J Seismol https://doi.org/10.1007/s10950-018-9762-9

ORIGINAL ARTICLE

New data on earthquake focal mechanisms in the Laptev Sea region of the Arctic-Asian seismic belt Alena I. Seredkina

&

Valentina I. Melnikova

Received: 28 November 2017 / Accepted: 14 May 2018 # Springer Science+Business Media B.V., part of Springer Nature 2018

Abstract We consider 16 earthquakes with Mw = 4.2– 5.2 that occurred in the south-eastern part of the Laptev Sea shelf, Lena River Delta, and North Verkhoyanye (Russia) in 1990–2014. Focal mechanisms, scalar seismic moments, moment magnitudes, and hypocentral depths of the seismic events have been calculated from the data on amplitude spectra of surface waves and P wave first-motion polarities. The obtained results sufficiently implement the existing dataset on reliable earthquake source parameters for the study region and prove the change of the stress-strain state of the crust from extension on the Laptev Sea shelf to compression on the continent providing finer spatial details of the deformation field in the transition zones such as Buor-Khaya Bay and the Lena River Delta. Keywords Earthquake . Focal mechanism . Surface waves . Amplitude spectra . Laptev Sea

1 Introduction The Laptev Sea region (70–75° N, 120–135° E) of the Arctic-Asian seismic belt is located in the transition area from the ocean to the continent and includes the southeastern fragment of the Laptev Sea shelf, Lena River Delta, and North Verkhoyanye (Figs. 1 and 2). On the A. I. Seredkina (*) : V. I. Melnikova Institute of the Earth’s Crust, Siberian Branch of Russian Academy of Sciences, Lermontov St., 128, Irkutsk, Russia 664033 e-mail: [email protected]

continental margin of the Laptev Sea, the narrow spreading zone of the Gakkel Ridge crossing the Eurasian Basin widens and forms a system of rifts observed in various geophysical fields (Andersen et al. 2010; Dodin and Surkov 2002; Kenyon et al. 2008; Verhoef et al., 1996). The configuration of the divergent North American-Eurasian plate boundary is accepted to develop here (Drachev and Shkarubo 2017; Drachev et al. 1998; Engen et al. 2003; Gaina et al. 2002; Grachev 2003; Grachev et al. 1973; Imaev et al. 2000; Parfenov and Kuz’min 2001; Zonenshain and Natapov, 1987; Zonenshain et al. 1990). In turn, plate boundaries are marked by high earthquake epicenter density and their inhomogeneous structure reveals in fragmentary character of seismicity (Imaev et al. 2000; Kanao et al. 2015; Sloan et al. 2011). Seismicity of the Laptev Sea region (Fig. 2) mostly confined to the crust (Avetisov 1993) reflects the structure of the present-day stress-strain state of the lithosphere and plays a key role in studying of different aspects (structural, tectonic, geodynamical etc.) of its development (Avetisov 1975, 1999; Fujita et al. 2009; Imaev et al. 2000; Jemsek et al. 1986). Some regional earthquakes are related to the Laptev Sea rift system consisted of a set of grabens and horsts separating them (Imaeva et al. 2017). The most seismoactive areas are the south-western coast of the Laptev Sea (Olenek Bay region) and the north-western part of the Verkhoyansk Range where some felt seismic events (M ≥ 4.5) have been localized. The largest of them are the Bulun earthquakes (М = 5.8–6.8) that occurred in 1927–1928 (Imaev et al. 1998). In total, earthquake epicenters trace

