scholarly journals t* for S waves with a continental ray path

1978 ◽  
Vol 68 (4) ◽  
pp. 1013-1030
Author(s):  
L. J. Burdick
Keyword(s):  
Body Wave ◽  
Body Waves ◽  
P Wave ◽  
Lateral Asymmetry ◽  
Deep Earthquake ◽  
Data Set ◽  
S Waves ◽  
Short Period ◽  
Wave Forms ◽  
Ray Path

abstract The purpose of this study was to determine t* for S waves with ray paths under the continental United States. The data set consists of long- and short-period body waves from the Borrego Mountain earthquake as observed in the northeastern U.S. The P wave forms are dominated by the sP phase and the SH wave forms by the sS. It is assumed that there are no losses in pure compression so that the relative attenuation rate of P and S waves is known. The initial source radiation is determined from the sP phase and the value of tβ* from the spectral content of the S wave. The results indicate that tβ* is 5.2 ± 0.7 sec along this ray path. Long- and short-period body waves from some deep South American events are used to test for lateral asymmetry of the Q distribution under the U.S. No lateral amplitude variation exists in this data, but this result is difficult to correlate with many previous results. The t°* value for a 600-km deep earthquake appears to be about 3 sec. A comparison of these values with values computed from current models of the Earth's Q distribution indicates that the models are slightly too high in Q overall and that more of the total body-wave attenuation occurs above 600 km than is indicated by the models.

Crustal structure in the Gangetic Plains of the Indian sub-continent from body waves

1974 ◽  
Vol 64 (3-1) ◽  
pp. 571-579
Author(s):  
R. K. Dube ◽  
J. C. Bhayana
Keyword(s):  
Crustal Structure ◽  
Body Wave ◽  
Bottom Layer ◽  
Body Waves ◽  
P Wave ◽  
Gangetic Plains ◽  
S Waves ◽  
Shear Wave Velocities ◽  
P Wave Velocity ◽  
Peninsular Shield

abstract Crustal structure in the Gangetic Plains of India has been investigated using body-wave data of earthquakes. A three-layered crust has been interpreted, consisting of a top layer of 2.7-km/sec P-wave velocity and 3.7-km thickness, an intermediate layer of 5.64-km/sec velocity and 15.2-km thickness and a bottom layer of 6.49-km/sec velocity and 21.4-km thickness. The average depth of the M discontinuity obtained is 40.3 km. The shear-wave velocities for the Sg, S*, Sn phases are 3.45, 3.85 and 4.61 km/sec, respectively. The velocities of both P and S waves are lower than those obtained for the Peninsular Shield of India.

The partition of radiated energy between P and S waves

1984 ◽  
Vol 74 (2) ◽  
pp. 361-376
Author(s):  
John Boatwright ◽  
Jon B. Fletcher
Keyword(s):  
Wave Energy ◽  
Body Wave ◽  
Focal Mechanisms ◽  
The Body ◽  
Body Waves ◽  
P Wave ◽  
Corner Frequency ◽  
S Wave ◽  
S Waves ◽  
Radiated Energy

Abstract Seventy-three digitally recorded body waves from nine multiply recorded small earthquakes in Monticello, South Carolina, are analyzed to estimate the energy radiated in P and S waves. Assuming Qα = Qβ = 300, the body-wave spectra are corrected for attenuation in the frequency domain, and the velocity power spectra are integrated over frequency to estimate the radiated energy flux. Focal mechanisms determined for the events by fitting the observed displacement pulse areas are used to correct for the radiation patterns. Averaging the results from the nine events gives 27.3 ± 3.3 for the ratio of the S-wave energy to the P-wave energy using 0.5 〈Fi〉 as a lower bound for the radiation pattern corrections, and 23.7 ± 3.0 using no correction for the focal mechanisms. The average shift between the P-wave corner frequency and the S-wave corner frequency, 1.24 ± 0.22, gives the ratio 13.7 ± 7.3. The substantially higher values obtained from the integral technique implies that the P waves in this data set are depleted in energy relative to the S waves. Cursory inspection of the body-wave arrivals suggests that this enervation results from an anomalous site response at two of the stations. Using the ratio of the P-wave moments to the S-wave moments to correct the two integral estimates gives 16.7 and 14.4 for the ratio of the S-wave energy to the P-wave energy.

