SPATIAL SCALE COORDINATES
EARTHQUAKE
STATISTICS
IN
GEOMAGNETIC
Khachikjan G. Mail Address: Institute of seismology, al-Farabi, 75a, 050060, Almaty, Kazakhstan E-mail:
[email protected] _______________________________________________________________ Key Words: seismicity, main geomagnetic field.
ABSTRACT
The integrated studies involving seismology, geodynamics and geomagnetism are essential for advances in understanding of the dynamics of our planet. In this report, the results obtained in this direction in the Institute of seismology of Kazakhstan are presented. To obtain these results, the data on earthquakes with M≥4.0 were taken from the NEIC catalogue, and for each of the epicenters the parameters of the main geomagnetic field were estimated with using the International Geomagnetic Reference Field (IGRF) model. Then, spatial scale distribution of earthquake epicenters in relation to geometry of the main geomagnetic field was investigated. It is found: 1) local peaks of seismic activity in the northern and southern hemispheres are better organized according to geomagnetic latitude or angle of geomagnetic inclination then the geographic latitude; 2) the western and southern boundaries of the now-forming Somalian plate are defined very well using traced earthquakes, if one presents the map of epicenters not in the usual geographic coordinates but in a plot of geomagnetic inclination versus geographic longitude; 3) spatial location of three main seismotectonic areas: orogeny, subduction zones, and rift systems show systematization according to the angle of geomagnetic declination (D): for earthquake epicenters occurred in orogeny, D values are rather small (vary from ~ -1.0 to +5.0); for epicenters occurred in subduction zones, D values are rather large and positive, while they are rather large and negative for epicenters occurred in rift systems. It is concluded that geographical distribution of seismic zones at the Earth is controlled by the geometry of its main magnetic field.
INTRODUCTION It is believed at present that the results of plate-tectonic forces are well evident in earthquakes activity: epicenters are located mainly within narrow bands, which coincide closely with the tectonic plate boundaries. For generation of plate tectonic forces the convection in the mantle is responsible (Bercovichi, 1998), which in turn is related to the processes into the fluid outer core (e.g. Hide et al., 2007). Simultaneously, the processes into the fluid outer core are responsible for generation of the Earth’s main magnetic field (Glatzmaier and Roberts, 1995). Taking this into account, one may expect a coupling between spacial scale distribution of seismic zones and parameters of the main geomagnetic field. On the other hand, there are a lot of evidences that a few days/hours before the seismic shock the electromagnetic phenomena related to earthquakes are evident in the near space plasma parameters over the area of earthquake preparation and in magnetically conjugate area as well (e.g. Galper et al.,1995; Molchanov et al., 1995; Ondoh, 2008; Ouzounov and Freund, 2004; Sakhar et al., 2007; Ruzhin et al.,1998; Pulinets and Boyarchuk, 2004 and references in herein). To explain electromagnetic phenomena related to earthquakes, a model of lithosphere– atmosphere-ionosphere–magnetosphere coupling is under developing. This model is based partly on a concept of Global Electric Circuit (GEC) and suggests that an earthquake is an element of this circuit (e.g. Makarova and Shirochkov, 1998; Pulinets, 2009 and references in herein). It is known that a problem with operating of GEC is non conductive troposphere and stratosphere. It is considered at present that a source of ions in the near ground troposphere is air ionization produced due to release of
radioactive gases from the soil, while the principal source of atmospheric ions away from the boundary layer is galactic cosmic rays (Rycroft et al., 2000). The intensity of galactic cosmic rays reaching the top of the atmosphere is a function of the earth’s magnetic field (Shea and Smart, 2006). Therefore, one again may expect a coupling between special distribution of seismic zones and geometry of the main geomagnetic field. The results presented in below confirm these expectations.
DATA AND METHOD The data on earthquakes with magnitude М≥4.0 occurred at the globe in 1973-2006 (more than 241000 events) were taken from the NEIC catalogue [1]. Spacial scale distribution of epicenters is shown in figure 1, where the location of the now-forming Somalian plate is marked. It is seen that in geographic coordinates, the western and southern boundaries of Somalian plate are not well defined by traced earthquake epicenters.
