The northeastern part of the Baltic Shield is characterized by a long and complex history of development, and the observed structure reflects the cumulative effect of multiple transformations (Zagorodnyi and Radchenko 1983; Petrov et al. 1986; Radchenko et al. 1992; Mitrofanov et al. 1995; Petrov 1999). A significant part of the study region is composed of Precambrian rocks (Radchenko et al. 1992; Mitrofanov et al. 1995). These rocks, whose age is 3.2–2.65 Ga (Kroner and Compston 1990; Bayanova et al. 2002), form the Murmansk, Kola, Belomorian, and Karelian Archaean megablocks, which all together make up a collage structure of the Earth’s crust in the region (Fig. 1). The Archaean Keivy structure embedded in the structure of the Kola megablock is distinguished among the secondary structures owing to the peculiarities of its structure (Mitrofanov et al. 1995). In addition, the following structural zones are significant: the Archaean greenstone belts, i.e., Kolmozero - Voronje, Yona, and Tersky- Allarechka, and the Early Proterozoic mobile belts, i.e., the Lapland granulate belt and Pechenga – Varguza rift belt, as well as the giant Paleozoic alkaline intrusions (Khibiny, Lovozero). The Early Proterozoic Pechenga – Varguza rift belt inherits the development region of the Archean Tersky - Allarechka belt. The Upper Archaean – Early Proterozoic period is characterized by linear tectonics and formation of belt structures, which were zones of repeated manifestation of mafic magmatism. All belt structures are divided by vast fields of infra - supracrustal, deep metamorphic sialic complexes. Magmatic formations including those of mafic – ultramafic composition play an important role in the development and formation of the crust in the region. The major part of the presently known mafic – ultramafic intrusions reveals spatial correlation with the zones of deep persistent fractures and rift genesis.

Fig. 1.

Geological structural map of the Kola region of the Baltic Shield (from Mitrofanov et al. 1995). Domains: Mur – Murmansk, Kol – Kola, Bel – Belomorian, Ter – Tersky, Ke – Keivy, In – Inary. Belts: Jon – Yona, K-V – Kolmozero-Voronja (Archean greenstone belts); LGB – Lapland, KGB – Kandalaksha-Kolvitsa (granulate belts); Pe – Pechenga, Im-V – Imandra-Varzuga, S-K – North-Karelian (Early Proterozoic rocks); (1) contours of Paleozoic intrusions (a-nepheline syenite, b-ultramafic alkaline rocks); (2) Upper Proterozoic sedimentary rocks. Early Proterozoic: (3) granite, granodiorite and diorite; (4) charnokite and granite (a), alkalinegranite (b); (5) volcanic-sedimentary rocks; (6) anorthosite and gabbro-anorthosite (in Keivy – Archean), gabbro, pyroxenite, peridotite. Early Proterozoic (or Archean?): (7) basic and intermediate granulite; (8) acid granulite. Late Archean: (9) granodiorite, diorite and enderbite; (10) alumina and super-alumina gneiss and schist; (11) acid gneiss; (12) fragments of greenstone belts (gneiss, amphibolites and komatiite); (13) fragments of banded iron formation (gneiss, amphibolites and ferruginous quartzite); (14) gneiss and schist; (15) gneiss and amphibolites; (16) granodiorite and diorite; (17) plagiogranite and granite-gneiss; (18) kyanite-garnet-biotite gneiss; (19) granite-gneiss, gneiss, migmatites and, rarely, amphibolites. (20) strike and dip; (21) subvertical faults and gentry dipping thrusts that separate the Proterozoic domains; (22) subvertical faults and thrusts. Encircled numbers indicate type-sections: 1 – Keivy, 2 – Kolmozero, 3 – Ura-Guba, 4 – Kaskama, 5 – Korva, 6 – Ar-Varench, 7 – Voche-Lambina, 8 – Iona, 9 – Kovdozero, 10 – Tersky

Regular seismological observations on the territory of the Murmansk region have been carried out since 1956 (Panasenko 1969). In general, the region belongs to areas of weak seismicity, but earthquakes occur relatively seldom, which can cause the surface damage, such as ruptures and landslides. The greatest number of seismic events both ancient and modern occurred in the Murmansk and Kandalaksha regions, where, along with weak earthquakes, there were also seven-point earthquakes. In the process of studying various types of deformations in the Quaternary sediments of the valley of the Pechenga River (the western part of the Murmansk block), traces of strong ancient earthquakes were noted, which were observed both in loose sediments and in crystalline rocks (Nikolaeva 2006). It also shows that in the range of 8.5–8.7 thousand years ago there was a manifestation of a strong seismic pulse with magnitude of 7.1 and above. Earlier it was noted that this area was already exposed to earthquakes about 8950 ± 150 years ago (Mitrofanov et al. 1995). In addition, seismic events occurring in the western part of the Murmansk block in 1772 and 1819 with a magnitude of 4.6 and 4.1 are described, respectively (Ahjos and Uski 1992). As appears from historical records, an earthquake was observed in the Kandalaksha area on May 14, 1626; the earthquake magnitude varied from 5.1 to 6.5 and the ground vibration magnitude was up to 7.5 (Stepanov 2001). In the modern age (the period of instrumental observations: February 2, 1960) a series of events with M > 4, which induced ground vibration with a magnitude higher than 5 was registered (Panasenko 1969).

