High speeds and high strain rates during earthquakes: friction, fault damage and fluids

Date & time

1–2pm 16 November 2017

Location

Jaeger 1 Seminar Room, RSES

Speakers

Tom Mitchell (University College London)

Event series

Contacts

 Adele Morrison

Fault zones in the upper crust (<15 km) accommodate large-scale tectonic movements, and are the focal point of major earthquakes.  During an earthquake, stored elastic strain is converted into kinetic energy with a significant portion of this being released in the form of seismic waves, which leads to strong ground shaking that has the potential to cause major destruction and loss of human life. However, not all the elastic energy is discharged as seismic waves; a portion of the energy is dissipated as a combination of frictional heating, co-seismic chemical processes and fault fracture generation at depth. Such processes cannot be measured directly during an earthquake as they occur deep in the Earths crust. Hence, Earth scientists take to looking at surface outcrops of both active and inactive fault zones that have been brought to the surface over millions of years in order to deduce paleo-earthquake conditions. 

In recent years, much debate has been sparked by the identification of so-called ‘pulverized rocks’ described on various crustal-scale faults, a type of intensely damaged fault rock which has undergone minimal shear strain, and the occurrence of which has been linked to damage induced by transient high strain-rate stress perturbations during earthquake rupture.  Damage induced by such transient stresses, whether compressional or tensional, likely constitute heterogeneous modulations of the remote stresses that will impart significant changes on the strength, elastic and fluid flow properties of a fault zone immediately after rupture propagation, at the early stage of fault slip.  In this presentation, I will review a variety of co-seismic fault processes that occur on timescales significantly shorter than that of geological timescales we are used to as Earth Scientists.  In particular I will demonstrate laboratory and field examples of two dynamic mechanisms that have been proposed for the generation of pulverized rocks; (i) compressive loading by high-frequency stress pulses due to the radiation of seismic waves and (ii) explosive dilation in tension in rocks containing pressurized pore fluids. 

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