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Nanoseismic technologies for nanoearthquakes

15.05.2006


Researchers of the Institute of Ecological Problems of the North, Ural Branch, Russian Academy of Sciences, and the Schmidt Institute of Physics of Earth, Russian Academy of Sciences, have developed technology that allows to register nanoearthquakes - seismic events of minimal possible magnitudes (-4, -5). Registration of such events allows to quickly and accurately make up seismic activity maps of small-scale territories.



Investigation of seismic activity in quiet areas (including revelation and assessment of degree of fracture activity) is of great practical importance. People are laying pipelines, building nuclear power-stations, radioactive waste storages and other potentially dangerous constructions, including tower buildings. However, the earth is vibrating from time to time even in the quietest areas. Certainly, typical shaking of the earth in the middle of the platform is much weaker than that in active regions, but it can also cause trouble if minifracture happens to go under a nuclear power plant. Nanoearthquakes can serve the indicator of seismic well-being in a certain area, but observations over nanoearthquakes require special technology.

Seismic activity of the territory is judged by the earthquake recurrence diagram. In a double logarithmic scale, the diagram represents a straight line, the slope of which characterized seismic activity. The diagram is drawn up based on observation results. Strong earthquakes are the easiest to register but they have to be awaited for a long time. To obtain reliable information about weaker and more frequent quakes, which slightly exceed the noise level and occur 10 to 20 times within half an hour, the researchers have to spend several days per each spot. Only three hours are needed to measure the most feeble events, but they are difficult to be distinguished from disturbances caused by the wash, transport motion or the life of a big settlemen. Besides, existence of such feeble vibrations was assumed only theoretically. However, Russian geophysicists have managed to record them.


They solved technical difficulties by placing seismic sensors simultaneously in three spots: at the very measurement point and 1 to 3 kilometers to the north and south from it, and then – in the latitudinal direction – to the east and to the west. Registration of microquakes was carried out in the 0.5-30 Hz band, the majority of man-caused noise being sieved out at this frequency. The above method also allows to ignore surface vibrations and to take into account only quakes coming from the depth. The researchers made records in each of the three points for three hours, and then they compared the records, thus counting the coherence function between the points. As seismic noise level is approximately the same in a small territory, the method allows to quantitatively compare seismic activity in the points located 1 to 3 kilometers from each other, and to single out feeble earthquakes, the energy of which is billion of times less than that of earthquakes registered by ordinary seismic stations. Therefore, the researchers have confirmed that nanoearthquakes do exist.

With the help of two simultaneously operating portable digital seismometric stations, the researchers rather quickly took a reading from more than 20 points in the region of the Northern Dvina estuary not far from Arkhangelsk. The region was not selected at random – the researchers had previously investigated it and found an active fracture in the the earth’s crust. Recording of nanoearthequakes allowed to more precisely define position and structure of this fracture.

Making an assessment of the new nanoseismic technology, its developers emphasize its simplicity and economy, which are very important for reconnaissance work, particularly in almost impassable conditions. Such technology will help to investigate global processes in the solid covering of earth and the role of seismically quiet areas in these processes.

Sergey Komarov | alfa
Further information:
http://www.informnauka.ru

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