New observations of magnetic, electric and electromagnetic field variations, possibly related to recent volcanic and seismic events, have been obtained on Mt. Unzen in Japan, Reunion Island in Indian Ocean, the Long Valley volcanic caldera in California, and for faults in China and Russia, California and several other locations. For volcanic events, contributions from different physical processes can be identified during the various eruption stages. Slow processes (weeks to months) include near-surface thermal demagnetization effects, piezomagnetic effects, and effects from rotation/displacement of magnetized material. Rapid processes (seconds to days) include piezomagnetic effects from instantaneous stress redistribution with explosive eruptions and electrokinetic effects from rupture of high pressure fluid compartments commonly encountered in volcanic regions. For seismic events, the observed coseismic offsets are instantaneous, provided care has been taken to ensure sensors are insensitive to seismic shaking and are in regions of low magnetic field gradient. Simple piezomagnetic dislocation models based on geodetically and seismically determined fault parameters generally match the observed signals in size and sign. Electrokinetic effects resulting from rupture of fluid filled compartments at hydrostatic to lithostatic pore pressures can generate transient signals in the frequency band 100 Hz to 0.01 Hz. However, large-scale fluid driven processes are not evident in near-field measurements in the epicentral region minutes to weeks before large earthquakes. The subset of ionospheric disturbances generated by trapped atmospheric pressure waves (also termed gravity waves and/or acoustic waves, traveling ionospheric disturbances or TID's) that are excited by earthquakes and volcanic eruptions are common and propagate to great distances. These are known and expected consequences of earthquakes, volcanic explosions (and other atmospheric disturbances), that must be identified and their effects removed from VLF/ULF electromagnetic field records before associating new observations of ionospheric disturbances with earthquake activity.

Review of electric and magnetic fields accompanying seismic and volcanic activity

A. M. Freudenthal London: John Wiley. 1966. Pp. xv + 492. Price £5 13s. Although written in modern idiom with a currently fashionable title, the book deals with a subject formerly called 'Strength of materials', and covers what engineers are expected to know about stresses in simple structures and mechanical properties of materials. The Introduction admirably discusses how the theoretical and experimental approaches to the behaviour of mechanical systems are to be reconciled with each other.

Introduction to the Mechanics of Solids

We formulate a relaxed linear elastic micromorphic continuum model with symmetric Cauchy force stresses and curvature contribution depending only on the micro-dislocation tensor. Our relaxed model is still able to fully describe rotation of the microstructure and to predict nonpolar size effects. It is intended for the homogenized description of highly heterogeneous, but nonpolar materials with microstructure liable to slip and fracture. In contrast to classical linear micromorphic models, our free energy is not uniformly pointwise positive definite in the control of the independent constitutive variables. The new relaxed micromorphic model supports well-posedness results for the dynamic and static case. There, decisive use is made of new coercive inequalities recently proved by Neff, Pauly and Witsch and by Bauer, Neff, Pauly and Starke. The new relaxed micromorphic formulation can be related to dislocation dynamics, gradient plasticity and seismic processes of earthquakes. It unifies and simplifies the understanding of the linear micromorphic models.

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A unifying perspective: the relaxed linear micromorphic continuum