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Fig. 1 Seismotectonics of the Laptev Sea and adjacent areas after (Imaeva et al. 2016; Parfenov and Kuz’min 2001) with modifications. The location of the study region is shown as a pink square in the inset. The lithospheric plate boundaries are marked after (Bird 2003)

fragmentarily the sublatitudinal structural-tectonic elements of the Olenek sector and the sublongitudinal strike of the complex deformation zone in the Lena River Delta (Imaeva et al. 2017). Though seismological data obtained at different stages of instrumental observations vary significantly in their reliability, the geodynamical pattern of the study region is widely discussed. For example, the existence of the incipient mantle plume (Grachev 2003) or the Laptev Sea microplate (Avetisov 1999; Imaeva et al. 2017) is proposed to explain the present-day extension of the continental lithosphere on the Laptev Sea shelf (Imaeva et al. 2016). A lot of papers are concerned with the kinematics of movements between the North American and Eurasian plates and the location of their pole of

rotation (Argus and Heflin 1995; Cook et al. 1986; DeMets et al. 1990, 2010; Franke et al. 2000; Merkourier and DeMets 2008; Minster and Jordan, 1978; Pitman and Talwani 1972). Special attention is paid to regional seismotectonics, deformation mechanisms and development of geostructures under different geodynamical conditions (Fujita et al. 2009; Imaev et al. 2000; Imaeva et al. 2017; Kanao et al. 2015; Parfenov and Kuz’min 2001). The latest results of the complex studies of active structures of the Arctic-Asian and Okhotsk-Chukotka seismic belts are published in Imaeva et al. (2017). In spite of significant advantage in the interpretation of the geodynamical pattern of the Laptev Sea region, its geology and geophysical characteristics are not well-

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study area and may facilitate the development of the well-grounded geodynamical models.

2 Data and methods

Fig. 2 Epicenters of the earthquakes with M ≥ 4.0 that occurred in the study region in 1927–2017 according to the International Seismological Centre (2017). Red circles mark the locations of earthquakes considered in this study. Topography and bathymetry are based on the ETOPO1 database (Amante and Eakins 2009)

studied. In particular, it is concerned with earthquake focal mechanisms which must be determined as sufficiently reliable for further geodynamical analysis. Due to the sparse and irregular distribution of seismic stations, focal mechanisms based only on first-motion P wave polarities are poorly constrained (Fujita et al. 2009). Moreover, source parameters obtained by different approaches are inconsistent with each other in many cases (Avetisov 1991, 1993; Franke et al. 2000; Fujita et al. 2009). Calculations of earthquake seismic moment tensor (SMT) can reduce the aforementioned ambiguity. But nowadays, there are few SMT assessments as the seismological agencies such as the Global CMT Project (Global CMT Web Page 2017) provide information only about relatively large seismic events (Mw > 5.0) which are rare in the study region (Fig. 2). In this study, we calculate SMTs (focal mechanisms, scalar seismic moments, moment magnitudes, and hypocentral depths) for 16 intermediate earthquakes that occurred in 1990–2014 on the basis of surface wave data. The obtained results sufficiently implement the existing SMT dataset and allow us to trace the changes in the seismotectonic deformation pattern within the

In order to solve the tasks formulated above, we used the information on 16 earthquakes with mb = 4.3–5.5 (International Seismological Centre 2017), which occurred in the study region from 1990 to 2014 (Fig. 2, Table 1). The data used for determination of source parameters were the surface wave records of the earthquakes at the IRIS, GEOFON, GEOSCOPE, and GLISN broadband digital stations (Incorporated Research Institutions for Seismology, 2018). The records with low signal/noise ratio and polarization anomalies of surface waves were rejected from the inversion. In total, we used the records from 49 seismic stations (Fig. 3). The positions of the stations for each seismic event were selected in such a way that they are located in different azimuths from the epicenter (Fig. 4). To extract fundamental modes of Love and Rayleigh waves, we applied a frequency-time analysis (FTAN) procedure (Levshin et al. 1972, 1989). First of all, for each earthquake, a bandpass filter in the period range 20–100 s was applied. Then, we selected a period range in which surface waves were clearly seen (Table 1) and in the determined period range applied multiple narrowband Gaussian filters to isolate the fundamental mode of surface waves from overtones, body waves, and other noise. Surface wave amplitude spectra were calculated in the period range from 30 to 80 s for the entire set of the events. Examples of the initial and filtered records normalized with respect to the maximum observed amplitude are shown in Fig. 4. Figure 5 illustrates the method of the signal analysis for the particular case of the September 15, 1996 earthquake recorded at the AlaArcha station (AAK, Kyrgyzstan). The FTAN diagrams are shown together with our estimates of the corresponding group velocity dispersion curves of Rayleigh and Love waves indicated by thick pink lines. The group velocity dispersion curves were used to design a timedependent filter to extract the fundamental mode with minimum noise contamination. To estimate source parameters of the earthquakes, we used the inversion of surface wave amplitude spectra, based on the method described in Bukchin (1990) and Lasserre et al. (2001). The reliability of this method in source mechanism determination of large Mw ≥ 5.2

