Surface waves confined to a thin low-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.

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

Tectonophysics ◽

10.1016/j.tecto.2005.11.015 ◽

2006 ◽

Vol 416 (1-4) ◽

pp. 113-131 ◽

Cited By ~ 5

Author(s):

Antonella Pontevivo ◽

Hans Thybo

Keyword(s):

Surface Waves ◽

Upper Mantle ◽

Low Velocity ◽

Low Velocity Layer ◽

Velocity Layer

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

The Leading Edge ◽

10.1190/tle40080601.1 ◽

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.

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Does CO2 cause partial melting in the low-velocity layer of the mantle?: Comment and reply

Geology ◽

10.1130/0091-7613(1976)4<787:dccpmi>2.0.co;2 ◽

1976 ◽

Vol 4 (12) ◽

pp. 787 ◽

Cited By ~ 8

Author(s):

David H. Eggler

Keyword(s):

Partial Melting ◽

Low Velocity ◽

Low Velocity Layer ◽

Velocity Layer

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

Earth and Planetary Science Letters ◽

10.1016/j.epsl.2020.116642 ◽

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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Focal mechanisms of earthquakes related to the 29 November 1975 Kalapana, Hawaii, earthquake: The effect of structure models

Bulletin of the Seismological Society of America ◽

10.1785/bssa0710030713 ◽

1981 ◽

Vol 71 (3) ◽

pp. 713-729 ◽

Cited By ~ 1

Author(s):

R. S. Crosson ◽

E. T. Endo

Keyword(s):

Main Shock ◽

Horizontal Layer ◽

Focal Mechanisms ◽

Local Network ◽

The South ◽

Low Velocity ◽

Tectonic Model ◽

Low Velocity Layer ◽

Using Data ◽

Velocity Layer

abstract Initial focal mechanism determinations for the 29 November 1975 Kalapana, Hawaii, earthquake indicated discrepancy between the mechanism determined from teleseismic data by Ando and the mechanism determined using data from the local U.S. Geological Survey network surrounding the epicenter region. The resolution of this difference is crucial to correctly understand this earthquake, as well as to understand the tectonics of the south flank of Kilauea volcano. When a model with a low-velocity layer at the base of the crust is used for projection back to the focal sphere for the local network mechanisms, the discrepancy vanishes. To further investigate this result, focal mechanisms were determined using several contrasting models for a set of well-recorded earthquakes. A large number of these earthquakes have mechanisms identical to the main shock when the low-velocity layer model is used. Dispersion of P and T axes is also minimized by use of this model. A low-angle slip direction, favored for the main shock and typical of most other solutions, exhibits remarkable stability normal to the east rift zone of Kilauea. Our results suggest a tectonic model, similar in nature to that proposed by Ando, in which the south flank of Kilauea consists of a mobile block of crust which is relatively free to move laterally on a low-strength zone at about 10 km depth. Forceful injection of magma along the rift zones provides the loading stress which is released by catastrophic failure in the weak, horizontal layer in a cycle of perhaps 100 yr.

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