A new approach to self-potential (SP) data interpretation for the recognition of a buried causative SP source system is presented. The general model considered is characterized by the presence of primary electric sources or sinks, located within any complex resistivity structure with a flat air-earth boundary. First, using physical considerations of the nature of the electric potential generated by any arbitrary distribution of primary source charges and the related secondary induced charges over the buried resistivity discontinuity planes, a general formula is derived for the potential and the electric field component along any fixed direction on the ground surface. The total effect is written as a sum of elementary contributions, all of the same simple mathematical form. It is then demonstrated that the total electric power associated with the standing natural electric field component can be written in the space domain as a sum of cross-correlation integrals between the observed component of the total electric field and the component of the field due to each single constitutive elementary charge. By means of the cross-correlation bounding inequality, the concept of a scanning function is introduced as the key to the new interpretation procedure. In the space domain, the scanning function is the unit strength electric field component generated by an elementary positive charge. Next, the concept of charge occurrence probability is introduced as a suitable function for the tomographic imaging of the charge distribution geometry underground. This function is defined as the cross-correlation product of the total observed electric field component and the scanning function, divided by the square root of the product of the respective variances. Using this physical scheme, the tomographic procedure is described. It consists of scanning the section, through any SP survey profile, by the unit strength elementary charge, which is given a regular grid of space coordinates within the section, at each point of which the charge occurrence probability function is calculated. The complete set of calculated grid values can be used to draw contour lines in order to single out the zones of highest probability of concentrations of polarized, primary and secondary electric charges. An extension to the wavenumber domain and to three-dimensional tomography is also presented and discussed. A few simple synthetic examples are given to demonstrate the resolution power of the new SP inversion procedure.

Introduction to ground surface self-potential tomography†

Pre-earthquake signals: Underlying physical processes

Macroseismic Rotation Effects and Micromotions.- Development of Earthquake Rotational Effect Study.- Sources of Rotation and Twist Motions.- Some Examples of Rotation Effects: the Tulbagh Earthquake, South Africa.- Theory of Continua and Fields of Defects.- Deviations from Symmetry and Elasticity: Asymmetric Continuum Mechanics.- Degenerated Asymmetric Continuum Theory.- Continuum with Rotation Nuclei and Defects: Dislocation and Disclination Densities.- Towards a Discrete Theory of Defects.- Fault Dynamics and Related Radiation.- A Review on Friction.- Soliton Physics.- Rotation Motions, Seismic Source Models, and Asymmetry of Fracture.- Rotational Motions Excited by Earthquakes.- Ground Rotational Motions Recorded in Near-Source Region of Earthquakes.- Fracture-Band Geometry and Rotation Energy Release.- Rotation Motions: Recording and Analysis.- Glacier Motion: Seismic Events and Rotation/Tilt Phenomena.- Rotational Energy and Angular Momentum of Earthquakes.- Bend-Rotation Wave as a Mechanism of Macroseismic Effects.- Solitary Waves in Crustal Faults and their Application to Earthquakes.- Seismic Rotation Waves: Spin and Twist Solitons.- Earth Rotation, Elasticity and Geodynamics: Earthquake Wave Rotary Model.- Effects Related to Medium Structures and Complexity of Wave Propagation.- Seismic Rotation Waves in the Continuum with Nonlinear Microstructure.- Tectonic Solitons Propagating Along the Fault.- Complexity of Rotation Soliton Propagation.- Micromorphic Continuum with Defects and Taylor-Bishop-Hill Theory for Polycrystals: Anisotropic Propagation of Seismic Waves and the Golebiewska Gauge.- Seismic Ray Theory for Structural Medium based on Kawaguchi and Finsler Geometry.- From Non-Local to Asymmetric Deformation Field.- Earthquake Hazard in the Valley of Mexico: Entropy, Structure, Complexity.- Seismic Rotational Motions: Recording Techniques and Data Analysis.- Note on the Historical Rotation Seismographs.- Ring Laser Gyroscopes as Rotation Sensors for Seismic Wave Studies.- Rotational Motions in Seismology: Theory, Observation, Simulation.- Absolute Rotation Measurement Based on the Sagnac Effect.- Design of Rotation Seismometer and Non-Linear Behaviour of Rotation Components of Earthquakes.- Rotation and Twist Motion Recording - Couple Pendulum and Rigid Seismometers System.- Equation of Pendulum Motion Including Rotations and its Implications to the Strong-Ground Motion.- Strong Motion Rotation Sensor.- High-Resolution Wide-Range Tiltmeter: Observations of Earth Free Oscillations Excited by the 26 December 2004 Sumatra -Andaman Earthquake.- Fiber Optic Sensors for Seismic Monitoring.- Rotations and Engineering Seismology.- Deriving Seismic Surface Rotations for Engineering Purposes.- Effects of Torsional and Rocking Excitations on the Response of Structures.

