Amplitude of<i>P</i>waves recorded at New Zealand stations from shallow earthquakes at less than 30°

Gibowicz, S. J.

doi:10.1080/00288306.1972.10422336

Cited by 13 publications

(5 citation statements)

References 21 publications

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“…Studies of amplitudes of P-wave first arrivals (Vanek 1968;Gibowicz 1972) indicate that amplitudes generally decrease fairly steadily with distance until about lo", increase to a maximum near 20" (the so-called ' 20" discontinuity '), and then decrease again (cf. inset to Fig.…”

Section: Fig 1 Ray Pathsmentioning

confidence: 99%

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P-Wave Scattering in the Earth's Crust and Upper Mantle

1975

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The codas which follow P-phase onsets on seismic records can be interpreted in terms of waves scattered from random inhomogeneities in the Earth's crust and upper mantle. In particular, the so-called precursors to PP have travel times and slownesses consistent with the interpretation of these signals as waves scattered from direct P-waves incident at the Earth's surface at distances greater than about 20" from either the source or receiver. Significant differences between recordings at the Warramunga array of PP precursors from two Novaya Zemlya explosions are extremely difficult to account for on any interpretation previously suggested, but are fully consistent with the random variations to be expected on the proposed scattering mechanism.The present interpretation accounts for the major observed features of P codas, including their durations and the variations in slowness along the trains. Precursors to P'P' can also be interpreted as scattering phenomena. Thus the interpretation removes the need to postulate sharp reflecting discontinuities in the uppermost few hundred kilometres of the upper mantle.

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Section: Fig 1 Ray Pathsmentioning

confidence: 99%

“…Below each trace, the unlabelled arrows indicate the trains of larger-amplitude arrivals commonly identified as ' precursors to PP '. Inset: Amplitude-distance curves for North Island(----) and South Island (------) of New Zealand (afterGibowicz, 1972), and for south-eastern Europe (. .…”

mentioning

confidence: 99%

P-Wave Scattering in the Earth's Crust and Upper Mantle

1975

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“…The density was derived from these velocities using the Nafe-Drake curve (Nafe & Drake 1957). Attenuation models were estimated from previous Q studies in the region (Hatherton 1970;Mooney 1970Gibowicz 1972Chadwick 1990). The results are summarised in Table 1. The major computational task is to find the eigenfrequencies and eigenfunctions of the Rayleigh modes (Takeuchi & Saito Table 1.…”

Section: Methodsmentioning

confidence: 99%

Seismic source parameters in New Zealand from broad-band data

Nformi

Mao

Gubbins

1996

Geophysical Journal International

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The Leeds Tararua array of nine broad-band, three-component seismometers was deployed in southern North Island, New Zealand, from 1991 January to 1992 September and recorded many local events. We selected 111 events with local magnitude 12.5 and good station coverage, and attempted to find their source mechanisms by using conventional first motions and by matching of the vertical component waveforms to Rayleigh-mode synthetics. 90 source mechanisms were found: 32 from the method of first motions, 18 by waveform matching, and 40 using both methods. A comparison of mechanisms for events using both methods suggests that waveform matching is reliable even for small events where the coverage on the focal sphere does not allow a firstmotion mechanism to be found. The mechanisms are of all types but with a predominance of normal faulting with one of the planes aligned approximately NE-SW. There are many dip-slip events. Deep (60-80 km) events are mostly thrusts, and there is some evidence of a trend from normal faulting in the shallow part of the subducted slab and upper seismic plane to thrusts in the deeper part, although the pattern is not clear cut.A complex stress pattern is to be expected in this region, which is near the southern end of the Tonga-Kermadec-Hikurangi subduction zone. The magnitudemoment relation ML = (0.84 f 0.23) log M o -(7.53 f 1.48) is found for the region, where ML is the local magnitude and M , the seismic moment in N m.

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“…For P it would be about 0.017. Gibowicz (1972) for New Zealand got values for P from 0.014 to 0.0055. Jeffreys & Shimshoni (1966) attributed the variation of amplitude (apart from the factor in distance) to variation of velocity with depth, and this explanation is not wholly excluded.…”

Section: The Modified Lomnitz Lawmentioning

confidence: 99%