J Seismol Table 1 Parameters of the study earthquakes (1990–2014) Date*, year month day

t0*, h min s Epicenter* φ°, Е

h*, km

mb* Epicentral distance range, degrees

Period range, s

λ°, N

Number of stations

1990.03.13

00:32:59

134.90 73.33 15.0

5.5

24.09–64.05

30–60

8

1991.03.01

01:57:05

126.85 72.15 39.4

5.2

23.19–47.11

30–60

7

1993.03.24

22:43:29

130.40 71.69 20.3

4.8

29.98–55.98

30–60

8

1995.01.31

12:43:43

132.28 72.71 33.0

4.3

22.73–47.47

30–60

10

1996.09.15

00:21:23

126.38 72.36 10.0

4.5

23.02–50.21

30–60

14

1998.08.23

09:59:03

129.73 72.77 10.0

4.5

23.32–48.15

30–60

10

2001.06.08

04:59:02

123.92 72.70 34.0

4.7

22.86–51.36

30–55

10

2003.12.07

09:16:12

134.82 74.10 10.3

5.1

23.31–55.65

30–80

15

2005.08.15

21:24:32

134.03 74.60 10.0

4.4

26.51–46.08

30–60

8

2007.07.19

06:18:44

125.80 72.24

8.3

4.4

22.82–52.10

30–60

11

2009.10.07

00:29:54

134.28 73.37 10.0

4.5

22.84–53.10

30–60

14

2010.07.12

10:06:43

123.79 72.98 13.3

4.9

23.60–54.08

30–80

15

2010.11.23

00:57:09

133.61 74.76 19.8

4.9

26.24–52.19

30–60

8

2011.05.20

04:51:45

121.35 72.86 11.4

4.8

22.60–49.25

30–60

13

2013.04.11

20:01:09

129.62 74.11 10.0

4.5

24.98–52.07

30–55

13

2014.12.12

07:42:57

130.46 74.21 10.0

4.4

25.36–40.31

30–60

9

*Parameters are taken from the ISC-catalog (International Seismological Centre 2017)

(Bukchin et al. 1994; Gomez et al. 1997a, b; Lasserre et al. 2001) and medium Mw ≥ 4.3 seismic events that occurred in regions with different tectonic settings, including the Baikal rift (Seredkina and Melnikova 2014), Transbaikalia (Melnikova et al. 2017), Siberian platform (Seredkina et al. 2015), and the Taimyr Peninsula (Seredkina and Kozmin 2017), has already been demonstrated. Assumptions of the method (instantaneous pure double-couple point seismic source with known origin time and epicentral location (Bukchin 1990) and

Fig. 3 Locations of the used seismic stations

medium with weak lateral inhomogeneity (Babich et al. 1976; Woodhouse 1974)) allow us to completely define the seismic source by its double-couple depth, seismic moment M0, and focal mechanism parameters (strike, dip, and slip angles of the nodal planes or directions of principal compression and tension axes). The focal mechanisms and hypocentral depths are determined by a systematic exploration of the 4D parametric space, and M0 is estimated by minimizing the residual between the observed and calculated surface wave amplitude spectra for each combination of all the other parameters (Lasserre et al. 2001). The moment magnitude Mw was calculated from the seismic moment according to Hanks and Kanamori (1979). Using only surface wave amplitude spectra does not provide a unique focal mechanism solution: four equivalent solutions exist due to the sign ambiguity of moment tensor components and to the symmetry of the radiation patterns of surface wave amplitude spectra with respect to the epicenter (Mendiguren 1977). To constrain the uniqueness of the solution (Lasserre et al. 2001), we additionally used P wave first-motion polarities from ISC (International Seismological Centre 2017) and Obninsk bulletins (Seimological Bulletin of the Geophysical Survey of RAS, 2017).

