Normal modes of continental surface waves

1958 ◽  
Vol 48 (1) ◽  
pp. 33-49 ◽  
Author(s):  
Jack Oliver ◽  
Maurice Ewing
Keyword(s):  
Surface Waves ◽  
Normal Modes ◽  
Wave Dispersion ◽  
Low Velocity ◽  
Surface Wave Dispersion ◽  
Mode Dispersion ◽  
Seismic Surface Waves ◽  
Low Velocity Layer ◽  
Rayleigh Mode ◽  
Velocity Layer

Abstract When the path between epicenter and station traverses only continental structure, the dispersion of the entire train of directly arriving seismic surface waves can be explained as the result of normal mode propagation in a crust-mantle system in which the velocity increases in some manner with depth within the crust. At least four modes, the Rayleigh mode, Sezawa's M2 mode, and the first two Love waves, may appear prominently on the seismogram. The characteristics of the higher-mode dispersion curves permit the explanation of the Lg phase of Press and Ewing, B䳨's Lg1 and Lg2, and, in some cases, Caloi's Sa without recourse to a low-velocity layer in the crust or mantle. Speculation on changes in these curves for less simplified models indicates that the remaining cases of Sa as well as Leet's C or coupled wave may be explained by classical theory. The occurrence of the higher-mode waves is widespread; they are found on the four continents for which data are available. Higher-mode data, particularly when combined with information from the fundamental modes, make surface-wave dispersion, previously a useful tool, a much more potent method for the study of crustal structure.

Simulation of scattered seismic surface waves on mountainous onshore areas: Understanding the “ground roll energy cone”

2021 ◽  
Vol 40 (8) ◽  
pp. 601-609
Author(s):  
Ivan Javier Sánchez-Galvis ◽  
Jheyston Serrano ◽  
Daniel A. Sierra ◽  
William Agudelo
Keyword(s):  
Surface Waves ◽  
Synthetic Data ◽  
Real Data ◽  
Small Scale ◽  
Low Velocity ◽  
Near Surface ◽  
Seismic Surface Waves ◽  
Ground Roll ◽  
Low Velocity Layer ◽  
Velocity Layer

The accurate simulation of seismic surface waves on complex land areas requires elastic models with realistic near-surface parameters. The SEAM Phase II Foothills model, proposed by the SEG Advanced Modeling (SEAM) Corporation, is one of the most comprehensive efforts undertaken by the geophysics community to understand complex seismic wave propagation in foothills areas. However, while this model includes a rough topography, alluvial sediments, and complex geologic structures, synthetic data from the SEAM consortium do not reproduce the qualitative characteristics of the scattering energy that is generally interpreted as the “ground roll energy cone” on shot records of real data. To simulate the scattering, a near-surface elastic model in mountainous areas ideally must include the following three elements: (1) rough topography and bedrock, (2) low-velocity layer, and (3) small-scale heterogeneities (size approximately Rayleigh wavelength). The SEAM Foothills model only includes element (1) and, to a lesser extent, element (2). We represent a heterogeneous near surface as a random medium with two parameters: the average size of the heterogeneities and fractional fluctuation. A parametric analysis shows the influence of each parameter on the synthetic data and how similar it is compared to real data acquired in a foothills area in Colombia. We perform the analysis in the shot gather panel and dispersion image. Our study shows that it is necessary to include the low-velocity layer and small-scale distributed heterogeneities in the shallow part of the SEAM model to get synthetic data with realistic scattered surface-wave energy.

