During the quadrennial term 1979–1982, major advances have been made in our knowledge of the rheology of the oceanic lithosphere by the skillful combination of experimental and theoretical rock mechanics, seismology and marine geophysics in increasingly sophisticated models for the flexure of the oceanic lithosphere at seamounts and island chains, along transform faults, and at subduction zones. The relative simplicity of plate bending geometry, thermal history, and mineralogical and chemical compositions of the oceanic plates in these settings make the geophysical observations very powerful constraints on the in situ rheology of the oceanic lithosphere. In the first part of this paper, I review the laboratory work on materials appropriate to the oceanic lithosphere with emphasis on contributions during the quadrennial period and the need for future work. The important results of flexure models incorporating realistic material properties are then summarized.

Rheology of the lithosphere

▶ Gathers original articles on topics in earth and planetary sciences ▶ Coverage includes geomagnetism, aeronomy, space science, seismology, volcanology, geodesy and planetary science ▶ Official journal of the Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan, and The Japanese Society for Planetary Sciences

「Earth,Planets and Space」のオープンアクセス出版

Electrokinetic phenomena are responsible for several electrical properties of fluid-saturated porous materials. Geophysical applications of these phenomena could include the use of streaming potentials for mapping subsurface fluid flow, the study of hydrothermal activity of geothermal areas, and in the context of earthquake prediction and volcanic activity forecasting, for example. The key parameter of electrokinetic phenomena is the ξ potential, which represents roughly the electrical potential at the mineral/water interface. We consider silica-dominated porous materials filled with a binary symmetric 1:1 electrolyte such as NaCl. When in contact with this electrolyte, the silica/water interface gets an excess of charge through chemical reactions. Starting with these chemical reactions, we derive analytical equations for the ξ potential and the specific surface conductance. These equations can be used to predict the variations of these parameters with the pore fluid salinity, temperature, and pH (within a /pH range of 6–8). The input parameters to these equations fall into two categories: (1) mineral/fluid interaction geochemistry (including mineral surface site density and surface equilibrium constants of mineral/fluid reactions), and (2) pore fluid /pH, salinity, and temperature. The ξ potential is shown to increase with increasing temperature and pH and to decrease with increasing salinity. The proposed model is in agreement with available experimental data. The application of this model to electric potentials generated in porous media by fluid flow is explored in the companion paper.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999JB900089

