Crustal Stress Studies
Using
Microearthquakes and Boreholes

BY


BJÖRN LUND

UPPSALA UNIVERSITY 2000

Dissertation for the Degree of Doctor of Philosophy in Geophysics presented at Uppsala University in 2000

ABSTRACT

Lund, B. 2000. Crustal stress studies using microearthquakes and boreholes. Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 517. 75 pp. Uppsala. ISBN 91-554-4671-X.

The state of stress in the crust is essential for the understanding of geodynamic processes such as the transmission of plate driving forces through the lithosphere and the triggering of earthquakes. This thesis describes the development and application of new methods for the estimation of crustal stresses. The first study revisits the stress state of the Swedish upper crust using data from the deep boreholes in Siljan. The analysis of wellbore image and other logging data enables us to constrain orientations and magnitudes of in situ stress. We find a strike-slip faulting regime at depth with the maximum horizontal stress in a WNW-ESE direction and $S_H - S_h \sim 60$ MPa at 5 km depth. The remaining three studies in this thesis are concerned with stress estimation from microearthquakes. We utilize earthquake data from the Icelandic SIL network in southwest Iceland. The first microearthquake study develops a stress tensor inversion scheme for focal mechanisms, including new methods for the selection of fault plane from the nodal planes as well as a novel technique to account for the errors in the focal mechanisms. The second study introduces a new method to assess the similarity of focal mechanisms for earthquakes with closely located hypocenters. The method is also promising as a monitoring instrument of earthquake repeating patterns. Finally, these new methods are applied to one year of seismicity before the November 13, 1998, $M_L = 5.0$ Ölfus earthquake. During the year we find significant temporal variations in microearthquake repeating patterns, and a large anomaly ending at the main event. The estimated background state of stress is well constrained, stress inversions over time show a rather stable, subhorizontal direction of $\sigma_3$ with more variations in $\sigma_1$ and $\sigma_2$.

Björn Lund, Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden

© Björn Lund 2000

ISSN 1104-232X
ISBN 91-554-4671-X

Printed in Sweden by Eklundshofs Grafiska AB, Uppsala 2000