Observations of Q from the northwest Pacific subduction zone recorded at teleseismic distances

1995 ◽  
Vol 85 (1) ◽  
pp. 237-253 ◽  
Author(s):  
D. S. Sharrock ◽  
I. G. Main ◽  
A. Douglas
Keyword(s):  
Subduction Zone ◽  
Upper Mantle ◽  
Wave Attenuation ◽  
High Sensitivity ◽  
P Wave ◽  
Corner Frequency ◽  
Northwest Pacific ◽  
Short Period ◽  
Imperfect Knowledge ◽  
Average Standard Error

Abstract Twenty-seven earthquakes in the northwest Pacific, with mb ≧ 5.4, are used to study upper-mantle attenuation. The data were recorded at seismometer arrays in Australia, Canada, India, and Scotland. The short-period instruments used in the arrays are sufficiently broadband, and the signal-to-noise enhancement resulting from beamforming the data is good enough, that seismic noise is above system noise out to between 3 and 5 Hz, and in some cases to 8 Hz. The instrument response has been removed and a power law (ω−2) farfield source correction has been assumed and applied. The remaining fall-off in the spectra above 1 Hz (which is above the corner frequency fc) is assumed to be due to attenuation, including losses due both to anelasticity and to scattering. An ω−3 spectral fall-off, preferred by some, would result in lower estimates of attenuation. Conservative estimates of the average attenuation along the whole path have been made from the spectral fall-off. Despite the imperfect knowledge of the source spectra, the high sensitivity of spectral amplitudes to attenuation at high frequencies yields quite reliable and stable attenuation estimates. The scatter in the data is small, resulting in estimates of mean t* (tAV*) with an average standard error of only 21%, where tAV* is defined as the ratio of the travel time T to the effective mean quality factor QAV. Only 14 of the 64 measurements show a tAV* above 0.5 sec within the given error bounds. All estimates of tAV* and QAV refer to P-wave attenuation only. The results for tAV* from the northwest Pacific subduction zone cannot be explained simply by the traditional high-Q subducting slab in a low-Q upper mantle. A low-Q region in the uppermost 100 km (approximately) of the subduction zone has been identified. Source-corrected tAV* yield upper-mantle QAV estimates of about 1200 for predominantly oceanic paths (Australia and India) and 1500 for mixed continental and oceanic paths (Canada and Scotland).

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.

Short-period P-wave attenuation along various paths in North America as determined from P-wave spectra of the SALMON nuclear explosion

1976 ◽  
Vol 66 (5) ◽  
pp. 1609-1622 ◽  
Author(s):  
Zoltan A. Der ◽  
Thomas W. McElfresh
Keyword(s):  
United States ◽  
North America ◽  
Wave Attenuation ◽  
Nuclear Explosion ◽  
P Wave ◽  
Western United States ◽  
Source Spectrum ◽  
Wave Spectra ◽  
Short Period ◽  
Q Values

abstract Average Q values were determined for ray paths to various LRSM stations from the SALMON nuclear explosion by taking ratios of observed P-wave spectra to the estimated source spectrum. Most Q values for P-wave paths throughout eastern North America are in the range 1600 to 2000 while those crossing over into the western United States are typically around 400 to 500. These differences in Q for intermediate distances can sufficiently explain the differences in the teleseismic event magnitudes observed, 0.3 to 0.4 magnitude units, in the western versus the eastern United States, if one assumes that the low Q layer under the western United States is located at depths less than 200 km.

scholarly journals The April 29, 1965, Puget Sound earthquake and the crustal and upper mantle structure of western Washington

1977 ◽  
Vol 67 (3) ◽  
pp. 693-711 ◽  
Author(s):  
Charles A. Langston ◽  
David E. Blum
Keyword(s):  
Upper Mantle ◽  
Source Parameters ◽  
Puget Sound ◽  
P Wave ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Low Velocity Zone ◽  
Crustal Model ◽  
Short Period ◽  
Velocity Zone

abstract Simultaneous modeling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both ray and layer matrix formulations for point dislocations imbedded in layered media. The source parameters obtained are: dip 70° to the east, strike 344°, rake −75°, 63 km depth, average moment of 1.4 ± 0.6 × 1026 dyne-cm, and a triangular time function with a rise time of 0.5 sec and falloff of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15-km-thick low-velocity zone with a 2.5-km/sec P-wave-velocity contrast lower boundary situated at approximately 56-km depth. Ray calculations which allow for sources in dipping structure indicate that the inferred high contrast value can trade off significantly with interface dip provided the structure dips eastward. The effective crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short-period teleseismic observations. The inferred reflection from the base of the low-velocity zone is recovered from short-period P and S waves. An apparent attenuation is also observed for pP from comparisons between the short- and long-period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.

