Coupled large earthquakes in the Baikal rift system: Response to bifurcations in nonlinear resonance hysteresis

The current lithospheric geodynamics and tectonophysics in the Baikal rift are discussed in terms of a nonlinear oscillator with dissipation.The nonlinear oscillator model is applicable to the area because stress change shows up as quasi-periodic inharmonic oscillations at rifting attractor structures （RAS）.The model is consistent with the space-time patterns of regional seismicity in which coupled large earthquakes,proximal in time but distant in space,may be a response to bifurcations in nonlinear resonance hysteresis in a system of three oscillators corresponding to the rifting attractors.The space-time distribution of coupled MLH ＞ 5.5 events has been stable for the period of instrumental seismicity,with the largest events occurring in pairs,one shortly after another,on two ends of the rift system and with couples of smaller events in the central part of the rift.The event couples appear as peaks of earthquake ‘migration＇ rate with an approximately decadal periodicity.Thus the energy accumulated at RAS is released in coupled large events by the mechanism of nonlinear oscillators with dissipation.The new knowledge,with special focus on space-time rifting attractors and bifurcations in a system of nonlinear resonance hysteresis,may be of theoretical and practical value for earthquake prediction issues.Extrapolation of the results into the nearest future indicates the probability of such a bifurcation in the region,i.e.,there is growing risk of a pending M ≈ 7 coupled event to happen within a few years.

Gravity and GPS Measurements at South-West Part of Baikal Rift

Measurements of non-tidal variations of gravity （Ag）, which were obtained from 1992 to 2012 at the Talaya seismic station （located in the south-western part of the Baikal region）, are interpreted together with GPS （Global Position System） observation data, which were obtained from 2000 to 2013 at the same station. An absolute gravimeter was used for gravity observation. The strongest precision requirements concern tidal gravity prediction. It is the reason why we investigated tidal effect by LCR （LaCoste ＆ Romberg） gravimeter from 1996 to 1997 at the same station. We use tested tidal model for correction of gravity change. The linear component of gravity variations corresponds to changes in the elevation of the site. The correlation coefficient is close to the normal value of the vertical gradient of gravity. At this site, coseismic gravity variations at the time of the Kultuk earthquake （August 27, 2008, Mw = 6.5） were caused by a combined effect of the change of the site＇s elevation and deformation of the crust. Our estimations of the coseismic effects are consistent with results obtained by modeling based on the available seismic data.

俄罗斯贝加尔湖区伸展构造及与中国东部伸展构造对比

在晚白垩世—始新世夷平面基础上,由于断裂作用形成了贝加尔裂谷系。断裂作用最大幅度超过10km。在裂谷系中心部位发育的断层长度最大、最深、最早,并以准对称形式向四周扩展。贝加尔裂谷系是在地幔隆起和印度—欧亚大陆碰撞双重作用下形成。贝加尔裂谷系与中国东部新生代断陷盆地和汾渭裂谷系同时形成,并有密切的成因联系。它们的形成不仅受太平洋板块的俯冲和印度—欧亚大陆碰撞的制约和影响,而且位于中国西南部的地幔流发散中心,呈扇状向太平洋区流动,可能是它们在更深层次上的共同场源基础。

贝加尔裂陷新构造特征

贝加尔裂谷系的断裂发育阶段和沉积作用过程基本一致，除沉积作用前的前裂谷断裂已经活化外，整个裂谷系的发育及演化过程大体可划分为两个阶段，即始新世中晚期—上新世早期（Ｅ２－３２—Ｎ１２）的开始阶段和上新世中晚期—第四纪（Ｎ２－３２—Ｑ）的主要阶段。前者属塑性形变，地形反差小，断裂活动弱；后者属脆性形变，地形反差大，断裂活动强。贝加尔裂谷拗陷是贝加尔裂谷系中发育最早、规模最大的水下拗陷，新构造运动表现明显，升降幅度自西南向东北增大。区内及其四周断块山体及盆地多显示了自西向东翘起的特点。夷平面及冲、洪积扇发生大量拱曲和拗曲。各方向断裂力学属性组合特征。

