Determination of a Seismic Wave Velocity from the Travel-Time Curve

Gerver, M. L.; Markushevich, V. M.

doi:10.1111/j.1365-246x.1966.tb03498.x

Cited by 113 publications

(60 citation statements)

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“…If a LVZ exists, this gap is the only possible shadow zone that could be caused by it. As described by Gerver [1966], the velocities in the LVZ cannot be found explicitly via inversion, however, the velocities above and below the LVZ can be calculated using Gerver's formula, and the maximum width of the LVZ can be estimated assuming a constant velocity within it [Aki and Richards, 1980 To discriminate between the two models obtained from the travel time inversions, waveforms are needed from epicentral distances of less than 2200 km. Since most of the waveforms from the TBSE are from distances greater than that, we cannot evaluate further structure above -300-km depth, and we are left with the possibility that a narrow LVZ might exist between a depth of 160 and 185 km in the P wave structure.…”

Section: Data and Travel Time Inversionmentioning

confidence: 99%

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

et al. 1999

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Abstract. The upper mantle seismic velocity structure beneath southern Africa is investigated using travel time and waveform data which come from a large mine tremor in South Africa (mb 5.6) recorded by the Tanzania broadband seismic experiment and by several stations in southern Africa. The waveform data show upper mantle triplications for both the 410-and 670-km discontinuities between distances of 2100 and 3000 km. Auxiliary travel time data along similar profiles obtained from other moderate events are also used. P wave travel times are inverted for velocity structure down to -800-km depth using the Wiechert-Herglotz technique, and the resulting model is evaluated by perturbing it at three depth intervals and then testing the perturbed model against the travel time and waveform data. The results indicate a typical upper mantle P wave velocity structure for a shield. P wave velocities from the top of the mantle down to 300-km depth are as much as 3% higher than the global average and are slightly slower than the global average between 300-and 420-km depth. Little evidence is found for a pronounced low-velocity zone in the upper mantle. A high-velocity gradient zone is required above the 410-km discontinuity, but both sharp and smooth 410-km discontinuities are permitted by the data. The 670-km discontinuity is characterized by high-velocity gradients over a depth range of *-80 km around 660-km depth. Limited S wave travel time data suggest fast S wave velocities above -150-km depth. These results suggest that the bouyant support for the African superswell does not reside at shallow depths in the upper mantle.

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Section: Data and Travel Time Inversionmentioning

confidence: 99%

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

et al. 1999

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“…This is not the case. Gerver and Markushevitch (1966) showed that only the maximum thickness of the L VZ can be constrained from travel time data. Our approach will tend to estimate that a LVZ haa a thickness less than the maximum bound provided by the travel time data.…”

Section: Discussionmentioning

confidence: 99%

Seismic velocity structure and microearthquake source properties at The Geysers, California, geothermal area

O’Connell¹

1986

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The method oC progressive hypocenter-velocity inversion has been extended to incorporate S-wave arrival time data and to estimate S-wave velocities in addition to P-wave velocities.Synthetic tests demonstrate that the joint use oC P and S-wave arrival time data has the Collowing advantages over the-use-oC-P---w,av.e-data-alone;-{-l-}--P--wav-e-v·elacity-and-slowness~gradient-structure are more accurately estimated; (2) hypocenter mislocation errors are substantially reduced, especially hypocentral depth; (3) convergence oC progressive inversions to local minima is more detectable using RMS data, misfits oC P and S-wave data; (4) velocity model and hypocenter estimates are more accurately determined when station corrections are used; (5) errors in linearized resolution and error estimated are reduced; and (6) complete elastic properties are estimated providing greater constraints Cor geologic interpretation oC velocity structure. Adding S-wave data to progressive inversion does not completely eliminate hypocenter-velocity tradeoffs, but they are substantially reduced.Results oC a P and S-wave progressive hypocente~velocity inversion at The Geysers show that the top oC the steam reservoir is clearly defined by a large decrease oC Vp/V. at the condensation zone-production zone contact. The depth interval oC maximum steam production coincides with minimum observed VpfV., and Vp/V. increases below the shallow primary production zone suggesting that reservoir rock becomes more fluid saturated. 2The moment tensor inversion method wast applied to three· microearthquakes at The Geysers. Estimated principal stress orientations were comparable to those estimated using Pwave first motions as constraints. Well constrained principal stress orientations were obtained for one event for which the 17 P-first motions could not distinguish between normal-slip and strike-slip mechanisms. The moment tensor estimates of principal stress orientations were obtained using far fewer stations than required for first-motion focal mechanism solutions. The three ,focal mechanisms lobtain!!d' here support the hypothesis that' focal mechanisms are a Junction of,depth at. The Geysers.'Progressive inversion as developed here and the moment tensor inversion method provide a complete approach for determining earthquake locations, P and S-wave velocity structure, and earthquake source mechanisms.

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“…The lower amplitude first arrivals in this range result from the delay of the geometric arrivals due to the LVZ. For travel-time inversion studies the delayed geometric arrivals are the important phases for identification (SLICHTER, 1932;GERVER and MARKUSHEVITCH, 1966;AKI and RICHARDS, 1980). The incorrect identification of the geometric arrivals would pose interpreta tion problems for both I-D inversions and tomographic studies using travel times from this distance range.…”

Section: Discussionmentioning

confidence: 99%

Slant-stack velocity analysis for one-dimensional upper mantle structure using short-period data from MAJO

Erdoĝan

Nowack

1993

PAGEOPH

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Abstract-In this paper, regional P-wave upper mantle structure is investigated using slant-stack velocity analysis of short-period earthquake data recorded at station MAJO (Matsushiro, Japan). Shallow earthquakes from [1980][1981][1982][1983][1984][1985][1986] within 35° of MAJO are used to construct a common receiver gather. Processing of the wavefie1d data includes focal depth and static time corrections, as well as deterministic deconvolution, in order to equalize pulse shapes and align wavelets on the first arrivals. The processed wavefield data are slant stacked and iteratively downward continued to obtain a regional upper mantle velocity model. The model includes a low velocity rone between 107 and 220 km. Beneath the L VZ, the velocity increases smoothly down to the discontinuity at 40 I km. In the transition zone, the velocity model again increases linearly, although there is some suggestion of further complexity in the downward continued wavefield data. At the base of the transition zone, a second velocity discontinuity occurs at 660 km, with a linear velocity gradient below. In addition to slant-stack analysis, travel times and synthetic seismograms are computed and compared with the processed and unprocessed wavefield data.

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Determination of a Seismic Wave Velocity from the Travel-Time Curve

Cited by 113 publications

References 0 publications

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

Seismic velocity structure and microearthquake source properties at The Geysers, California, geothermal area

Slant-stack velocity analysis for one-dimensional upper mantle structure using short-period data from MAJO

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