Principles of Seismology

Udías, Agustín

doi:10.1017/cbo9781139164306

Cited by 166 publications

(97 citation statements)

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“…which can be calculated through the ray theory within stratified media (e.g., Udías, 2000). Here we examine how the single quantity α i provides good approximation on traveltimes for ray-paths with variable slant distances by calculating the difference between the exact and approximated traveltimes ( Fig.…”

Section: Representing Sound Speed By α α αmentioning

confidence: 99%

Temporal variation of sound speed in ocean: a comparison between GPS/acoustic and in situ measurements

2008

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The GPS/acoustic technique applied to seafloor geodesy intrinsically measures integrated sound speed along a trajectory of an acoustic signal as well as the position of a seafloor transponder array. We present here a generalized expression of sound speed variation in terms of a traveltime residual normalized to the vertical component. With this expression, residual traveltimes to any seafloor transponders will have a same value regardless of their depths and slant angles. This is valid even for the case having horizontal gradient in sound speed structure; the gradient affects only on positioning of a transponder array and not on the estimate of sound speed just beneath the observation point. We monitored temporal variation of this quantity through a GPS/acoustic survey and compared it with in situ expendable bathythermograph (XBT) measurements periodically carried out during the survey. We found that the relative change of the two independent measurements are in good agreement within 5% of the typical amplitude of temporal variation.

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Section: Representing Sound Speed By α α αmentioning

confidence: 99%

Temporal variation of sound speed in ocean: a comparison between GPS/acoustic and in situ measurements

2008

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“…Statistically, the b-value is the slope on the log N * M regression line and is a constant parameter that determines the rate of fall in the frequency of occurrence of events with increasing magnitude. High b-values indicate a large number of small earthquakes expected in regions of low strength and large heterogeneity, and low b-values indicate high resistance (asperity) and homogeneity of the constituent rock-mass (TSAPANOS, 1990;UDIAS, 1999;WASON et al, 2002). In natural situations, b-values are found to lie between 0.5 and 1.5, depending on the tectonics of a region.…”

Section: Introductionmentioning

confidence: 99%

Seismic b-Value and the Assessment of Ambient Stress in Northeast India

Khan

Ghosh

Chakraborty

et al. 2010

Pure Appl. Geophys.

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Abstract-Seismicity data of northeast India, recorded between 1986 and 1999 by a local network, are analysed for estimation of bvalues. Based on the obtained values, viz. low (b B 0.5), moderate (0.5 \ b B 0.7) and high (b [ 0.7), the study area is classified into different seismic-domains. An assessment of stress level is also carried out in identifying seismic-domains. Seismic activities, though mostly confined in some sectors, are presumably triggered by mutual interaction of the Shillong Plateau, Mikir Hills, IndoBurman Ranges and the easternmost part of the Himalayas, and the contributions from deep-seated fractures cannot be ignored. The results resemble the seismic character of a foreland setting adjacent to a convergent margin. The b-values estimated for 240 square grids of dimension 0.6°9 0.6°over five seismic domains indicate wide variation. An analysis of cumulative seismic moment release (M O ) in different layers also indicates an anomaly in reference to the total seismic-energy budget of the five zones. The lower b-value and higher M O recorded at relatively lower depth (*30 km) towards the southwest of the study area might be associated with upward bulging of a strong lithosphere. The bulging is perhaps regionally compensated by the downward flexing of the descending Indian lithosphere beneath the Upper Assam area; features unequivocally observed in any foreland setup. Towards the north and east of the study area, random variations of in both b-value and M O along the converging zone suggest a varied tectonic environment with active interaction between the tectonic elements in these areas.

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“…Because the frequency of a surface wave controls the depth range over which particle motion is excited, high-frequency signals contain information on the shallow part of the earth (e.g. Udías 1999), making ambient-noise studies ideal to assess the structure of the crust and upper mantle. Recordings of earthquake-generated teleseismic surface waves are dominated by lowerfrequency signal and thus sensitive to larger depths; their combination with ambient-noise data results in a data set that is sensitive to a broader depth range than ever previously achieved (Yao et al 2006(Yao et al , 2008Ritzwoller et al 2011;Köhler et al 2012;Zhou et al 2012).…”

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confidence: 99%

Two-receiver measurements of phase velocity: cross-validation of ambient-noise and earthquake-based observations

Kästle¹,

Soomro

Weemstra

et al. 2016

Geophys. J. Int.

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S U M M A R YPhase velocities derived from ambient-noise cross-correlation are compared with phase velocities calculated from cross-correlations of waveform recordings of teleseismic earthquakes whose epicentres are approximately on the station-station great circle. The comparison is conducted both for Rayleigh and Love waves using over 1000 station pairs in central Europe. We describe in detail our signal-processing method which allows for automated processing of large amounts of data. Ambient-noise data are collected in the 5-80 s period range, whereas teleseismic data are available between about 8 and 250 s, resulting in a broad common period range between 8 and 80 s. At intermediate periods around 30 s and for shorter interstation distances, phase velocities measured from ambient noise are on average between 0.5 per cent and 1.5 per cent lower than those observed via the earthquake-based method. This discrepancy is small compared to typical phase-velocity heterogeneities (10 per cent peak-to-peak or more) observed in this period range.We nevertheless conduct a suite of synthetic tests to evaluate whether known biases in ambient-noise cross-correlation measurements could account for this discrepancy; we specifically evaluate the effects of heterogeneities in source distribution, of azimuthal anisotropy in surface-wave velocity and of the presence of near-field, rather than far-field only, sources of seismic noise. We find that these effects can be quite important comparing individual station pairs. The systematic discrepancy is presumably due to a combination of factors, related to differences in sensitivity of earthquake versus noise data to lateral heterogeneity. The data sets from both methods are used to create some preliminary tomographic maps that are characterized by velocity heterogeneities of similar amplitude and pattern, confirming the overall agreement between the two measurement methods.

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Principles of Seismology

Cited by 166 publications

References 0 publications

Temporal variation of sound speed in ocean: a comparison between GPS/acoustic and in situ measurements

Temporal variation of sound speed in ocean: a comparison between GPS/acoustic and in situ measurements

Seismic b-Value and the Assessment of Ambient Stress in Northeast India

Two-receiver measurements of phase velocity: cross-validation of ambient-noise and earthquake-based observations

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