P–Coda Evidence for a Layer of Anomalous Velocity in the Crust Beneath Leduc, Alberta

1972 ◽  
Vol 9 (7) ◽  
pp. 845-856 ◽  
Author(s):  
P. G. Somerville ◽  
R. M. Ellis
Keyword(s):  
Synthetic Seismograms ◽  
Precambrian Basement ◽  
Upper Crust ◽  
Spectral Ratios ◽  
Low Velocity ◽  
Crustal Model ◽  
Short Period ◽  
Large Velocity ◽  
Theoretical Results ◽  
Velocity Zone

Previous seismic studies of crustal structure using short-period P-coda recorded in the vicinity of Leduc in central Alberta have indicated that serious discrepancies exist between the experimental observations and those based on a horizontally layered model of the crust in both the time and frequency domains.Using vertical-radial spectral ratios and synthetic seismograms, a modified crustal model has been derived which gives better agreement between experimental and theoretical results. This model involves the insertion of a layer several kilometers thick having large velocity contrast with respect to the surrounding media at the base of the Precambrian basement (12 km deep). The new crustal model is discussed in the light of evidence for a low velocity zone in the upper crust in certain continental regions.

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.

Synthetic seismograms and the response of an anisotropic attenuating medium

1977 ◽  
Vol 14 (5) ◽  
pp. 1062-1076 ◽  
Author(s):  
K. F. Sprenke ◽  
E. R. Kanasewich
Keyword(s):  
Upper Mantle ◽  
Normal Values ◽  
Transverse Isotropy ◽  
Uniaxial Anisotropy ◽  
Vertical Axis ◽  
Synthetic Seismograms ◽  
Spectral Ratios ◽  
Matrix Formulation ◽  
Crustal Model ◽  
Short Period

The Haskell–Thomson matrix formulation has been modified to yield synthetic seismograms and theoretical spectral ratios in a system of parallel anisotropic lossy layers. Transfer function matrices are derived for the case of transverse isotropy with a unique vertical axis. Attenuation is included in the analysis through the use of complex elastic coefficients. Short period spectral ratios and time syntheses are found to be significantly affected by the presence of uniaxial anisotropy for a typical continental crustal model while long period spectral ratios are shown to be sensitive to anisotropy in the upper mantle. Normal values of Q for the crust produce almost no change in teleseismic spectral ratios at useful frequencies. Synthetic seismograms for a continental crust indicate that crustal reverberations of teleseismic impulses are attenuated within 5 s of onset

A linear gradient crustal model for south Hawaii

1981 ◽  
Vol 71 (5) ◽  
pp. 1503-1510
Author(s):  
Fred W. Klein
Keyword(s):  
Travel Time ◽  
Focal Mechanism ◽  
Low Velocity ◽  
Linear Gradient ◽  
Low Velocity Zone ◽  
Focal Mechanism Solutions ◽  
Local Earthquakes ◽  
Velocity Gradients ◽  
Crustal Model ◽  
Velocity Zone

abstract A new crustal model with linear velocity gradients within layers does as good a job of locating earthquakes on south Hawaii as any model yet published. Incorporating linear gradients means the model can be simpler and free of artificial velocity discontinuities. Using travel-time residuals from local earthquakes and consistency of focal mechanism solutions as tests, it is seen that a low-velocity zone at the base of the crust is not required.

Evaluation of the relation between near-surface geological units and ground response in the vicinity of Long Beach, California