Identification of multiple underground explosions using the relative amplitude method

1988 ◽  
Vol 78 (2) ◽  
pp. 885-897
Author(s):  
R. A. Clark ◽  
R. G. Pearce
Keyword(s):  
Body Wave ◽  
The Body ◽  
P Wave ◽  
Relative Amplitude ◽  
Time Data ◽  
Large Measure ◽  
Source Discrimination ◽  
Nuclear Explosions ◽  
Short Period ◽  
Wave Forms

Abstract The relative amplitude method is applied to the few available good quality teleseismic P-wave seismograms from five presumed double nuclear explosions and one known multiple chemical explosion, under the “naive” assumption that the observed multiple arrivals correspond to P, pP, and sP from a single earthquake—an interpretation which is indeed consistent with the body-wave arrival time data in most cases. The purpose is to investigate the ability of relative amplitudes to identify correctly such multiple events for which established discrimination criteria may give earthquake-like or ambiguous results. For five of the examples, observed relative amplitudes from only four azimuthally well-distributed array seismograms are sufficient to exclude the single-earthquake interpretation. Deliberate attempts to simulate earthquake teleseismic P wave-forms using multiple explosions are restricted to simulation studies, and one of these is analyzed here using the same approach. We conclude that relative amplitudes can act as a valuable aid to source discrimination in cases where complexity gives rise to fallibility of conventional discriminants, even where only a small number of well-distributed teleseismic short-period array seismograms are available, their signal-to-noise ratios being maximized by suitable array design and careful choice of array site. The network need not be dense, since closely spaced observations of the focal sphere generally embody a large measure of redundancy.

scholarly journals Inner core anisotropy measured using new ultra-polar PKIKP paths

2020 ◽  
Vol 223 (2) ◽  
pp. 1230-1246
Author(s):  
Henry Brett ◽  
Arwen Deuss
Keyword(s):  
Seismic Anisotropy ◽  
Inner Core ◽  
Rotation Axis ◽  
Body Waves ◽  
P Wave ◽  
Mantle Structure ◽  
Data Set ◽  
Seismic Stations ◽  
Ray Path ◽  
Ray Paths

SUMMARY We measure the seismic anisotropy of the inner core using PKPbc-PKPdf and PKPab-PKPdf differential traveltimes, as a function of the angle ζ between the Earth’s rotation axis and the ray path in the inner core. Previous research relied heavily on body waves originating in the South Sandwich Islands (SSI) and travelling to seismic stations in Alaska to sample inner core velocities with low ζ (polar paths). These SSI polar paths are problematic because they have anomalous travel time anomalies, there are no ultra-polar SSI paths with ζ < 20° and they only cover a small part of the inner core. Here we improve constraints on inner core anisotropy using recently installed seismic stations at high latitudes, especially in the Antarctic, allowing us to measure ultra-polar paths with ζ ranging from 20°–5°. Our new data show that the SSI’s polar events are fast but still within the range of velocities measured from ray paths originating elsewhere. We further investigate the effect of mantle structure on our data set finding that the SSI data are particularly affected by fast velocities underneath the SSI originating from the subducted South Georgia slab, which is currently located just above the core mantle boundary. This fast velocity region results in mantle structure being misinterpreted as inner core structure and we correct for this using a P-wave tomographic model. We also analyse the effect of velocity changes on the ray paths within the inner core and find that faster velocities significantly change the ray path resulting in the ray travelling deeper into the inner core and spending more time in the inner core. To remove this effect, we propose a simple but effective method to correct each event-station pair for the velocity-dependent ray path changes in the inner core, producing a more reliable fractional traveltime measurement. Combining the new ultra-polar data with mantle and ray path corrections results in a more reliable inner core anisotropy measurement and an overall measured anisotropy of 1.9–2.3 per cent for the whole inner core. This is lower than previous body wave studies (3 per cent anisotropy) and in better agreement with the value of inner core anisotropy measured by normal modes (2 per cent anisotropy). We also identify regional variation of anisotropic structure in the top 500 km of the inner core, which appears to be more complex than simple hemispherical variations. These regional variations are independent of the SSI data and are still present when these data are excluded. We also find a potential innermost inner core with a radius of 690 km and stronger anisotropy.

