Abstract
In this paper, an observational space–time ensemble of sea surface elevations is investigated in search of the highest waves of the sea state. Wave data were gathered by means of a stereo ...camera system, which was installed on top of a fixed oceanographic platform located in the Adriatic Sea (Italy). Waves were measured during a mature sea state with an average wind speed of 11 m s
−1
. By examining the space–time ensemble, the 3D wave groups have been isolated while evolving in the 2D space and grabbed “when and where” they have been close to the apex of their development, thus exhibiting large surface displacements. The authors have selected the groups displaying maximal crest height exceeding the threshold adopted to define rogue waves in a time record, that is, 1.25 times the significant wave height (
H
s
). The records at the spatial positions where such large crests occurred have been analyzed to derive the empirical distributions of crest and wave heights, which have been compared against standard statistical linear and nonlinear models. Here, the maximal observed wave crests have resulted to be outliers of the standard statistics, behaving as isolated members of the sample, apparently uncorrelated with other waves of the record. However, this study has found that these unexpectedly large wave crests are better approximated by a space–time model for extreme crest heights. The space–time model performance has been improved, deriving a second-order approximation of the linear model, which has provided a fair agreement with the empirical maxima. The present investigation suggests that very large waves may be more numerous than generally expected.
Microseisms generated by storms primarily propagate as surface and P waves. However, there has been only one report of S wave microseisms excited by strong storms in the North Atlantic, not from ...typhoons in the Pacific. In this paper, we present the first observations for S wave microseisms caused by typhoons in Western Pacific, via analyzing ambient seismic waveform recorded by a large‐aperture seismic array in China. The results show that three super typhoons clearly generated S wave microseisms at periods of 5–10 s. The strength of S wave microseism is found to be correlated with P wave. The excitation mechanism for S wave microseism observed in regions with flat seafloor may be related to the interaction between P waves and sub‐ocean sedimentary structure.
Plain Language Summary
Besides causing property damages and life losses, ocean storms can also generate seismic waves named as microseisms, which could also be used as signals for studying interior of the Earth. Microseisms mostly propagate as surface waves and P waves. Yet S waves from storms are only detected in the North Atlantic Ocean shown by previous studies. However, there are no reports of S wave microseism by storms in the Pacific Ocean, though the typhoons there are strong and frequent. In this study, we observe and locate S wave microseisms generated by three super typhoons in Western Pacific by processing continuous ambient seismic waveforms recorded in China. The source regions of P and S waves are located very close to each other in the deep ocean near typhoons. Our results show that observability of S wave microseisms depends on P wave energy, which is controlled by strength of typhoons. It is demonstrated that S wave microseisms not only can be generated in the regions with steep topography but also can be excited in the flat seafloor regions. This work is helpful for understanding the excitation mechanism of the S waves.
Key Points
We observed body‐wave (P, SV, and probable SH) microseisms from typhoons in the Western Pacific
We found that S wave strength is related to P wave power, which in turn relies on typhoon intensity
S wave microseisms could be generated in the regions with flat seafloor or bathymetric gradients
Single seismometer structure
Because of the lack of direct seismic observations, the interior structure of Mars has been a mystery. Khan
et al.
, Knapmeyer-Endrun
et al.
, and Stähler
et al.
used ...recently detected marsquakes from the seismometer deployed during the InSight mission to map the interior of Mars (see the Perspective by Cottaar and Koelemeijer). Mars likely has a 24- to 72-kilometer-thick crust with a very deep lithosphere close to 500 kilometers. Similar to the Earth, a low-velocity layer probably exists beneath the lithosphere. The crust of Mars is likely highly enriched in radioactive elements that help to heat this layer at the expense of the interior. The core of Mars is liquid and large, ∼1830 kilometers, which means that the mantle has only one rocky layer rather than two like the Earth has. These results provide a preliminary structure of Mars that helps to constrain the different theories explaining the chemistry and internal dynamics of the planet.
