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Michel Campillo - One of the best experts on this subject based on the ideXlab platform.
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spatiotemporal correlation analysis of noise derived seismic body waves with ocean wave climate and Microseism sources
Geochemistry Geophysics Geosystems, 2020Co-Authors: Pierre Boue, Lise Retailleau, Michel CampilloAbstract:Seismic signals can be extracted from ambient noise wavefields by the correlation technique. Recently, a prominent P‐type phase was observed from teleseismic noise correlations in the secondary Microseism period band. The phase is named Pdmc in this paper, corresponding to its origin from the interference between the direct P waves transmitting through the deep mantle and the core (P and PKPab waves). We extract the phase by correlating noise records from two seismic networks in the Northern Hemisphere and locate the Microseism sources that are efficient for the Pdmc construction in the South Pacific. We investigate the spatiotemporal links of the Pdmc signal with global oceanic waves and Microseism sources. Interestingly, the correlation with wave height is higher in several regions surrounding the effective source region, rather than in the effective source region. The Pdmc amplitude is highly correlated with the power of the effective Microseism sources. Also, it is apparently correlated with ineffective sources in the Southern Hemisphere and anticorrelated with sources in the Northern Hemisphere. We ascribe the correlation with the ineffective southern sources to the spatiotemporal interconnections of the southern sources. The anticorrelation with northern sources can be explained by the reverse seasonal patterns between the southern and northern sources and by that the northern sources impede the signal construction. The signal construction from noise correlations relies on the competition between the effective and ineffective sources, not just on the power of the effective sources. This principle should be valid in a general sense for noise‐derived signals.
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Ambient seismic noise imaging of the lowermost mantle beneath the North Atlantic Ocean
Geophysical Journal International, 2020Co-Authors: Lise Retailleau, Pierre Boue, Michel CampilloAbstract:Body waves can be extracted from correlation functions computed from seismic records even at teleseismic distances. Here we use P and PcP waves from the secondary Microseism frequency band that are propagating between Europe and the Eastern United States to image the core-mantle boundary (CMB) and D" structure beneath the North Atlantic. This study presents the first 3-D image of the lower mantle obtained from ocean-generated Microseism data. Robustness of our results is evaluated by comparing images produced by propagation in both directions. Our observations reveal complex patterns of lateral and vertical variations of P-wave reflectivity with a particularly strong anomaly extending upward in the lower mantle up to 2600 km deep. We compare these results with synthetic data and associate this anomaly to a Vp velocity increase above the CMB. Our image aims at promoting the study of the lower mantle with Microseism noise excitations.
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locating Microseism sources using spurious arrivals in intercontinental noise correlations
Journal of Geophysical Research, 2017Co-Authors: Pierre Boue, Lise Retailleau, Laurent Stehly, Michel CampilloAbstract:The accuracy of Green's functions retrieved from seismic noise correlations in the Microseism frequency band is limited by the uneven distribution of Microseism sources at the surface of the Earth. As a result, correlation functions are often biased as compared to the expected Green's functions, and they can include spurious arrivals. These spurious arrivals are seismic arrivals that are visible on the correlation and do not belong to the theoretical impulse response. In this article, we propose to use Rayleigh wave spurious arrivals detected on correlation functions computed between European and United States seismic stations to locate Microseism sources in the Atlantic Ocean. We perform a slant stack on a time-distance gather of correlations obtained from an array of stations that comprises a regional deployment and a distant station. The arrival times and the apparent slowness of the spurious arrivals lead to the location of their source, which is obtained through a grid search procedure. We discuss improvements in the location through this methodology as compared to classical back-projection of Microseism energy. This method is interesting because it only requires an array and a distant station on each side of an ocean, conditions that can be met relatively easily.