汾渭与贝加尔裂谷系相似性的统计分析

在野外观测与遥感综合解译的基础上，充分利用区内已积累的地质与地球物理资料，应用统计分析方法，从７个方面分别计算分析了汾渭与贝加尔裂谷系的相似性系数。显著性统计检验表明，两裂谷系在几何形态、三维构造格局、历史与现今活动性各方面都存在显著的相似性。结合空间测量与两裂谷系现今构造应力场的实测资料分析，推断汾渭与贝加尔裂谷系高度相似性产生的动力学机制与东亚大陆这一地带南、北块体间不等速西向运动所造成的大规模Ｓ形张裂作用存在成因联系。

Prevalent thickening and local thinning of the mantle transition zone beneath the Baikal rift zone and its dynamic implications

The Baikal rift is the most seismically active continental rift in the world and is significant for studying the dynamics of continental rifts, although its precise dynamic mechanisms remain controversial. We calculated receiver functions (1748) from Global Seismographic Network seismic stations TLY and ULN and stacked receiver functions in different bins. Here we present discontinuities at depths of 410km and 660km and thickness of the mantle transition zone (MTZ) beneath the study area. The MTZ structure shows an obvious thickening (292km) in the Baikal rift zone except for an area of limited thinning (230km), whereas it is basically normal (250km) beneath the Mongolian area, to the southeast of the Baikal rift. Combining these results with previous findings, we propose that the large-scale thickening beneath the Baikal rift zone is likely to be caused by the Mesozoic collision between the Siberian Platform and the Mongolia-North China Block or magmatic intrusion into the lower crust, which would result in crust and lithosphere thickening. Thus, the lower crust becomes eclogitized and consequently detached into the deep mantle because of negative buoyancy. The detachment not only induces asthenosphere upwelling but also accelerates mantle convection of water detached from the subducted slab, which would increase mantle melting, while both processes promote the development of the rift. Our preliminary results indicate that the detachment and the consequent hot upwelling have an important influence on the development of the Baikal rift, and a small-scale mantle upwelling indicated by the located thinning may have destroyed the lithosphere and promoted this development.

贝加尔裂谷系及邻区断裂现今活动变化及成因

已有的地质和地球物理方法无法对年、月、旬等现今时间尺度上的断层活动变化进行有效预测。运用基于地震活动定量指数运算程序的地理信息技术可以解决这个问题。这种方法被应用于贝加尔裂谷系（BRS）及邻区的研究当中。研究发现，断层活动变化发生在以几年为周期的时间尺度上，这无法用区域应力场的变化进行合理解释。沿着穿越贝加尔裂谷系的剖面编制了活动断层图以及地震活动定量指数曲线。提出的这种方法可以根据地震活动定量指数对活动断层进行仔细的分类，从而为解决地震中期预报相关问题开辟了重要途径。这种方法还被用来研究断层现今活动的时空变化和形成机制。研究发现，在实时尺度上断层活动具有高频发生的特征，这种现象可能是由于脆性岩石圈板内和块体内部运动产生的慢变形波的干扰导致的。利用变形波的通过速度可以对活动断层进行分组，各组具有不同的地质和地球物理特征参数，同时还可以对变形波波前的方向以及某一区域内主要断层在实时间隔内（地质上的瞬间）的活动情况进行预测。

蒙古Khuvsgul湖HDP09湖芯中地下玄武岩的K-Ar年龄:对贝加尔裂谷起始时间的估计（英文）

HDP09 core drilled in Lake Khuvsgul,Mongoria,at 50°52'48 'N,100°26'30' E where the water depth is 222.25 m reached to the depth of ~60 m below lake floor in 2006.The bottom part of the core consists of alkali basalt.This basalt consists of the basement of the lake Khuvsgul based on its bulk chemistry and core position plotted on the seismic profile.K-Ar age of the basalt is(8.5±0.2) Ma,which is concordant with the on-land basalt distributed in the eastern part of the lake,and implies the maximum age of the Lake Khuvsgul formation.