Science
, abf2966, abf8966, abi7730, this issue p.
434
, p.
438
, p.
443
see also abj8914, p.
388
Data from the InSight mission on Mars help constrain the structure and properties of the martian interior.
For 2 years, the InSight lander has been recording seismic data on Mars that are vital to constrain the structure and thermochemical state of the planet. We used observations of direct (
P
and
S
) and surface-reflected (
PP
,
PPP
,
SS
, and
SSS
) body-wave phases from eight low-frequency marsquakes to constrain the interior structure to a depth of 800 kilometers. We found a structure compatible with a low-velocity zone associated with a thermal lithosphere much thicker than on Earth that is possibly related to a weak
S
-wave shadow zone at teleseismic distances. By combining the seismic constraints with geodynamic models, we predict that, relative to the primitive mantle, the crust is more enriched in heat-producing elements by a factor of 13 to 20. This enrichment is greater than suggested by gamma-ray surface mapping and has a moderate-to-elevated surface heat flow.
Love Wave Tomography of the United States Hariharan, Anant; Dalton, Colleen A.
Geophysical research letters,
28 December 2022, Volume:
49, Issue:
24
Journal Article
Peer reviewed
Open access
Love wave phase velocity maps provide essential constraints on radial anisotropy and deformation in the crust and upper mantle. However, the phenomenon of overtone interference causes scatter and ...systematic bias in the velocity measurements and impedes efforts to image small‐scale anisotropic variations. We develop an approach for identifying Love wave measurements that are biased by overtone interference, demonstrate its efficacy with EarthScope USArray data, and determine the first earthquake‐derived Love wave phase velocity maps for the entire conterminous U.S. in the period range 35–75 s. We show that radial anisotropy in parts of the crust and most of the lithospheric mantle is necessary to reconcile these maps with Rayleigh wave phase velocities. Our results convey the impact and geographic variability of overtone interference, offer an easy‐to‐implement method to ameliorate this impact, and present high‐resolution constraints on radial anisotropy beneath North America.
Plain Language Summary
Measurements of Love wave phase speeds at different frequencies provide unique information that can help understand how the Earth's interior is deforming. These measurements are difficult to make because other seismic waves interfere with the one we are attempting to measure, resulting in low‐quality and biased measurements. Here, we calculate the times at which the different waves arrive and use these times to identify and remove measurements impacted by interference. The resulting cleaned data set increases the precision of derived images of the Earth's interior and reduces bias in these images. Our images suggest shear waves polarized in vertical and horizontal directions are required to have different speeds.
Key Points
We describe an approach to eliminate overtone interference and improve the quality of Love wave measurements
We present the first earthquake‐derived Love wave phase velocity maps of the conterminous U.S., at periods up to and including 75 s
Our maps require radial anisotropy in many regions of the crust and upper mantle beneath the U.S.
SUMMARY
We develop a new method to monitor and locate seismic velocity changes in the subsurface using seismic noise interferometry. Contrary to most ambient noise monitoring techniques, we use the ...ballistic Rayleigh waves computed from 30 d records on a dense nodal array located above the Groningen gas field (the Netherlands), instead of their coda waves. We infer the daily relative phase velocity dispersion changes as a function of frequency and propagation distance with a cross-wavelet transform processing. Assuming a 1-D velocity change within the medium, the induced ballistic Rayleigh wave phase shift exhibits a linear trend as a function of the propagation distance. Measuring this trend for the fundamental mode and the first overtone of the Rayleigh waves for frequencies between 0.5 and 1.1 Hz enables us to invert for shear wave daily velocity changes in the first 1.5 km of the subsurface. The observed deep velocity changes (±1.5 per cent) are difficult to interpret given the environmental factors information available. Most of the observed shallow changes seem associated with effective pressure variations. We observe a reduction of shear wave velocity (–0.2 per cent) at the time of a large rain event accompanied by a strong decrease in atmospheric pressure loading, followed by a migration at depth of the velocity decrease. Combined with P-wave velocity changes observations from a companion paper, we interpret the changes as caused by the diffusion of effective pressure variations at depth. As a new method, noise-based ballistic wave passive monitoring could be used on several dynamic (hydro-)geological targets and in particular, it could be used to estimate hydrological parameters such as the hydraulic conductivity and diffusivity.