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global oceanic Microseism sources as seen by seismic arrays and predicted by wave action models
Geochemistry Geophysics Geosystems, 2012Co-Authors: Gregor Hillers, Nicholas E Graham, Michel Campillo, Sachin Kedar, Maurice Landes, N M ShapiroAbstract:We analyze global Microseism excitation patterns between July 2000 and June 2001. Seismological observations are compared with modeling results to isolate robust activity features of relevant source processes. First, we use observations of Microseism source locations estimated by Landes et al. (2010) based on array processing of ambient noise correlations. Second, we construct synthetic activity patterns by coupling sea state estimates derived from wave action models to the excitation theory for Microseisms. The overall spatiotemporal evolution of both estimates is characterized by a seasonal character that is associated with strong activity during winter months. The distribution of landmass causes seasonal changes on the Northern Hemisphere (NH) to exceed the variability on the Southern Hemisphere (SH). Our systematic comparison of the two estimates reveals significant Microseism excitation along coastlines and in the open ocean. Since coastal reflections are not accounted for in the modeling approach, the consistent mismatch between near-coastal observations and predictions suggests that relevant Microseism energy arriving at the networks is generated in these areas. Simultaneously, systematic coincidence away from coastlines verifies the open ocean generation hypothesis. These conclusions are universal and robust with respect to the seismic network locations on the NH. The spatially homogeneous resolution of our synthetics provides a valuable resource for the assessment of the global Microseism weather. Similar to previously identified hot spot areas in the North Atlantic, the modeled distributions hypothesize regions of strong localized activity on the SH, which are only partially confirmed by the analyzed data sets.
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a study of the seismic noise from its long range correlation properties
Journal of Geophysical Research, 2006Co-Authors: Michel Campillo, L Stehly, Nikolai M ShapiroAbstract:[1] We study the origin of the background seismic noise averaged over long time by cross correlating of the vertical component of motion, which were first normalized by 1-bit coding. We use 1 year of recording at several stations of networks located in North America, western Europe, and Tanzania. We measure normalized amplitudes of Rayleigh waves reconstructed from correlation for all available station to station paths within the networks for positive and negative correlation times to determine the seasonally averaged azimuthal distribution of normalized background energy flow (NBEF) through the networks. We perform the analysis for the two spectral bands corresponding to the primary (10–20 s) and secondary (5–10 s) Microseism and also for the 20–40 s band. The direction of the NBEF for the strongest spectral peak between 5 and 10 s is found to be very stable in time with signal mostly coming from the coastline, confirming that the secondary Microseism is generated by the nonlinear interaction of the ocean swell with the coast. At the same time, the NBEF in the band of the primary Microseism (10–20 s) has a very clear seasonal variability very similar to the behavior of the long-period (20–40 s) noise. This suggests that contrary to the secondary Microseism, the primary Microseism is not produced by a direct effect of the swell incident on coastlines but rather by the same process that generates the longer-period noise. By simultaneously analyzing networks in California, eastern United States, Europe, and Tanzania we are able to identify main source regions of the 10–20 s noise. They are located in the northern Atlantic and in the northern Pacific during the winter and in the Indian Ocean and in southern Pacific during the summer. These distributions of sources share a great similarity with the map of average ocean wave height map obtained by TOPEX-Poseidon. This suggests that the seismic noise for periods larger than 10 s is clearly related to ocean wave activity in deep water. The mechanism of its generation is likely to be similar to the one proposed for larger periods, namely, infragravity ocean waves.
Lise Retailleau - One of the best experts on this subject based on the ideXlab platform.
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multi phase seismic source imprint of tropical cyclones
EGU21, 2021Co-Authors: Lise Retailleau, Lucia GualtieriAbstract:The coupling between the ocean activity driven by winds and the solid Earth generates seismic signals recorded by seismometers worldwide. The 2–10 s period band, known as secondary Microseism, represents the largest background seismic wavefield. While moving over the ocean, tropical cyclones generate particularly strong and localized sources of secondary Microseisms that are detected remotely by seismic arrays. We assess and compare the seismic sources of P, SV, and SH waves associated with typhoon Ioke (2006) during its extra-tropical transition. To understand their generation mechanisms, we compare the observed multi-phase sources with theoretical sources computed with a numerical ocean wave model, and we assess the influence of the ocean resonance (or ocean site effect) and coastal reflection of ocean waves. We show how the location and lateral extent of the associated seismic source is period- and phase-dependent. This information is crucial for the use of body waves for ambient noise imaging and gives insights about the sea state, complementary to satellite data. The authors locate the maximum seismic energy imprint and lateral extent of the seismic sources generated by Typhoon Ioke. Based on this data set, they present a new tool to shed light on the generation mechanism of secondary Microseisms body waves.
