1970 11 21 Nature - Vol 228 No 5273 - Earthquake Lightning

Magazine Overview

Title: NATURE
Issue: VOL. 228
Date: November 21, 1970
Publisher: Nature Publishing Group
Country: United Kingdom
Document Type: Magazine Issue

Articles

"Earthquake Lightning"

This article, authored by David Finkelstein and James Powell, explores the phenomenon of "earthquake lightning," suggesting a causal link between seismic strains and atmospheric electric discharges. While acknowledging theories like streaming potential from subterranean water flow, the authors dismiss them as requiring seismic stresses far greater than typically occur during earthquakes. They also rule out electrostatic generation by dust as significant for these events.

The primary hypothesis presented is the "seismoelectric effect," where the piezoelectric properties of the Earth's crust generate electric fields. Quartz is identified as a key piezoelectric constituent, but its effectiveness depends on long-range crystalline order or texture. The presence of such order in quartz-rich rocks, likely resulting from stress history in tectonic units, is implied. The authors cite the North Idu peninsula earthquake of November 26, 1930, as a well-documented instance of seismoelectricity, noting the prevalence of quartz-rich lava flows and dikes in the region, including a significant quartz diorite plug near the Hakone fault.

Calculations suggest that field measurements during earthquakes in this region should reveal ground voltage differences capable of causing atmospheric electric discharges. The authors express surprise that while minute piezomagnetic fields of seismic origin have been explored, gross piezoelectric fields have received little attention.

Furthermore, the article predicts low-frequency electromagnetic radiation from seismoelectric waves (10 Hz to below 1 Hz) and from transient stress changes. Seismic waves are described as acting as an effective 1 km³ antenna carrying currents of 1 to 10 A, with a spectral maximum near 1.5 Hz. Radiation into the atmospheric cavity (fundamental frequency ~7 Hz) is expected. While radiation from 1.5 Hz seismic waves would be low power, higher frequency transients could be more intense.

Electrical precursors to earthquakes, resulting from stress changes near earthquake foci, are also discussed. The authors suggest that the tradition of earthquake prediction in Japan, sometimes aided by clear sky lightning, could be placed on a more systematic basis with improved electric measuring instruments.

"Crack Growth under Creep Conditions"

Authored by M. J. Siverns and A. T. Price, this article addresses the challenge of quantifying the behavior of engineering components with crack-like defects operating at elevated temperatures, specifically under creep conditions. It builds upon the established success of linear elastic fracture mechanics in predicting behavior in fast fracture and fatigue crack growth.

The study introduces a convenient technique for measuring crack length at elevated temperatures using an electrical resistance method, which has been refined over three years. The research focused on samples of 2.25 per cent Cr, 1 per cent Mo steampipe steel, quenched to simulate a weld's heat-affected zone, and tested in air at 565° C.

Edge-notched tensile specimens were subjected to dead loads, producing nominal stresses from 10 to 44 hbar and causing fracture within approximately 1000 hours. The results, presented in Figure 1, establish a relationship between the opening mode stress intensity factor (K) and crack growth rate (á) described by the equation á = constant K^n, where 'n' is approximately 5.5. This exponent is comparable to that relating minimum creep rate to stress for similar materials.

Correlations of growth rate with nominal applied stress or net section stress yielded approximately parallel lines. While acknowledging the limitations of applying linear elastic fracture mechanics to creep situations, the authors consider the findings to be a useful empirical relationship. Preliminary tests suggest that relationships for high-temperature crack propagation can be established, analogous to those successful in lower-temperature applications.

Mathematical Appendices

Spherical Harmonic Series Rotation

An appendix by Hemendra K. Acharya details the mathematical transformations required to rotate spherical harmonic series from one polar axis to another. This is necessary for studies involving displacements of magnetic patterns in latitude. The rotation is specified by an angle and an axis, and can be represented as a product of matrices. The article provides formulas for transforming coefficients (a_nm and b_nm) of magnetic potential (v) and gravitational potential (U) under rotation.

Recurring Themes and Editorial Stance

The issue highlights the interdisciplinary nature of scientific inquiry, bridging geophysics, physics, and materials science. The articles on "Earthquake Lightning" and "Crack Growth under Creep Conditions" both emphasize the importance of developing quantitative models and experimental techniques to understand complex natural phenomena and engineering challenges. The editorial stance appears to favor rigorous scientific investigation, proposing new hypotheses and developing methodologies to address previously intractable problems.