Dr Ake Fagereng (Cardiff University) will give the ESE Departmental Seminar on 24 February: ‘Controls on fault slip style in natural, experimental, and numerical faults’.
Join us online on 24 February by clicking “Livestream” on the seminar page at 12pm.
The geophysically determined slip speeds of natural faults range from steady plate boundary creep (mm/yr) through to earthquake slip (m/s). In comparison, geological descriptions of faults and shear zones range from localized displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness, indicating a large range of possible strain rates in natural faults. Linking these two observational datasets is critical for understanding the physical controls on how faults slip.
We take a tour of natural faults and shear zones that are characterised by a mixture of spatially localised and distributed deformation styles. From shallow drill core on the active Hikurangi subduction margin, to rocks exhumed from across the base of the subduction thrust seismogenic zone, mutually cross-cutting faults and ductile fabrics imply temporal switching of rheology, commonly associated with fluid pressure fluctuations and low effective stress. Low effective stress can also be demonstrated from the deep extent of crustal transform faults, and inferred for frictional-viscous deformation along oceanic transforms. Except at very shallow depths, slip appears governed by competition between pressure solution creep and frictional sliding.
There must be certain conditions that produce earthquakes, creep, and slip at intermediate velocities, across a broad range of temperatures. In experiments, changes in the ratio of weak to strong components can bring about distinct changes in rheology. In numerical models, where we can explore bulk rheology at a larger scale than experiments, we suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors controlling bulk rheology. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity.
Biography
Åke Fagereng received his PhD in geology from Otago University, New Zealand, in 2010. He went on to a position as Lecturer and then Senior Lecturer at the University of Cape Town, South Africa, before moving to Cardiff University in 2014, where he is currently a Reader in Structural Geology.
His research focuses on structure and rheology of faults and shear zones, and correlations between geological and geophysical observations of fault slip style. He has worked on exhumed subduction systems in New Zealand, Japan, Namibia, and the European Caledonides, and ocean drilling of the active New Zealand margin. He also studies exhumed strike-slip faults in Namibia and Cyprus, and active rifting in Malawi. He thinks that metamorphic petrology is important and worries about the effect of fluids and melts on rheology.
He holds an ERC Starting Grant, and also receives funding from the UK Global Challenges Research Fund and the National Environmental Research Council. He was awarded the South African National Research Foundation’s President’s Award in 2013, and the Outstanding Young Scientist Award of the EGU Tectonics and Structural Geology Division in 2016. He is an Associate Editor for Geophysical Research Letters, edited a book, Geology of the Earthquake Source, for the Geological Society of London, and was recently appointed to the Editorial Board of Seismica.