The epicenter of the 2018/11/09 Jan Mayen earthquake is located at 71.623°N, 11.240°W along a prominent geological structure, the West Jan Mayen Fracture Zone (WJMFZ) that crosses the Mid Atlantic Ridge. This active fracture zone passes immediately northeast of the Jan Mayen island. Since 1900, this zone has experienced several 6+ earthquakes (Figure 1), with the dominant left-lateral strike-slip mechanism (Figure 2).
The largest recent earthquakes along this fracture zone are the 2018/11/09 (Mw 6.8) and 2012/08/30 (Mw 6.7) earthquakes. These two nearby events exhibit similar characteristics in terms of magnitude and mechanism. In order to examine any likely connection between them, we determined and analyzed the finite-fault slip distributions of both events using the body waveform inversion method of Kikuchi and Kanamori (Kikuchi et al., 1993).
1. Slip Inversion of the 2018/11/09 (Mw 6.8) Earthquake
We used 83 P and 73 SH waveforms recorded at 86 seismic stations in distance ranges of 24° to 96°. Our preferred slip solution infers a seismic moment of 2.02E+19 Nm, equivalent to a moment magnitude of 6.8 (Figure 3).
The rupture initiated from the western end of the failed fault segment and unilaterally propagated eastward. The computed strike of 111° coincides with the trend of the WJMFZ. The dip angle of 78° infers a slight southward inclination. The average rake angle of -8° suggests an almost pure left-lateral strike-slip mechanism.
Figure 4 shows that slip concentrated in two main asperities, a shallow and a narrow deep slip patch (Figure 4). The main asperity extends 15 km along the strike and 10 km along the dip. This asperity covers depth ranges of 5 to 15 km and exhibits a maximum slip of ~2.1 m. The deep asperity is a strip located in depth ranges of 20 to 25 km. It extends 25 km along the strike and 5 km along the dip. The maximum slip of this asperity is ~2.0 m.
Our analysis suggests that the rupture reached the ocean bottom for a length of about 10 km that starts from the epicenter and extends eastward. The slip distribution suggests ocean bottom dislocation of about 0.5 m.
2. Slip Inversion of the 2012/08/30 (Mw 6.7) Earthquake
We used 163 P and 41 SH waveforms from 164 seismic stations in teleseismic ranges of 24° to 96° in a similar procedure as we applied for the analysis of the 2018 earthquake (Figure 5). The computed seismic moment of this event is 1.4E+19 Nm, equivalent to a moment magnitude of 6.69. Our analyses show that the rupture initiated from the western part of the failed fault segment and unilaterally propagated eastward.
Our preferred slip inversion solution indicates strike/dip/rake angles of 111°/75°/-4°, which are very close to the corresponding values for the 2018 earthquake. The slip distribution of the 2012 earthquake infers 2 sets of shallow and deep asperities, similar to the 2018 earthquake (Figure 6). The main asperity, with a maximum slip of 1.2 m, extends 15 km along the strike and 10 km along the dip. This asperity is located at a depth range of 5 to 15 km. The deep asperity, with a maximum slip of about 0.8 m, extends 5 km along the strike and 5 km along the dip of the causative fault. This asperity is located at depth ranges of 20 to 25 km. There is a clear correspondence in the depth ranges of the shallow and deep asperities of both the 2012 and 2018 earthquakes.
Our computations suggest that the rupture of the 2012 earthquake reached the ocean bottom with dislocation values less than 0.3 m. The suggested ocean bottom rupture locates between the epicenter and a point 25 km towards the east.
3. Spatial Proximity of the Slip Distributions
Figure 7 shows the slip distribution of the 2012 (yellow) and 2018 (red) earthquakes projected on the ocean bottom. Due to their steep dip angles, the projected slip contours cover a narrow band. The overlapping contours suggest a possible affinity between these two models. In order to reveal any probable tie, the slip contours are flattened by pivoting around the strike of the fault at the ocean bottom. Figure 8 shows that the slip distributions are in fact complementary. These two events have filled the seismic gap along the WJMFZ between 10°W and 11.3°W. The gap is observable in Figures 2.
4. Concluding Remarks
Analysis of the 2012 and 2018 Jan Mayen earthquakes along with the long-term seismicity and mechanism of the events along the WJMFZ may infer the following remarks:
- The seismogenic layer along the segment of WJMFZ between 10°W and 11.3°W is about 25 km thick and consists of two sub-layers separated at depths of 15 to 20 km.
- The main and shallow asperities are located between 5 and 15 km depths.
- The deep asperities are located between 20 and 25 km depths.
- For both 2012 and 2018 earthquakes, rupture of the deep asperities are delayed temporally and shifted (eastward) spatially relative to their shallow counterparts. This results in slip distributions with step patterns.
- The ruptures initiated from westernmost parts of the failed fault segments and propagated eastward unilaterally.
- The 2018 earthquake most likely ruptured the juxtaposing segment to the active ridge, provided that the available ridge map is correct.
- It seems that the 2012 and 2018 earthquakes have filled the seismic gap (10°W to 11.3°W) along the WJMFZ.
We thank Hadi Ghofrani from the University of Western Ontario for reviewing the report.
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