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Earthquake source asymmetry, structural media and rotation effects

A group of distinguished scientists contributes to the foundations of a new discipline in Earth sciences: earthquake thermodynamics and thermodynamics of formation of the Earth's interior structures. The predictive powers of thermodynamics are so great that those aspiring to model earthquake and the Earth's interior will certainly wish to be able to use the theory. Thermodynamics is our only method of understanding and predicting the behavior of many environmental, atmospheric, and geological processes. The need for Earth scientists to develop a functional knowledge of thermodynamic concepts and methodology is therefore urgent. Sources of an entropy increase the dissipative and self-organizing systems driving the evolution and dynamics of the Universe and Earth through irreversible processes. The non-linear interactions lead to the formation of fractal structures. From the structural phase transformations the important interior boundaries emerge. Non-linear interactions between the defects in solids lead the authors to develop the physics of continua with a dense distribution of defects. Disclinations and dislocations interact during a slow evolution as well as during rapid dynamic events, like earthquakes. Splitting the dynamic processes into the 2D fault done and 3D surrounding space brings a new tool for describing the slip nucleation and propagation along the earthquake faults. Seismic efficiency, rupture velocity, and complexity of seismic source zone are considered from different points of view, fracture band earthquake model is developed on the basis of thermodynamics of line defects, like dislocations. Earthquake thermodynamics offers us a microscopic model of earthquake sources. Physics of defects helps the authors decscribe and explain a number of precursory phenomena caused by the buildup of stresses. Anomalies in electric polarization and electromagnetic radiation prior to earthquakes are considered from this point of view. Through the thermodynamic approach, the authors arrive at the fascinating question of posssibility of earthquake prediction. In general, the Earth is considered here as a multicomponent system. Transport phenomena as well as wave propagation and shock waves are considered in this system subjected also to chemical and phase transformations.

Earthquake thermodynamics and phase transformations in the Earth's interior

The paper introduces a new model earthquake process based on the theory of micromorphic continua. The processes in a focal region are described by deformations of microstructure in time. It is assumed that the fracturing processes as well as phase transformation of metamorphic phenomena have caused in the past certain non-reversible changes which determine the microstructure of focal region. These internal microstructural elements form the attaching points around which the couple stresses arise. The properties of focal region are determined by the constitutive equations. The micromorphic mechanics considers the existence of body couples as determined by a regional stresses and looks after a response field of stresses, stress moments and strains in the focal region. Further, it is explained how microdislocation field is connected with microdeformations and micromorphic structure. In the considered earthquake structure model a microanisotropy is assumed through the tensor of microinertia. This tensor describes a distribution of microelements. Simple solutions of wave processes in a focal region are presented. The dispersion of waves is discussed.

Earthquake processes in a micromorphic continuum

Returning to the old problem of observed rotation effects, we present the recording system and basic elements of
the theory related to the rotation fi eld and its association with seismic waves. There can be many different causes
leading to observed/recorded rotation effects; we can group them as follows: generation of micro-displacement
motion due to asymmetry of source processes and/or due to interaction between seismic body/surface waves and
medium structure; interaction between incident seismic waves and objects situated on the ground surface. New
recording techniques and advanced theory of deformation in media with defects and internal (e.g., granular) structure
make it possible to focus our attention on the fi rst group, related to microdisplacement motion recording, which
includes both rotation and twist motions. Surface rotations and twists caused directly by the action of emerging
seismic waves on some objects situated on the ground surface are considered here only in the historical aspects
of the problem. We present some examples of experimental results related to recording of rotation and twist
components at the Ojcow Observatory, Poland, and L'Aquila Observatory, Italy, and we discuss some prospects
for further research.

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Roman Teisseyre

Papers

Earthquake source asymmetry, structural media and rotation effects

Earthquake thermodynamics and phase transformations in the Earth's interior

Earthquake processes in a micromorphic continuum

Seismic rotation waves: basic elements of theory and recording

Theory of earthquake premonitory and fracture processes