Seismic Hazard in Norway Due to Lage Earthquakes
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Seismic hazard in Norway due to large earthquakes Martine Johnsen Master of Science, Geodynamics Department of Earth Science UNIVERSITY OF BERGEN 01.06.2015 Abstract There are many faults located in Norway, posing as potential earthquake sources. Even though the seismicity is characterized as low to moderate, earthquakes of magnitude 3.5 or lower occur regularly. The seismic hazard following such a small event is very low; however, the occurrence of small earthquakes may promote the possibility of larger events. This thesis is based on the potential seismic hazard in Norway due to large earthquakes, and this study performs simulations of three or four different earthquake scenarios on 13 faults located in Norway, in areas where cities and towns are situated nearby, and these magnitudes are: the highest possible magnitude for each fault based on the fault length (ranges from MW 7.7 to MW 6.9), MW 7.0, MW 6.5 and MW 6.0. The simulations show the expected ground motion in peak ground acceleration (PGA), and the PGA caused by the earthquake scenarios are quite high, with a maximum of 317.6 cm/s2. This could correspond to shaking of intensity VIII on the Modified Mercalli Intensity Scale. The MW 6.5 and MW 6.0 scenarios would result in PGA between 191.4 and 91.11 cm/s2, which may indicate intensity VI – VII. The area outside the 50 cm/s2 contour line would be exposed to PGA below 50 cm/s2 and the shaking in this area could correspond to intensity IV – VI. A stochastic simulation code, EXSIM12, was used in performing these simulations. The difference in maximum PGA for scenarios of similar magnitude shows that the distribution of the PGA and the maximum value are dependent on several factors, for example the length and width of the fault, as well as the number of iterations per site, and whether the simulations are performed with random or fixed hypocenter location. Both increasing the number of iterations per site and changing from random to fixed hypocenter location result in smoother distribution of the PGA and lower maximum value for the earthquake scenarios. ii Acknowledgements First, I would like to express my greatest appreciation to my supervisor, Mathilde Bøttger Sørensen, Associate Professor at the University of Bergen, for providing great guidance and helpful comments during the process. I am particularly thankful for her patience and support in regards to EXSIM12 and working with GMT plots. Secondly, I would like to thank my co-supervisors Kuvvet Atakan and Haakon Fossen for guidance and discussion of my work. I would also like to give thanks to my fellow students and everyone who have provided the literature that my study builds on, especially Randi Karin Tveit who has provided the research that my thesis brings further. Finally, I would like to thank Gro Helene Carlsen, Jens Harald Johnsen, Karen Johnsen and Arnstein Stangeland Waldeland for supporting me through the Master study, and helping me stay focused during the final stages of writing and editing the thesis. iii Table of Contents 1 Introduction ................................................................................................................................................................ 1 2 Tectonic and geological background ................................................................................................................ 4 2.1 Tectonic background and seismicity in Norway ................................................................................ 4 2.1.1 The Caledonian Orogeny .................................................................................................................... 4 2.1.2 Seismicity in Norway ........................................................................................................................... 6 2.2 The Oslo Rift, Eastern Norway .................................................................................................................. 8 2.3 Hordaland, Western Norway .................................................................................................................. 11 2.4 Nordland, Northern Norway ................................................................................................................... 15 3 Method ....................................................................................................................................................................... 19 3.1 Literature search .......................................................................................................................................... 19 3.2 Stochastic finite fault modeling ............................................................................................................. 20 3.2.1 SMSIM ..................................................................................................................................................... 22 3.2.2 FINSIM .................................................................................................................................................... 24 3.2.3 EXSIM ...................................................................................................................................................... 25 4 Data ............................................................................................................................................................................. 28 4.1 Faults and magnitudes .............................................................................................................................. 28 4.2 Input parameters in EXSIM12 ................................................................................................................ 37 5 Results ........................................................................................................................................................................ 41 5.1 Ground motion simulations ..................................................................................................................... 41 5.2 Earthquake Scenarios and Peak Ground Motion Acceleration ................................................. 41 5.2.1 Oslo Rift Zone, Eastern Norway ................................................................................................... 44 5.2.2 Hordaland, Western Norway ......................................................................................................... 51 5.2.3 Nordland, Northern Norway ......................................................................................................... 61 5.3 Seismograms .................................................................................................................................................. 70 5.3.1 Oslo .......................................................................................................................................................... 70 5.3.2 Bergen ..................................................................................................................................................... 71 6 Discussion ................................................................................................................................................................. 72 6.1 Discussion of the results ........................................................................................................................... 72 6.2 Influence of fault length and width ...................................................................................................... 74 6.3 Number of iterations .................................................................................................................................. 79 6.3.1 Nesna Fault ........................................................................................................................................... 81 6.3.2 Northern Sunnhordland Fault ...................................................................................................... 84 6.3.3 Discussion of the results simulated with 20 iteration per site ........................................ 87 6.4 Location of epicenter .................................................................................................................................. 88 6.4.1 Nesna Fault ........................................................................................................................................... 89 iv 6.4.2 Northern Sunnhordland Fault ...................................................................................................... 92 6.4.3 Discussion of the results simulated with fixed hypocenter location ............................ 95 6.5 Earthquake recurrence in Norway ....................................................................................................... 96 6.6 Comparison of my results with the results from Tveit (2013) and Probabilistic Seismic Hazard Assessments ................................................................................................................................................. 97 7 Conclusion ............................................................................................................................................................. 108 8 Recommendations for future studies ......................................................................................................... 110 9 References 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