Frictional and Fracture Energy Estimation for the Intraplate Seismicity Recorded in Southern Norway During the Period 2000-2019

Abstract:

Small seismic events (0 ≤ M_w ≤ 3.8) recorded in southern Norway in the period 2000-2019 were used to calculate the individual coseismic temperature increase using the Kanamori’s equations with a simple stress-drop model. The relationship between temperature and moment magnitude exhibit a linear behavior, which strongly depends of the initial tectonic stress and the static stress drop; the temperature increases reach a maximum of ~62° C for events of M_{w =}3.7. The temperature values were subsequently used to estimate the thermal energy in 2D and 3D cumulative patterns in southwestern Norway as well as in the northern part of the North Sea. Additionally, the amount of fracture energy that is available regarding merely local seismic activity was calculated. The total frictional energy achieves a value of ~1.4 x 10¹² [J]. Likewise, the fracture energy, calculated through experimental curves, reach a value of ~5.0 x10⁵ [J]. The temperature increases are proposed as an additional constituent that explain the weakness areas related to the high amount of low-grade intraplate seismicity, mainly regarding the anisotropic thermal expansion coefficient of rocks and the thermal weakening processes produced continuously by local earthquakes. At the same time, the low fracture energy could explain the relative absence of new nucleation zones. The results were correlated with existing geological information, considering lineaments distribution and heat flux data. Areas with high values of thermal energy seems to be spatially linked with both high heat flux zones and high density of lineaments mainly offshore to the south of the Møre basin and in western Norway.