Definition
Focal mechanisms are geometrical or mathematical representations of the faulting during an earthquake. In very simple terms, the latter consists of the relative motions of two blocks of Earth called walls along a planar surface called fault. Figure 1 shows that the description of an earthquake rupture needs three angles. The strike angle φ is the azimuth (with respect to North) of the trace of the fault on a horizontal plane such as the Earth’s surface; the dip angle δ characterizes the steepness of the fault, and the rake or slip angle λ, the direction of motion, within the fault plane and relative to the horizontal, of the hanging wall relative to the foot wall. A full description of the earthquake rupture requires an additional scalar, related to the length of slip of the fault walls, and thus to the size or magnitude of the event.
(a) and (b) Before and after sketches of the displacement of fault blocks during an earthquake. The...
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Bibliography
Aki, K., and Richards, P. G., 1980. Quantitative Seismology. New York: W.H. Freeman, p. 932.
Antolik, M., Dreger, D. S., and Romanowicz, B., 1999. Rupture processes of large deep-focus earthquakes from inversion of moment rate functions. Journal of Geophysical Research, 104, 863–894.
Bridgman, P. W., 1945. Polymorphic transition and geological phenomenon. American Journal of Science, 243A, 90–97.
Dziewonski, A. M., Chou, A.-T., and Woodhouse, J. H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. Journal of Geophysical Research, 86, 2825–2852.
Ekström, G., 1994. Anomalous earthquakes on volcano ring-fault structures. Earth and Planetary Science Letters, 128, 707–712.
Frohlich, C., and Apperson, K. D., 1992. Earthquake focal mechanisms, moment tensors and the consistency of seismic activity near plate boundaries. Tectonics, 11, 279–296.
Gilbert, J. F., and Dziewonski, A. M., 1975. An application of normal mode theory to the retrieval of structural parameters and source mechanisms from seismic spectra. Philosophical Transaction of Royal Society London, 278A, 187–269.
Isacks, B. L., and Molnar, P., 1971. Distribution of stresses in the descending lithosphere from a global survey of focal-mechanism solutions of mantle earthquakes. Reviews of Geophysics and Space Physics, 9, 103–174.
Isacks, B. L., Oliver, J., and Sykes, L. R., 1968. Seismology and the newglobal tectonics. Journal of Geophysical Research, 73, 5855–5899.
Kagan, Y. Y., 1991. 3-D rotation of double-couple earthquake sources. Geophysical Journal International, 106, 709–716.
Kanamori, H., Ekström, G., Dziewoński, A. M., Barker, J. S., and Sipkin, S. A., 1993. Seismic radiation by magma injection: an anomalous seismic event near Tori Shima, Japan. Journal of Geophysical Research, 98, 6511–6522.
Kawakatsu, H., 1991. Insignificant isotropic components in the moment tensor of deep earthquakes. Nature, 351, 50–53.
Kirby, S. H., Stein, S., Okal, E. A., and Rubie, D., 1996. Deep earthquakes and metastable mantle phase transformations in subducting oceanic lithosphere. Reviews of Geophysics and Space Physics, 34, 261–306.
Knopoff, L., and Gilbert, J. F., 1959. Radiation from a strike-slip earthquake. Bulletin of the Seismological Society of America, 49, 163–178.
Li, X., Shao, G., and Ji, C., 2009. Rupture process of the M w = 8.1 Samoa earthquake constrained by joint inverting teleseismic body, surface waves and local strong motion. Eos Transactions, American Geophysical Union, 90, (52), U21D–03, [abstract].
Nettles, M., and Ekström, G., 2009. Ice loss and glacial earthquakes in Greenland and Antarctica. Eos Transactions, American Geophysical Union, 90(52), G52B–01 [abstract].
Okal, E. A., 1996. Radial modes from the great 1994 Bolivian earthquake: no evidence of an isotropic component to the source. Geophysical Research Letters, 23, 431–434.
Stein, S., and Wysession, M. E., 2003. An Introduction to Seismology, Earthquakes and Earth Structure. Malden: Blackwell, 498 pp.
Sykes, L. R., 1967. Mechanisms of earthquakes and nature of faulting on the mid-oceanic ridges. Journal of Geophysical Research, 72, 2131–2153.
Vvedenskaya, A. V., 1956. Opredelenie polej smeshchenii pri zemletryasenyakh s pomoshchyu teorii dislokatsii. Izv Akad Nauk SSSR, Ser Geofiz, 6, 277–284 [in Russian].
Wilson, J. T., 1965. A new class of faults and their bearing on continental drift. Nature, 207, 343–347.
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Okal, E.A. (2011). Earthquake, Focal Mechanism. In: Gupta, H.K. (eds) Encyclopedia of Solid Earth Geophysics. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi.org/10.1007/978-90-481-8702-7_158
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