We retrieve P diving waves by applying seismic interferometry to ambient‐noise records observed at Long Beach, California, and invert travel times of these waves to estimate 3‐D P wave velocity ...structure. The ambient noise is recorded by a 2‐D dense and large network, which has about 2500 receivers with 100 m spacing. Compared to surface wave extraction, body wave extraction is a much greater challenge because ambient noise is typically dominated by surface wave energy. For each individual receiver pair, the cross‐correlation function obtained from ambient‐noise data does not show clear body waves. Although we can reconstruct body waves when we stack correlation functions over all receiver pairs, we need to extract body waves at each receiver pair separately for imaging spatial heterogeneity of subsurface structure. Therefore, we employ two filters after correlation to seek body waves between individual receiver pairs. The first filter is a selection filter based on the similarity between each correlation function and the stacked function. After selecting traces containing stronger body waves, we retain about two million correlation functions (35% of all correlation functions) and successfully preserve most of body wave energy in the retained traces. The second filter is a noise suppression filter to enhance coherent energy (body waves here) and suppress incoherent noise in each trace. After applying these filters, we can reconstruct clear body waves from each virtual source. As an application of using extracted body waves, we estimate 3‐D P wave velocities from these waves with travel time tomography. This study is the first body wave tomography result obtained from only ambient noise recorded at the ground surface. The velocity structure estimated from body waves has higher resolution than estimated from surface waves.
Key Points
We extract body waves from ambient noise recorded by a dense network
Two signal‐processing filters are applied to retrieve clear body waves
We use the extracted body waves to estimate 3‐D P wave velocities
Sea ice inhibits the development of wind‐generated surface gravity waves which are the dominant factor in upper ocean mixing and air‐sea fluxes. In turn, sea ice properties are modified by wave ...action. Understanding the interaction of ice and waves is important for characterizing both air‐sea interactions and sea ice dynamics. Current leading theory attributes wave attenuation primarily to scattering by ice floes. Here we use new in situ wave measurements to show that attenuation is dominated by dissipation with negligible effect by scattering. Time series of wave height in ice exhibit an “on/off” behavior that is consistent with switching between two states of sea ice: a relatively unbroken state associated with strong damping (off), possibly caused by ice flexure, and very weak attenuation (on) across sea ice that has been broken up by wave action.
Plain Language Summary
Waves created by wind at the ocean surface are strongly attenuated when they travel across ice‐covered regions. Until now, this effect was thought to be the result of reflection of waves off pieces of ice. Using new measurements of wave directions, we show that waves do not come from a broad range of directions, and scattering must be weak. Instead, we find that attenuation is highly variable and related to the size of ice floes. We hypothesize that attenuation may be caused by cyclic deformation of the ice. When the waves are large enough to break the ice up, this deformation stops, and the attenuation is much less. This finding is important for forecasting waves in ice‐infested waters as well as predicting seasonal sea ice extent.
Key Points
Wind waves attenuate across the Antarctic sea ice with a narrow directional distribution
Scattering of waves by ice floes plays a negligible role in wave attenuation
Observed wave attenuation is consistent with ice breakup modulating the dissipation strength
Electron resonant interaction with whistler mode waves is traditionally considered as one of the main drivers of radiation belt dynamics. The two main theoretical concepts available for its ...description are quasi‐linear theory of electron scattering by low‐amplitude waves and nonlinear theory of electron resonant trapping and phase bunching by intense waves. Both concepts successfully describe some aspects of wave‐particle interactions but predict significantly different timescales of relativistic electron acceleration. In this study, we investigate effects that can reduce the efficiency of nonlinear interactions and bridge the gap between the predictions of these two types of models. We examine the effects of random wave phase and frequency variations observed inside whistler mode wave packets on nonlinear interactions. Our results show that phase coherence and frequency fluctuations should be taken into account to accurately model electron nonlinear resonant acceleration and that, along with wave amplitude modulation, they may reduce acceleration rates to realistic, moderate levels.