Surface waves across the Pacific

1966 ◽  
Vol 56 (5) ◽  
pp. 1067-1091 ◽  
Author(s):  
Masanori Saito ◽  
Hitoshi Takeuchi
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Shear Wave Velocity ◽  
Group Velocity Dispersion ◽  
Wave Dispersion ◽  
Low Velocity ◽  
Oceanic Region ◽  
The Pacific ◽  
Low Velocity Layer ◽  
Velocity Layer

Abstract Making use of Rayleigh and Love wave dispersion data, Santô divided the Pacific into seven regions. From his map and compiled group velocity dispersion curves, upper mantle structure in the Pacific in which the depths of the low velocity layer and the shear wave velocity are changing systematically from continent to ocean is obtained. In orogenic regions such as Japan and its surroundings, extremely low velocity layer in which the shear wave velocity is about 4.3 km/sec is just under the Moho. In the oceanic side of this region, the layer is overlain by the normal mantle material with shear wave velocity of about 4.6 km/sec and in the pure oceanic region this extremely low velocity layer disappears. The so-called ‘low velocity layer’ which is believed to begin at the depth of about 60 km under the ocean is present in the oceanic region but the shear wave velocity in the layer may be a little higher than that obtained by earlier works.

Low-Frequency Seismic Surface Waves in the Upper Mississippi Embayment

1994 ◽  
Vol 65 (2) ◽  
pp. 137-148 ◽  
Author(s):  
James Dorman ◽  
Robert Smalley
Keyword(s):  
Surface Waves ◽  
Broad Band ◽  
Low Frequency ◽  
Shear Velocity ◽  
Low Velocity ◽  
Surface Wave Dispersion ◽  
Mississippi Embayment ◽  
Basin Surface ◽  
Upper Mississippi Embayment ◽  
Seismic Surface Waves

Abstract Low-frequency seismic surface waves lasting about 6 minutes were recorded at Memphis following the magnitude 4.6 Risco, Missouri earthquake of May 4, 1991. The motion following S included a very long, sinusoidal train of Love waves with periods of 3 to 5 seconds and weaker groups of Rayleigh waves of periods between 2 and 7 seconds arriving early and late. The unusual Risco surface waves travel a source-receiver path internal to the upper Mississippi embayment, a shallow basin containing soft, young sediments overlying rigid carbonate rocks. In contrast to the strong Risco surface waves, the magnitude 4.8 Cape Girardeau, Missouri earthquake of September 26, 1990, which occurred near the edge of the basin, produced relatively weak surface waves at Memphis. The Risco and Cape Girardeau earthquakes are the largest regional earthquakes ever recorded on long-period and broad-band seismographs within the embayment. They show that (1) the sedimentary basin has a profound effect on low-frequency seismic surface waves; (2) the velocity dispersion of a Love wave mode and two Rayleigh wave modes between periods of 2 and 7 sec is well explained by the layering of low-velocity embayment sediments overlying the high-velocity Knox dolomite; (3) because of their strong dispersion, the characteristic basin surface waves can shake the entire embayment for several minutes following any large intra-basin earthquake; (4) excitation of this characteristic basin disturbance seems to be inefficient for strong earthquakes marginal or external to the basin. Lacking direct measurements of shear velocity in the young embayment clastic section, we find that a simple non-linear relationship between shear velocity and logged compressional velocity makes the sediment physical properties compatible with the observed surface wave dispersion.

Test of the upper mantle low velocity layer in Siberia with surface waves

2006 ◽  
Vol 416 (1-4) ◽  
pp. 113-131 ◽  
Author(s):  
Antonella Pontevivo ◽  
Hans Thybo
Keyword(s):  
Surface Waves ◽  
Upper Mantle ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
Velocity Layer

Surface waves confined to a thin low-velocity layer

1976 ◽  
Vol 6 (5) ◽  
pp. 539-546 ◽  
Author(s):  
I. A. Molotkov ◽  
S. S. Sardarov
Keyword(s):  
Surface Waves ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
Velocity Layer

scholarly journals Pervasive low-velocity layer atop the 410-km discontinuity beneath the northwest Pacific subduction zone: Implications for rheology and geodynamics

2021 ◽  
Vol 554 ◽  
pp. 116642
Author(s):  
Guangjie Han ◽  
Juan Li ◽  
Guangrui Guo ◽  
Walter D. Mooney ◽  
Shun-ichiro Karato ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Northwest Pacific ◽  
Low Velocity ◽  
Low Velocity Layer ◽  
Velocity Layer
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