SOME FEATURES OF GLOBAL SEISMICITY

BURMIN V. Yu.

Schmidt’s Institute of Physics of the Earth, Russian Academy of Sciences

I. FREQUENCY-MAGNITUDE RELATIONSHIP FOR GLOBAL SEISMICITY

First, we build the recurrence curve of earthquakes. The straight-line Gütenberg-Richter relationship is: lgN = a - gM,

Where N - average of earthquakes, the magnitude of which lies in an interval of [M-DM, M+DM]; a and g - parameters of recurrence.

As demonstrates the analysis of the recurrence curve of earthquakes built on the data of the catalog, representing the events with a magnitude of 4.5 and more. Thus, the global network registers without the miss of event with a magnitude greater than 4.5. In this connection the further analysis is conducted for events with a magnitude greater than 4.5. Since the magnitude of 4.5, the frequency-magnitude relationship is practically linear with small downwards since the magnitude of 8.0. The slope of the frequency-magnitude relationship is -1.04 and a = 9.85.

Figure 1. Frequency-magnitude relationship
for global seismicity.

It is easy to be convinced that the power contribution of weak earthquakes (M < 4.5) in global seismicity is rather minute. The fraction of energy of gentle earthquakes does not surpass 0.333 % from the common seismic energy for period of from 1973 to 2007, which has compounded about 10²⁵ ergs.

II. DISTRIBUTION OF EARTHQUAKES IN DEPTH

Let us look, as the earthquakes in depth have arranged. Along with the distribution of change of number of earthquakes for earthquakes with miscellaneous magnitudes, the distribution of free seismic energy has built. In the Fig. 2а is distribution of a velocity curve of P-waves for the upper and middle mantle and distribution of number of earthquakes in depth.

From the distribution in the figure, it is well visible, that most of earthquakes take place in earth crust in the depth of 0-10 km and 30-40 km. Then there is a monotonic waning of number of earthquakes with depth, down to the depth of 300 km. On an interval of depths of 300-450 km the number of earthquakes remains to an approximate constant, and then there is an increase of number of earthquakes up to the depth of 600 km. The ambassador of 600 km is a sharp decay of seismicity on number of earthquakes.

Diverse picture have be seen in a Fig. 2b and 2c. On Fig. 2b, the distribution of the seismic energy in miscellaneous depths is shown. In this figure the distribution curve does not such smoothly vary, as in the Fig. 6а. The splashes of seismic energy in the depths of 220, 270, 400, 640 and 670 km are especially appreciable, i.e. there, where the splashes on the distribution of number of earthquakes in depth miss.

The even more expressive picture has be seen in the Fig. 2c, where the energy distribution with depth, coming on one earthquake in a ten-km layer is shown. Here, splashes of a maximum liberated energy has more and, that is interesting in depths of 220, 270, 400, 640 and 670 km they have dated for the applicable boundaries. Moreover, from the distribution in the Fig. 2b follows, that in depths of 640 and 670 km the energy per one earthquake is more than in depth of 40 km, where there is an overwhelming number of earthquakes. It means that in the zone C of mantle of the Earth in depths of 640 and 670 km take place more intensive earthquakes, than in zones A and B.

III. ANNUAL RELATION OF GLOBAL SEISMICITY

For analyzing the annual relation of global seismicity the distribution of change of number of earthquakes from one year to one year for 4 intervals of magnitudes have built: 7.5-9, 6.5-7.5, 5.5-6.5 and 4.5-5.5 (Fig. 3а), change of common seismic energy (Fig. 3b) for period 1973 -2007. From consideration of this distribution, it is visible that for earthquakes with magnitudes of from 7.5 up to 9 some periodicity or repetition of the form in intervals 1973-1989 and 1990-2007 (Fig. 3а) has watched. Thus, the general number of earthquakes in the second period is more little, than in maiden. That can be told and about earthquakes with magnitudes of from 6.5 up to 7.5. For earthquakes with magnitudes of from 5.5 up to 6.5 such relations to observe is difficult. The distribution for earthquakes with magnitudes of from 4.5 up to 5.5 interestingly behaves. It has the obvious tendency to ascending number of earthquakes with years. It have been connected to two reasons. First is really increased of number of gentle earthquakes from one year by one year. Second, bound with increased of number and sensitivity of seismic stations.

