Современный геодинамический режим межплитной границы Евразии и Северной Америки: индикаторные признаки по данным сейсмологии Арктики

The present paper is devoted to revealing earthquakes with the essential (NDC) component within the sector of the circumpolar area: from -7° till 120°E and from 72° till 90° N (Fig. 1). The research was based on the data taken from CMT catalog containing seismic moments and seismic moment tensors.

Fig. 1. Seismic zones and fault plain solutions of strong earthquakes occurred in the North Atlantics and Arctic for the period 1977-2011 (based on the CMT Catalog data)

u_n = M_pq * G_np,q ,                                               (1)

where u n ( n = 1, 2, 3) is the component of displacement; G np,q is the derivative of Green function, which gives displacement component number p caused by single forces along the q direction; * denotes a convolution. The seismic moment tensor is symmetrical and has 6 independent components characterizing deformation field at the source. S. Yunga (1979) assumed that a type of deformation type (shear, tension or compression) might be recognized by means of computation the Lode - Nadai index:

LN = 3 V 2 / ( V 1 – V 3 ),                                           (2)

where V ₁ ≥ V ₂ ≥ V ₃ are eigenvalues of the tensor M . The LN index allows estimate a type of deformation:

if -0.3 < LN < 0.3 the shear deformation dominates, if LN > 0.3 (or < -0.3) the compressive (tension) strain dominates.

We calculated LN values for all Arctic earthquakes listed into the CMT Catalog (Fig. 1). To measure a partial contribution of the NDC earthquakes to a seismic energy release in a different areas we used a relation of their total seismic moment to the seismic moment of all earthquakes occurred in the aimed area. The seismic moment is defined by M 0 = μSu , where μ is the shear modulus of rock (Pa), S is the area of the rupture along the fault (m²), and u is the average displacement on S (m). Physically, seismic moment is a work which was made by an earthquake during displacement of the rock blocks. One can define M 0 of an earthquake from the moment tensor.

Proceedings of the MSTU, Vol. 15, No. 2, 2012   pp.435-438

• 3.    Results

Distribution of recent strong earthquakes in the Northern Atlantics and Arctic MORBs is shown in Fig. 2. The share of the NDC earthquakes and their contribution to a seismic energy release is indicated in Table. As it is revealed, 25.5 % of the recent earthquakes within NW and Northern segment of the interplate boundary, divided the Eurasian and North American lithosphere plates, have essential value of the NDC component. The total contribution of the NDC earthquakes in a regional seismic moment is 9.4 % only. On the shelf plate margin the significance of the NDC earthquakes is lesser as to MORBs (20 % of total ability and only 1.6 % in energy release). In the Mon Ridge rifting area the NDC earthquakes contribute up to 48.6 % of energy release.

90°W              105°W    135°W  18°W   150°E      135°E

Fig. 2. Spatial distribution of earthquakes occurred in West and Central Arctic for 1977-2011 depending on their absolute values of the Lode-Nadai index (2)

Table. Part (%) of NDC earthquakes and their contribution to total seismic moment in seismic zones at the North West and Arctic margin of the Eurasian plate for period 1977-2011

Note: 1 – total numbers of recorded earthquakes; 2 – percent of NDC earthquakes (| LN | > 0.3); 3 – total seismic moment (10¹⁶ Nm); 4 – part (%) of the seismic moment corresponds to NDC earthquakes.

(252 events with the magnitudes mb from 3 to 5.5). The CMT catalog contains seismic moment tensors for 21 of those earthquakes. Their total seismic moment is equal to 299.3∙10¹⁶ Nm and 7 of the earthquakes have essential NDC component (0.3< | LN | < 0.62). Total seismic moment of NDC earthquakes from the storm is equal to 71.1∙10¹⁶ Nm, i.e. these 7 events generated 23.8 % of the storm seismic moment. Thus, this parameter is a reliable marker for distinguishing the volcanic area in other parts of the Gakkel Ridge, where the NDC earthquakes control about 10 % of seismic moment (Table). As for the Gakkel Ridge as a whole, one can see that 76 % of earthquakes were caused by shear dislocations under tangential stresses (Table). DC earthquakes generated more than 90 % of total seismic moment in the ridge. A bit more NDC earthquakes are observed in the Knipovich Ridge and SFZ (Fig. 2, Table).

The major contribution of volcanic processes and rifting to the local seismicity was revealed in the Mon Ridge: the NDC earthquakes generate here > 48 % of total seismic moment. As opposed to this zone the Knipovich Ridge together with crossing it SFZ has the minimal contribution of NDC earthquakes to the total seismic moment (5 % overall).

In contrast to the MORBs, the shelf plate margin has less essential NDC earthquakes. The major parts of earthquakes (4 of 5) and total seismic moment (more than 98 %) of the continental margin are controlled by shear dislocations under tangential stresses (Table). This fact corresponds to traditional view on seismicity.

The above estimation of spatial distribution of NDC earthquakes made for West and Central Arctic part of MOR has led to the following unexpected result. Instead of dominating earthquake sources provoked by the Earth crust spreading or explosion processes in volcanic apparatuses we have seen prevalence of shear strains along the full length of the Mon, Knipovich, and Gakkel ridges bounding northwest part of the Eurasian lithospheric plate.

Thus, in the present time (at least during 100 years of instrumental seismic monitoring in Arctic) major geodynamic process is not a divergent movement of the Eurasian and North American plates but a counterclockwise rotation of the Eurasian plate. According to this point of view the interplate border, made in the Arctic by the Gakkel and Knipovich ridges, has to be treated as a transform boundary.

5. Conclusion

Tectonic situation and earthquake genesis in the Knipovich and Gakkel Ridges, which form the NW and Northern segment of border between Eurasian and North American lithospheric plates, require more detailed investigation. The revealed pattern of the NDC earthquakes spatial distribution in these areas do not correspond to the geodynamic models which explain that a regional seismicity is controlled by spreading of ocean bottom in the North Atlantic and in Arctic, because the major contribution to the total seismic moment in the Arctic MORBs recently is linked with NDC earthquakes caused by shear dislocations.