Imaging of seismic scatterers beneath the Gauribidanur (GBA) array
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Seismic array
Geophysical Imaging
A great earthquake of Mw 9.0 occurred on March 11, 2011 off the coast of Tohoku region, Northeast Honshu, Japan. Strong ground motions from the earthquake were recorded at 4 stations of a small seismic array, with an aperture of about 500 m, located 120 km away from the epicenter. Peak ground acceleration exceed the full scale of 2g on the horizontal components, and was larger than 1g even on the vertical component. Two prominent bursts and at least two following smaller bursts are identified on the strong-motion records which lasted for longer than 200 s. We have performed semblance analysis to estimate the rupture propagation during the earthquake using coherent seismograms at frequencies of 0.5–2 Hz. The rupture seems to consist of at least four stages. Rupture propagated in a northerly direction in the beginning 50 s forming the first burst, then proceeded to the southwest from the epicenter in the next 50 s during the second burst. The rupture further extended southwests in the following 40 s, and finally migrated to the south for about 30 s. A small seismic array makes it possible to observe rupture propagation during a large earthquake even with a small number of stations.
Epicenter
Seismogram
Seismic array
Earthquake rupture
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A passive seismic experiment is carried out at the non-volcanic highly degassing site of Mefite d'Ansanto located at the northern tip of the Irpinia region (southern Italy), where the 1980 MS 6.9 destructive earthquake occurred. Between 2020 and 2021, background seismic noise was recorded by deploying a broadband seismic station and a seismic array composed of seven 1 Hz three-component sensors. Using two different array configurations, we were allowed to explore in detail the 1-20 Hz frequency band of the seismic noise wavefield as well as Rayleigh wave phase velocities in the 400-800 m/s range. Spectral analyses and array techniques were applied to one year of data showing that the frequency content of the signal is very stable in time. High frequency peaks are likely linked to the emission source, whereas at low frequencies seismic noise is clearly correlated to meteorological parameters. The results of this study show that small aperture seismic arrays probe the subsurface of tectonic CO2-rich emission areas and contribute to the understanding of the link between fluid circulation and seismogenesis in seismically active regions.
Seismic Noise
Passive seismic
Seismic array
Rayleigh Wave
Vertical seismic profile
Ambient noise level
Seismometer
Seismic survey
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We discuss several outstanding aspects of seismograms recorded during >4 weeks by a spatially dense Nodal array, straddling the damage zone of the San Jacinto fault in southern California, and some example results. The waveforms contain numerous spikes and bursts of high-frequency waves (up to the recorded 200 Hz) produced in part by minute failure events in the shallow crust. The high spatial density of the array facilitates the detection of 120 small local earthquakes in a single day, most of which not detected by the surrounding ANZA and regional southern California networks. Beamforming results identify likely ongoing cultural noise sources dominant in the frequency range 1–10 Hz and likely ongoing earthquake sources dominant in the frequency range 20–40 Hz. Matched-field processing and back-projection of seismograms provide alternate event location. The median noise levels during the experiment at different stations, waves generated by Betsy gunshots, and wavefields from nearby earthquakes point consistently to several structural units across the fault. Seismic trapping structure and local sedimentary basin produce localized motion amplification and stronger attenuation than adjacent regions. Cross correlations of high-frequency noise recorded at closely spaced stations provide a structural image of the subsurface material across the fault zone. The high spatial density and broad frequency range of the data can be used for additional high resolution studies of structure and source properties in the shallow crust.
Seismogram
Seismic array
Ambient noise level
Seismic Noise
Microseism
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Slowness
Seismic array
Caldera
Ambient noise level
Seismic Noise
Epicenter
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Seismometer
Seismic array
Misorientation
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Seismic ground motion at a site is strongly affected by the underground structure, especially the S-wave velocity structure below. During the 1995 Hyogo-ken Nanbu earthquake, it is found that the basin structure with the thickness of about 1km played a major role to form the damage belt in Kobe. We estimate the S-wave velocity structure at Higashinada ward, Kobe where the damage belt runs through from west to east. We deployed two different sized arrays in southeastern part of Higashinada ward. The smaller array, named KUMM array, has a diameter of 200 meters, while the larger array, named M-array, has a diameter of 2000 meters. We recorded microtremors at each array with ten three-components velocity sensors simultaneously. Independently, a seismic refraction experiment was carried out on December 12 and 14, 1995 in Hyogo and Osaka prefectures. We also recorded seismic waves from the explosion in the Osaka port by using the same KUMM array as the microtremor observation. Phase velocities of Rayleigh waves not only for microtremors but also for explosion-induced seismic waves are used to estimate the S-wave structure. Estimated S-wave velocity structure is very close to the PS logging data which is obtained at a site close to the array. Seismic refraction survey using explosions is often carried out to estimate an underground structure, in which only initial motions are used and later arrivals are considered to be noise. In this paper, we show a possibility to use phase velocities of Rayleigh wave, which is induced by an explosion, just as the case of microtremors.
Microtremor
Rayleigh Wave
Seismic refraction
Seismic Noise
Microseism
Seismic array
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Epicenter
Seismic array
Earthquake rupture
Magnetic dip
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In many subsurface industrial applications, fluids are injected into or withdrawn from a geologic formation. It is of practical interest to quantify precisely where, when, and by how much the injected fluid alters the state of the subsurface. Routine geophysical monitoring of such processes attempts to image the way that geophysical properties, such as seismic velocities or electrical conductivity, change through time and space and to then make qualitative inferences as to where the injected fluid has migrated. The more rigorous formulation of the time-lapse geophysical inverse problem forecasts how the subsurface evolves during the course of a fluid-injection application. Using time-lapse geophysical signals as the data to be matched, the model unknowns to be estimated are the multiphysics forward-modeling parameters controlling the fluid-injection process. Properly reproducing the geophysical signature of the flow process, subsequent simulations can predict the fluid migration and alteration in the subsurface. The dynamic nature of fluid-injection processes renders imaging problems more complex than conventional geophysical imaging for static targets. This work intents to clarify the related hydrogeophysical parameter estimation concepts.
Multiphysics
Geophysical Imaging
Geophysical fluid dynamics
Exploration geophysics
Subsurface Flow
Electrical Resistivity Tomography
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We conducted a small-aperture seismic array observation at Suwanose-jima volcano for the period from 1 October to 18 November 2010. The seismic array consists of 13 elements of short-period seismometers and 5 data loggers. We perform array analysis for continuous seismic records observed during this period to investigate the wave-field properties of several episodes of volcanic tremors and 30 explosion earthquakes. Averages of the slowness values and back azimuths of volcanic tremors are estimated to be 0.8-1.4 s/km and 0°-40°, respectively. This suggests that the tremor wave consists of body and surface waves that propagate from the active crater. The array analyses and particle motions of explosion earthquakes indicate that the initial parts of the waveforms of explosion earthquakes are S waves that come from the direction of the active crater. The slowness values of the S waves of the explosion earthquakes are 0.3-0.5 s/km. Variation of the slowness value reflects the depth change of the explosion sources.
Slowness
Seismometer
Seismic array
Geophone
Epicenter
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