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spatiotemporal correlation analysis of noise derived seismic body waves with ocean wave climate and Microseism sources
Geochemistry Geophysics Geosystems, 2020Co-Authors: Pierre Boue, Lise Retailleau, Michel CampilloAbstract:Seismic signals can be extracted from ambient noise wavefields by the correlation technique. Recently, a prominent P‐type phase was observed from teleseismic noise correlations in the secondary Microseism period band. The phase is named Pdmc in this paper, corresponding to its origin from the interference between the direct P waves transmitting through the deep mantle and the core (P and PKPab waves). We extract the phase by correlating noise records from two seismic networks in the Northern Hemisphere and locate the Microseism sources that are efficient for the Pdmc construction in the South Pacific. We investigate the spatiotemporal links of the Pdmc signal with global oceanic waves and Microseism sources. Interestingly, the correlation with wave height is higher in several regions surrounding the effective source region, rather than in the effective source region. The Pdmc amplitude is highly correlated with the power of the effective Microseism sources. Also, it is apparently correlated with ineffective sources in the Southern Hemisphere and anticorrelated with sources in the Northern Hemisphere. We ascribe the correlation with the ineffective southern sources to the spatiotemporal interconnections of the southern sources. The anticorrelation with northern sources can be explained by the reverse seasonal patterns between the southern and northern sources and by that the northern sources impede the signal construction. The signal construction from noise correlations relies on the competition between the effective and ineffective sources, not just on the power of the effective sources. This principle should be valid in a general sense for noise‐derived signals.
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Ambient seismic noise imaging of the lowermost mantle beneath the North Atlantic Ocean
Geophysical Journal International, 2020Co-Authors: Lise Retailleau, Pierre Boue, Michel CampilloAbstract:Body waves can be extracted from correlation functions computed from seismic records even at teleseismic distances. Here we use P and PcP waves from the secondary Microseism frequency band that are propagating between Europe and the Eastern United States to image the core-mantle boundary (CMB) and D" structure beneath the North Atlantic. This study presents the first 3-D image of the lower mantle obtained from ocean-generated Microseism data. Robustness of our results is evaluated by comparing images produced by propagation in both directions. Our observations reveal complex patterns of lateral and vertical variations of P-wave reflectivity with a particularly strong anomaly extending upward in the lower mantle up to 2600 km deep. We compare these results with synthetic data and associate this anomaly to a Vp velocity increase above the CMB. Our image aims at promoting the study of the lower mantle with Microseism noise excitations.
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locating Microseism sources using spurious arrivals in intercontinental noise correlations
Journal of Geophysical Research, 2017Co-Authors: Pierre Boue, Lise Retailleau, Laurent Stehly, Michel CampilloAbstract:The accuracy of Green's functions retrieved from seismic noise correlations in the Microseism frequency band is limited by the uneven distribution of Microseism sources at the surface of the Earth. As a result, correlation functions are often biased as compared to the expected Green's functions, and they can include spurious arrivals. These spurious arrivals are seismic arrivals that are visible on the correlation and do not belong to the theoretical impulse response. In this article, we propose to use Rayleigh wave spurious arrivals detected on correlation functions computed between European and United States seismic stations to locate Microseism sources in the Atlantic Ocean. We perform a slant stack on a time-distance gather of correlations obtained from an array of stations that comprises a regional deployment and a distant station. The arrival times and the apparent slowness of the spurious arrivals lead to the location of their source, which is obtained through a grid search procedure. We discuss improvements in the location through this methodology as compared to classical back-projection of Microseism energy. This method is interesting because it only requires an array and a distant station on each side of an ocean, conditions that can be met relatively easily.
Peter D Bromirski - One of the best experts on this subject based on the ideXlab platform.
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Are Deep-Ocean-Generated Surface-Wave Microseisms Observed on Land?