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Fig. 4 Azimuthal distributions of the seismic stations relative to the earthquake epicenters with examples of the filtration (initial normalized velocity waveforms are marked with blue lines,

filtered—with green and red lines for Love and Rayleigh waves, respectively). Solid lines mark the stations where Rayleigh waves are filtered, dashed lines—Love waves

For every study seismic event, the result of the joint inversion coincided with one of the equivalent solutions calculated only from the amplitude spectra. We estimated the quality of the results obtained by the normalized joint residual function (ε) characterizing the deviation of the synthetic surface wave amplitude spectra from the observed one and the ratio between the P wave first-motion polarities which are inconsistent with the calculated radiation pattern and the total number of the polarities. To estimate the resolution of the parameters, we also calculated a partial residual function fixing one of the sought parameters (depth, for instance—εh) (Lasserre et al. 2001).

The crustal structure beneath the seismic stations was specified by the 3SMAC model (Nataf and Ricard 1996). We approximated the crust in the vicinity of the source by the 3SMAC and CRUST 2.0 models (Bassin et al. 2000) whichever provided the lower values of the normalized joint residual function. It is necessary to note that the choice of the crust model does not significantly affect the calculated parameters that have been previously shown in the regions with available detailed information about the crust structure (Melnikova et al. 2013; Seredkina and Kozmin 2017). To describe the upper mantle structure and to calculate the attenuation of surface waves, we used the PREM model (Dziewonski and Anderson 1981).

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Fig. 5 An example of the FTAN procedure for the vertical (LHZ) and transversal (LHT) components of the AAK station (Δ = 38.59°, azm = 348°) for the September 15, 1996 mb = 4.5 earthquake. a FTAN diagrams together with group velocity curves (pink lines) before and after filtration. The background colors

show the relative amplitude of the signal with the corresponding periods and group velocities: the minimum and maximum amplitudes are marked with red and blue colors, respectively. b Raw and filtered velocity waveforms. The color legend is the same as in Fig. 4. c Raw and filtered amplitude spectra

The inversion of earthquake focal mechanisms is widely applied to estimate average characteristics of the stress-strain field of the crust (Angelier, 2002; Angelier and Mechler 1977; Carey-Gailhardis and Mercier 1987; Delvaux 1993, 2012; Delvaux and Sperner 2003; Gephart and Forsyth, 1984; Maury et al. 2013; Zoback 1992). In all the schemes, it is assumed that the stress-strain field is homogeneous in space and time, and that slip and tangential strain vectors are in the same direction. In this study, the program Win-Tensor (improved version 5.8.6 of 23/11/2016) (Delvaux and Sperner 2003) was used to calculate the stress regimes of the crust in the Laptev Sea region. For the inversion, we implemented the right dihedra method (Angelier 1984) and a dynamic rotational optimization procedure (Delvaux and Sperner 2003). This software allows us to reconstruct four parameters of the deviatoric stress tensor: orientation of three principal stress axes (maximum compression σ1, intermediate σ2, minimum compression σ3) and the ratio of principal stress differences R = (σ2–σ3)/(σ1–σ3) defining the shape of the stress