Key Points
Chorus wave phase randomly fluctuates between subpackets inside longwave packets
Wave phase coherence significantly influences the efficiency of electron acceleration via nonlinear resonant interaction
Fluctuations of wave frequency within longwave packets similarly reduce the efficiency of nonlinear resonant electron acceleration
The multiple-taper correlation (MTC) algorithm for the estimation of teleseismic receiver functions (RFs) has desirable statistical properties. This paper presents several adaptations to the MTC ...algorithm that exploit its frequency-domain uncertainty estimates to generate stable RFs that include moveout corrections for deeper interfaces. Narrow-band frequency averaging implicit in spectral cross-correlation restricts the MTC-based RF estimates to resolve Ps converted phases only at short delay times, appropriate to the upper 100 km of Earth's lithosphere. The Ps conversions from deeper interfaces can be reconstructed by the MTC algorithm in two ways. Event cross-correlation computes a cross-correlation of single-taper spectrum estimates for a cluster of events rather than for a set of eigenspectrum estimates of a single P coda. To extend the reach of the algorithm, pre-stack moveout corrections in the frequency domain preserves the formal uncertainties of the RF estimates, which are used to weight RF stacks. Moving-window migration retains the multiple-taper approach, but cross-correlates the P-polarized motion with time-delayed SH and SV motion to focus on a Ps phase of interest. The frequency-domain uncertainties of bin-averaged RFs do not translate directly into the time domain. A jackknife over data records in each bin stack offers uncertainty estimates in the time domain while preserving uncertainty weighting in the frequency-domain RF stack.
Electromagnetic whistler‐mode chorus and electrostatic electron cyclotron harmonic (ECH) waves can contribute significantly to auroral electron precipitation and radiation belt electron acceleration. ...In the past, linear and nonlinear wave‐particle interactions have been proposed to explain the occurrences of these magnetospheric waves. By analyzing Van Allen Probes data, we present here the first evidence for nonlinear coupling between chorus and ECH waves. The sum‐frequency and difference‐frequency interactions produced the ECH sidebands with discrete frequency sweeping structures exactly corresponding to the chorus rising tones. The newly generated weak sidebands did not satisfy the original electrostatic wave dispersion relation. After the generation of chorus and normal ECH waves by hot electron instabilities, the nonlinear wave‐wave interactions could additionally redistribute energy among the resonant waves, potentially affecting to some extent the magnetospheric electron dynamics.
Plain Language Summary
Whistler‐mode chorus and electron cyclotron harmonic emissions are two distinct magnetospheric waves responsible for auroral electron precipitation and radiation belt electron acceleration. How these magnetospheric waves are generated has remained an outstanding question. They were usually explained as a result of linear and nonlinear wave‐particle interactions in early studies. By analyzing the high‐resolution data of Van Allen Probes, we present here the first evidence for nonlinear coupling between chorus and electron cyclotron harmonic emissions. Such nonlinear wave‐wave interactions could transfer energy among the resonant waves and affect the magnetospheric electron dynamics. This new finding will be of high interest to the communities of space plasma physics and magnetospheric physics.
Key Points
Nonlinear resonant interactions between chorus and electron cyclotron harmonic waves occurred around the magnetic equator
Newly generated ECH sidebands exhibited frequency sweeping structures and failed to satisfy the original linear dispersion relation
After the hot electron instabilities, the nonlinear wave‐wave interactions could additionally redistribute energy among the resonant waves
The Tien Shan is one of the most active intracontinental orogenic belts worldwide and shows intensive seismicity and tectonic activity that reflects laterally heterogeneous structures. Here, we ...employ multimode surface wave tomography to construct an anisotropic 3-D shear-wave velocity model, which achieves a resolution of 2.0°. The new tomographic model reveals pronounced velocity contrasts between the central Tien Shan, which shows low velocity, and the eastern Tien Shan, which shows high velocity in the depth range of 50–300 km. We interpret that this velocity contrast as potentially resulting from the difference in the uplift mechanism of the Tien Shan. A comprehensive analysis suggested that the uplift of the central Tien Shan might be related to northward subduction of the Tarim lithosphere while the low-velocity anomaly beneath the central Tien Shan might be related to subsequent mantle upwelling. In contrast, the Tarim block in the south collided with the Junggar Basin in the north, which may have led to the uplift of the eastern Tien Shan. The Tarim Basin is mainly characterized by high-velocity anomalies of SV waves but low-velocity anomalies of SH waves beneath the western Tarim Basin, which may be related to the Tarim mantle plume.