2013Co-Authors: Peter D Bromirski, Peter Gerstoft, Ralph A Stephen, Woods HoleAbstract:Recent studies attribute land double-frequency (DF) Microseism observations to deepwater generation. Here we show that near-coastal generation is generally the dominant source region. This determination is based on observations at land and ocean seismic stations, buoys, gravity wave hindcasts, and on beamforming results from continental seismic arrays. Interactions between opposing ocean wave components generate a pressure excitation pulse at twice the ocean wave frequency that excites pseudo-Rayleigh (pRg) wave DF Microseisms. pRg generated in shallow coastal waters have most of their energy in the solid Earth (“elastic ” pRg) and are observed by land-based and seafloor seismometers as DF Microseisms. pRg generated in the deep-ocean have most of their energy in the ocean (“acoustic ” pRg) and are continuously observed on the ocean-bottom, but acoustic pRg does not efficiently transition onto continents. High amplitude DF signals over the [0.2,0.3] Hz band observed on the deep seafloor are uncorrelated with continental observations, and are not clearly detectable at individual continental stations or by land seismic-array beamforming. Below 0.2 Hz, modeling an
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are deep ocean generated surface wave Microseisms observed on land
Journal of Geophysical Research, 2013Co-Authors: Peter D Bromirski, Ralph A Stephen, Peter GerstoftAbstract:[1] Recent studies attribute land double-frequency (DF) Microseism observations to deep water generation. Here we show that near-coastal generation is generally the dominant source region. This determination is based on observations at land and ocean seismic stations, buoys, gravity-wave hindcasts, and on beamforming results from continental seismic arrays. Interactions between opposing ocean wave components generate a pressure excitation pulse at twice the ocean wave frequency that excites pseudo-Rayleigh (pRg) wave DF Microseisms. pRg generated in shallow coastal waters have most of their energy in the solid Earth (“elastic” pRg) and are observed by land-based and seafloor seismometers as DF Microseisms. pRg generated in the deep ocean have most of their energy in the ocean (“acoustic” pRg) and are continuously observed on the ocean bottom, but acoustic pRg does not efficiently transition onto continents. High-amplitude DF signals over the [0.2, 0.3] Hz band observed on the deep seafloor are uncorrelated with continental observations and are not clearly detectable at individual continental stations or by land seismic-array beamforming. Below 0.2 Hz, modeling and some observations suggest that some deep water-generated elastic pRg energy can reach continental stations, providing that losses from scattering and transition across the continental-shelf boundary to the shore are not substantial. However, most observations indicate that generally little deep-ocean-generated DF Microseism energy reaches continental stations. Effectively, DF land observations are dominated by near-coastal wave activity.
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Microseisms and hum from ocean surface gravity waves
Journal of Geophysical Research, 2012Co-Authors: James Traer, Peter D Bromirski, Peter Gerstoft, Peter M ShearerAbstract:[1] Ocean waves incident on coasts generate seismic surface waves in three frequency bands via three pathways: direct pressure on the seafloor (primary Microseisms, PM), standing waves from interaction of incident and reflected waves (double-frequency Microseisms, DF), and swell-transformed infragravity wave interactions (the Earth's seismic hum). Beamforming of USArray seismic data shows that the source azimuths of the generation regions of hum, PM and DF Microseisms vary seasonally, consistent with hemispheric storm patterns. The correlation of beam power with wave height over all azimuths is highest in near-coastal waters. Seismic signals generated by waves from Hurricane Irene and from a storm in the Southern Ocean have good spatial and temporal correlation with nearshore wave height and peak period for all three wave-induced seismic signals, suggesting that ocean waves in shallow water commonly excite hum (via infragravity waves), PM, and DF Microseisms concurrently.
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global trends in extremal Microseism intensity
Geophysical Research Letters, 2010Co-Authors: R C Aster, Daniel E. Mcnamara, Peter D BromirskiAbstract:[1] Globally ubiquitous seismic background noise peaks near 7 and 14 s period are generated via distinct mechanisms that transfer storm-generated gravity wave energy to the seismic wave field. We utilize continuous digital ground motion data recorded by the Global Seismographic Network and precursor instrumentation to chronicle Microseism power extreme events for 1972–2009. Because most land-observed Microseism surface-wave energy is generated at or near coasts, Microseism metrics are particularly relevant to assessing changes in coastal ocean wave energy. Extreme Microseism winter storm season event counts reveal the widespread influence of the El Nino Southern Oscillation (ENSO). Individual station and ensemble slopes trend positive for this study period for Northern Hemisphere stations. The double-frequency Microseism is particularly volatile, suggesting that the weaker single-frequency Microseism directly generated by ocean swell at coasts is likely a more representative seismic proxy for broad-scale ocean wave energy estimation.
