The deepest crust in Europe found in the Jyväskylä region

In 2012 and 2013, the University of Helsinki’s Institute of Seismology completed the Kokkola–Kymi (KOKKY) Deep Seismic Sounding profile, ranging from northwest to southeast, in the form of a measuring campaign spanning two summers.

The profiling was based on the use of measuring devices temporarily installed in the field, which registered the seismic waves generated by quarry blasts, road construction sites and other human activity. The equipment, which covered a long zone extending from the coast of the Bothnian Bay all the way to the Russian border, was able to register seismic events from the area surrounding the city of Vaasa to the Karelian Isthmus. A handful of permanent stations in the Finnish National Seismic Network were also available to increase the accuracy of the temporal and spatial elements of the observations.

Based on the travel times of the seismic waves observed in the project, it was possible to model the crustal structure of the zone under study.

The sounding direction was perpendicular to most prior structural seismic soundings and the direction in which the Svecofennian orogeny was formed in what is now Finland 1.7–1.9 billion years ago. The Polish Academy of Sciences contributed to the interpretation of the observations made in the project, which utilised seismic refraction profiling.

The benefit of such projects relying on seismic sources is their very low cost and negligible permit-related formalities compared to projects in which seismic signals are generated on purpose. At the same time, the quality of observation data varies greatly. In the KOKKY project, there were a total of 63 explosive sources, of which 25 were included in the final analysis.

The findings corroborate prior assumptions of high seismic velocities in the lower crust in Central and Southern Finland. In Central Finland, around the city of Jyväskylä, the crust is the deepest in Europe. Thanks to the success of the project, other research projects based on easily movable seismic equipment have been done and have been planned.

P-wave velocities in the KOKKY sounding line. Vp distribution = P-wave velocity distribution, BB = Bothnian schist belt, CFGC = Central Finland Granitoid Complex, SA = Saimaa area, HVLC = high-velocity lower crust. Also shown in the image are the locations of the seismic sources as well as those of the parallel and intersecting prior seismic sounding lines. Image: Tiira et al. (2020).

The findings corroborate prior assumptions of high seismic velocities in the lower crust in Central and Southern Finland. In Central Finland, around the city of Jyväskylä, the crust is the deepest in Europe. Thanks to the success of the project, other research projects based on easily movable seismic equipment have been done and have been planned.

Top figure: KOKKY profile and the seismic events used in the analysis. The previous sounding lines of the region (FIRE1, FIRE2, FIRE3A, SVEKA81) are marked in the image. BB = Bothnian schist belt, CFGC = Central Finland Granitoid Complex, SA = Saimaa area, WRB = Wiborg rapakivi batholith, BZ = the boundary zone of BB and CFGC. Image:: Tiira et al. (2020)

References:

Tiira, T. Janik, T., Skrzynik, T., Komminaho, K., Heinonen, A., Veikkolainen, T., Väkevä, S., Korja, A., 2020. Full-Scale Crustal Interpretation of Kokkola–Kymi (KOKKY) Seismic Profile, Fennoscandian Shield. Pure and Applied Geophysics, 17.3.2020. doi: 10.1007/s00024-020-02459-3.

More details:
Fennoscandian Shield

The results, published in the Pure and Applied Geophysics journal, indicate that the average thickness of the upper crust in the sounding line is 22 km. The velocity of longitudinal seismic waves (P-waves) is 5.9–6.2 km/s close to the uppermost crust and 6.25–6.4 km/s in the lower part of the upper crust. At its thinnest, the middle crust is only 14 km thick in Central Finland, while in the northwest part of the sounding line its thickness is 25 km and in the southeast 20 km. However, P-wave velocity was a stable 6.6–6.9 km/s in all parts of the middle crust, 7.35–7.4 km/s in the lower crust and 8.2–8.25 km/s in the uppermost mantle. The velocity of S-waves, or transverse seismic waves, ranged between 3.4 and 4.85 km/s in line with the P-wave velocity variation.

The boundary between the Earth’s crust and mantle (known as Moho) is located 43 kilometres deep in the southeast part of the sounding line, 54 km deep in the northwest and as deep as 63 km in the middle region.