Seismogenic Earth

Earth’s thin skin of brittle rocks plays an important role in controlling the deformation we observe at the surface, from small faults and fractures to continental-scale plate boundaries.  This thin layer controls the generation of sudden-onset dynamic events such as rock-falls, shallow earthquakes, and the timing of volcanic eruptions.  These catastrophic events occur when rocks are at relatively low temperatures and pressures near Earth’s surface, and their occurrence defines a seismogenic layer where dynamic rupture is possible.   

In turn, this dynamic rupture provides the energy source for the vibrations and waves that lead to strong ground motion during earthquakes and the consequent loss, damage, and destruction. The deformation may not be 100% seismogenic, it may be combined with non-seismic deformation mechanisms such as slow, steady ‘creep’.  

Like our bones, the behaviour of rocks and minerals in the brittle or semi-brittle field is strongly controlled by their microscopic structure, the nature and properties of the fluids within the pores, and chemically-enhanced ageing processes.  All of these affect the material’s strength and toughness on different spatial and temporal scales.  We investigate these processes on several scales, including live laboratory experiments carried out in an X-ray transparent deformation cell imaged in a synchrotron.  

The properties of seismogenic rocks are important because they control the hazard on different scales posed by brittle failure events such as rock-falls, volcanic eruptions and earthquakes.  They also control the likelihood of seismicity being induced by subsurface engineering projects such as CO2 storage, or the production of heat and power from geothermal energy.  These applications are discussed in more detail in our Edinburgh Earth Resources pages.

Visit our Edinburgh Earth Resources research group 

Current projects

Our group examines the fundamental mechanical, hydraulic, chemical and geophysical properties of rocks under pressures simulating the brittle upper crust. We are actively developing and applying state-of-the-art experimental techniques to document the evolution of the microstructure in controlled deformation experiments in synchrotron micro-tomographs and observe and locate tiny earthquakes recorded as acoustic emissions during the tests.  

The X-ray transparent deformation cell with acoustic monitoring is a world first and is enabling us to piece together the fundamental grain-scale processes of micro-fracture and the co-operative behaviour that leads to spontaneous localisation of deformation of ultimate catastrophic failure of the sample (Cartwright-Taylor et al, 2020).  These observations have helped explain the predictability of the timing of catastrophic failure as a function of the density of pre-existing flaws in the material, notably when to expect ‘surprise’ sudden-onset failure (Vasseur et al, 2017).  

We have also developed new techniques to characterise the effects of material heterogeneities, including faults and fractures, on the propagation of seismic waves and to locate new source ruptures using the scattered part of the wavefield in the tail of the seismogram (Zhao, et al, 2019; Singh et al, 2019; Zhao and A. Curtis, 2019).

Projects completed in the period 2015-2019 

Techniques and facilities

The Rock Physics Laboratory houses a number of deformation cells and our own in-house X-ray CT imaging facility.  It has triaxial deformation rigs to house samples from 38-100mm diameter enable deviatoric loading of rock samples to failure at confining pressures of up to 70 MPa. As well as stress and strain measurements, we can monitor acoustic emissions (full waveform and trigger hit rate for P and S waves) and measure acoustic velocities using our Applied Seismology AE system.  

We have developed two miniature X-ray transparent triaxial rock deformation rigs to enable 4D experiments using synchrotron X-ray microtomography to complement our conventional rock deformation capability. Our first cell, Mjölnir, enables investigations of brittle-style rock deformation and failure, as well as coupled thermal, chemical and mechanical processes relevant to a range of Earth subsurface environments. It operates with cylindrical samples up to 3.2 mm outside diameter and up to 10 mm length and can attain up to 50 MPa confining pressure and in excess of 600 MPa axial load.  The addition of heaters extends the experimental range to temperatures up to 140 °C.

More recently developed is our Stòr Mjölnir apparatus which takes samples of 5-10 mm outside diameter and 12.5 to 25 mm length and can attain 50 MPa confining pressure and between 500 MPa and 2 GPa axial load at room temperature. The Stòr Mjölnir is equipped with AE sensors that enable AE waveforms and hit-rates to be recorded simultaneously as x-ray microtomographic imaging. Furthermore, the rate of AE hits can be used to control the loading of the sample to optimise x-ray imaging close to the point of failure.  Both cells are transparent enough to acquire full 3D datasets in a few seconds to minutes on synchrotron imaging beamlines and thus enables full 4D investigations of deformation processes. They are constructed from readily available materials and components.  A detailed technical description of Mjölnir is published in Butler et al (2020) below.

Further information about our Rock Physics Laboratory

Publications