Crustal structure and intraplate seismicity in Nordland, Northern Norway: insight from seismic tomography

Abstract

Summary The Nordland region, Northern Norway, situated in an intraplate continental setting, has the highest seismicity rate in mainland Norway. However, the exact cause of seismicity in this region is still debated. Better understanding of factors that influence the seismicity in Nordland can help increase knowledge of intraplate seismicity in general. Here, we address this problem with the aid of a new high-resolution 3-D VP and VP/VS ratio images of the crust in Nordland using seismic travel-time tomography. These images show the existence of a localized, 10 - 15 km Moho step that runs parallel to the coast. The north-south extent of this step coincides with the region that exhibits the highest rates of seismicity. Focal mechanisms of selected earthquakes computed in this study are dominated by normal and oblique-normal, indicating a coast-perpendicular extension. The coast-perpendicular extensional stress regime deviates from the regional compression imposed by the ridge push from the North Atlantic. This deviation is thought to stem from the additional interference with local flexural stress caused by sediment redistribution and glacial isostatic adjustment, and possibly exacerbated by gravitational potential energy stress associated with the Moho step. The deformation due to the extensional regime is localized on pre-existing faults and fractures along the coastline. The tomography result shows that two distinct seismic swarms occurred in the coastal area with low VP and variable VP/VS ratio anomalies, pointing towards fractured crust and possibly the presence of fluids. The existence of fluids here can change the differential stress and promote seismic rupture.

The Moho Architecture and Its Role for Isostasy—Insights From the Lofoten‐Vesterålen Rifted Margin, Norway

“…The change in Moho depth appears to be in large parts accommodated by an abrupt step located beneath the southern tip of the Lofoten, between ∼68.00°N and 68.25°N. This step is also observed in other models of the region (Maystrenko et al., 2017; Shiddiqi et al., 2022), but at slightly different locations and with different steepness. The crust along the Lofoten‐Vesterålen archipelago and Lofoten Ridge is not in full isostatic equilibrium, which is supported by all models studied here: while the Moho depth increases from ∼20 to ∼40 km, the corresponding averaged topography only shows very minor variations of typically less than 300 m. Unrealistic density changes would be required to compensate these crustal thickness variations.…”

“…In coast-near areas north of ∼67°N, shallower Moho in the range of ∼35-40 km depth is observed. Earthquake tomography (Figure 2b, Shiddiqi et al, 2022) 2d). While the background Moho depths from our model are similar to those from the tomographic model, the RFs do not recover the two deep Moho anomalies (features 3 and 6).…”

“…The model is compared to models from Shiddiqi et al. (2022) and Maystrenko et al. (2017), represented as hatched and stippled black lines, respectively.…”

“…Their density model includes lateral density changes in the crust and in the mantle lithosphere, which are not considered here. Most Moho depth estimates by Shiddiqi et al (2022) (Figure 8b) on the mainland suggest crustal isostasy with a compensation depth of ∼40 km. However, the very thick crust in the Uppermost (Breivik et al, 2017(Breivik et al, , 2020Drivenes et al, 1984;Goldschmidt-Rokita et al, 1988;Mjelde et al, 1993) and gravity inversion profiles (Tsikalas et al, 2005) closest to the seismometers used for RF inversion.…”

“…(2017), (b) Shiddiqi et al. (2022) and (c) Combined RF estimates from Ben Mansour et al. (2018), Ottemöller and Midzi (2003), Olsson et al.…”