Workreport 2019-10



Co-Seismic Secondary Fracture Displacements Under Different Stress Conditions


Fälth, B., Lönnqvist, M., Hökmark, H.



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In this report, we present results from simulations of co-seismic secondary fracture displacements at the spent nuclear fuel repository site in Olkiluoto, Finland. The objective of the work is to expand the catalogue of secondary displacement results by considering a wider range of earthquake rupture scenarios than those considered in previous similar studies (Fälth and Hökmark 2011, 2012, 2015). We simulate earthquake ruptures on primary fault planes and calculate co-seismic secondary displacements induced on 300 m diameter target fractures located at different distances from the earthquake fault slip plane.

Our model catalogue includes 29 synthetic earthquakes occurring under present-day stress conditions, thermal stress conditions, forebulge stress conditions and endglacial stress conditions. The earthquakes are of both single fault type and multiple fault type and have moment magnitudes in the range Mw 5.8 – Mw 7.3. We generate more than 3200 secondary fracture displacement results at about ten fault-fracture distances in the range from 0 m (target fractures intersecting the primary fault) to 4500 m and consider more than ten different target fracture orientations. We also examine the potential effects of inhomogeneous fault properties and of abrupt fault rupture arrest. We observe the following:

  • The secondary fracture displacements generated under thermal stress conditions are similar to those generated under present-day stress conditions.
  • There is no significant difference in the secondary displacements generated by single fault- and multiple fault earthquakes.
  • For reverse stress conditions, gently dipping target fractures tend to slip more than steeply dipping fractures. For forebulge stress conditions the trend is the opposite.
  • For all earthquake scenarios considered here, and for fractures without connection to the damage zone, no fracture displacement exceeds 56 mm. Based on our calculation results we estimate that the displacement on a fracture that is connected to the primary fault slip plane or to the damage zone may become about two times the displacement of a non-connected fracture.

We conclude that, for a wide range of modelling assumptions, and for strong earthquake sources, our calculated secondary displacements are kept within some tens of millimetres on 300 m diameter fractures. About 90% of the gently dipping fractures (dip < 45°) slip less than 30 mm. If we consider fractures without connection to the primary fault slip plane or to the damage zone, 96% of them slip less than 30 mm.

We note that there are uncertainties in the results related to potential effects of model discretization and of variations in the rock mass stiffness. The additional safety margins required to account for these items are estimated to 25% and 5%, respectively. In addition, primary fault non-planarity/inhomogeneity may locally increase secondary displacements. On the other hand, the assumption of target fracture planarity should contribute to an overestimation of the displacements. We estimate that the calculated fracture displacements could be reduced by possibly 25% if roughness on the target fracture surface is considered.


Safety assessment, Earthquake, Fracture, Secondary displacement


WR 2019-10_web (pdf) (6.7 MB)


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