Introduction

Despite our long traditions of using rock as a construction material, traditions probably as old as civilization itself, it was only during the last century that the need to understand and measure the forces acting in the rock became urgent. The foundation of tall buildings and long bridges, the excavation of long tunnels and deep mines and the drilling of deep wells can only be safely carried out with knowledge of how the rock responds to changes in the stress field. The advancement of civil, mining and petroleum engineering spurred interest in the measurement of rock stresses and from the relief methods of the 1930s the development of new stress measurement techniques has been constantly increasing. At the same time, in geology and geophysics, the need for quantitative estimates of the development of folds and intrusions, the movement of earthquake generating faults etc., accentuated the need for estimates of in situ stress from the deep crust. Further interest in deep stress estimates was triggered by the plate tectonic revolution. Plate tectonics has been very successful in describing various tectonic processes by a single concept, but the forces that drive the plates are still not well understood. Which forces act on the plates, where do they act, what are their magnitudes and how and where are they transmitted? Some of these questions can be answered through the study of mechanical stresses in the lithosphere. The recent interest in stress transfer and stress triggering of earthquakes is a field which requires detailed knowledge of crustal stresses, preferably both before and after large earthquakes. Accurate estimates of the background stress field and the perturbations caused by smaller groups of events will contribute to the understanding of earthquake generating mechanisms and, hopefully one day, the forecasting of earthquakes.

This thesis describes some new developments in the estimation of crustal stresses from earthquake focal mechanisms and a new method for the monitoring of one aspect of earthquake faulting, based on focal mechanisms and locations. An application of an integrated stress measurement strategy, utilizing two deep boreholes to measure crustal stress, is also included in the thesis.

The thesis is divided into two parts. The first part starts with an introduction to the stress tensor concept and to some of the properties and applications of the stress tensor. Included in this section is also a very brief summary of rock fracture, friction and pore pressure. In the next section I introduce the methods used to infer stress from earthquake focal mechanisms and include some comments on the use of P and T axes as stress orientation indicators. This section also contains a discussion on the distribution of focal mechanism data, pertaining to stress tensor inversion. The third major section of this part of the thesis describes some of the methods used for inferring stresses from deep boreholes. I have concentrated on hydraulic fracturing, borehole breakouts and induced tensile fracturing. Since all of the data for the seismological part of this thesis come from the Icelandic SIL network, I have included a very brief introduction to the tectonics of Iceland and the operation of the SIL network. After SIL follows the summaries of the papers and a few concluding remarks.

The second, or main, part of the thesis consists of four papers. Paper I describes the estimation of the orientation and magnitude of the state of stress in Siljan, Sweden, using data from the deep boreholes Gravberg-1 and Stenberg-1. All stress related data obtained from the two boreholes have been compiled and analyzed using an integrated stress measurement strategy. Papers II, III and IV are all based on microearthquake data from Ölfus in southwest Iceland. Paper II extends an earlier stress inversion method by introducing a new nodal plane selection criterion, and through the use of acceptable focal mechanisms it allows for errors in the mechanisms. Paper III introduces a new technique for assessing the similarity of focal mechanisms, which is also discovered to be very valuable as a monitor of earthquake repeating patterns. In Paper IV we apply the methods developed in Paper II and III in a study of one year of seismicity prior to the November 13, 1998, magnitude 5.0 earthquake in Ölfus.