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Workreport 2019-5

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Name:

Microearthquakes During the Construction and Excavation of the Final Repository for the Spent Nuclear Fuel at Olkiluoto in 2002-2018

Writer:

Kaisko, O., Malm, M.

Language:

English

Page count:

68

Summary:

Posiva Oy is constructing an underground disposal facility for spent nuclear fuel in Olkiluoto island, Eurajoki municipality, in the south-western coast of Finland. Posiva has been monitoring the natural background seismicity in order to characterize the baseline conditions at the site since 2002. In addition, the monitoring has focused on the changes in stability of the bedrock and safeguarding the facility during the excavation since 2004. In the end of 2018, the repository area with its surroundings was monitored with 18 permanent stations equipped with accelerometers and/or geophones.

The Olkiluoto area has distinctly low seismicity with no observations of natural earthquakes within 5 km radius from the site. Also, induced microearthquakes were only rarely recorded during the years 2003 ‑ 2016. Seismicity related to the final disposal facility construction became more intense in 2017, when the excavation was conducted simultaneously in several locations at the repository level. Since that, the number of probable microearthquakes has shown correlation with excavation intensity by decreasing significantly together with ceased blasting in the end of 2018.

This study describes the microearthquake observations at the Olkiluoto site during the years 2002 - 2018. Altogether 412 probable microearthquakes were detected and analysed with Posiva Oy’s local microseismic network during this period. In this study, the earthquakes and rockfalls occurring in 2002 ‑ 2016 were reidentified and the reported microearthquakes lacking location were reanalyzed. An updated spectral source model and the associated source parameters were computed for the 400 microearthquakes in 2014 - 2018 and reliable moment tensor solutions could be derived for 51 earthquakes.

Based on the updated spectral source models, the earthquake magnitudes ranged from ML = -3.2 to M= ‑0.5 with average source radii from 2 to 69 m, and displacements from 0.1 to 123 μm. The static and dynamic stress drops were from 3.7·10-4 to 8.4·10-1 MPa and 2.4·10-4 to 9.6·10-1 MPa. Energy released as seismic waves during an event was estimated to be from 3.7·10-4 to 1.5·102 J and seismic potency and moment from
4.9·10-6 to 4.1·10-2 m3 and from 1.6·105 to 1.4·109 Nm, respectively.
The events occurring within the bedrock further away from the excavated volumes had generally smaller source radii, relatively larger displacements on the source, larger stress drops and higher radiated energy especially on the P‑wave than the events close to free surfaces at tunnel and shaft openings.

The majority of the moment tensor solutions with subvertical pressure axes indicate that the microearthquakes have mostly occurred in the tunnel walls and have been induced by the perturbed stress field caused by the tunnel openings. Even though majority of the events have occurred when the excavation has passed a known brittle fault zone, the estimated fault planes of these events do not always correspond to the modelled orientations of the faults in the geological structural model. These events could be also associated to smaller fractures within the damage zones of the faults.

The moment tensor solutions of the microearthquakes induced by silica injection within otherwise undisturbed bedrock are concluded to reflect the natural stress field, yet not being of natural origin. The events occurred on the brittle fault zone OL‑BFZ020a, with nodal planes corresponding to the modelled fault plane. The source mechanisms show reverse mechanism with small explosive component, which is explained by the combination of the mechanism expected under natural stress field within the uppermost crust affected by increased pore pressure. Based on the analysis of the events, the natural pressure, tension and intermediate stress axes have trend/plunge values of 294º/16º, 56º/63º and 198º/23º at the site.

Several microearthquakes can be associated with the brittle fault zones DSM-BFZ002, DSM-BFZ006, OL‑BFZ019c, OL-BFZ020a, OL‑BFZ034, OL-BFZ045, OL-BFZ063, OL-BFZ084, OL-BFZ135, OL-BFZ265, OL-BFZ297, OL‑BFZ300, OL‑BFZ346, and OL-BFZ353. However, based on the results of this study it can be stated that these structures have not been reactivated in the sense of releasing continuously stresses through natural mechanisms. The brittle fault zones DSM-BFZ006, OL‑BFZ020a, OL-BFZ045, OL-BFZ084, and OL-BFZ346 have hosted microearthquakes not only beside the tunnels but also within the bedrock, which could indicate their ability for future reactivation.

Keywords:

Induced seismicity, microearthquake, source parameters, moment tensor solution

File(s):

WR 2019-05_web (pdf) (5.9 MB)
Appendix_A_Microearthquakes_at_Olkiluoto_in_2002_2018 (xlsx) (172.4 KB)
Appendix_B_Excavation_blasts_at_Olkiluoto_in_2015-2018 (xlsx) (236.1 KB)


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