High-frequency P-wave attenuation determination using multiple-window spectral analysis method

1989 ◽  
Vol 79 (4) ◽  
pp. 1054-1069
Author(s):  
Tianfei Zhu ◽  
Kin-Yip Chun ◽  
Gordon F. West
Keyword(s):  
Spectral Analysis ◽  
High Frequency ◽  
Wave Attenuation ◽  
P Wave ◽  
Analysis Method ◽  
Window Method ◽  
Short Period ◽  
Spectral Analysis Method ◽  
Eastern Kazakhstan ◽  
Multiple Window

Abstract Yellowknife array (Northwest Territories, Canada) recordings of nuclear explosions detonated at French Tuamotu, South Pacific and Shagan River, Eastern Kazakhstan (USSR) test sites are used to derive t* values via the application of the spectral decay method. An important factor that limits the reliability of this widely used method is the degree of accuracy with which one is able to determine the signal spectral shape. For transient, pulse-like, short-period teleseismic phases, the conventional single-window spectral estimate methods may not be appropriate due to the trade-off betgween the leakage resistance and variance. A recently developed, multiple-window spectral analysis method is used in this study to effectively control spectral leakage, a capability that is especially important when analyzing seismic data characterized by a rapid high-frequency fall-off rate, such as the French Tuamotu explosion data. We compare the t* estimates obtained using a conventional single-window method with those obtained using the multiple-window method and show that the latter are more reliable. The t* (0.5 to 4.5 Hz) found by the multiple-window method for the Tuamotu-Yellowknife path is 0.66 sec. For the Eastern Kazakhstan-Yellowknife path, the multiple-window t* estimate is 0.42 sec in the 0.5 to 4.5 Hz range; a smaller value is obtained at higher frequencies (4.5 to 8 Hz).

A source theory for complex earthquakes

1975 ◽  
Vol 65 (5) ◽  
pp. 1385-1405 ◽  
Author(s):  
R. R. Blandford
Keyword(s):  
Fault Plane ◽  
Amplitude Ratio ◽  
P Wave ◽  
Corner Frequency ◽  
Transform Faults ◽  
Shallow Earthquakes ◽  
Short Period ◽  
Wave Forms ◽  
The Moment ◽  
Third Moment

Abstract Earthquake source theories of Haskell, Brune, and Savage have been drawn upon to develop a description of an earthquake as a major slip accompanied by many smaller tensional and slip events. We find natural explanations of several previously unexplained observations, such as: Robustness of the MS: mb discriminant, P corner frequency higher than S corner frequency for shallow earthquakes, High-frequency P/S amplitude ratio higher for shallow earthquakes than previous theories predict, Increase of complexity as a function of third moment, Small mb relative to MS for transform faults. (This can also be explained by emergent short-period P wave forms or by a low-Q region underlying the source). The theory predicts that MS:mb populations of earthquakes and explosions will not converge at small magnitudes. The theory also suggests that the standard interpretations of corner-frequency measurements can provide only a lower limit to fault-plane dimensions and, in combination with the moment, only an upper limit on stress drop.

Upper mantle P-wave model catalogue

1976 ◽  
Vol 13 (10) ◽  
pp. 1481-1486 ◽  
Author(s):  
George A. McMechan ◽  
Judith J. Sinclair
Keyword(s):  
Upper Mantle ◽  
Epicentral Distance ◽  
Vertical Component ◽  
Wave Model ◽  
Synthetic Seismograms ◽  
P Wave ◽  
P Wave Velocity ◽  
Short Period ◽  
Distance Travel ◽  
Ray Parameter

This note is a description of a catalogue that contains tables of parameters and synthetic seismograms for 50 upper-mantle P-wave velocity–depth profiles. The table for each model contains values of ray parameter, epicentral distance, travel time, velocity, and bottoming depth for a number of representative rays. Short-period synthetic seismograms are computed at 1 °intervals from 10° to 30° by the quantized ray theory algorithm and are vertical component traces for a surface focus point source. The catalogue is designed as a comprehensive reference and so includes a wide variety of mantle models.

scholarly journals Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations

2021 ◽  
Vol 225 (3) ◽  
pp. 2097-2119
Author(s):  
Brandon P VanderBeek ◽  
Manuele Faccenda
Keyword(s):  
Subduction Zone ◽  
Upper Mantle ◽  
Large Scale ◽  
Seismic Anisotropy ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Zone Model ◽  
Upper Mantle Anisotropy ◽  
Delay Times ◽  
Mantle Anisotropy

SUMMARY Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic P-wave imaging remains largely ignored. Unmodelled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities. Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy. However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as synthetic testing has been restricted to the recovery of simplified block-like structures using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm). Here, we present a modified parametrization for imaging arbitrarily oriented hexagonal anisotropy and test the method by reconstructing geodynamic simulations of subduction. Our inversion approach allows for isotropic starting models and includes approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Synthetic seismic data are created by propagating teleseismic waves through an elastically anisotropic subduction zone model created via petrologic-thermomechanical modelling. Delay times across a synthetic seismic array are measured using conventional cross-correlation techniques. We find that our imaging algorithm is capable of resolving large-scale features in subduction zone anisotropic structure (e.g. toroidal flow pattern and dipping fabrics associated with the descending slab). Allowing for arbitrarily oriented anisotropy also results in a more accurate reconstruction of isotropic slab structure. In comparison, models created assuming isotropy or only azimuthal anisotropy contain significant isotropic and anisotropic imaging artefacts that may lead to spurious interpretations. We conclude that teleseismic P-wave traveltimes are a useful observable for probing the 3-D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies particularly in subduction settings.

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