In connection with changes in the stress-strain state of the Earth's crust, various physical and mechanical processes, including destruction, take place in the rocks and are accompanied by tectonic earthquakes. Different models have been proposed to describe earthquake preparation and occurrence, depending on the mechanisms and the rates of geodynamic processes. One of the models considers crustal stretching that is characteristic of formation of rift structures. The model uses the data on rock samples that are stretched until destruction in a special laboratory installation. Based on the laboratory modeling, it is established that the samples are destroyed in stages that are interpreted as stages of preparation and occurrence of an earthquake source. The preparation stage of underground tremors is generally manifested by a variety of temporal (long-, medium-and short-term) precursors. The main shortcoming of micro-modeling is that, considering small sizes of the investigated samples, it is impossible to reveal a link between the plastic extension of rocks (taking place in the earthquake hypocenter) and the rock rupture. Plasticity is the ability of certain rocks to change shape and size irreversibly, while the rock continuity is maintained, in response to applied external forces. In order to take into account the effect of plastic deformation of rocks on earthquake preparation and occurrence, we propose not to refer to the diagrams showing stretching of the rock samples, but use a typical diagram of metal stretching, which can be obtained when testing a metal rod for breakage (Fig. 1). The diagram of metal stretching as a function of the relative elongation (to some degree of approximation and taking into account the coefficient of plasticity) can be considered as a model of preparation and occurrence of an earthquake source in case of rifting. The energy released in the period immediately preceding the earthquake contributes to the emergence of its precursors. This article discusses various earthquake precursors with reference to the energy model of tectonic earthquake preparation and occurrence in conditions of crustal stretching.

Abstract:On 27 August 2008, a strong earthquake occurred in the Southern Baikal area. Its focus was located within the Baikal rift zone which characterized with high seismicity. The general location and approximate strength of the earthquake were thus expectable, whereas the shock occurred at an unexpected time, as usual. The article provides a brief review of the geological environment and macro-seismic effects of the earthquake. It also reviews a set of seismic and hydrogeochemical indicators which are interpreted as precursors of the given earthquake. Considering such precursors, the fact that the Baikal deepwater helium content had changed before the main shock was recorded for the first time in the region under study.Keywords: earthquake, magnitude of earthquake, seismic zones, forecast of earthquakes, earthquake precursors. Recommended by Р. М. СеменовИнститут земной коры СО РАН, 664033, Иркутск, ул. Лермонтова, 128, Россия Аннотация: 27 августа 2008 года на юге Байкала произошло сильное землетрясение. Его очаг располагался в пределах высокосейсмичной Байкальской рифтовой зоны, так что его общее местоположение и примерная сила не были неожиданными. Внезапным, как всегда, оказалось время возникновения подземного толчка. В статье кратко рассматриваются геологические условия возникновения землетрясения, его макросейсмические послед-ствия и некоторые сейсмолого-гидрогеохимические данные, рассматриваемые как предвестники. Среди них впервые для данного района зафиксированы изменения содержаний гелия в глубинной воде Байкала, предше-ствующие основному толчку.Ключевые слова: землетрясение, магнитуда, сейсмические зоны, предсказание землетрясений, предвестники землетрясений.

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The large earthquake which shook the southern part of Lake Baikal on August 27, 2008 has received much scientific and public interest. The occurrence of the event in the high-seismicity Baikal rift was not surprising but its origin time was, as usual, unexpected. We discuss the geological conditions of the earthquake, its macroseismic effects, and some data interpreted as its precursors. A special focus is made on preseismic changes in dissolved helium in Lake Baikal. Studying helium patterns in a deep freshwater lake in an active seismic area is a pioneering experience that appears to have no match in the world earthquake prediction practice.

This paper presents the results of study of seismically induced variations in helium concentration in groundwater in the southern Baikal region. Unlike previous studies, data were obtained not only in Baikal deepwater but also in artesian wells in the southern Baikal region. The correlation coefficients between the data obtained at these observation stations, indicator functions, and an integrated indicator functions were calculated. They provide an objective assessment of time variations in concentrations due to changes in the stress–strain state of the Earth’s crust related to earthquake preparation and hence permit one to predict the time of an earthquake.