Figure-1. The map of earthquake epicenters with М 4.0 occurred at the globe during 1973-2006 [1]. For each of the epicenters presented in figure 1, the geomagnetic coordinates were calculated with using the well-known relations: sin = sin sin р + cos cos p cos ( p), sin Ф = cos sin ( - p)/cos , where and are geographical latitude and longitude, and and Ф are geomagnetic latitude and longitude, respectively; р and p are geographical coordinates of geomagnetic dipole poles. Also, for each of the epicenters, the parameters of the main geomagnetic field were estimated with using the International Geomagnetic Reference Field (IGRF) model, which computer codes are available at [2]. The IGRF is a series of mathematical models describing the Earth’s main magnetic field and its secular variation. The model for each of the geomagnetic epoch comprises a set of spherical harmonic coefficients (called Gauss’s coefficients) g nm и hnm, in a truncated series expansion of a geomagnetic potential function of internal origin: N
n
V r , , , t R Rr g nm t cos m hnm t sin m Pnm (cos ) n 1 m 1
where r, θ, λ are geocentric coordinates (r is the distance from the centre of the Earth, θ is the colatitude, i.e. 90° - latitude, and λ is the longitude), R is a reference radius (6371.2 km); and are the coefficients at time t and are the Schmidt semi-normalized associated Legendre functions of degree n and order m. The main field
coefficients are functions of time and for the IGRF the change is assumed to be linear over five-year intervals. Because of the time variation of the field, a good accuracy of the model can only be produced for times when there is global coverage by satellites measuring the vector field. This occurred in 1979-1980 years, when data of satellite MAGSAT were used, and from 1999, when data of Ørsted, CHAMP were used. It is considered [3] that the root mean square error for estimation of a module value of field is ~ 10 nT for 1980 and 2000 epochs. A 10 nT global rms vector error corresponds to global rms values of about 5, 5, and 7 nT for south, east, and radial geomagnetic components, respectively.
RESULTS Distribution of number of earthquakes depending on geographic and geomagnetic latitudes and angle of geomagnetic inclination Figure 2 presents histograms for number of earthquakes in dependence on geographic latitude (2a), geomagnetic latitude (2b) and angle of geomagnetic inclination (2c).
Figure-2. Distribution of number of earthquakes versus geographic, geomagnetic latitudes, and angle of geomagnetic inclination (a-c, respectively). Latitudinal variations of seismicity have been discussed by many authors (e.g. Levin and Chirkov, 1999; Fridman and Klimenko, 2002; and references in herein). It was found that epicenters density strongly decreases from equator to poles, described as a first approximation a curve of a square of distance up to an axis of rotation of the Earth. This result allowed authors to suggest that the mechanism of earthquake generation is related to the irregularity of the earth’s rotation speed. Also, two local peaks of epicenters density seismicity located not symmetrically according to equator were revealed. In the northern hemisphere the peak was located near 30-390N, whereas in the southern hemisphere it was located near 0-90S. Fridman and Klimenko (2002) showed that asymmetry in peaks location takes place for both, the shallow and deep earthquakes. The results in figure 2a are in close agreement with Levin and Chirkov (1999). It is evident that number of earthquakes with M≥4.0 strongly decreases from equator to poles. The local peaks in northern and southern hemispheres are also evident in figure 2a, but they are better defined in figure 2b, where the distribution of number of earthquakes in dependence on geomagnetic latitude is shown. There is an asymmetry in
peaks location according to geomagnetic equator: the center of hemispherical symmetry is displaced to the geomagnetic latitude ~ 5-100N. It is interesting to notice that during calculation ofd geomagnetic coordinates, the magnetic field was taken as centered dipole with symmetrically located poles. For epoch 2001 for example, the geomagnetic poles are chosen to be located at 79.397°S, 108.392°E and 79.397°N, 71.608°W. This is not in agreement with the real situation. According to the IGRF model for epoch 2001, the geomagnetic poles at the earth’s surface are located at 79.0°N, 254.9°Е and 64.7°S, 138.6°E. One may easily estimate that the center of the line connecting two geomagnetic poles is projected at latitude ~7.2°N that closely coincides with the center of symmetry in figure 2b. In figure 2c, the distribution of number of earthquakes depending on the angle of geomagnetic inclination (I) is shown. This plot differs seriously from that one in figure 2a. There are two clear peaks in number of earthquakes depending on geomagnetic inclination. In the southern hemisphere (negative inclinations), where geomagnetic field lines come upward from the earth’s surface, the maximum of seismicity occurs at I=~300. In the northern hemisphere (positive inclinations), where geomagnetic field lines come dawn into the surface, the maximum of seismicity occurs at I=~600. To more visualize a relationship between spatial scale distribution of epicenters and geomagnetic inclination, let us consider figure 3, where the map of epicenters is presented in a plot of geomagnetic inclination versus geographic longitude. Surprisingly, in figure 3, the western and southern boundaries of the nowforming Somalian plate are defined very well using traced earthquakes, while it was not evident in figure 1.