ellipsoid. The type of the stress regime can be expressed numerically using a stress regime index R′ (Delvaux et al. 1997) varying from 0.0 to 3.0 which expresses as follows: R′ = R when σ1 is subvertical (R′ = 0–1, extensional stress regime), R′ = 2 − R when σ2 is vertical (R ′ = 1 − 2, strike-slip stress regime), R′ = 2 + R when σ3 is subvertical (R′ = 2 − 3, compressional stress regime). To represent the stress tensor calculation results, it is preferred to use the average horizontal principal stress (S Hmax ) azimuth and the stress regime index R′ (Delvaux and Sperner 2003; Heidbach et al. 2010; Zoback 1992). These parameters completely describe the orientation of the stress axes in terms of the maximum horizontal principal compressional stress—SHmax, and the minimum horizontal principal compressional stress—Shmin, assuming SV to be vertical. To estimate the quality of the obtained results, the stress tensors were assigned a quality rank varying from A (highest) to E (lowest). A-quality indicates that the SH orientation is accurate to within ± 15°, B-quality to within ± 20°, C-quality to within ± 25°, and D-quality to within ± 40°, and E-quality marks insufficient or

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widely scattered stress information (Heidbach et al. 2010). The tensors assigned to A-, B-, or C-quality are typically considered reliable for use in stress analysis and the interpretation of geodynamic processes (Heidbach et al. 2010; Zoback 1992).

Centre 2017) demonstrate complex and ambiguous distribution (Fig. 6a), we have averaged them in the 10° solid angle sectors according to the procedure described in Lasserre et al. (2001). It should be noted that in the sector between 270 and 310°, a number of compressions and dilatations are approximately equal and the predominant polarity cannot be attributed to the sector. The smoothed polarities agree with the radiation pattern corresponding to the calculated focal mechanism (Fig. 6b).The partial residual function has a rather sharp minimum at the depth of 3 km (Fig. 6c). Comparison of synthetic and observed intermediate-period spectra for Rayleigh and Love waves (Fig. 6d) demonstrates good data fitting.

3 Results and discussion The obtained results are presented in Table 2. An example of the inversion for the May 20, 2011 earthquake is presented in Fig. 6. As the original P wave first-motion polarities from ISC bulletin (International Seismological Table 2 Focal parameters of the study earthquakes Date, year month day

M0·1016, N m

Mw

h, km

ε

Nodal planes Strike°

Dip°

Slip°

1990.03.13

6.70

5.2

11–12

190 311

70 36

− 60 − 144

0.291

1991.03.01

3.50

5.0

21

2 165

23 68

106 83

0.263

1993.03.24

0.60

4.5

9–10

329 201

41 62

46 121

0.217

1995.01.31

0.84

4.6

37

135 232

79 57

− 34 − 167

0.264

1996.09.15

2.70

4.9

4–5

170 299

12 82

140 81

0.301

1998.08.23

0.24

4.2

33–35

302 200

29 83

14 118

0.260

2001.06.08

0.76

4.5

10–12

205 101

84 24

67 165

0.260

2003.12.07

2.20

4.9

6

30 157

28 72

− 40 − 112

0.232

2005.08.15

0.64

4.5

7

205 334

15 80

− 40 − 102

0.237

2007.07.19

0.44

4.4

37–38

79 344

45 85

7 135

0.231

2009.10.07

1.10

4.6

11

143 25

35 72

− 147 − 60

0.248

2010.07.12

2.40

4.9

6–7

325 123

9 82

112 87

0.278

2010.11.23

0.41

4.4

3

107 288

45 45

− 91 − 89

0.242

2011.05.20

1.30

4.7

3

295 105

15 75

− 80 − 93

0.218

2013.04.11

0.33

4.3

36

20 178

25 67

− 70 − 99

0.243

2014.12.12

0.78

4.6

33

177 351

62 28

− 87 − 96

0.286

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(c)

N

N

NP1: strike=295° dip=15° 90° slip=-80° 270°

270°

P 90° NP2: strike=105° dip=75° slip=-93°

T 180°

0.30

Partial residual function, εh

(a)

0.28

3km k 3m 0.26 0.24

0.22

180° 0

Normalized amplitude

ABKT

1

Normalized amplitude Normalized amplitude

40

45

50

55

0 30

60

KBS

1

35

40

45

50

55

40

45

50

55

SUMG

1

35

40

45

50

55

40

45

50

55

60

50

55

60

35

40

55

60

50

55

60

50

55

60

50

55

60

45 50 Period (s)

