Source directionality of ambient seismic noise inferred from three‐component beamforming
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Abstract The increased use of ambient seismic noise for seismic imaging requires better understanding of the ambient seismic noise wavefield and its source locations and mechanisms. Although the source regions and mechanisms of Rayleigh waves have been studied extensively, characterization of Love wave source processes are sparse or absent. We present here the first systematic comparison of ambient seismic noise source directions within the primary (~10–20 s period) and secondary (~5–10 s period) microseism bands for both Rayleigh and Love waves in the Southern Hemisphere using vertical‐ and horizontal‐component ambient seismic noise recordings from a dense temporary network of 68 broadband seismometers in New Zealand. Our analysis indicates that Rayleigh and Love waves within the primary microseism band appear to be mostly generated in different areas, whereas in the secondary microseism band they arrive from similar backazimuths. Furthermore, the source areas of surface waves within the secondary microseism band correlate well with modeled deep‐water and near‐coastal source regions.Keywords:
Microseism
Seismic Noise
Seismometer
Rayleigh Wave
Ambient noise level
A group of resonant vertical seismometers, each tuned to cover a part of the spectrum of microseism frequencies, has been operated for about one year. These instruments (a) clearly distinguish between simultaneous microseisms from two separate sources; (b) show an improved signal‐to‐noise ratio for microseisms from a single storm, permitting earlier detection of storm onsets; (c) show clearly the increase in period of frontal microseisms as cold fronts move seaward from the east coast of North America; (d) record only the envelope of the oscillations, which greatly facilitates measurement of intensity as a function of time; and (e) appear to be very useful tools in continued attempts at hurricane location by means of microseism amplitude studies. The performance of the instruments is demonstrated by seven case histories in which microseismic readings of seismometers tuned to different frequencies are related to the meteorological conditions which are apparently responsible for the microseismic activity.
Microseism
Seismometer
Seismic Noise
Envelope (radar)
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The design of horizontal and vertical component seismometers for recording ocean-generated microseisms is first described. Both instruments have a reasonably constant response to ground displacements in the 0·05 to 0·5 c/s range. Two three-component stations are in operation using these seismometers. The construction of the vaults and the methods of maintaining stable air conditions inside them are described. The seismometer outputs from the remote station are telemetered to the central recording laboratory by converting each to a frequency-modulated signal and transmitting over a G.P.O. landline. A brief description of the construction and performance of the equipment used is given. The six outputs are monitored on a graphic recorder and also sampled once per second for digital recording on punched paper tape. Some results of the analyses carried out during the winter of 1963/64 are presented. Auto spectra are computed and related to the spectra of sea waves, either deduced from the prevailing weather conditions over the North Atlantic or measured from weather ship wave recordings. The Rayleigh wave constant is evaluated for the two stations and related to local geology. The coherence and phase between components at one station and between stations as functions of frequency are explained with reference to the proportions of Rayleigh and Love waves present in microseisms and to the direction of arrival of the waves.
Microseism
Seismometer
Rayleigh Wave
SIGNAL (programming language)
Seismogram
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Ambient seismic noise techniques are an excellent choice for imaging the subsurface in areas that are seismically quiet or otherwise unsuitable for active source experiments due to geographic isolation or environmental sensitivity. Recently, decades-long time series were made available for download from the Incorporated Research Institutions for Seismology (IRIS) from permanent network installations, allowing access to long, uninterrupted recordings from seismometers around the world. This has spurred the development of an entire field of applications for passive seismic noise analysis. Over the continental United States, the USArray project has advanced to provide station coverage in relatively dense and regularly spaced arrays, but along the Aleutian Island arc in Alaska and other geographically isolated but seismically active locations, the hazards associated with volcanic eruptions and the difficulty of accessing stations for repair or replacement throughout most of the year has allowed only for sparse coverage. The analysis of ambient seismic recordings generally suits one of two purposes. The first involves the parameterization of the source of each component of the ambient seismic noise spectrum and focuses on both the spatial locations and mechanisms of generation. The second purpose of looking at ambient seismic noise is to create velocity models of the subsurface below the array. The assumptions required for the traditional approach to analysis of ambient seismic noise, namely beamforming and the spatial autocorrelation
Seismometer
Ambient noise level
Seismic Noise
Passive seismic
Seismic array
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Abstract The increased use of ambient seismic noise for seismic imaging requires better understanding of the ambient seismic noise wavefield and its source locations and mechanisms. Although the source regions and mechanisms of Rayleigh waves have been studied extensively, characterization of Love wave source processes are sparse or absent. We present here the first systematic comparison of ambient seismic noise source directions within the primary (~10–20 s period) and secondary (~5–10 s period) microseism bands for both Rayleigh and Love waves in the Southern Hemisphere using vertical‐ and horizontal‐component ambient seismic noise recordings from a dense temporary network of 68 broadband seismometers in New Zealand. Our analysis indicates that Rayleigh and Love waves within the primary microseism band appear to be mostly generated in different areas, whereas in the secondary microseism band they arrive from similar backazimuths. Furthermore, the source areas of surface waves within the secondary microseism band correlate well with modeled deep‐water and near‐coastal source regions.