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the near coastal Microseism spectrum spatial and temporal wave climate relationships
Journal of Geophysical Research, 2002Co-Authors: Peter D Bromirski, F. K. DuennebierAbstract:[1] Comparison of the ambient noise data recorded at near-coastal ocean bottom and inland seismic stations at the Oregon coast with both offshore and nearshore buoy data shows that the near-coastal Microseism spectrum results primarily from nearshore gravity wave activity. Low double-frequency (DF), Microseism energy is observed at near-coastal locations when seas nearby are calm, even when very energetic seas are present at buoys 500 km offshore. At wave periods >8 s, shore reflection is the dominant source of opposing wave components for near-coastal DF Microseism generation, with the variation of DF Microseism levels poorly correlated with local wind speed. Near-coastal ocean bottom DF levels are consistently ∼20 dB higher than nearby DF levels on land, suggesting that Rayleigh/Stoneley waves with much of the mode energy propagating in the water column dominate the near-coastal ocean bottom Microseism spectrum. Monitoring the southward propagation of swell from an extreme storm concentrated at the Oregon coast shows that near-coastal DF Microseism levels are dominated by wave activity at the shoreline closest to the seismic station. Microseism attenuation estimates between on-land near-coastal stations and seismic stations ∼150 km inland indicate a zone of higher attenuation along the California coast between San Francisco and the Oregon border.
Anya M Reading - One of the best experts on this subject based on the ideXlab platform.
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impacts of the cryosphere and atmosphere on observed Microseisms generated in the southern ocean
Journal of Geophysical Research, 2020Co-Authors: Ross J Turner, Mark Hemer, Anya M ReadingAbstract:The Southern Ocean (in the region 60-180° E) south of the Indian Ocean, Australia, and the West Pacific is noted for the frequent occurrence and severity of its storms. These storms give rise to high-amplitude secondary Microseisms from sources, including the deep ocean regions, and primary Microseisms where the swells impinge on submarine topographic features. A better understanding of the varying Microseism wavefield enables improvements to seismic imaging and development of proxy observables to complement sparse in situ wave observations and hindcast models of the global ocean wave climate. We analyze 12-26 years of seismic data from 11 seismic stations either on the East Antarctic coast or sited in the Indian Ocean, Australia, and New Zealand. The power spectral density of the seismic wavefield is calculated to explore how the time-changing Microseism intensity varies with (i) sea ice coverage surrounding Antarctica and (ii) the Southern Annular Mode (SAM) climate index. Variations in sea ice extent are found to be the dominant control on the Microseism intensity at Antarctic stations, which exhibit a seasonal pattern phase-shifted by 4-5 months compared to stations in other continents. Peaks in extremal intensity at East Antarctic stations occur in March-April, with the highest peaks for secondary Microseisms occurring during negative SAM events. This relationship between Microseism intensity and the SAM index is opposite to that observed on the Antarctic Peninsula. This work informs the complexity of Microseism amplitudes in the Southern Hemisphere and assists ongoing interdisciplinary investigations of interannual variability and long-term trends.
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matched field processing of three component seismic array data applied to rayleigh and love Microseisms
Journal of Geophysical Research, 2018Co-Authors: M Gal, Anya M Reading, Nicholas Rawlinson, Vera SchultepelkumAbstract:We extend three-component plane wave beamforming to a more general form and devise a framework, which incorporates velocity heterogeneities of the seismic propagation medium and allows us to estimate accurately sources that do not follow the simple plane wave assumption. This is achieved by utilizing fast marching to track seismic wave fronts for given surface wave phase velocity maps. The resulting matched field processing approach is used to study the surface wave locations of Rayleigh and Love waves at 8 and 16Â s based on data from four seismic arrays in the western United States. By accurately accounting for the path propagation effects, we are able to map Microseism surface wave source locations more accurately than conventional plane wave beamforming. In the primary Microseisms frequency range, Love waves are dominant over Rayleigh waves and display a directional radiation pattern. In the secondary Microseisms range, we find the general source regions for both wave types to be similar, but on smaller scales differences are observed. Love waves are found to originate from a larger area than Rayleigh waves and their energy is equal or slightly weaker than Rayleigh waves. The energy ratios are additionally found to be source location dependent. Potential excitation mechanisms are discussed which favor scattering from Rayleigh-to-Love waves.