55

60

45 WMQ

1

0

0 35

50

45 PET

30

60

ULN

1

0

40

0 30

60

35

1

0 35

30

60

MAKZ

1

0

30

Normalized amplitude

35

ILULI

1

0

30

30

35

40

45

50

55

30

60

35

40

45

XAN

1

0 30

Normalized amplitude

Depth (km )

FFC

1

0 30

35

40

45 AAK

1

Love waves ABKT

1

35

40

45

50

55

0

MDJ

1

30

60

35

40

45

50

55

35

40

50

55

60

45 50 Period (s)

55

60

45

40

45 SUMG

0 30

35

40

TLY

1

35

1

0 30

30

60

PET

1

0

ILULI

1

0

0 30

Normalized amplitude

10 15 20 25 30 35 40

Rayleigh waves

(d)

Normalized amplitude

5

45 50 Period (s)

55

60

30

35

40

0 30

35

40

Fig. 6 An example of the inversion procedure and results. a Original P wave first-motion polarities from ISC bulletin (International Seismological Centre 2017). b Focal mechanism solution with the smoothed polarities. c Partial residual function

as a function of depth. d Normalized synthetic (gray lines) and observed (black lines) intermediate-period spectra for Rayleigh and Love waves

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It should be noted that focal mechanisms were firstly calculated for 12 earthquakes. For other 4 seismic events, we compared the obtained focal parameters with all the available data including routine determinations from the GCMT catalog (Global CMT Web Page 2017) and the results of special studies (Franke et al. 2000; Fujita et al. 2009). To estimate the difference between the solutions quantitatively, we calculated the Kagan rotation angle (Ф) representing a 3D rotation angle by which a double-couple earthquake source can be rotated into another one (Kagan 1991, 2007). The minimum value for Ф is zero, when both solutions are identical. We used the focal mechanisms obtained in this study as reference double-couple solutions in the calculations. In the case of the December 7, 2003 earthquake, the Ф value is 8° and the GCMT and our focal mechanism estimates are almost identical (Fig. 7). For the March 13, 1990 and March 1, 1991 earthquakes, the focal mechanisms obtained in this study and the solutions from the GCMT catalog (Global CMT Web Page 2017) and Fujita et al. (2009), respectively, agree rather well with each other (Ф = 37° and 31°), while the comparison of our determinations of focal parameters and the results by Franke et al. (2000) for the same earthquakes and the March 24, 1993 seismic event demonstrate wide scattering in fault plane solutions (Ф > 55°) and a difference in movement kinematic in the sources (Fig. 7). In the case of the March 1, 1991 earthquake for which both the solutions by Franke et al. (2000) and Fujita et al. (2009) are assumed to be well-constrained by first-motion polarities, the observed discrepancies can be caused by changes in faulting character corresponding to different stages of rupture development. The mechanism determined in this study appears to be the most reliable being the result of joint inversion of intermediate-period (surface wave amplitude spectra) and short-period (firstmotion polarities) data. In the case of the March 24, 1993 earthquake, the focal mechanism by Franke et al. (2000) is poorly resolved due to lack of the initial data that can be the reason of the obtained inconsistency of the solutions. As far as the hypocentral depths are concerned, the majority of the studied seismic events are observed in the depth range 3–12 km, i.e., in the upper crust, and the March 1, 1991 earthquake has a hypocentral depth of 21 km (Table 2). An interesting feature of the obtained focal depth distribution is the occurrence of 5 earthquakes in the uppermost mantle (33–38 km), assuming that the crustal thickness varies from 22 km under the

