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3 results

  • 2 - Stress, Strain, and Seismic Waves

  • Edward S. Krebes, University of Calgary
  • Book:
    Seismic Wave Theory
    Published online:
    15 March 2019
    Print publication:
    28 March 2019, pp 51-111

  • Summary

    This chapter begins with coverage of the quantitative concepts used to describe the deformation of solids by seismic waves, namely the concepts of stress, strain, and dilatation. This is followed by the derivation of equations for describing seismic wave motion in the subsurface, namely, the equation of motion, conservation of energy, kinetic and strain-energy density, intensity or energy flux, the stress–strain relation, isotropy, hydrostatic stress, elastic constants (which are related to the nature of the medium in which waves travel), the wave equations, compressional and shear waves, plane harmonic waves, displacement potentials, Helmholtz equations, near-field and far-field waves, mean values, and the acoustic wave equation. The chapter ends with examples that discuss seismic waves produced by a buried explosive charge and by a directed point force, and discussions of the moment tensor and apparent velocities.

Seismic Wave Theory
  • Edward S. Krebes
  • Published online:
    15 March 2019
    Print publication:
    28 March 2019
  • Perfect for senior undergraduates and first-year graduate students in geophysics, physics, mathematics, geology and engineering, this book is devoted exclusively to seismic wave theory. The result is an invaluable teaching tool, with its detailed derivations of formulas, clear explanations of topics, exercises along with selected answers, and an additional set of exercises with derived answers on the book's website. Some highlights of the text include: a review of vector calculus and Fourier transforms and an introduction to tensors, which prepare readers for the chapters to come; and a detailed discussion on computing reflection and transmission coefficients, a topic of wide interest in the field; a discussion in later chapters of plane waves in anisotropic and anelastic media, which serves as a useful introduction to these two areas of current research in geophysics. Students will learn to understand seismic wave theory through the book's clear and concise pedagogy.

  • Accurate surface exposure dating with lichens

  • William B. Bull
  • Journal:
    Quaternary Research / Volume 90 / Issue 1 / July 2018
    Published online by Cambridge University Press:
    26 March 2018, pp. 1-9

  • Lichenometry accurately dates exposure times of glacial moraines and landslides when measuring the longest axis of the largest crustose lichen on many blocks, as demonstrated by numerous examples. In Sweden, the sizes of Rhizocarpon subgenus Rhizocarpon describe five pulses of glacial moraine creation in 120 yr. Six historic California earthquakes, between AD 1800 and 1906, caused many landslides that constrain lichen growth as linear with a dating accuracy of±0.5 yr. Crustose lichen sizes date earthquake-created additions to Sierra Nevada talus with an accuracy of±5 yr. The oldest lichen ages are 400 yr for Lecanora sierrae, 800 yr for Lecidea atrobrunnea, and 1100 yr for Acarospora chlorophana and Rhizocarpon subgenus Rhizocarpon. Lichen sizes also record differing spatial attenuation of ground shaking from the magnitude (Mw) ~7.9 San Andreas earthquake of AD 1857 and the more distant, smaller San Jacinto AD 1800 earthquake, which both caused Sierra Nevada rockfalls. AD 1800 seismic shaking was relatively stronger than that of AD 1857 farther north, perhaps expressing stronger Love and Rayleigh styles of surface waves from the north-trending AD 1800 surface rupture that were particularly efficient in causing rockfalls at greater distances.