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full wavefield decomposition of high frequency secondary Microseisms reveals distinct arrival azimuths for rayleigh and love waves
Journal of Geophysical Research, 2017Co-Authors: M Gal, Anya M Reading, S P Ellingsen, Keith D Koper, Relu BurlacuAbstract:In the secondary Microseism band (0.1–1.0 Hz) the theoretical excitation of Rayleigh waves (Rg/LR), through oceanic wave-wave interaction, is well understood. For Love waves (LQ), the excitation mechanism in the secondary Microseism band is less clear. We explore high-frequency secondary Microseism excitation between 0.35 and 1 Hz by analyzing a full year (2013) of records from a three-component seismic array in Pilbara (PSAR), Australia. Our recently developed three-component waveform decomposition algorithm (CLEAN-3C) fully decomposes the beam power in slowness space into multiple point sources. This method allows for a directionally dependent power estimation for all separable wave phases. In this contribution, we compare quantitatively Microseismic energy recorded on vertical and transverse components. We find the mean power representation of Rayleigh and Love waves to have differing azimuthal distributions, which are likely a result of their respective generation mechanisms. Rayleigh waves show correlation with convex coastlines, while Love waves correlate with seafloor sedimentary basins. The observations are compared to the WAVEWATCH III ocean model, implemented at the Institut Francais de Recherche pour l'Exploitation de la Mer (IFREMER), which describes the spatial and temporal characteristics of Microseismic source excitation. We find Love wave energy to originate from raypaths coinciding with seafloor sedimentary basins where strong Rayleigh wave excitation is predicted by the ocean model. The total power of Rg waves is found to dominate at 0.35–0.6 Hz, and the Rayleigh/Love wave power ratio strongly varies with direction and frequency.
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source locations of teleseismic p sv and sh waves observed in Microseisms recorded by a large aperture seismic array in china
Earth and Planetary Science Letters, 2016Co-Authors: M Gal, Keith D Koper, Relu Burlacu, Q Liu, Fuyun Wang, Changqiao Zou, Yunhao Wei, Anya M ReadingAbstract:Transversely polarized seismic waves are routinely observed in ambient seismic energy across a wide range of periods, however their origin is poorly understood because the corresponding source regions are either undefined or weakly constrained, and nearly all models of Microseism generation incorporate a vertically oriented single force as the excitation mechanism. To better understand the origin of transversely polarized energy in the ambient seismic wavefield we make the first systematic attempt to locate the source regions of teleseismic SH waves observed in Microseismic (2.5–20 s) noise. We focus on body waves instead of surface waves because the source regions can be constrained in both azimuth and distance using conventional array techniques. To locate Microseismic sources of SH waves (as well as SV and P waves) we continuously backproject the vertical, radial, and transverse components of the ambient seismic wavefield recorded by a large-aperture array deployed in China during 2013–2014. As expected, persistent P wave sources are observed in the North Atlantic, North Pacific, and Indian Oceans, mainly at periods of 2.5–10 s, in regions with the strong ocean wave interactions needed to produce secondary Microseisms. SV waves are commonly observed to originate from locations indistinguishable from the P wave sources, but with smaller signal-to-noise ratios. We also observe SH waves with about half or less the signal-to-noise ratio of SV waves. SH source regions are definitively located in deep water portions of the Pacific, away from the sloping continental shelves that are thought to be important for the generation of Microseismic Love waves, but nearby regions that routinely generate teleseismic P waves. The excitation mechanism for the observed SH waves may therefore be related to the interaction of P waves with small-wavelength bathymetric features, such as seamounts and basins, through some sort of scattering process.
Peter Gerstoft - One of the best experts on this subject based on the ideXlab platform.
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Are Deep-Ocean-Generated Surface-Wave Microseisms Observed on Land?
