Seismicity and seismotectonic deformations of the crust in the Southern Baikal basin

This paper addresses the seismicity of the Southern Baikal basin, where the M _w = 6.0 earthquake of 1999 was the strongest over the period of instrumental observations in this region. Focal mechanisms of background earthquakes and aftershocks are analyzed in relation to faults mapped on flanks of and within the basin. Based on a supplemented catalog of focal mechanisms, the value and direction of seismotectonic strain are evaluated. The results show that the territory to the west of the transverse Angara fault (the Mishikhinskaya depression) experiences deformation of pure extension, while the E-W basin segment west of the fault is subjected to deformation of extension with shear (the transtension regime). The crustal deformation directions as determined from GPS measurements and seismological observations are found to agree well. The average seismotectonic strain rate of the crust amounts to 2.95 × 10^?9 yr^?1, which is about an order of magnitude smaller than the value obtained from geodetic observations.     

Observation of a medium-term earthquake precursor in Irkutsk

Complex geophysical investigations of the earth dam of the Irkutsk hydroelectric power plant (HEP) revealed strong negative anomalies of the natural electric field (NEF). These anomalies are associated with focal mechanisms of earthquakes, in which subhorizontal compressive stresses prevail. The sources of such earthquakes are located in the south of the Baikal rift zone. Their epicentral distances are comparable with the range of the action of precursors of earthquakes with M > 5. NEF anomalies correlate with an increase in the filtration at the earth’s dam base in the Angara fault zone along the Angara River bed. In the shape of their plots, manifestations of these NEF anomalies are similar to variations in the ratios of velocities of longitudinal and transverse waves before earthquakes in the Garm region.     

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.     

The low seismic activity of the Korean Peninsula surrounded by high earthquake countries

Although the Korean Peninsula is locatednear several great earthquake regions suchas NE China and SW Japan, it has neversuffered from catastrophic earthquakes forthe last 2000 years according to historicaland instrumental records. We investigatedthe low seismicity of Korea based on thehypothesis of the Baikal-Korea Plate (BKP)or Amurian Plate movement which isinitiated by the Baikal Rift Zone spreadingin a southeastward motion with acounter-clockwise rotation due to thecollision of the Indian Plate against theEurasian Plate. Many disastrous earthquakesof NE China, SW Japan and Sakhalin releaselarge amounts of seismic energy along theboundary of the Baikal-Korea Plate. It isnecessary to compute the released seismicenergy along the presumed boundary of theBaikal-Korea Plate compared to the KoreanPeninsula in order to estimate themicro-plate boundary. The total energyreleases (1900–1999) from the majordisastrous earthquakes (M6.0) alongthe Baikal-Korea plate are about10³–10⁴ times as much as theKorean Peninsula (M3.0). The focalmechanisms for the intra-continentalearthquakes near and/or along theBaikal-Korea Plate boundary of NE China, SW Japan, Sakhalin and Mongolia mostlyrepresent the horizontal motions of theright-lateral strike slip type, indicatingthat the Baikal-Korea Plate is acounter-clockwise and transcurrent motion. The relative displacement vectors of GPS(global positioning system) also indicatedthat the Baikal-Korea Plate movescounter-clockwise around the KoreanPeninsula. These factors may indicate thatthe Korean Peninsula is not located at thePlate boundary, but just within a margin ofthe Baikal-Korea Plate which movessoutheastward with a counter-clockwiserotation from the Baikal Rift Zone in NEAsia. Therefore there is no enoughaccumulated strain to generate largeearthquakes in the Korean Peninsula and itmakes the Korean Peninsula free fromseismic hazard of large catastrophicearthquakes.     

2013年芦山MS7.0地震序列断层结构及震源区应力场特征

使用芦山地震序列2013年4月20日至5月20日一个月的地震震相数据和M_S4.0以上地震的波形数据, 通过双差定位方法得到了3398个地震的精定位结果, 利用时间域全波形反演方法得到17个地震的矩张量解。 综合分析地震双差定位结果和芦山地震序列中强地震震源机制解, 发现芦山地震发震构造由主震断层和次级反冲断层组成, 主震断层为一走向北东、 倾向北西、 倾角约为45°的高角度逆冲断层, 次级反冲断层与主震断层走向相同, 倾向相反, 两条断层均未出露地表。 主震和余震震源机制解均为逆冲型, 几乎没有走滑分量。 震源区主压应力方位为北西向, 与发震断层走向近乎垂直。     

The August 27, 2008, M w = 6.3 Kultuk earthquake (South Baikal): The stress-strain state of the source area from the aftershock data