Figure 2 shows the disposition of the seismogenic zones of the Kola Peninsula, identified by the authors (Baranov et al. 2011) for the period of 9-13 thousand years ago. The most reconstructed paleoseismic dislocations are in the selected zones. The presented scheme is based on the results of a long-term research conducted in the northeastern part of the Baltic Shield in order to carry out reconstructions of paleoseismic dislocations (Nikolaeva 2001, 2002, 2006). The zones have a linear view with intersection nodes, stretching from the north-west to the south-east (Murmansk and Kandalaksha), and from the south-west to the northeast (Tuloma and Khibiny–Kharlovka). Directly paleoseismic dislocations tend to appear in zones of long-living deep faults and in areas where crust breaks, shifts and subsidence of blocks are observed both in crystalline rocks and in sedimentary ones. The uplift of the region’s crust and intraplate vertical displacements of individual blocks with different speeds occurred during the deglaciation period (9–13 thousand years ago), and the resulting detente of interblock stresses caused earthquakes (Nikolaeva and Yevzerov 2018). Currently the uplift of the shield is still ongoing and as for tectonic regime, the role of vertical motions is waning. In this case, horizontal compressive stresses come to the first place, the detente of which also gives rise to seismic events.

Fig. 2.

Ancient seismogenic zones of the Kola Peninsula according to paleoreconstructions for a period of 9–13 thousand years ago (from Baranov et al. 2011). Numerals in circles indicate zones: 1 – Murmansk; 2 – Kandalaksha; 3 – Khibiny-Kharlovka; 4 – Tuloma

Figure 3 shows the location of seismogenic zones based on the seismic monitoring data over the period of 1956–1993 (Vinogradov et al. 2016). The scheme (Fig. 3) includes 6 zones distinguished by the results of seismic monitoring (1 – Murmansk ; 2 – Kandalaksha; 3 – Khibiny–Lovozero; 4 – Kuusamo–Poria Guba; 5 – Belomorie; 6 – Botni–Finnmark). Epicenters of earthquakes for that period were predominantly recorded within these zones. It should be noted that in the first ten years of instrumental measurements, increased seismicity was recorded in the Murmansk and Kondalaksha zones, but after 1966 an increase in seismicity was observed in the Khibiny-Lavozero, Kuusamo- Poria Guba and Botni-Finnmark zones. If we consider the time interval from the beginning of the region’s seismic monitoring, since 1956, the Kandalaksha zone is the most seismically active region of the Kola Peninsula. Until 1969 only the Khibiny massif area was reliably distinguished by seismic activity within the limits of the Khibiny-Lovozero zone which has become a belt of increased seismic activity only for the period 1970–1990.

Fig. 3.

Seismogenic zones according to the regional monitoring network data in 1956–1995 in the Kola Peninsula (from Vinogradov et al. 2016). Numerals in circles indicate zones: 1 – Murmansk ; 2 – Kandalaksha; 3 – Khibiny–Lovozero; 4 – Kuusamo–Poria Guba; 5 – Belomorie; 6 – Botni–Finnmark

The decrease in the earthquake frequency was recorded in the range of 1993–2013, but the shape of seismogenic zones also changed (Vinogradov et al. 2016). It should be noted that during this period the reconstruction of the network of seismic observations began: the analog seismic stations were replaced with the digital ones, and additional stations were installed. New opportunities for the separation of natural and induced seismic events have appeared. Therefore, the number of reliably established natural earthquakes has decreased. Figure 4 shows the seismogenic zones disposition for the specified period. It is noted that the seismicity in the Murmansk zone has noticeably decreased, but in the zones Kandalaksha and Khibino-Lavozero remained at the same level (Vinogradov et al. 2016). At the same time, the Kuusamo- Poria Guba zone disappeared, but the Kandalaksha–Varenga zone appeared. Thus, the spatial evolution of the selected zones is observed, within which the largest number of seismic events were recorded for one or another period of time. In addition, the authors of the work (Vinogradov et al. 2016) concluded that, in general, in the region there is an increase in seismicity.

Fig. 4.