1979 ◽  
Vol 69 (5) ◽  
pp. 1603-1622
Author(s):  
A. M. Rogers ◽  
J. C. Tinsley ◽  
W. W. Hays ◽  
K. W. King
Keyword(s):  
Strong Motion ◽  
Test Site ◽  
Peak Ground Velocity ◽  
Long Beach ◽  
Spectral Ratios ◽  
Low Velocity ◽  
Ground Response ◽  
Near Surface ◽  
Long Period ◽  
Short Period

abstract Simulataneous recordings of Nevada Test Site nuclear events were made at sites underlain by alluvium in the Long Beach, California, area and at sites underlain by rock in the Palos Verdes and Pasadena areas. These data show peak-ground-velocity alluvium-to-rock ratios as large as 7 and spectral ratios as high as 11 in the period band from 0.2 to 6 sec. Comparison of the low-strain nuclear-explosion data and the San Fernando earthquake strong-motion data at three sites indicates that the alluvium-to-rock spectral ratios derived from the nuclear explosions are similar to those derived from the earthquake. Significant trends exist in the short-period data, indicating higher ground response at sites underlain at the near-surface by materials that have high void ratios and lower ground response with increasing thickness of Quaternary deposits. These results suggest that the short-period response is primarily controlled both by near-surface low-velocity layers and by attenuation in the Quaternary sediments. Comparison of the data of this study with data collected in other areas indicates that the long-period response increase with either increasing depth to basement or with alluvium thickness, when this thickness is greater than 400 m. From previous theoretical studies and these results, ground response in the long-period band is related to those underlying geological structures and major velocity contrasts that control the development of surface waves.

Seismic Evidence for a Low-Velocity Zone in the Upper Crust Beneath Mount Vesuvius

Science ◽  
1996 ◽  
Vol 274 (5287) ◽  
pp. 592-594 ◽  
Author(s):  
A. Zollo ◽  
P. Gasparini ◽  
J. Virieux ◽  
H. le Meur ◽  
G. de Natale ◽  
...  
Keyword(s):  
Upper Crust ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Velocity Zone

Noise attenuation in shallow holes

1966 ◽  
Vol 56 (3) ◽  
pp. 619-632
Author(s):  
Eduard J. Douze
Keyword(s):  
Background Noise ◽  
Seismic Waves ◽  
Noise Attenuation ◽  
Signal To Noise ◽  
Low Velocity ◽  
Wind Noise ◽  
Low Velocity Zone ◽  
Short Period ◽  
Normal Background ◽  
Velocity Zone

abstract Significant improvements in the performance of short-period seismograph recordings are sometimes obtained in shallow holes (<300 m). Wind noise attenuates rapidly with depth and becomes insignificant at depths of 60 m or less. In the presence of low-velocity weathered layers, the normal background noise decays rapidly with depth and significant improvements in signal-to-noise ratios are obtained. In the absence of a low-velocity zone, only a small attenuation in the background noise level is obtained. Little or no wind noise is converted into traveling seismic waves.

The Multiscale Structure of the Longmen Shan Central Fault Zone from Local and Teleseismic Data Recorded by Short-Period Dense Arrays

2020 ◽  
Vol 110 (6) ◽  
pp. 3077-3087
Author(s):  
Yafen Huang ◽  
Hongyi Li ◽  
Xin Liu ◽  
Yuting Zhang ◽  
Min Liu ◽  
...  
Keyword(s):  
Fault Zone ◽  
Receiver Functions ◽  
Seismic Array ◽  
Trapped Waves ◽  
Low Velocity ◽  
Surface Rupture ◽  
P Waves ◽  
Low Velocity Zone ◽  
Short Period ◽  
Velocity Zone

ABSTRACT The Longmen Shan fault zone (FZ), which consists of the back-range, the central, and the front-range faults, acts as the boundary between the Sichuan basin and eastern Tibet. In this study, local and teleseismic waveforms recorded by a 2D small aperture seismic array (176 temporary short-period seismometers) deployed by China University of Geosciences (Beijing) from 22 October to 20 November 2017 and a dense linear seismic array of 16 stations deployed by Geophysical Exploration Center, China Earthquake Administration during July 2008 are used to study the FZ structure by analyzing FZ-trapped waves (FZTWs), the radial-to-vertical amplitude ratio, and travel-time delays. Based on power density spectra analysis, FZTWs from local events with larger amplitudes and longer wavetrains are clearly observed at stations 6002–6003, 6013–6025, and W025–W032. The dispersion measured from trapped waves is quite weak. The near-surface shear velocity structure estimated from the radial-to-vertical amplitude ratios of local initial P waves shows a low-velocity zone around the surface rupture trace. The slight time delay of direct P waves examined from local and teleseismic events indicates a relatively shallow slow structure beneath the arrays. Through the comprehensive analysis of the central FZ, our results suggest a shallow low-velocity zone with a width of ∼150–160  m along the surface rupture trace. Moreover, our P-wave receiver functions reveal that the Moho depth beneath the Longmen Shan FZ is approximately 45 km, and receiver functions at stations located within the surface rupture zone show more complicated waveforms than those off the surface rupture.

