Book contents
- Frontmatter
- Contents
- Preface
- 1 Seismology, the science of earthquakes
- 2 Fundamental equations of an elastic medium
- 3 Elastic waves
- 4 Normal mode theory
- 5 Reflection and refraction
- 6 Ray theory. Media of constant velocity
- 7 Ray theory. Media of variable velocity
- 8 Ray propagation in a spherical medium
- 9 Travel times and the structure of the Earth
- 10 Surface waves
- 11 Wave propagation in layered media
- 12 Wave dispersion. Phase and group velocities
- 13 Free oscillations of the Earth
- 14 Anelasticity and anisotropy
- 15 Focal parameters of earthquakes
- 16 The source mechanism
- 17 The seismic moment tensor
- 18 Models of fracture
- 19 Methods of determination of source mechanisms
- 20 Seismicity, seismotectonics, and seismic risk
- 21 Seismographs and seismograms
- Appendix 1 Vectors and tensors
- Appendix 2 Cyclindrical and spherical coordinates
- Appendix 3 Bessel and Legendre functions
- Appendix 4 Fourier transforms
- Appendix 5 Parameters of the Earth
- Appendix 6 The interior of the Earth
- Appendix 7 Important earthquakes
- Appendix 8 Problems and exercises
- Bibliography
- References
- Index
19 - Methods of determination of source mechanisms
- Frontmatter
- Contents
- Preface
- 1 Seismology, the science of earthquakes
- 2 Fundamental equations of an elastic medium
- 3 Elastic waves
- 4 Normal mode theory
- 5 Reflection and refraction
- 6 Ray theory. Media of constant velocity
- 7 Ray theory. Media of variable velocity
- 8 Ray propagation in a spherical medium
- 9 Travel times and the structure of the Earth
- 10 Surface waves
- 11 Wave propagation in layered media
- 12 Wave dispersion. Phase and group velocities
- 13 Free oscillations of the Earth
- 14 Anelasticity and anisotropy
- 15 Focal parameters of earthquakes
- 16 The source mechanism
- 17 The seismic moment tensor
- 18 Models of fracture
- 19 Methods of determination of source mechanisms
- 20 Seismicity, seismotectonics, and seismic risk
- 21 Seismographs and seismograms
- Appendix 1 Vectors and tensors
- Appendix 2 Cyclindrical and spherical coordinates
- Appendix 3 Bessel and Legendre functions
- Appendix 4 Fourier transforms
- Appendix 5 Parameters of the Earth
- Appendix 6 The interior of the Earth
- Appendix 7 Important earthquakes
- Appendix 8 Problems and exercises
- Bibliography
- References
- Index
Summary
Parameters and observations
The determination of the source mechanism of earthquakes consists in finding the parameters of the model used in its representation, whose number depends on its complexity. The localization of the source is given by four parameters (φ, λ, h, and t0), the coordinates of the epicenter, depth of focus, and origin time (section 15.2). Generally, in determinations of focal mechanisms, these parameters are assumed to be known, although in some methods some (the focal depth) or all (the centroid) are determined anew. The size given by the magnitude M is independently determined, but, in many methods, the seismic moment M0 is evaluated as part of the mechanism.
The simplest models of source mechanism are those with point sources. For a point shear dislocation, the orientation of the mechanism is given by the angles φ, δ, and λ (section 15.1). For the equivalent double-couple force model, the orientation is given by those of the X and Y axes, that is, by the angles ⊗x, φy, and φy, or by the P and T axes, angles ⊗P, φP, and φT (section 16.2). If the source is not a shear fracture, it can be represented by five components of the moment tensor Mij (for a deviatoric source) or six if there are changes in volume (Chapter 17). Including its size, a shear-fracture point source or DC source is given by four parameters (M0, φ, δ, and λ) plus four for its location, that is a total of eight parameters.
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- Information
- Principles of Seismology , pp. 359 - 375Publisher: Cambridge University PressPrint publication year: 2000