Streaming potential in porous media: 1. Theory of the zeta potential

Following the 2009 L'Aquila earthquake, the Dipartimento della Protezione Civile Italiana (DPC), appointed an International Commission on Earthquake Forecasting for Civil Protection (ICEF) to report on the current state of knowledge of short-term prediction and forecasting of tectonic earthquakes and indicate guidelines for utilization of possible forerunners of large earthquakes to drive civil protection actions, including the use of probabilistic seismic hazard analysis in the wake of a large earthquake. The ICEF reviewed research on earthquake prediction and forecasting, drawing from developments in seismically active regions worldwide. A prediction is defined as a deterministic statement that a future earthquake will or will not occur in a particular geographic region, time window, and magnitude range, whereas a forecast gives a probability (greater than zero but less than one) that such an event will occur. Earthquake predictability, the degree to which the future occurrence of earthquakes can be determined from the observable behavior of earthquake systems, is poorly understood. This lack of understanding is reflected in the inability to reliably predict large earthquakes in seismically active regions on short time scales. Most proposed prediction methods rely on the concept of a diagnostic precursor; i.e., some kind of signal observable before earthquakes that indicates with high probability the location, time, and magnitude of an impending event. Precursor methods reviewed here include changes in strain rates, seismic wave speeds, and electrical conductivity; variations of radon concentrations in groundwater, soil, and air; fluctuations in groundwater levels; electromagnetic variations near and above Earth's surface; thermal anomalies; anomalous animal behavior; and seismicity patterns. The search for diagnostic precursors has not yet produced a successful short-term prediction scheme. Therefore, this report focuses on operational earthquake forecasting as the principle means for gathering and disseminating authoritative information about time-dependent seismic hazards to help communities prepare for potentially destructive earthquakes. On short time scales of days and weeks, earthquake sequences show clustering in space and time, as indicated by the aftershocks triggered by large events. Statistical descriptions of clustering explain many features observed in seismicity catalogs, and they can be used to construct forecasts that indicate how earthquake probabilities change over the short term. Properly applied, short-term forecasts have operational utility; for example, in anticipating aftershocks that follow large earthquakes. Although the value of long-term forecasts for ensuring seismic safety is clear, the interpretation of short-term forecasts is problematic, because earthquake probabilities may vary over orders of magnitude but typically remain low in an absolute sense (< 1% per day). Translating such low-probability forecasts into effective decision-making is a difficult challenge. Reports on the current utilization operational forecasting in earthquake risk management were compiled for six countries with high seismic risk: China, Greece, Italy, Japan, Russia, United States. Long-term models are currently the most important forecasting tools for civil protection against earthquake damage, because they guide earthquake safety provisions of building codes, performance-based seismic design, and other risk-reducing engineering practices, such as retrofitting to correct design flaws in older buildings. Short-term forecasting of aftershocks is practiced by several countries among those surveyed, but operational earthquake forecasting has not been fully implemented (i.e., regularly updated and on a national scale) in any of them. Based on the experience accumulated in seismically active regions, the ICEF has provided to DPC a set of recommendations on the utilization of operational forecasting in Italy, which may also be useful in other countries. The public should be provided with open sources of information about the short-term probabilities of future earthquakes that are authoritative, scientific, consistent, and timely. Advisories should be based on operationally qualified, regularly updated seismicity forecasting systems that have been rigorously reviewed and updated by experts in the creation, delivery, and utility of earthquake information. The quality of all operational models should be evaluated for reliability and skill by retrospective testing, and they should be under continuous prospective testing against established long-term forecasts and alternative time-dependent models. Alert procedures should be standardized to facilitate decisions at different levels of government and among the public. Earthquake probability thresholds should be established to guide alert levels based on objective analysis of costs and benefits, as well as the less tangible aspects of value-of-information, such as gains in psychological preparedness and resilience. The principles of effective public communication established by social science research should be applied to the delivery of seismic hazard information.

/pdf/operational-earthquake-forecasting-state-of-knowledge-and-4t0v3rfrup.pdf

OPERATIONAL EARTHQUAKE FORECASTING. State of Knowledge and Guidelines for Utilization

Deformation of granitoids at low metamorphic grade. I: Reactions and grain size reduction

The ζ potentials and the streaming potential coefficients of silicate mineral-water and rock-water systems were measured for purposes of estimating the magnitude of electrokinetic effects induced by water diffusion in the earth's interior. In rock-water systems the adsorption reaction of H+ at a solid surface is proved to be a main factor determining the ζ potential. For typical crustal rocks the ζ potential in water of pH higher than 2 is commonly negative and increases in absolute magnitude as supporting electrolyte concentration decreases and as temperature increases. A physicochemical theory was developed on the above features of the ζ potential on the basis of the adsorption reaction at the solid-liquid interface. The experimental and theoretical results obtained in the present study of the ζ potential and the streaming potential coefficient in rock-water systems are applied to some geophysical problems: electromagnetic variations induced by water diffusion prior to earthquakes and the self-potential anomaly associated with hydrothermal convection.

Experimental and theoretical basis of electrokinetic phenomena in rock‐water systems and its applications to geophysics

A postprocessor has been developed to calculate space/time distributions of electrokinetic potentials resulting from histories of underground conditions (pressure, temperature, vapor saturation, concentrations of dissolved species, flow rate, etc.) computed by unsteady multidimensional geothermal reservoir simulations. Electrokinetic coupling coefficients are computed by the postprocessor using formulations based on experimental work reported by Ishido and Mizutani [1981]. The postprocessor was applied to both numerical modeling of natural self-potential (SP) anomalies in geothermal fields and production-induced SP changes. The essential features of the SP anomalies and SP changes which have actually been observed in real geothermal fields are reproduced reasonably well in these calculations for both single-phase (liquid) and two-phase (vapor/liquid) geothermal reservoirs. The postprocessor was also applied to predicting the magnitude of electrical earthquake precursory signals caused by dilatant strains taking place within a narrow vertical fault zone. If the rate of porosity increase is high enough to induce substantial pore pressure decrease, two-phase flow develops in the fault zone and produces observable signals at the ground surface if a high conductivity channel connects the near-surface region and the deep fault zone.

