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Recent publications

Workreport 2016-47



Results of Monitoring at Olkiluoto in 2015, Rock Mechanics


Johansson, E., Haapalehto, S., Malm, M., Saari, J., Kallio, U., Koivula, H., Lahtinen, S., Rouhiainen, P., Saaranen, V.



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The rock mechanics monitoring at Olkiluoto concentrates on the assessment of potential tectonic movements and stability of the bedrock. The rock mechanics monitoring programme 2015 consisted of seismic, GPS, surface levelling and temperature measurements at Olkiluoto and vicinity and displacement measurements, temperature measurements and visual tunnel observations made in the ONKALO.

The Posiva's microseismic network consists of 17 seismic stations and 21 triaxial sensors. Four stations are in the ONKALO. The network operated continuously in the ONKALO area, but in 2015, there was about 2 hours long break in monitoring of seismic semi-regional area due to power cut of the link mast. Altogether 203 explosions and earthquakes were located in 2015. Almost 30% (56) of the observed explosions happened inside the seismic ONKALO block, and total of 80 explosions were located inside the seismic semi-regional area. The magnitudes of the explosions inside the semi-regional area range from ML = -0.7 to ML = 1.7. Four microearthquakes occurred inside the seismic ONKALO block in 2015. The magnitudes of the located events were from ML = -2.1 to ML = -1.9. The estimated dislocations were about 1 μm, and the radiuses of the faults less than 10 meters. According to the seismic monitoring the rock mass has been stable in 2015.

The local GPS network consists of 18 stations of which 17 permanent stations operate continuously. The non-permanent Pyrekari station (GPS12) was successfully observed once during the year. The control markers of the station GPS22, established in 2014, were also measured. The permanent data from April 2013 to November 2015 was analysed using the L3 ionospheric free linear combination. The estimated horizontal velocity components relative to the station GPS2 (reference station) were small (maximum 0.3 mm). Larger vertical velocities were estimated at stations west from GPS2: up to 1.3 mm/y relative to GPS2. The rms of the residuals varied between 0.3 and 1.0 mm in horizontal and between 1.7 and 6.0 mm in vertical. The vertical velocities are more uncertain due to the poorer vertical accuracy of the GPS measurements in the northern latitudes. The daily baseline lengths were also analysed. Roughly 60 % of the change rates were below 0.2 mm/y and the maximum rate was 0.5 mm/y (GPS6-GPS9). The change rates were on average slightly larger compared to the earlier results based on the campaign data. The relatively short time span of the data typically overestimates the change rates.

The surface levelling network currently consists of 87 fixed measuring points. The whole levelling network was measured in 2015. In the GPS network loop, differences from the previous measurement were small. Largest height difference was 0.62 mm.  The height difference between Olkiluoto Island and Lapijoki was close to the results of 2003 and 2011. Difference between the years was less than one millimetre. The analysis of the levelling results showed that the bedrock over the Olkiluoto strait was stable.  The height changes were within ±0.3 mm between 2014 and 2015. In the VLJ loop, the largest movement was at +0.28 mm at GPS9A. In the ONKALO area height changes are analysed relative to the benchmark 03216. Between 2014 and 2015, the largest movement was +0.83 mm at the benchmark 14301. The benchmark 06220 has a unique deformation rate: its height has been changed 2.08 mm since 2006.

The continuous displacement measurements in the technical rooms of the ONKALO and in the demonstration tunnel 3 and manual measurements in the POSE niche showed that the displacement behaviour has been stable during 2015 and no significant changes have taken place.

Temperature measurements collected in 2015 confirmed the previous results and indicated relatively uniform distributions of temperature in all depths across the site. Thermal gradient is around 1.3 - 1.4 ºC/100 m below 300 m.

Only two tunnel damage observations were recorded in 2015. This is due to low excavation activity in 2015 but it also confirms the stable rock mechanical conditions in the ONKALO.


Olkiluoto, rock mechanics, monitoring, crustal movements, deformation studies, seismic network, microearthquake, GPS measurements, precise levelling, temperature measurements, extensometer measurements, tunnel observations.


Results of Monitoring at Olkiluoto in 2015, Rock Mechanics (pdf) (21 MB)


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