Interferometric body-wave retrieval from ambient noise after polarization filtering: application to shallow reflectivity imaging

Geophysics ◽  
2021 ◽  
pp. 1-58
Author(s):  
Deepankar Dangwal ◽  
Michael Behm
Keyword(s):  
Wave Energy ◽  
Ambient Noise ◽  
Body Wave ◽  
Body Waves ◽  
P Wave ◽  
Receiver Array ◽  
Short Period ◽  
Wave Imaging ◽  
Polarization Filtering ◽  
The Impact

Interferometric retrieval of body waves from ambient noise recorded at surface stations is usually challenged by the dominance of surface-wave energy, in particular in settings dominated by anthropogenic activities (e.g., natural resource exploitation, traffic, infrastructure construction). As a consequence, ambient noise imaging of shallow structures such as sedimentary layers remains a difficult task for sparse and irregularly distributed receiver networks. We demonstrate how polarization filtering can be used to automatically extract steeply inclined P-waves from continuous three-component recordings and in turn improves passive body-wave imaging. Being a single-station approach, the technique does not rely on a dense receiver array and is therefore well suited for data collected during surveillance monitoring for tasks such as reservoir hydraulic stimulation, CO_2 sequestration, and wastewater disposal injection. We apply the method on a continuous dataset acquired in the Wellington oilfield (Kansas, US), where local and regional seismicity, and other forms of ambient noise provide an abundant source of both surface- and body-wave energy recorded at 15 short-period receivers. We use autocorrelation to derive the shallow (lt; 1 km) reflectivity structure below the receiver array and validate our workflow and results with well logs and active seismic data. Raytracing analysis and waveform modeling indicates that converted shear waves need to be taken into account for realistic ambient noise body-wave source distributions, as they can be projected on the vertical component and might lead to misinterpretation of the P-wave reflectivity structure. Overall, our study suggests that polarization filtering significantly improves passive body-wave imaging on both autocorrelation and interstation crosscorrelation. It reduces the impact of time-varying noise source distributions and is therefore also potentially useful for time-lapse ambient noise interferometry.

A teleseismic body-wave analysis of the May 1980 Mammoth Lakes, California, earthquakes

1983 ◽  
Vol 73 (2) ◽  
pp. 419-434
Author(s):  
Jeffery S. Barker ◽  
Charles A. Langston
Keyword(s):  
Body Wave ◽  
Moment Tensor ◽  
Normal Fault ◽  
Strike Slip ◽  
Body Waves ◽  
P Wave ◽  
Moment Tensors ◽  
Double Couple ◽  
The Moment ◽  
Mammoth Lakes

abstract Teleseismic P-wave first motions for the M ≧ 6 earthquakes near Mammoth Lakes, California, are inconsistent with the vertical strike-slip mechanisms determined from local and regional P-wave first motions. Combining these data sets allows three possible mechanisms: a north-striking, east-dipping strike-slip fault; a NE-striking oblique fault; and a NNW-striking normal fault. Inversion of long-period teleseismic P and SH waves for the events of 25 May 1980 (1633 UTC) and 27 May 1980 (1450 UTC) yields moment tensors with large non-double-couple components. The moment tensor for the first event may be decomposed into a major double couple with strike = 18°, dip = 61°, and rake = −15°, and a minor double couple with strike = 303°, dip = 43°, and rake = 224°. A similar decomposition for the last event yields strike = 25°, dip = 65°, rake = −6°, and strike = 312°, dip = 37°, and rake = 232°. Although the inversions were performed on only a few teleseismic body waves, the radiation patterns of the moment tensors are consistent with most of the P-wave first motion polarities at local, regional, and teleseismic distances. The stress axes inferred from the moment tensors are consistent with N65°E extension determined by geodetic measurements by Savage et al. (1981). Seismic moments computed from the moment tensors are 1.87 × 1025 dyne-cm for the 25 May 1980 (1633 UTC) event and 1.03 × 1025 dyne-cm for the 27 May 1980 (1450 UTC) event. The non-double-couple aspect of the moment tensors and the inability to obtain a convergent solution for the 25 May 1980 (1944 UTC) event may indicate that the assumptions of a point source and plane-layered structure implicit in the moment tensor inversion are not entirely valid for the Mammoth Lakes earthquakes.

