Abstract
This paper deals with the interpretation of travel times of seismic waves from earthquakes and explosions. The location of seismic sources comes ultimately from travel time data but the same data are used in evaluating the wave speeds within the Earth itself, so the two problems of source location and velocity determination cannot be separated. In the early work of Jeffreys and Bullen a large set of travel times from earthquakes was used to refine the locations as well as to calculate the travel times of waves as a function of distance. Since that time instruments have improved and more data has become available but the J-B model is still in date. The principal problem with improving travel time models has been that the Earth is not spherically symmetric and it is simply not possible to fit the data with a spherically symmetric model. For example Herrin et al (22) give an improved set of travel time tables that are considered to be a better fit to the oceanic regions of the world. This reflects improved station coverage rather than any new understanding about Earth. It is clear that further developments must take account of lateral variations within the Earth.
Considerable efforts have been made recently towards finding lateral variations immediately beneath arrays of seismometers (e.g. Aki et al.(1)). These studies are often restricted to regions where there happens to be an array of seismometers, such as at NORSAR or LASA. The velocity models derived from the data are in many instances rather ambiguous. A more serious problem is that all the travel time anomalies are assumed to arise from lateral variations beneath the array whereas we know that the most inhomogeneous parts of the Earth are near sources and so a possible bias in the results will come from source effects. While the idea of finding lateral variations without relocating the sources is an attractive one, there is no sure alternative to finding locations and velocity models simultaneously, as Jeffreys & Bullen did for spherically symmetric models.
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Gubbins, D. (1981). Source Location in Laterally Varying Media. In: Husebye, E.S., Mykkeltveit, S. (eds) Identification of Seismic Sources — Earthquake or Underground Explosion. NATO Advanced Study Institutes Series, vol 74. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-8531-5_29
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DOI: https://doi.org/10.1007/978-94-009-8531-5_29
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