RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5002, doi:10.2205/2008ES000303, 2008

5. Relationship Between Seismicity and Components of Tidal Force

[26]  Relationship between tidal force and frequency of earthquakes was investigated. Tidal force was calculated from a model of the gravitational interaction between the Earth and Moon. We neglect the solar component of the tidal force because, on average, it is more than two times less of the lunar component. The disregard of this component does not hinder the establishment of the fact of existence of statistical relations between seismicity and earth tides.

2008ES000303-fig05
Figure 5
[27]  Astrometry data were used for synchronization of tidal force with earthquakes. Characteristics of tidal force were examined: vertical (radial) component F r, north-south F SN and east-west F WE horizontal components, the module of a vector of horizontal components F h and the module of the full vector of tidal force |F|. Time diagrams of tidal vector components for latitude 320 are shown on Figure 5. In figure we see fast components with daily and semidiurnal the periods and slow a components with the fortnightly period. On the top of the diagram a dashed line shows envelope A r for characteristic F r. This characteristic we shall name a daily variation of tidal force.

[28]  Two phases: positive ("+'') and negative ("-'') were considered for each characteristic of tidal force. The phase was considered positive if the characteristic exceeds its average value for the long-term period. Otherwise the phase was considered negative. The phase changes a sign one or two times per day for the first five characteristics and two times per month for the daily variation A r. Duration of each phase depends on a sort of the characteristic and from latitude of a place of supervision and does not depends on a longitude.

[29]  For revealing correlations between seismicity and tidal force the special statistical measure was used

eq002.gif

where f+ = N+/T+ and f- = N-/T- are the frequencies of earthquakes for "+'' and "-'' phases of tidal force correspondingly, N+ and N- are the numbers of earthquakes for "+'' and "-'' phases, T+ and T- are the total duration of intervals for "+'' and "-'' phase. The sign of s variable specifies in what phase ("+'' or "-'') earthquakes occur more often. Measure of s possesses statistical properties: if N+ and N- are sufficiently great, then value s has standard normal distribution and so its big values (for example, 2 or 3) are "significant'', i.e. argue in favor of correlation between seismicity and tidal force.

[30]  Earthquakes with the magnitudes mge 2 from 01.01. 1999 till 30.01.2005 were used to estimate the correlations. The maps of prevailing seismic activity for characteristics of tidal force Fr, FSN, FWE, Fh, |F| and A r were analyzed. For this purpose the territory under study is divided on square (in terms of degree measure) cells. If the number of earthquakes within the cell is more than 10, then the frequencies of earthquakes corresponding to "+'' and "-'' phases of the tidal characteristic are calculated. Thus signs "+'' or "-'' are assigned to the cells depending on prevalence either of two frequencies.

[31]  Under close examination of the results some "positive'' or "negative'' cells form clusters of the cells. Such clusters are approximated by ellipses. Frequencies f+, f- and measure s are calculated for the seismic events which fit in the ellipses.

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Figure 6
[32]  As a result of the analysis the area AI which appeared seismically sensitive to tidal force A r is shown on Figure 6. This area includes 178 events, the ratio frequencies of earthquakes in "+'' and "-'' phases f+/f- = 0.6, s = -3.5. The size of statistics s in this area is high enough, that testifies to essential distinction between frequencies of earthquakes for two compared phases. Thus, frequency of earthquakes for negative phase tidal force A r is raised (in comparison with positive phase of this characteristic). Similar correlations for other characteristics of tidal force were not revealed.

[33]  The interpretation of this result is based on the assumption that a unidirectional action of the tectonic and tidal pressures increases probability the earthquake. We shall notice that vertical tidal force in a negative phase causes compression of the earth crust. Setting the intensity of earthquakes during a negative phase of characteristic A r is raised, it is necessary to conclude, that area AI is in a condition of primary vertical tectonic compression.


RJES

Citation: Gitis, V., E. Yurkov, B. Arora, S. Chabak, N. Kumar, and P. Baidya (2008), Analysis of seismicity in North India, Russ. J. Earth Sci., 10, ES5002, doi:10.2205/2008ES000303.

Copyright 2008 by the Russian Journal of Earth Sciences

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