2013Co-Authors: Peter D Bromirski, Peter Gerstoft, Ralph A Stephen, Woods HoleAbstract:Recent studies attribute land double-frequency (DF) Microseism observations to deepwater generation. Here we show that near-coastal generation is generally the dominant source region. This determination is based on observations at land and ocean seismic stations, buoys, gravity wave hindcasts, and on beamforming results from continental seismic arrays. Interactions between opposing ocean wave components generate a pressure excitation pulse at twice the ocean wave frequency that excites pseudo-Rayleigh (pRg) wave DF Microseisms. pRg generated in shallow coastal waters have most of their energy in the solid Earth (“elastic ” pRg) and are observed by land-based and seafloor seismometers as DF Microseisms. pRg generated in the deep-ocean have most of their energy in the ocean (“acoustic ” pRg) and are continuously observed on the ocean-bottom, but acoustic pRg does not efficiently transition onto continents. High amplitude DF signals over the [0.2,0.3] Hz band observed on the deep seafloor are uncorrelated with continental observations, and are not clearly detectable at individual continental stations or by land seismic-array beamforming. Below 0.2 Hz, modeling an
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are deep ocean generated surface wave Microseisms observed on land
Journal of Geophysical Research, 2013Co-Authors: Peter D Bromirski, Ralph A Stephen, Peter GerstoftAbstract:[1] Recent studies attribute land double-frequency (DF) Microseism observations to deep water generation. Here we show that near-coastal generation is generally the dominant source region. This determination is based on observations at land and ocean seismic stations, buoys, gravity-wave hindcasts, and on beamforming results from continental seismic arrays. Interactions between opposing ocean wave components generate a pressure excitation pulse at twice the ocean wave frequency that excites pseudo-Rayleigh (pRg) wave DF Microseisms. pRg generated in shallow coastal waters have most of their energy in the solid Earth (“elastic” pRg) and are observed by land-based and seafloor seismometers as DF Microseisms. pRg generated in the deep ocean have most of their energy in the ocean (“acoustic” pRg) and are continuously observed on the ocean bottom, but acoustic pRg does not efficiently transition onto continents. High-amplitude DF signals over the [0.2, 0.3] Hz band observed on the deep seafloor are uncorrelated with continental observations and are not clearly detectable at individual continental stations or by land seismic-array beamforming. Below 0.2 Hz, modeling and some observations suggest that some deep water-generated elastic pRg energy can reach continental stations, providing that losses from scattering and transition across the continental-shelf boundary to the shore are not substantial. However, most observations indicate that generally little deep-ocean-generated DF Microseism energy reaches continental stations. Effectively, DF land observations are dominated by near-coastal wave activity.
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Correspondence
2013Co-Authors: Key Points, Peter Gerstoft, R. Menon, P. Gerstoft, William S. HodgkissAbstract:• Averaging is not always helpful in ambient noise processing and can induce bias • Higher-mode Rayleigh and P waves contaminate coherences in the Microseism band • Attenuation retrieved from data is mostly due to interference and not intrinsi
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Microseisms and hum from ocean surface gravity waves
Journal of Geophysical Research, 2012Co-Authors: James Traer, Peter D Bromirski, Peter Gerstoft, Peter M ShearerAbstract:[1] Ocean waves incident on coasts generate seismic surface waves in three frequency bands via three pathways: direct pressure on the seafloor (primary Microseisms, PM), standing waves from interaction of incident and reflected waves (double-frequency Microseisms, DF), and swell-transformed infragravity wave interactions (the Earth's seismic hum). Beamforming of USArray seismic data shows that the source azimuths of the generation regions of hum, PM and DF Microseisms vary seasonally, consistent with hemispheric storm patterns. The correlation of beam power with wave height over all azimuths is highest in near-coastal waters. Seismic signals generated by waves from Hurricane Irene and from a storm in the Southern Ocean have good spatial and temporal correlation with nearshore wave height and peak period for all three wave-induced seismic signals, suggesting that ocean waves in shallow water commonly excite hum (via infragravity waves), PM, and DF Microseisms concurrently.
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global p pp and pkp wave Microseisms observed from distant storms
Geophysical Research Letters, 2008Co-Authors: Peter Gerstoft, Peter M Shearer, Nicholas Harmon, Jian ZhangAbstract:Microseisms are the continuous background vibrations of the Earth observed between earthquakes. Most Microseism studies have focused on low frequency energy (0.05–0.5 Hz) propagating as surface waves, but in the Microseism spectrum there is also energy that propagates as body waves (P-waves). Using array analysis on southern California stations we show that these body waves are generated in the ocean from distant storms and propagate deep within the Earth's mantle and core as P, PP and PKP phases. Comparisons with ocean wave hindcast data identify several distinct source regions in both the northern and southern hemispheres. Analyses of these body waves demonstrate that Microseisms often have a strong P-wave component originating from distant locations.