On the chart, introduced in a Fig. 3b the increase of seismic energy, since 1990 is well tracked. Thus abnormal on liberated number of energy are 1994-1996, 1998, 2000, 2004 and 2007.

Figure 3. a) Distribution of change of number of earthquakes from one year by one year;
b) Distribution of change of common seismic energy from one year by one year.

IV. SEASONAL DEPENDENCE OF GLOBAL SEISMICITY

The relation of seismic activity to a season also represents an interesting picture. For consideration of this relation, we have divided all earthquakes into 4 groups, depending on depth of the centres.

In the first group has come crust’s earthquake with depth of the centres up to 40 km. From the chart of distribution of the seismic energy of shallow earthquakes depending on month of year for period from 1973 to 2007 it is well visible (Fig. 4a), that the noticeable increase of seismicity is necessary at the end of year - November and December. In January and February, the seismicity decreases sharply. Then, in March the seismic activity of earthquakes has sharply augmented, and in April wanes, systematically increasing up to July sharply. In July and August the seismicity is levelled down of February and then, in September increases, practically twice sharply.

Characteristically, that the December maximum coincides a minimum of spacing interval from the Earth up to the Sun at its motion on an ecliptic, March and September with a vernal and autumnal solstice. Apparently, the increase of seismicity has connected to change of tidal forces, which are operational on the Earth on the part of the Sun.

Figure 4. The seasonal dependence of global seismicity. a) crust’s earthquake. b) earthquakes with depth in the interval of from 40 up to 300 km. c) earthquakes with depth in the interval of from 300 up to 450 km. d) earthquakes with depth in the interval of from 440 up to 700 km.

For earthquakes with depth of the interval from 40 up to 300 km, the minima of the liberated seismic energy in February, April, September and Novembers (Fig. 4b) have notably allocated. Thus, the relative increase of seismicity in January month and summer months is characteristic.

For an interval of depths of from 300 up to 450 km the maxima of the liberated seismic energy are necessary for January and August (Fig. 4c). The minima of seismic activity for these depths have for September and December.

In the Fig. 4d, the chart of distribution of the liberated seismic energy for the steepest earthquakes with depths of from 450 up to 700 km is shown. We see that the maximum of number of earthquakes is necessary for June, i.e. that month, when the Earth is on maximum deleting from the Sun. Except for this month the maxima in March and August take place. An interesting feature of this distribution is that in December is selected (allocated) least of seismic energy of steep earthquakes, in difference of earthquakes, for which the month December is characterized by maximum activity, both on number of earthquakes, and quantity of liberated energy.

Analyzing the seasonal charts of distribution of the liberated seismic energy introduced in the Figs. 4a, b, c and d, it is possible with confidence to tell, that for all intervals of depth quietest is February, and most by fissile - March. At the same time, for depths of from 0 up to 300 km fissile is December. For depths of from 300 up to 450 km fissile is January and August.

V. CONCLUSIONS

Despite of different natures of the development of seismicity for different intervals of depths we see that for the last 35 years, the seismicity does not only remain to a constant, but also the nature of it essentially varies. Therefore, from the analysis of global seismicity the conclusion follows, that for last eighteen years has taken place essential increase of seismic activity of the Earth, and it is not so much in number of earthquakes, as in seismic energy. It means that in the upper shells of the Earth there are increasing geodynamic processes, which one-catch zones A, B and C and most intensively are exhibited in earth crust and in depths of from 450 up to 700 km. Thus, the seasonal dependence of seismicity lets see that the essential influential factor in geodynamic processes is the position of the Earth concerning planets of the solar system and Sun in that or diverse seasons.