Laptev Sea shelf (Piskarev et al. 2003) according to gravimetric data to 26–30 km under Buor-Khaya Bay and Lena River Delta (seismological data) (Avetisov and Guseva 1991; Kogan 1974). Our results of the hypocentral depth determinations confirm the detailed field studies of weak seismicity (Avetisov 1991; Kovachev et al. 1994) indicated the occurrence of seismic events up to the depth of 82 km. Thus, the tectonic processes in this area are not confined to the crust but also take place in the uppermost mantle. Previously, Doser and Yarwood (1994) noted an important role of deep crustal or even mantle earthquakes observed along continental rift margins in rifting process evolution. To trace changes in the seismotectonic deformation pattern within the study area in more details, we also selected the SMT solutions of earthquakes occurred before 1990 (Table 3) (Global CMT Web Page 2017). Epicenters of the April 23, 1977 and January 1, 1988 earthquakes are located at the Laptev Sea shelf and the February 1, 1980 event is localized in Olenek Bay (Fig. 7). All the solutions were calculated from the data on body waves (Dziewonski et al., 1981) assuming the fixed centroid depths and demonstrate normal fault movements in their sources. For the earthquakes that occurred within the study area before 1990, we do not consider focal mechanisms estimated using the firstmotion polarities (Avetisov 1975, 1991, 1993; Franke et al. 2000; Fujita et al. 2009) as the solutions obtained for the same earthquake by different authors can vary significantly or be poorly constrained in some cases that are illustrated in Fig. 7 and discussed in previous studies (Avetisov 1993; Franke et al. 2000; Fujita et al. 2009). Totally, for the further analysis, we add only three extra solutions to our dataset of the source parameters (Fig. 8). Geological-structural observations and data on earthquake focal mechanisms show that the geodynamical pattern of the Laptev Sea region is characterized by a change of regimes of seismotectonic deformations of the crust. It is assumed that the rifting under which the structures of the eastern Laptev Sea shelf are developed is replaced by inhomogeneous tectonic stress field (transtension and transpression) in the north-western part of the Verkhoyansk Range. From the seismological point of view, this important geodynamical conclusion is based on both a few focal mechanisms of individual strong (M > 5.0) earthquakes (Imaev et al. 2000; Franke et al. 2000; Fujita et al. 2009) and their statistical analysis (Imaeva et al. 2016, 2017).

J Seismol Fig. 7 Comparison of the obtained focal mechanisms with the available data. The Kagan rotation angle (Ф) is indicated under each solution

The calculated focal parameters of the medium regional seismic events allow us to trace the changes in the seismotectonic deformation pattern of the crust in finer details and with more confidence than in previous

works. We can divide the considered earthquakes into four groups according to movement kinematics in their sources (Fig. 8). The first group includes all the events that occurred on the Laptev Sea shelf to the north of 73°

Table 3 Focal parameters of the earthquakes occurred within the study region before 1990 according to Global CMT Web Page (2017) Date, year month day

1977.04.23 1980.02.01 1988.01.01

t0, h min s

14:49:13.2 17:30:30.6 14:36:12.4

M0·1017, N m

Epicenter φ°, Е

λ°, N

133.62

74.67

123.00 129.14

72.86 74.39

0.25 1.02 0.58

Mw

4.9 5.3 5.1

h, km

10 18 15

Nodal planes Strike°

Dip°

Slip°

180

45

− 90

0

45

− 90 − 107

114

36

315

55

− 78

175

29

− 122

31

65

− 73

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Fig. 8 Focal mechanisms of the earthquakes occurred before (pink) (Table 3) and after (red) (Table 2) 1990. The earthquake groups discussed in the text are marked with dashed contours and Latin numbers. The length and color of the stress symbols denote the horizontal deviatoric stress magnitude. Red outward arrows indicate minimum horizontal principal compressional stress (Shmin) and blue inward arrows mark maximum horizontal principal compressional stress (SHmax). The vertical stress is symbolized by a solid circle, blue for extensional regimes, and green for strikeslip regimes. The obtained stress regimes: I—SHmax = 174°, Shmin = 84°, R′ = 0.57 (extension), tensor quality rank is B; II—SHmax = 43°, Shmin = 133°, R′ = 1.53 (strike-slip), tensor quality rank is C