Seismic zones location according to the regional seismic monitoring data in 1993-2013 in the Kola Peninsula (from Vinogradov et al. 2016). Numerals in circles indicate zones: 1 – Murmansk; 2 – Kandalaksha; 3 – Khibiny–Lovozero; 4 – Kandalaksha–Varenga

The results of seismic monitoring show that the activity of seismogenic zones is not constant and there is no sinchronicity in its manifestation in different zones (Fig. 3, 4). An increase in seismicity can be observed in some areas, while in others, attenuation. In addition, over time, new seismogenic zones may appear, exciting territories where no seismic activity had previously been observed. The observed variations of the seismic activity of the region allow us to distinguish two periodicities in its spatial structure: 2-3-year-old and 8-9-year-old, which overlap each other (Vinogradov et al. 2016). Therefore, it can be assumed that in 2019-2020 there will be an increase in the number of recorded natural earthquakes in the Murmansk region.

In addition to tectonic earthquakes, induced seismic events occur due to the disturbance of the natural equilibrium in massifs, which occurs because of the active development of the region’s mining and industrial complex on the Kola Peninsula. A tectonic rockburst occurs directly in the mine working, in which the magnitude level is higher than that of an induced earthquake, the center of the latter is located in the massif outside the working area. Because of this, tectonic rockbursts cause much greater damage than induced earthquakes. The largest induced earthquake in the region with a magnitude of M = 4.0 was recorded in 1999, which caused the destruction of the underground workings of the Umbozero mine (the Lovozero massif) over an area of 650 km² with the development of a crack to a surface 0.5 km long (Lovchikov 2013).

Previously, seismic events were never recorded in the Lovozero massif, but in the 90s earthquakes with a magnitude of M > 1 began to occur and their number increased annually. Moreover, a relationship is established between the emergence and increase of induced seismic activity of the Lovozero massif with the commissioning and increase in production at the Umbozero mine (Lovchikov 2013). Due to the change in the stress state of massif, a series of rockburst with a magnitude of M = 2.2-2.6 followed. Further continuation of the mining operations led to the relaxation of the potential energy in the massif, which caused a tectonic rockburst with a magnitude of M = 5.1 (local scale of the Richter magnitude) in 1999 (Lovchikov 2013). When exploitation of the mine was stopped the number of seismic phenomena within the massif has decreased markedly in recent years. In addition to the Umbozero mine, strong seismic events (rockburst and induced earthquakes) also occurred at the Kirovsk mine in the Khibiny, but the energy level of these events was lower.

In the Late Archaean, consolidation of the Earth’s crust transformed the region into a relatively stable continental structure, and by that time, the study region was in a stable state (Bel’kov 1987). Hence, we can admit that the region could have been subjected to overall uniform compression owing to remote forcing. In the Early Proterozoic, the main front of tangential stresses was directed to the northeast (Zagorodnyi and Radchenko 1983; Radchenko et al. 1992). The Murmansk megablock was in a stable position. It is not improbable that it was subjected to stress directed from northeast to southwest. Hence, we admit that, in the Early Proterozoic, the region was subjected to monoaxial compression by uniformly distributed forces from the southwest and northeast.

We assume that the northeastern part of the Baltic Shield over the entire period of the geological history of the region represented an inhomogeneous elastic body subjected to the impact of volume forces and specific stresses at its boundaries (Filatova 2014). We also admit that (a) the tectonic magmatic activity established in the Early Precambrian was of the intraplate type; (b) the configuration of contact boundaries between the Archaean megablocks did not change strongly over the entire geological history. The region considered here consists of a few finite subregions. Each of them is considered uniformly isotropic and linearly elastic with linear elastic constants (Poisson coefficient μ and Young modulus E). Each Archaean megablock is a separate subregion. The zones of deep fractures dividing the Archaean megablocks are considered as subregions with a width of 15–30 km. We specify the condition that the region is in the equilibrium state and the components of the stress tensor σ_ij in the case of the plane problem satisfy the equilibrium conditions (Muskhelishvili 1966):

.

where β_x and β_y are volume forces. We used the method of boundary elements to numerically solve this boundary problem with respect to stresses. The numerical solution is constructed using previously obtained analytical solutions for simple singular problems so as to satisfy the specified boundary conditions at each element of the contour (Crouch and Starfield 1983). We consider the upper horizontal surface of the solid medium model.

