Structural control on the directional amplification of seismic noise (Campo Imperatore, central Italy)
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Abstract:
Seismic signals propagating across a fault may yield information on the internal structure of the fault zone. Here we have assessed the amplification of seismic noise (i.e., ambient vibrations generated by natural or anthropogenic disturbances) across the Vado di Corno Fault (Campo Imperatore, central Italy). The fault zone is considered as an exhumed analogue of the normal faults activated during the L'Aquila 2009 earthquake sequence. Detailed structural geological survey of the footwall block revealed that the fault zone is highly anisotropic and is affected by a complex network of faults and fractures with dominant WNW–ESE strike. We measured seismic noise with portable seismometers along a ∼500 m long transect perpendicular to the average fault strike. Seismic signals were processed calculating the horizontal-to-vertical spectral ratios and performing wavefield polarization analyses. We found a predominant NE–SW to NNE–SSW (i.e., ca. perpendicular to the average strike of the fault-fracture network) amplification of the horizontal component of the seismic waves. Numerical simulations of earthquake-induced ground motions ruled out the role of topography in controlling the polarization and the amplitude of the waves. Therefore, the higher seismic noise amplitude observed in the fault-perpendicular direction was related to the measured fracture network and the resulting stiffness anisotropy of the rock mass. These observations open new perspectives in using measures of ambient seismic noise, which are fast and inexpensive, to estimate the dominant orientation of fracture networks within fault zones.Keywords:
Seismic Noise
Seismometer
Seismic anisotropy
Ambient noise level
Microseism
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
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Seismogram
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abstract Enhancement of P waves is obtained by the suppression of microseisms with a combination of vertical linear strain seismograph and vertical inertial seismograph outputs. Significant suppression is observed at sites where the predominant microseisms consist of essentially single-mode Rayleigh waves in contrast to that observed at a site where the microseismic noise is compositionally more complex. A description is given of the vertical linear strain seismograph used in these investigations. Consideration is given to the use of a vertical transverse strain seismograph at sites where the microseismic noise field is complex.
Microseism
Seismometer
Strain (injury)
Rayleigh Wave
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Microseism
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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
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Ambient noise level
Seismic Noise
Passive seismic
Seismic array
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Summary A comparison study between borehole arrays, broadband seismometers and surface geophones is undertaken using a new passive seismic dataset from the Fox Creek, Alberta, area. Induced seismicity and microseismic events are compared based on waveform character, arrival time accuracy, and frequency content. In addition, using cross-correlation for detection, the effectiveness of each of the instruments at picking up events is compared. It is concluded that the broadband seismometers are most effective for induced seismicity monitoring, while the borehole array is most suitable for the microseismic monitoring.
Geophone
Microseism
Seismometer
Passive seismic
Vertical seismic profile
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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.
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Seismic Noise
Seismometer
Rayleigh Wave
Ambient noise level
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abstract Seismometers in spherical aluminum pressure housings have been weighted to float stably at midwater depths in the ocean, and thus record water motions in a frequency band of 0.02 to 5 cps. Simultaneous records made with a midwater instrument at 1.2-km depth and a bottom instrument at 4.6-km depth showed coherence at spectral power peaks of leaky organ-pipe frequencies and additional coherence peaks at frequencies down to 0.025 cps. Twenty organ-pipe modes can be tentatively identified. The spectral power can be attributed almost entirely to microseismic motions in wave-guide modes. We conclude that the forcing functions for microseisms are broad enough so that deep ocean-bottom and midwater microseism spectral peak frequencies are characteristic of local bathymmetry.
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Forcing (mathematics)
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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.
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Seismic Noise
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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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