Seismotectonics

My work on seismotectonics and earthquake mechanics includes participation in many EU projects on seismic risk. I use field observations of active and inactive faults, as well as analytical and numerical models, to understand the local stresses around and within fault zones that determine (a) the condition for slip (not all slips/displacements generate earthquakes –  many slips are aseismic) and (b) the associated permeability changes in and around the fault zone.

I have participated in many EU projects on seismic risk, focusing on the large transform zones in Iceland. These projects combined structural and geodetic studies of active and inactive parts of these large fault zones with analytical and numerical modelling with a view of increasing our success in forecasting earthquakes.

One of the main findings in recent years is that every fault zone has mechanical properties that differ from those of the adjacent host rock. It follows that a fault zone should be treated as an elastic inclusion, that is, a zone of solid material with (commonly widely) different properties from those of the hosting material. The elastic inclusion, or zone, is composed of two main hydromechanical units: a central core, characterised by breccias and gouge, and a damage zone, characterised by fractures whose frequency varies with distance from the fault core. The variation in fracture frequency within the damage zone means that the effective mechanical properties (mainly Young’s modulus or stiffness) also varies within the damage zone, resulting in many damage zones being composed of mechanically distinct subzones.

It follows that the local stresses inside a fault zone may be quite complex. To study them, I use numerical modelling. These models, primarily finite element and boundary element, make it possible to forecast the likely state of stress within a fault zone of given mechanical properties and remote (regional) local loading (stress, displacement) boundary conditions. From the models, we can infer if and then where the fault is most likely to slip, producing earthquakes and changes in its temporary permeability.

Bergerat, F., Angelier, J., Gudmundsson, A. and Torfason, H., 2003.
Push-ups, fracture patterns, and palaeoseismology of the Leirubakki Fault, South Iceland.
J. Struct. Geol. 25, 591-609.

Gudmundsson, A., 2004.
Effects of Young’s modulus on fault displacement.
C. R. Geoscience 336, 85-92.

Gudmundsson, A., 2007.                                                                                            Infrastructure and evolution of ocean-ridge discontinuities in Iceland.                                    J. Geodynamics 43, 6-29.

Gudmundsson, A., Simmenes, T.H., Larsen, B., Philipp, S.L., 2010.                          Effects of internal structure and local stresses on fracture propagation, deflection, and arrest in fault zones.                                                                                                       Journal of Structural Geology doi:10.1016/j.jsg.2009.08.013

Gudmundsson, A. 2011.                                                                                                  Rock Fractures in Geological Processes.                                                                       Cambridge University Press.