scholarly journals Triplicated P-wave measurements for waveform tomography of the mantle transition zone

Solid Earth ◽  
2012 ◽  
Vol 3 (2) ◽  
pp. 339-354 ◽  
Author(s):  
S. C. Stähler ◽  
K. Sigloch ◽  
T. Nissen-Meyer
Keyword(s):  
Transition Zone ◽  
Epicentral Distance ◽  
Body Wave ◽  
Source Parameters ◽  
Body Waves ◽  
P Wave ◽  
Theoretical Modelling ◽  
Finite Frequency ◽  
Mantle Transition Zone ◽  
Band Limited

Abstract. Triplicated body waves sample the mantle transition zone more extensively than any other wave type, and interact strongly with the discontinuities at 410 km and 660 km. Since the seismograms bear a strong imprint of these geodynamically interesting features, it is highly desirable to invert them for structure of the transition zone. This has rarely been attempted, due to a mismatch between the complex and band-limited data and the (ray-theoretical) modelling methods. Here we present a data processing and modelling strategy to harness such broadband seismograms for finite-frequency tomography. We include triplicated P-waves (epicentral distance range between 14 and 30°) across their entire broadband frequency range, for both deep and shallow sources. We show that is it possible to predict the complex sequence of arrivals in these seismograms, but only after a careful effort to estimate source time functions and other source parameters from data, variables that strongly influence the waveforms. Modelled and observed waveforms then yield decent cross-correlation fits, from which we measure finite-frequency traveltime anomalies. We discuss two such data sets, for North America and Europe, and conclude that their signal quality and azimuthal coverage should be adequate for tomographic inversion. In order to compute sensitivity kernels at the pertinent high body wave frequencies, we use fully numerical forward modelling of the seismic wavefield through a spherically symmetric Earth.

Upper-mantle discontinuity from amplitude data of P′P′ and its precursors

1973 ◽  
Vol 63 (2) ◽  
pp. 587-597
Author(s):  
Ta-Liang Teng ◽  
James P. Tung
Keyword(s):  
Upper Mantle ◽  
Body Waves ◽  
P Wave ◽  
Specific Reference ◽  
Amplitude Data ◽  
Surface Displacements ◽  
Short Period ◽  
Mantle Velocity ◽  
Mantle Discontinuity ◽  
Upper Mantle Discontinuity

abstract Recent observations of P′P′ and its precursors, identified as reflections from within the Earth's upper mantle, are used to examine the structure of the uppermantle discontinuities with specific reference to the density, the S velocity, and the Q variations. The Haskell-Thomson matrix method is used to generate the complex reflection spectrum, which is then Fourier synthesized for a variety of upper-mantle velocity-density and Q models. Surface displacements are obtained for the appropriate recording instrument, permitting a direct comparison with the actual seismograms. If the identifications of the P′P′ precursors are correct, our proposed method yields the following: (1) a structure of Gutenberg-Bullen A type is not likely to produce observable P′P′ upper-mantle reflections, (2) in order that a P′P′ upper-mantle reflection is strong enough to be observed, first-order density and S-velocity discontinuities together with a P-wave discontinuity are needed at a depth of about 650 km, and (3) corresponding to a given uppermantle velocity-density model, an estimate can be made of the Q in the upper mantle for short-period seismic body waves.