We consider the results of reconstructing the stress-strain state of the Earth’s crust in South Baikal from the focal mechanism data for the Kultuk earthquake of August 27, 2008 (M _w = 6.3) and its aftershocks. The source parameters of the main shock were determined by calculating the seismic moment tensor. The focal mechanism solutions of 32 aftershocks (M _w ≥ 2.3) were obtained through the deployment of a local seismic network at South Baikal. It is found that the main shock and first aftershocks (August–September) gave rise to the activation of latitudinal fragments of the segmented near-edge fault, and the sources of the consequent aftershocks were dominated by the NW-striking planes related to the small intrabasin structures. The calculations of seismotectonic deformations based on the data on the focal mechanisms of the earthquakes show that the area of activation is dominated by the transtension regime (with deformation in the form of extension with shear). The epicentral and hypocentral fields of the aftershocks and the mechanisms of their sources reflect the complex tectonic structure of the source zone of the Kultuk earthquake, which exhibits a clear subvertical zonality of the local seismically active volume and a wedge-shaped area of crustal destruction.     

Tectonic setting of the recent damaging seismic series in the Southeastern Betic Cordillera,Spain

In this work we analyze the tectonic setting of the recent damaging seismic series occurred in the Internal Zones of the eastern Betic Cordillera (SE Spain) and surrounding areas, the tectonic region where took place the 11th May 2011 Mw 5.2 Lorca earthquake. We revisit and make a synthesis of the seven largest and damaging seismic series occurred from 1984 to 2011. We analyze their seismotectonic setting, and their geological sources under the light of recent advances in the knowledge on active faults, neotectonics, seismotectonics and stress regime, with special attention focused on the Lorca Earthquake. These seismic series are characterized by two types of focal mechanisms, produced mainly by two sets of active faults, NNW–SSE to NNE–SSW small (no larger than 20–30 km) extensional faults with some strike slip component, and E–W to NE–SW large strike slip faults (more than 50 km long) with some compressional component (oblique slip faults). The normal fault earthquakes related to the smaller faults are dominant in the interior of large crustal tectonic blocks that are bounded by the large E–W to NE–SW strike-slip faults. The strike slip earthquakes are associated to the reactivation of segments or intersegment regions of the large E–W to NE–SW faults bounding those crustal tectonic blocks. Most of the seismic series studied in this work can be interpreted as part of the background seismicity that occurs within the crustal blocks that are strained under a transpressional regime driven by the major strike slip shear corridors bounding the blocks. The seismotectonic analysis and the phenomenology of the studied series indicate that it is usual the occurrence of damaging compound earthquakes of M  \(\sim \)  5.0 associated with triggering processes driven by coseismic stress transfer. These processes mainly occur in the seismic series generated by NNW–SSE to NNE–SSW faults. These mechanical interaction processes may induce a higher frequency of occurrence of this kind of earthquakes than considered in traditional probabilistic seismic hazard assessments and it should be taken into account in future seismic hazard assessments.     

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.     

张家口-渤海地震构造带的地壳形变研究

华北北部地区的张家口-渤海断裂带是控制现代强震的一条地震构造带。新近纪以来,在区域NEE向主压应力的作用下,新发育了一系列的NE向构造活动带,与张家口-渤海断裂带组成1组共轭的剪切破裂系统,控制了近代强震的发生。文中主要探讨了这条断裂带的地壳形变特征。长趋势GPS地壳形变图像反映了这条活动断裂带相对完整的左旋走滑活动。分期的地壳形变图像揭示了在中强地震前,沿该断裂带出现一系列的NE向梯度异常带,分别指示了唐山-河间、三河-涞水以及延怀-山西地震构造活动带的活动,表明了沿张家口-渤海断裂带出现了中强地震的中期前兆。研究认为,强震前地壳形变揭示的是深部蕴震层的应变活动信息,而强震之后比较杂乱的图像特征体现了盖层的调整运动     

Seismic investigations of crustal structure in east-central Minnesota

Relative arrival times from 120 mine blasts recorded on the Central Minnesota Seismic Array (CMSA) out to distances of 350 km have been statistically analysed and interpreted to yield a crustal velocity-depth function for the Minnesota area. Velocities increase continuously from 5.9 km s^?1 at the surface to 7.4 km s^?1 at 40 km depth, with a steepening of the velocity gradient at about 20 km. A long-range refraction profile extending from the Mesabi Iron Range in Northern Minnesota to the seismic array, has also been recorded. A striking feature of the profile is the absence of direct P waves in the distance range 60–120 km. Our preferred explanation for this shadow zone is a fault or fracture zone near x = 60 km. Reflections within the shadow zone have been analysed to give a Moho depth of 42 km under northeastern Minnesota. Teleseismic residuals have been computed for 85 earthquakes recorded on the CMSA. No systematic dependence of the residuals upon azimuth or distance was found. The average residuals agree quantitatively with time delays predicted from earlier upper crustal refraction studies (Mooney et al., 1970).     