scholarly journals Temporal changes in the distinct scattered wave packets associated with earthquake swarm activity beneath the Moriyoshi-zan volcano, northeastern Japan

2019 ◽  
Vol 71 (1) ◽  
Author(s):  
Yuta Amezawa ◽  
Masahiro Kosuga ◽  
Takuto Maeda
Keyword(s):  
Wave Packets ◽  
Scattered Wave ◽  
Temporal Changes ◽  
Low Velocity ◽  
Small Aperture ◽  
Low Velocity Zone ◽  
The North ◽  
Northeastern Japan ◽  
Short Period ◽  
Velocity Zone

AbstractWe investigated temporal changes in the waveforms of S-coda from triggered earthquakes around the Moriyoshi-zan volcano in northeastern Japan. Seismicity in the area has drastically increased after the 2011 off the Pacific coast of Tohoku earthquake, forming the largest cluster to the north of the volcano. We analyzed distinct scattered wave packets (DSW) that are S-to-S scattered waves from the mid-crust and appeared predominantly at the high frequency range. We first investigated the variation of DSW for event groups with short inter-event distances and high cross-correlation coefficients (CC) in the time window of direct waves. Despite the above restriction, DSW showed temporal changes in their amplitudes and shapes. The change occurred gradually in some cases, but temporal trends were much more complicated in many cases. We also found that the shape of DSW changed in a very short period of time, for example, within ~ 12 h. Next, we estimated the location of the origin of the DSW (DSW origin) by applying the semblance analysis to the data of the temporary small-aperture array deployed to the north of the largest cluster of triggered events. The DSW origin is located between the largest cluster within which hypocentral migration had occurred and the low-velocity zone depicted by a tomographic study. This spatial distribution implies that the DSW origin was composed of geofluid-accumulated midway in the upward fluid movement from the low-velocity zone to the earthquake cluster. Though we could not entirely exclude the possibility of the effect of the event location and focal mechanisms, the temporal changes in DSW waveforms possibly reflect the temporal changes in scattering properties in and/or near the origin. The quick change in DSW waveforms implies that fast movement of geofluid can occur at the depth of the mid-crust.

Causal absorption: Its effect on synthetic seismograms computed by the reflectivity method

1979 ◽  
Vol 69 (1) ◽  
pp. 17-25
Author(s):  
M. E. O'Neill ◽  
David P. Hill
Keyword(s):  
Phase Velocity ◽  
Upper Mantle ◽  
Synthetic Seismograms ◽  
Low Velocity ◽  
Frequency Dependent ◽  
Low Velocity Zone ◽  
Frequency Independent ◽  
Reflectivity Method ◽  
Independent Parameters ◽  
Velocity Zone

abstract Lomnitz's causal formulation of anelasticity is incorporated into the Fuchs-MüKennett reflectivity program. For waves passing through a highly attenuating upper-mantle low-velocity zone (Q = 35), the synthetic seismograms computed with Lomnitz's weakly frequency-dependent phase velocity, c, and specific attenuation, Q, differ noticeably from those computed with frequency-independent parameters. For these waves, the first pulse is larger and more abrupt and the peaks and troughs are somewhat later for causal absorption than for noncausal absorption.

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