Inversion of multicomponent, multiazimuth, walkaway VSP data for the stiffness tensor

Geophysics ◽  
2003 ◽  
Vol 68 (3) ◽  
pp. 1022-1031 ◽  
Author(s):  
Pawan Dewangan ◽  
Vladimir Grechka
Keyword(s):  
Elastic Stiffness ◽  
P Wave ◽  
Vacuum Field ◽  
Stiffness Tensor ◽  
Slowness Surface ◽  
Data Set ◽  
S Waves ◽  
Stiffness Coefficients ◽  
Vsp Data ◽  
Walkaway Vsp

Vertical seismic profiling (VSP), an established technique, can be used for estimating in‐situ anisotropy that might provide valuable information for characterization of reservoir lithology, fractures, and fluids. The P‐wave slowness components, conventionally measured in multiazimuth, walkaway VSP surveys, allow one to reconstruct some portion of the corresponding slowness surface. A major limitation of this technique is that the P‐wave slowness surface alone does not constrain a number of stiffness coefficients that may be crucial for inferring certain rock properties. Those stiffnesses can be obtained only by combining the measurements of P‐waves with those of S (or PS) modes. Here, we extend the idea of Horne and Leaney, who proved the feasibility of joint inversion of the slowness and polarization vectors of P‐ and SV‐waves for parameters of transversely isotropic media with a vertical symmetry axis (VTI symmetry). We show that there is no need to assume a priori VTI symmetry or any other specific type of anisotropy. Given a sufficient polar and azimuthal coverage of the data, the polarizations and slownesses of P and two split shear (S1 and S2) waves are sufficient for estimating all 21 elastic stiffness coefficients cij that characterize the most general triclinic anisotropy. The inverted stiffnesses themselves indicate whether or not the data can be described by a higher‐symmetry model. We discuss three different scenarios of inverting noise‐contaminated data. First, we assume that the layers are horizontal and laterally homogeneous so that the horizontal slownesses measured at the surface are preserved at the receiver locations. This leads to a linear inversion scheme for the elastic stiffness tensor c. Second, if the S‐wave horizontal slowness at the receiver location is unknown, the elastic tensor c can be estimated in a nonlinear fashion simultaneously with obtaining the horizontal slowness components of S‐waves. The third scenario includes the nonlinear inversion for c using only the vertical slowness components and the polarization vectors of P‐ and S‐waves. We find the inversion to be stable and robust for the first and second scenarios. In contrast, errors in the estimated stiffnesses increase substantially when the horizontal slowness components of both P‐ and S‐waves are unknown. We apply our methodology to a multiazimuth, multicomponent VSP data set acquired in Vacuum field, New Mexico, and show that the medium at the receiver level can be approximated by an azimuthally rotated orthorhombic model.

scholarly journals The February 9, 1971 San Fernando earthquake: A study of source finiteness in teleseismic body waves

1978 ◽  
Vol 68 (1) ◽  
pp. 1-29 ◽  
Author(s):  
Charles A. Langston
Keyword(s):  
Near Field ◽  
Dislocation Line ◽  
Strong Motion ◽  
Body Waves ◽  
P Waves ◽  
Long Period ◽  
Short Period ◽  
Wave Forms ◽  
San Fernando ◽  
The Time Domain

abstract Teleseismic P, SV, and SH waves recorded by the WWSS and Canadian networks from the 1971 San Fernando, California earthquake (ML = 6.6) are modeled in the time domain to determine detailed features of the source as a prelude to studying the near and local field strong-motion observations. Synthetic seismograms are computed from the model of a propagating finite dislocation line source embedded in layered elastic media. The effects of source geometry and directivity are shown to be important features of the long-period observations. The most dramatic feature of the model is the requirement that the fault, which initially ruptured at a depth of 13 km as determined from pP-P times, continuously propagated toward the free surface, first on a plane dipping 53°NE, then broke over to a 29°NE dipping fault segment. This effect is clearly shown in the azimuthal variation of both long period P- and SH-wave forms. Although attenuation and interference with radiation from the remainder of the fault are possible complications, comparison of long- and short-period P and short-period pP and P waves suggest that rupture was initially bilateral, or, possibly, strongly unilateral downward, propagating to about 15 km depth. The average rupture velocity of 1.8 km/sec is well constrained from the shape of the long-period wave forms. Total seismic moment is 0.86 × 1026 dyne-cm. Implications for near-field modeling are drawn from these results.

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