Main Feature and Potential Seismic Capability of Active Fault Belts in the Chengdu Depression

Chengdu fault depression is an important Quaternary basin in the piedmont of Longmenshan mountain.Formation and evolution of the fault depression are entirely controlled by the Longmenshan piedmont fault and the Longquanshan fault.Since the late Quaternary,Chengdu fault depression has been subjected to an NW-SE oriented compression.Many NE or NNE trending faults inside the depression or near its margins show thrust slip,resulting in moderate and strong earthquakes along the piedmont Longmenshan fault,the western slop Longquanshan fault,and Pujiang-Xinjin-Chengdu-Deyang fault.It is indicated that three faults as mentioned above have the potential capability for the occurrence of moderate earthquakes.     

RESTUDY ON HYPOCENTRAL LOCATION AND SEISMOGENIC TECTONIC OF THE JIUJIANG-RUICHANG MS5.7 EARTHQUAKE SEQUENCE,JIANGXI PROVINCE

We collected seismic records of 228 M_L≥1.0 Jiujiang-Ruichang M_S5.7 earthquake sequence from Dec.26, 2005 to Jun. 30, 2006. By using double-difference method combined with waveform cross-correlation, those earthquakes were relocated and finally the accurate source parameters of 224 earthquakes were obtained. The errors are about 0. 5km in horizontal and less than 2km in vertical direction, respectively. It was found that the depth of earthquake sequence concentrates in 8~14km range, and the epicenters are distributed along both NW and NE direction, and dominantly along NW direction. Combined with the focal mechanism, the distribution direction and the tectonic setting, we infer that the rupture of the NW-trending fault caused the M_S5. 7 main shock, and then the rupture probably encountered an asperity and triggered the M_S4. 8 strong aftershock. The NE-trending fault came into a seismically quiet period by stress adjustment in a short time, while the NW-trending fault released stress for a long time which caused a series of aftershocks. The M_S5. 7 main shock is caused by the NW striking Yangjisshan-Wushan-Tongjiangling Fault and the M_S4. 8 aftershock occurred on the NE striking Liujia-Fanjiapu-Chengmenshan Fault.     

RESEARCH ON GENESIS OF M4.2 AND M4.5 EARTHQUAKE SEQUENCES IN MARCH 2014 IN ZIGUI COUNTY,HUBEI PROVINCE

On 27^th and 30^th March 2014, an M4.2 and M4.5 earthquake sequence occurred in Zigui County, Hubei Province, and the earthquake sequence type is double seismic type. The two earthquake sequences occurred at the water unloading stage of the 175m trial impounding, and G-R relations showed the similar characteristics with that of the tectonic earthquakes. In order to verify the influences of dam reservoir on earthquake triggering, ETAS model was introduced, the results showed that the slow water level changes had little impact on the occurrence of earthquake. Double difference precision relocation results indicated that the two earthquake sequences occurred at the intersection part of a NE-striking fault and the NNW-striking Xiannvshan Fault, and the preferred direction of aftershock distribution was separately NE and NNW. Moment tensor inversion method and P wave initial motion method were used to determine the focal mechanisms of the two earthquakes, and the results indicated that the two earthquakes were controlled by the regional tectonic stress field and were of reverse-slip type. Comprehensive analysis showed that the M4.2 earthquake was caused by a small-scale fault striking NE with a big dip angle. From the hypocenter profile, it can be seen that the M4.2 earthquake sequence was restrained by an east-dip fault, and the M4.5 earthquake sequence was the product under the conjugate action of the NE-striking fault and the NNW-striking Xiannvshan fault.     