We show examples of teleseismic S waves from western Pacific earthquakes converting to surface waves near the western U.S. coastline. Many of these events originate in the Tonga‐Samoa region. We ...observe these surface wave conversions at USArray stations at relatively long periods (>10 s). The amplitudes vary considerably from station to station and appear highly amplified in the Yellowstone region. Two‐dimensional spectral element simulations successfully generate scattered Rayleigh waves from incident SV waves and models with surface topography at the coastline and crustal thickness variations across the margin, although simple models cannot explain the large Rayleigh wave amplitudes (greater than the direct S wave amplitude) observed in some regions, suggesting that the wave train is amplified by local structure or 3‐D focusing effects.
Plain Language Summary
We show new observations of body‐to‐surface wave conversions at the U.S. continental margin observed with USArray. Simulation results show how simple surface topography at the coastline can successfully generate scattered Rayleigh waves from incident SV waves. These converted surface waves may be an important source of signal‐generated noise in continental seismology, in particular in studies of seismic phases and parts of the wavefield between direct S and later‐arriving surface waves from the source.
Key Points
Body‐to‐surface wave conversions at the U.S. continental margin can be observed at USArray
Two‐dimensional spectral element simulations can generate S‐to‐Rayleigh wave scattering
The scattered wave train is amplified locally by 3‐D effects
Near-surface site characterization is of great significance in the fields of geotechnical engineering and resource exploration. In this paper, we propose a near-surface site characterization method ...based on the joint iterative analysis of first-arrival and surface-wave data (JIAFS). The proposed method combines the advantages of first-arrival traveltime tomography (FATT) and multichannel analysis of surface waves (MASW). First, the 1D S-wave velocity (
v
S
) models obtained by MASW are interpolated to construct the pseudo-2D
v
S
model. According to the available geological survey information and borehole data, the initial Poisson’s ratio (
σ
) model is estimated. Based on the estimated
σ
model, the pseudo-2D
v
S
model is converted to a referenced P-wave velocity (
v
P
) model which is utilized to constrain the progress of FATT. This helps FATT overcome the inherent defect that it cannot effectively identify velocity-inversion interfaces and low-velocity zones. On the other hand, the
v
P
model obtained by FATT can provide a favorable priori information to improve the reliability of the results of MASW. Then, the
v
P
and
v
S
models obtained by constrained FATT and MASW are used to update the
σ
model. In addition, the
v
P
and
v
S
models are also used as initial models in the next iterative analysis. Finally, through the iteration of this process, the two inversion methods can make use of their own advantages to improve each other, so we can establish accurate near-surface
v
P
,
v
S
and
σ
models under complex geological conditions. A velocity model including low-velocity zone is established for synthetic model test to analyze and verify the advantage of JIAFS. The
v
P
,
v
S
and
σ
models obtained by JIAFS can accurately identify the low-velocity zone and match the true models well. In addition, the proposed method is applied to the field seismic data acquired for oil and gas exploration in Northwest China. Compared with the results of individual inversions and borehole data, JIAFS can establish more reliable 3D
v
P
,
v
S
and
σ
models by interpolating the 2D inversion results, which reveals further details and enhances the geological interpretation significantly.
The modulation instability is a prominent explanation for the occurrence of nonlinear freak waves. The modulation instability could be described by the famous nonlinear Schrödinger (NLS) equation. ...The Peregrine breather solution of the NLS equation is usually adopted as a prototype in freak wave simulation. The freak waves based on the Peregrine breather solution (Peregrine breather waves) are characterised by huge wave crests, large wave heights and concentrated energies. In this paper, the Peregrine breather waves are experimentally generated in deep water. The specific experimental wave-making procedure is first proposed. The measured wave elevations show great agreement with the analytical solutions. Moreover, the modulation instability scenario is observed by wavelet analysis. The effects of wave parameters on wave energy distributions are first analysed. The results indicate carrier wave amplitude and wave height determine the width of focusing energy frequency and carrier wave frequency determines the duration of focusing energy.