Microseism
Seismic Noise
Seismometer
Rayleigh Wave
Ambient noise level
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In order to use ambient seismic noise for mapping Earth's structure, it is important to understand the spatio‐temporal characteristics of the noise field. This study uses data collected during four austral winter months of 2002 to investigate New Zealand's ambient seismic noise field in the double‐ocean‐wave‐frequency range (0.1–0.3 Hz). It is shown via beamforming analysis that there are two distinct dispersive waves in the data. These waves can be separated. Their estimated phase velocities (2.5–2 and 4–3 km/s in the frequency range 0.14–0.25 Hz) match well with fundamental and higher‐mode Rayleigh dispersion curves. Studies of double‐wave‐frequency microseisms elsewhere generally show the Rayleigh noise fields to be dominated by fundamental mode waves. The reason why higher‐mode signals are observed here may reflect a combination of long ocean wave periods, large waveheights, the direct deep water approach to narrow continental margins, and the proximity of the seismograph array to the source regions.
Rayleigh Wave
Microseism
Seismometer
Seismic Noise
Ambient noise level
Mode (computer interface)
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Abstract We performed a frequency‐dependent polarization analysis on ambient seismic energy recorded by 1768 USArray Transportable Array (TA) seismometers for the time period of 1 April 2004 through 31 October 2014. The seismic energy has strong seasonal variations in power and polarization at essentially all stations; however, the annual variation is much smaller. One year of data is sufficient to determine the average properties of the ambient seismic wavefield at a particular site. The average power and dominant period in the double‐frequency (DF) microseism band, defined here as periods of 2–10 s, vary significantly and coherently across North America. Proximity to a coastline generally leads to increased DF microseism amplitude, but site geology is much more important, with sedimentary basins having especially large DF amplitudes. The western U.S. as a whole has longer dominant DF periods than the central and eastern U.S., with the southeastern U.S. having the shortest dominant DF periods. Power spectral density estimates at many TA stations show a splitting of the DF microseism peak into two distinct subpeaks. This has been observed previously in data recorded by ocean bottom seismometers, with the shorter‐period DF peak attributed to the local sea and the longer‐period DF peak attributed to more distant, coastally generated microseisms. In the case of the land‐based TA data analyzed here, the DF splitting arises from simultaneous microseism generation at various source areas (Pacific Ocean, Atlantic Ocean, and Gulf of Mexico) with distinct, preferred excitation frequencies. DF microseism source properties derived from global models of ocean wave interaction support this interpretation.
Microseism
Seismometer
Seismic Noise
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Citations (86)
The increased use of ambient seismic noise for seismic imaging requires better understanding of the ambient seismic noise wavefield and its source locations and mechanisms. Although the source regions and mechanisms of Rayleigh waves have been studied extensively, characterization of Love wave source processes are sparse or absent. We present here the first systematic comparison of ambient seismic noise source directions within the primary (~10-20 s period) and secondary (~5-10 s period) microseism bands for both Rayleigh and Love waves in the Southern Hemisphere using vertical- and horizontal-component ambient seismic noise recordings from a dense temporary network of 68 broadband seismometers in New Zealand. Our analysis indicates that Rayleigh and Love waves within the primary microseism band appear to be mostly generated in different areas, whereas in the secondary microseism band they arrive from similar backazimuths. Furthermore, the source areas of surface waves within the secondary microseism band correlate well with modeled deep-water and near-coastal source regions. Key Points Rayleigh and Love wave source regions of the secondary microseism are co-located Rayleigh and Love wave source regions of the primary microseism differ strongly Observed and modeled source directions for the secondary microseism agree well ©2012. American Geophysical Union. All Rights Reserved.
Microseism
Rayleigh Wave
Seismic Noise
Seismometer
Ambient noise level
Love wave
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Rayleigh Wave
Ambient noise level
Seismic Noise
Seismic interferometry
Seismic Tomography
Group velocity
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Citations (0)
The increased use of ambient seismic noise for seismic imaging requires better understanding of the ambient seismic noise wavefield and its source locations and mechanisms. Although the source regions and mechanisms of Rayleigh waves have been studied extensively, characterization of Love wave source processes are sparse or absent. We present here the first systematic comparison of ambient seismic noise source directions within the primary (~10-20 s period) and secondary (~5-10 s period) microseism bands for both Rayleigh and Love waves in the Southern Hemisphere using vertical- and horizontal-component ambient seismic noise recordings from a dense temporary network of 68 broadband seismometers in New Zealand. Our analysis indicates that Rayleigh and Love waves within the primary microseism band appear to be mostly generated in different areas, whereas in the secondary microseism band they arrive from similar backazimuths. Furthermore, the source areas of surface waves within the secondary microseism band correlate well with modeled deep-water and near-coastal source regions. Key Points Rayleigh and Love wave source regions of the secondary microseism are co-located Rayleigh and Love wave source regions of the primary microseism differ strongly Observed and modeled source directions for the secondary microseism agree well ©2012. American Geophysical Union. All Rights Reserved.
Microseism
Rayleigh Wave
Seismic Noise
Seismometer
Ambient noise level
Love wave
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Direction of Approach of microseismic waves was investigated in Kyushu by means of vector seismographs. It was found that no microseismic wave comes from the west direction even when typhoons were situated in the south-west direction. The frequency of arrival directions was distributed partially for the direction of the Hyuga Sea where the continental margin is near to the coast.From these distributions and those at the Abuyama Observatory, it is reasonably concluded that the nearer the continental margin is to the coast, the more frequently microseismic waves are generated.
Microseism
Seismometer
Typhoon
Margin (machine learning)
Continental Margin
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