西秦岭北缘断裂带断层气浓度空间分布特征与强震危险性分析

以西秦岭北缘断裂带不同段断层气Hg、Rn浓度空间分布特征分析为基础、通过对历史大震背景、现今地震活动影像以及b值空间分布特征的对比分析,从地球化学的角度,对西秦岭北缘断裂带不同段落的活动习性进行深入探讨,并识别出该断裂带潜在大震危险区段。可为未来地震趋势判定及震情跟踪提供重要的基础资料。     

Influence of electromagnetic fields on the seismotectonic strain rate; relaxation and active monitoring of elastic stresses

We study the changes in the rate of seismotectonic strains in the Earth’s crust in two of the most seismically active Central Asian areas, the Garm region in Tadjikistan and the Northern Tien Shan, which are excited by high energy pulsed electromagnetic irradiation. It is shown that in both areas the average rates of strain release increase by a factor of 10–20. The increment of the seismotectonic strains initiated during the entire period of electromagnetic treatment is 1.3 × 10⁻⁶–6.3 × 10⁻⁶, which is comparable with the value of crustal strains observed during the preparation of strong earthquakes and amounts to 3–13% of the ultimate strains for rocks. Such a contribution to the process of quasi-plastic strains of the crust results in the additional relaxation of elastic stresses by 0.4–1.67 bar, which corresponds to approximately 1–17% of the stress released in the sources of strong earthquakes. The spatial variations in the changes of the strain rate during electromagnetic treatment are studied, and their correlation with the level of stresses in the Earth’s crust is revealed. It is shown that the change in the strain rate during the electromagnetic excitation can be used as the criterion for active monitoring of the stress state of the medium for the purposes of predicting the location, time, and magnitude of strong earthquakes.     

成都断陷区活动断裂带基本特征及其潜在地震能力的判定

成都断陷是龙门山前缘重要的第四纪构造盆地，龙门山前山断裂和龙泉山西坡断裂对断陷盆地的形成和演化有重要的控制作用。晚第四纪时成都断陷遭受北西－南东向的挤压，使断陷盆地边缘和内部的北东，北北东向断裂具逆冲运动性质，并导致龙门山前山断裂，龙泉山西坡断裂和蒲江－新津－成都－德阳断裂上中，强地震的发生，研究表明，上述３条断裂均具有发生中强地震的潜在能力。     

Double-difference relocation of earthquakes in central-western China, 1992–1999

The double-difference earthquake location algorithm was applied to the relocation of 10,057 earthquakes that occurred in central-western China (21^∘N to 36^∘N, 98^∘E to 111^∘E) during the period from 1992 to 1999. In total, 79,706 readings for P waves and 72,169 readings for S waves were used in the relocation. The relocated seismicity (6,496 earthquakes) images fault structures at seismogenic depths that are in close correlation with the tectonic structure of major fault systems expressed at the surface. The new focal depths confirm that most earthquakes (91%) in this region occur at depths less than 20 km.     

Analysis on the Characteristics of the Kalpin MS5.3 Earthquake Sequence of December 1, 2013 and Anomalies before the Earthquake

This paper introduces the basic parameters, focal mechanism solutions and earthquake sequence characteristics of the Kalpin M_S5.3 earthquake sequence of December 1, 2013, and analyzed seismic activity before the earthquake, the adjacent tectonic features and the precursory anomaly at fixed points within a range of 200km. Research indicates:(1) The earthquake occurred on Kalpin fault, the source rupture type is thrust faulting with sinistral strike-slip component. (2) The earthquake sequence is mainshock-aftershock type, with the aftershock distribution attenuating quickly and trending NE. (3) Abnormal seismic activity before the earthquake was characterized by seismically nesting quiescence of M_S2.0-4.0 earthquakes, seismic quiescence of M_S4.0 earthquakes and seismic belts of M_S3.0 earthquakes in the Kalpin block, abnormal enhancement zone of moderate earthquakes on Puchang fault and seismological parameters. (4) Anomalies of precursory observation data at fixed stations are mainly characterized by mutation. Apart from the borehole tiltmeter in Halajun, the spatial distribution of other abnormal precursors showed a phenomenon of migration from the near field to far field and from the epicenter to the peripheries.     

1999年山西大同Ms 5.6地震的震源断层

大同震区先后在 1989、1991和 1999年发生MS >5地震 ,利用大同遥测地震台网的记录资料进行比较精确的地震序列震源定位 ,结合宏观烈度分布和震源机制解资料 ,详细地分析对比了 3次子序列的异同。结果显示 ,1999年MS5 .6地震的震源断层是走向NWW、长 16km、宽12km、埋深 5km以下、倾角近直立的左旋走滑断层。而前 2个子序列是NNE为主的右旋走滑断层活动所致 ,表明地震破裂方向发生了变化。这种 2个以上方向先后出现、并且强弱有别的地震破裂是普遍存在的 ,表明震源环境的复杂程度与地震序列的类型有关。虽然震区存在NE向的大王村断裂和NW向的团堡断裂 ,但目前没有证据说明震源断层和 2条构造断层连通。 3次子序列的震源断层都是走滑断层 ,也和 2条构造正断层有别。 1999年的子序列可能属于新破裂。