N. Their focal mechanisms are normal faults with sublongitudinal orientation of nodal planes. The latter coincides with the strike of the rift system (Fig. 1). So, their sources were formed under the similar stress-strain field as under the spreading Gakkel Ridge (Heidback et al. 2010; Imaeva et al. 2016; Zoback 1992). A stress analysis of focal mechanisms has been carried out for this group as it is the most representative in the study region. To improve the homogeneity of the initial dataset, we have excluded the solution of the November 23, 2010 earthquake for which the orientation of the principal stress axes differs from the other seismic events in the group. The obtained stress tensor quality rank is B and, as expected, the implemented calculations show the predominance of extension (R′ = 0.57). The second group consists of three earthquakes in Buor-Khaya Bay. Two of them represent thrust faults proving more precisely the predominance of

subhorizontal compression revealed here from the data on group determinations of weak earthquake focal mechanisms (Avetisov 1991) and different assessments of source parameters of the largest during the instrumental observation period July 21, 1964 earthquake (Cook et al. 1986; Koz’min 1984). The January 31, 1995 seismic event has a strike-slip mechanism with a small normal fault component and can be regarded as a boundary between pure normal faults in the north and thrust faults in the west. The number of focal mechanism solutions in this group is too little to calculate a stress tensor. The continental part of the study area shows a complex and inhomogeneous structure of the stress-strain field that is expressed in various focal mechanisms of the earthquakes located in the Lena River Delta and combined into the third group. Here, we observe a mixture of thrust and strike-slip movements in the earthquake sources as opposed to the results of seismotectonic reconstructions by Imaeva et al. (2016) where the Lena River Delta is considered along with Olenek Bay and not extracted as a separate region. The obtained discrepancy can be caused by coarse lateral resolution of the statistical analysis of seismotectonic deformations (Imaeva et al. 2016) and usage of very diverse and sometimes poorly constrained with P wave polarities data on focal mechanisms. The orientation of nodal planes varies from near sublongitudinal in the east to NW in the west that corresponds to the strikes of the main faults (Bogdanov and Khain, 1998). Although this group consists of only 5 earthquakes with rather diverse focal mechanisms, we have calculated an average stress tensor to demonstrate the changes in the stress-strain field of the crust within the study region. The strike-slip regime is observed here (R ′ = 1.53) but the quality rank of the tensor is C (poor) and the solution can be regarded as only a preliminary one. We have only two earthquakes in the fourth group associated with Olenek Bay and observe that their sources have been formed under extension conditions which are typical for the whole Lena-Anabar segment extending to the west of the study area (Imaeva et al. 2016). As in the case of the second group, we have not enough data for further statistical analysis of the crust stresses in Olenek Bay.

4 Conclusions We have inverted source parameters of 16 earthquakes with medium magnitudes that occurred in the south-

J Seismol

eastern part of the Laptev Sea shelf, Lena River Delta, and North Verkhoyanye in 1990–2014. Focal mechanisms, hypocentral depths, scalar seismic moments, and moment magnitudes have been calculated from the data on surface wave amplitude spectra and P wave firstmotion polarities. The obtained results sufficiently implement the existing dataset on reliable earthquake source parameters for the study region and prove the change of the stress-strain state of the crust from extension on the Laptev Sea shelf to compression on the continent providing finer spatial details of the deformation field in the transition zones such as Buor-Khaya Bay and the Lena River Delta. They are of great value to further statistical analysis of modern deformation field of the crust and various geodynamical reconstructions including verification of the location of the pole of rotation between the Eurasian and North American lithospheric plates. Acknowledgements The authors are grateful to Jiri Zahradnik and anonymous reviewers for their valuable comments. The stress tensors were obtained using Win-Tensor, the software developed by Dr. Damien Delvaux (Royal Museum for Central Africa, Tervuren, Belgium).

Funding information This study was supported by the Russian Science Foundation, grant 17-77-10037.

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