–27 years prior to their occurrence. On the other hand, precursory time periods ranged from 440 to 1,768 days for main shocks with M 5.6–6.5 for the period from 1963 to 1988. Furthermore, the 6 August, 1988 main shock of M 7.5 in the Arakan Yoma fold belt was preceded by well-defined patterns of anomalous seismicity that occurred during 1963–1964, about 25.2 years prior to its occurrence. The pattern of anomalous seismicity in the form of earthquake swarms preceding major earthquakes in the northeast India region can be regarded as one of the potential seismic precursors. Database constraints have been the main barrier to searching for this precursor preceding smaller earthquakes, which otherwise might have provided additional information on its existence. The entire exercise indicates that anomalous seismicity preceding major shocks is a common seismic pattern for the northeast India region, and can be employed for long-range earthquake prediction when better quality seismological data sets covering a wide range of magnitudes are available. Anomalous seismic activity is distinguished by a much higher annual frequency of earthquake occurrence than in the preceding normal and the following gap episodes.     

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。