A deterministic system of ocean surface waves and flow in the oceanic boundary layer is key to understanding the dynamics of the upper ocean. For the description of such complex systems, a ...higher‐order shear‐current modified nonlinear Schrödinger equation is newly derived and then used to physically interpret the interplay between Stokes drift, Eulerian return flow due to a passing wave group, and an open‐ocean vertically sheared flow in the extreme sea wave generation. The conditions for the suppression or enhancement of the modulation instability in the rogue wave dynamics in the presence of a background flow are reported, whose relevance and influence to the Craik‐Leibovich type 2 instability in triggering a Langmuir‐type circulation is discussed. The findings highlight the need for future studies to establish and assess the energy transfer from waves to currents or in the reversing order, asserting a plausible physical mechanism for the dissipation of the surface wave energy through wave‐current interactions in the open ocean.
Plain Language Summary
The dynamics of the upper‐ocean involve many complex processes, including for instance the interplay between wind, waves, currents, and global circulation systems. Such interactions can give rise to instabilities and extreme events with far‐reaching consequences. In this letter, we use a newly derived weakly nonlinear wave framework accounting for the presence of shear currents to quantify the requirements to trigger modulation instability, giving rise to long‐crested rogue waves. Our investigation also provides combined conditions for the occurrence of both, modulation and Craik‐Leibovich (type 2) instabilities, and demonstrates the possibility of energy transfers between waves as well as between waves and currents in the ocean.
Key Points
An advanced shear‐current modified nonlinear Schrödinger‐type equation is derived for surface waves in a background open‐ocean flow
The interplay between Stokes drift, background flow, and Eulerian return flow by a wave group, in extreme waves generation is revealed
How a background flow suppresses the modulational instability is explained and its relevance to the CL2 instability is discussed
Wave loading on marine structures is the major external force to be considered in the design of such structures. The accurate prediction of the nonlinear high-order components of the wave loading has ...been an unresolved challenging problem. In this paper, the nonlinear harmonic components of hydrodynamic forces on a bottom-mounted vertical cylinder are investigated experimentally. A large number of experiments were conducted in the Danish Hydraulic Institute shallow water wave basin on the cylinder, both on a flat bed and a sloping bed, as part of a European collaborative research project. High-quality data sets for focused wave groups have been collected for a wide range of wave conditions. The high-order harmonic force components are separated by applying the ‘phase-inversion’ method to the measured force time histories for a crest focused wave group and the same wave group inverted. This separation method is found to work well even for locally violent nearly-breaking waves formed from bidirectional wave pairs. It is also found that the
$n$
th-harmonic force scales with the
$n$
th power of the envelope of both the linear undisturbed free-surface elevation and the linear force component in both time variation and amplitude. This allows estimation of the higher-order harmonic shapes and time histories from knowledge of the linear component alone. The experiments also show that the harmonic structure of the wave loading on the cylinder is virtually unaltered by the introduction of a sloping bed, depending only on the local wave properties at the cylinder. Furthermore, our new experimental results reveal that for certain wave cases the linear loading is actually less than 40 % of the total wave loading and the high-order harmonics contribute more than 60 % of the loading. The significance of this striking new result is that it reveals the importance of high-order nonlinear wave loading on offshore structures and means that such loading should be considered in their design.
Teleseismic S wave microseisms Nishida, Kiwamu; Takagi, Ryota
Science (American Association for the Advancement of Science),
08/2016, Volume:
353, Issue:
6302
Journal Article
Peer reviewed
Although observations of microseisms excited by ocean swells were firmly established in the 1940s, the source locations remain difficult to track. Delineation of the source locations and energy ...partition of the seismic wave components are key to understanding the excitation mechanisms. Using a seismic array in Japan, we observed both P and S wave microseisms excited by a severe distant storm in the Atlantic Ocean. Although nonlinear forcing of an ocean swell with a one-dimensional Earth model can explain P waves and vertically polarized S waves (SV waves), it cannot explain horizontally polarized S waves (SH waves). The precise source locations may provide a new catalog for exploring Earth's interior.
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