Seismic surveys on land must be designed so that the source‐generated noise, such as ground roll, is preferentially attenuated before P‐wave signal amplification and recording. The correct specification of spatial and frequency filters requires prior knowledge of the noise properties in the area. We show that the strong Rayleigh wave component of source‐generated noise has a wavelength range which is predictable on a regional scale, using widespread P‐wave velocity measurements in shallow upholes. This predictive capability decreases the number of noise analyses required to map the boundaries between areas with different Rayleigh wave properties. The case history presented is for northeastern Saudi Arabia, an area of roughly [Formula: see text]. The data comprise 80 noise analyses and a data base of over 10,000 up‐hole measurements of P‐wave velocities, supplemented by maps of topography and geologic outcrops. Examples show that the frequency‐wavenumber transforms of time‐offset records can be interpreted in detail in terms of Rayleigh wave dispersion and air wave coupling, dictated by the elastic properties of the very shallow layers. P‐wave velocities, measured in shallow upholes at noise analysis sites, are used to form initial estimates of the corresponding shear‐wave velocities and subsequently refined by matching the observed and predicted dispersion curves. Even without this refinement process, the initial S‐wave velocities can be used to estimate Rayleigh wave velocities at frequencies which typify the top and bottom of current vibrator sweeps (10 and 80 Hz). These velocities are mapped for the area and used to determine the wavelength range of Rayleigh waves. An effort is also made to map regions where Rayleigh wave scattering from surface topography is likely to occur.
A search of seismograms recorded at the Warramunga seismic array (WRA) from events occurring below the Izu-Bonin Islands shows an arrival on some, but not all, of the records, with an onset at 25-30 s after P , which is not predicted by the standard travel-time tables. The slowness and azimuth of the phase show that it is generated almost in line with P , and the variation of arrival time with the hypocentral depth of the earthquake indicates that its origin lies on the receiver side of the source. It appears, in fact, to be an S to P conversion at a depth of 650-700 km, which is seen only when the receiver is close to a node of the P radiation pattern and an antinode for S so that its amplitude compared with that of P is at a maximum.Finally, the duration of the phase indicates that it is not simply a refracted wave, but that it has a coda of scattered arrivals from lateral heterogeneity in the neighbourhood of 650 km below the Izu-Bonin Islands.
Earthquakes at consideiable depths in the upper mantle generally produce simple teleseismic P signals, whilst the converse is true for shallow earthquakes. This is probably because deep (greater than 300 km, say) hypocentres are remote from the strong variations in velocity, density and anelastic attenuation which occur in the crust or shallow upper mantle; consequently, many of those mechanisms which proceed via structural to signal complexity cannot be excited until many tens of seconds after the origin time, and the early P codas of deep earthquakes have a proportionately smaller amplitude and level of complexity. The corollary for shallow earthquake P codas is obvious.Here, we consider some complex examples from a suite of deep earthquake seismograms, originally assembled to study the distribution of anelastic attenuation in the source region.Their common denominator is that direct P has an anomalously low amplitude (in this case, ., 1975, Short period teleseismic S waves, Sleep, N., 1973. Teleseismic P wave transmission through slabs, Bull. seism. SOC. Am., 63, 1349-1 373. travel times, Bull. seism. SOC. A m . , 58, 1273-1291.Nature, 253,181-182.
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