/pdf/numerical-simulation-of-electrokinetic-potentials-associated-2lib9w3o8z.pdf

Numerical simulation of electrokinetic potentials associated with subsurface fluid flow

A postprocessor has been developed to calculate space/time distributions of electrokinetic potentials resulting from histories of underground conditions (pressure, temperature, flowrate, etc.) computed by multi-phase multicomponent unsteady multidimensional geothermal reservoir simulations. Electrokinetic coupling coefficients are computed by the postprocessor using formulations based on experimental work reported by Ishido and Mzutani (1981). The purpose of the present study is to examine whether or not self-potential anomalies actually observed in real geothermal fields are consistent with quantitative mathematical reservoir models constructed using conventional reservoir engineering data. The most practical application of the postprocessor appears to be modeling self-potential changes induced by field-wide geothermal fluid production. Repeat self-potential surveying appears to be promising as a geophysical monitoring technique to provide constraints on mathematical reservoir models, in a similar fashion to the use of repeat microgravity surveys.

/pdf/numerical-simulation-of-electrokinetic-potentials-associated-15utgy7g93.pdf

[1] The imaging of hydrothermal systems within volcanoes is critical in evaluating the nature and likelihood of future volcanic activity and hazard assessment. In this study, we present a conceptual model of the hydrothermal system in a volcanic edifice, as deduced from the relationship between electric self-potential (SP) and high-resolution resistivity structures. In order to develop a comprehensive model of water flow in volcanoes, we conducted the audiofrequency (10,000-0.3 Hz) magnetotelluric surveys in five large stratovolcanoes (Iwate, Iwaki, Nasu, Nantai, and Nikko-Shirane) in Japan and found that the obtained 2-D resistivity profiles have a close relationship to the previously reported SP data: good extensive conductors occur beneath areas without SP anomalies, whereas good localized conductors only occur beneath large spatial wavelength SP anomalies on the volcano side of the SP minimum. Also taking into account the locations of surface geothermal activity, the good conductors roughly correspond to the hydrothermal zone, whose upper limit is sealed by a low-permeability clay layer. The sealing layer separates an upper groundwater flow from a lower hydrothermal flow in the subsurface and controls the geothermal manifestations and river locations on the surface. We confirmed the feasibility of the proposed model based on numerical simulations of a hydrothermal system. The horizontal extent of the hydrothermal zone is highly heterogeneous even in a volcanic edifice. This heterogeneity can reflect the geological age of flanks that may be related to the occurrence of a previous large sector collapse.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2008JB005910

Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self‐potential and magnetotellurics

We describe in this paper the theoretical background for the electrokinetics in rocks and
in porous media, to be included in the special issue “Electrokinetics in Earth Sciences” of International Journal of Geophysics. We describe the methodology used for self-potential (SP) and for seismoelectromagnetic measurements, for both field and laboratory experiments and for modelling. We give a large bibliography on the studies performed in hydrology to detect at distance the water flow, to deduce the thickness of the aquifer and to predict the hydraulic conductivity. The observation of SP has also been proposed to detect fractures in boreholes, to follow the hydraulic fracturing, and to predict the earthquakes. Moreover, we detail the studies on geothermal applications.

/pdf/electrokinetics-in-earth-sciences-a-tutorial-k6q8fgdvv8.pdf

Tsuneo Ishido

Papers

Experimental and theoretical basis of electrokinetic phenomena in rock‐water systems and its applications to geophysics

Numerical simulation of electrokinetic potentials associated with subsurface fluid flow

Numerical simulation of electrokinetic potentials associated with subsurface fluid flow

Groundwater flow and hydrothermal systems within volcanic edifices: Delineation by electric self‐potential and magnetotellurics

Electrokinetics in Earth Sciences: A Tutorial