Seismic noise has been widely used to image Earth's structure in the past decades as a powerful supplement to earthquake signals. Although the seismic noise field contains both surface‐wave and ...body‐wave components, most previous studies have focused on surface waves due to their large amplitudes. Here, we use array analyses to identify body‐wave noise traveling as PKP waves. We find that by cross‐correlating the array‐stacked horizontal‐ and vertical‐component data in the time windows containing the PKP noise signals, we extract a phase likely representing PKS‐PKP, the differential phase between PKS and PKP. This phase can potentially be used for shear‐wave‐splitting analysis. Our results also suggest that the sources of body‐wave noise are extremely heterogeneous in both space and time, which should be accounted for in future studies using body‐wave noise to image Earth structure.
Plain Language Summary
Seismic noise is the vibration of Earth generated by activities other than earthquakes, such as wind and ocean waves. Signals extracted from seismic noise can be used to study Earth's interior structure in ways similar to how earthquake records have been analyzed. Most previous studies using seismic noise to study Earth structure used its surface‐wave component, that is, the waves propagating at Earth's surface, whereas the body‐wave component, that is, the waves traveling through Earth's interior, is less used because body‐wave noise is usually much weaker than surface‐wave noise. Here, we use data collected by a dense seismic array to identify body‐wave noise propagating as PKP waves, P waves that travel through Earth's core. We also find that PKS‐PKP, the differential phase between PKS and PKP, can be extracted from the records of time windows containing strong PKP energy. This phase can potentially be used to study the anisotropic properties of Earth's crust and mantle.
Key Points
PKS‐PKP is retrieved from cross‐component cross‐correlation of seismic noise
PKS‐PKP is enhanced by array stacking before cross‐correlation and using only the time windows with strong PKP energy
PKS‐PKP may be useful for shear‐wave splitting studies of crust and mantle anisotropy
Theoretical and experimental studies are carried out to investigate the characteristics of P-wave and S-wave propagation in soils with different degrees of saturation, which is quantified using ...Skempton's B-value. Based on the assumption of homogeneous pore fluid and Biot's theory, the characteristics of elastic wave propagation in nearly saturated soils are obtained and the effects of confining pressure, degree of saturation, and excitation frequency on the wave velocities are analyzed. It is found that the evolutions of P-wave, Biot-wave, and S-wave velocities with the degree of saturation are quite different. In experimental bender element tests, the P-wave signal in nearly saturated soil contains two parts. It is inferred that the first part of the P-wave signal is the fast P-wave component and the second part is a Biot-wave (i.e. slow P-wave of the second kind) as their characteristics agree well with theoretical predictions. It is also found that the relationship between the P-wave velocity, Vp, and the B-value depends on the characteristics of the pore fluid. For a pore fluid with de-aired water, the measured Vp is independent of the B-value, which is contrary to theoretical predictions. However, for a pore fluid consisting of tap water (i.e. water with air bubbles), the measured Vp generally increases with the B-value as predicted, and the relationship between the measured Vp and B-value agrees well with the prediction curve when the B-value is smaller than 0.85. When the B-value is larger than 0.85, the measured P-wave velocity is higher than the theoretical prediction, which is probably due to the non-uniform distribution of air bubbles in the pore fluid.
•Theoretical and experimental works are conducted to study the characteristics of elastic waves in nearly saturated sand.•The tests indicate that the first part of the P-wave signal is the fast P-wave component and the second part is a Biot-wave.•The relationship between Vp and the B-value depends on the pore fluid type.•The non-homogeneous distribution of air bubbles in pore fluid significantly affects the P-wave velocity.
PDF