Figure-3. Epicenters of earthquakes with М 4.0 presented in a plot of geomagnetic inclination versus geographic longitude. The western and southern boundaries of the now-forming Somalian plate are defined very well using traced earthquake epicenters.
Distribution of number of earthquakes depending on geomagnetic declination Figure 4 presents the global map of geomagnetic declination (D) for epoch 2000 [4]. Green lines in this figure show location of D=0 values. It is seen that the first D=0 line
stretches from the south to the north through the American continent, and the second D=0 line stretches from Scandinavia through Europe, Africa, and India to East Asia, where it produces a loop and then comes through Indonesia and Australia to Antarctica. The D=0 lines divide the areas with positive (red) and negative (blue) declination values.
Figure-4. Map of geomagnetic declination (D) for epoch 2000 [4] Green lines show zero, red – positive, and blue – negative D values.
Figure-5. (a) – Distribution of number of earthquakes in dependence on geomagnetic declination D; (b) - geographical location of epicenters which formed the main peak in (a) for 5.0 < D < -1.0, where green lines show location of D=0 for epoch 2000; (c, d) geographical location of epicenters with D≥5.0 and D≤-1.0, respectively.
Earlier, Khachikjan et al. (2007, 2008) have reported on a coupling between spacial scale distribution of declination angles and earthquakes with M≥4.0 occurred in different tectonic areas. They showed that in epicenters belonging to continental orogeny, Dvalues vary around zero; in epicenters belonging to subduction zones, D - values are rather large and positive, while they are rather large and negative in epicenters belonging to rift systems. The goal of the present study is to carry out a similar analysis for stronger earthquakes (M≥5.0) which occurred at the globe in 1973-2008 (56596 events). The results are presented in figure 5. A histogram in (5a) shows three maxima in number of earthquakes depending on D. The central maximum is located around D=0, in addition, two weaker maxima are located at larger positive and negative D values. Figure 5b shows geographical location of epicenters which formed the main peak in (5a) at 5.0 < D < -1.0). Green lines in 5b mark D=0 lines. Keeping in mind a geological map, one may understand that the main peak in (5a) is formed by earthquakes occurred in areas of continental orogeny. Figure 5c shows geographical location of epicenters with large and positive declinations (D≥5.0). It is not difficult to understand that these earthquakes belong mainly to subduction zones which are located along the Pacific cost and partly in Alpine-Himalayan and Scandinavian regions. Figure 5d shows geographical location of epicenters with large and negative declination angles (D≤-1.0). One may conclude that these events occurred mainly in the rift systems at the bottom of the oceans. The results in figure 5 for earthquakes with M≥5.0 are in full agreement with the results obtained earlier for earthquakes with M≥4.0.
DISCUSSION AND CONCLUSION The results in above paragraphs allow one to conclude that distribution of seismotectonic zones at the globe is related somehow to the geometry of the main geomagnetic field. On the definition, magnetic field may influence only electric currents or moving charge particles. Taking this into account, one may conclude that for generation of tectonic strains and deformations, which then results in earthquake activity, the electromagnetic forces may be responsible. It is suggested at present (e.g. Glatzmaier and Roberts, 1996; Aurnou et al., 1998) that the electromagnetic Lorenz force (Maxwell stresses) may be responsible for the rotation of the inner core with respect to the mantle (superrotation of the inner core). Also, (Karato, 1999) showed that the Maxwell-stress-induced flow at the boundary of the inner core results in a large strain that could cause the lattice preferred orientation of iron and, thus, the seismic anisotropy of the inner core that is observed in seismological observations. Khachikjan et all. (2007, 2008) presented very simple qualitative model for global distribution of magnetic part of Lorentz force, which may appear due to motion of charged particles in the geomagnetic field observed at the earth’s surface. They showed in particular that due to vertically moved charged particles, the horizontal components of Lorentz force appear, and these components change theirs direction along the line of D=0, where the main amount of earthquakes occurred (figure 5a). It is clear that much work remains to further verify this speculative assertion and to find the physical processes linking main geomagnetic field with seismotectonic structures.
ACKNOWLEDGMENTS I am very appreciated to Chairman of Organizing Committee Prof. Mithat Fırat ÖZER for financial grant which allowed me to participate in the International Earthquake Symposium Kocaeli 2009. Also, comments on the presentation by Prof. Şerif BARIŞ are appreciated very much.
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