Three time stages of the Kola region development were considered in the course of solving the formulated problem (3.0–2.8, 2.8–2.5, 2.5–1.6 Ga), and correspondingly, a certain basic model was specified at each of the stages that describes the study region with account for the geological structures formed by the corresponding time (Filatova 2014) (Fig. 5). With this in mind, we defined a rectangular contour for simulations that envelope the study region including the Murmansk, Kola, and Belomorian megablocks and the marginal region of the Karelian megablock in the contact zone with the Belomorian megablock. The area of the specified contour significantly exceeds the study region in order to exclude the influence of the contour boundaries in the simulations. Stress T was specified along the entire boundary in the numerical experiment. Since we do not have reliable data on the absolute value of forces in the region, we assume that their intensity T is equal to unity and obtain stresses in the simulations in the units of T. Stresses σ_xx , σ_yy, τ_xy were estimated for each basic model, which alloywed us to calculate the main stresses σ–_xx, σ–_yy, τ–_xy and the maximum absolute values of shear stresses (Muskhelishvili 1966):

.

The main stresses were calculated using the following relations (Muskhelishvili 1966):

where α is the angle between the axis of the main stress with the OX-axis, tg2α = 2τ_xy /(σ_xy– σ_xy).

Finally, the values of stresses |τ_xy|_max were normalized and presented as percentage of the maximum value over the region. Thus, after normalizing, the domains with anomalous shear stresses were considered as weak zones in the basement of the region. All the works were carried out using a scale of 1 : 1 000 000 and the initial geological chart of the region with a scale of 1:500000 (Mitrofanov 1996) . Simulations for several versions of the load applied to the region were performed. In the case of overall uniform compression of the region (Fig. 5a, 5b) and mono-axial compression along the southwest–northeast line (Fig. 5c), the structural peculiarities of the region caused by the development of the permeable zones of the Earth’s crust were pronounced most clearly.

The values of linear elastic constants (μ, E) for the rocks of the Archaean megablocks, greenstone belts, Keivy structure, and fracture zones were specified according to the data presented in (Filatova 2009). The Poisson coefficient in the simulations for the Archaean megablocks (Karelian, Murmansk, Kola, and Belomorian) and the Keivy structure was assumed equal to μ = 0.25, while in the weak zones it was μ = 0.3. The zones of deep fractures dividing the Archaean megablocks were considered weak. The Young modulus in each of the subregions was determined as the weighted mean value: in the Karelian, Murmansk, Kola, and Belomorian megablocks, it was taken equal to E = 6.2 · 10⁴ MPa, and in the Keivy structure it was E = 5.8 · 10⁴ MPa. In the weak (fracture) zones, the Young modulus was taken as one order of magnitude smaller. The regions of the Kolmozero-Voronja and Tersky – Allarechka greenstone belts, respectively, and the Pechenga – Varzuga rift belt, may be considered as weak zones because they represent mobile permeable structures.

Fig. 5.

Model block regions: approximation schemes of the Earth’s crust block structure in the northeastern part of the Baltic Shield for the period of 3.0–1.6 Ga (from Filatova 2014). (a) 3.0–2.8 Ga; (b) 2.8–2.5 Ga; (c) 2.5–1.6 Ga; (1) contours of the modern coastline; (2) contours of the study region; T is force

The basic model includes structural elements of the Archaean basement formed by the moment of termination of the Earth’s crust accretion in the region. The following structures are considered as subregions: the Murmansk, Kola, Belomorian, and Karelian megablocks, the Keivy structure, and also the fracture zones dividing the megablocks (Fig. 5a). The region is subject to overall uniform compression. The boundary conditions at the contour enveloping the study region were specified conventionally: bⁱ_s = (σⁱ_s)₀ and bⁱ_n = (σⁱ_n)₀. We assume that the region at its boundary is subject to equal normal stress (σⁱ_n)₀ = T, and the tangential stress is (σⁱ_s)₀ = 0. At the contact surface between subregions r₁ and r₂, at each of its points q, we specified the conditions of continuous force

.

The initial basic model is supplemented with the subregions marked by anomalous values of shear stresses at the first stage of investigation; they overlap the development territories of the Kolmozero-Voronja and Tersky-Allarechka greenstone belts (Fig. 5b). The region is also subject to overall uniform compression. The boundary conditions at the contour of the study region were specified in a manner similar to the first basic model (Fig. 5a). The conditions of continuous force were also specified at the contact surface between the subregions.

The configuration of the subregions in the initial basic model to a great extent resembles the second model (Fig. 5c). The region is subject to mono-axial compression by uniformly distributed force T from the southwest and northeast. The directions of compression were selected according to the direction of the gaping fault of the Pechenga-Varzuga rift system and with the direction of the general front of tectonic stresses in the region. If we take the condition that the Y-axis in the local coordinate system coincides with the direction of compression, we get

.

Let us make a transition to the unique coordinate system for all models and perform rotation of the coordinate axes by angle (Muskhelishvili 1966):

We eventually get the boundary conditions along the contour of the region in the following form (Muskhelishvili 1966):

The conditions of continuous forces were maintained at the contact surface between the subregions.