Antarctic icequakes triggered by the 2010 Maule earthquake in Chile
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Keywords:
Rayleigh Wave
Microseism
Passive seismic
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やや長周期地震動の波動特性を精度良く理解するために不可欠なS 波速度構造を求めることを目的として, 京葉臨海地帯の地震観測サイトにおいて微動のアレー観測を実施した. 微動の上下動成分のアレー観測によって周期約4.5秒までの位相速度を求め, 遺伝的アルゴリズムによる逆解析によって, 深い地盤のS波速度構造を求めた. このサイトで得られた地震記録のうちで表面波の卓越する記録を用いて, 非定常スペクトル解析やセンブランス解析を行い, 波動の伝播速度を算出した. この値が, 微動アレー観測より推定したS波速度構造から計算される理論値で説明できることを確認した. さらに異なる伝播経路からやってくる複数の表面波の存在を指摘した.
Microtremor
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Abstract It is now established that the primary microseism, the secondary microseisms, and the hum are the three main components of seismic noise in the frequency band from about 0.003 Hz to 1.0 Hz. Monthly averages of seismic noise are dominated by these signals in seismic noise. There are, however, some temporary additional signals in the same frequency band, such as signals from tropical cyclones (hurricanes and typhoons) in the ocean and on land, stormquakes, weather bombs, tornadoes, and wind-related atmospheric pressure loading. We review these effects, lasting only from a few hours to a week but are significant signals. We also attempt to classify all seismic noise. We point out that there are two broad types of seismic noise, the propagating seismic waves and the quasi-static deformations. The latter type is observed only for surface pressure changes at close distances. It has been known since about 1970 but has not been emphasized in recent literature. Recent data based on co-located pressure and seismic instruments clearly show its existence. Because the number of phenomena in the first type is large, we propose to classify all seismic noise into three categories: (1) propagating seismic waves from ocean sources, (2) propagating seismic waves from on-land sources, and (3) quasi-static deformation at ocean bottom and on land. The microseisms and the hum are in the first category although there are differences in the detailed processes of their excitation mechanisms. We will also classify temporary signals by these categories.
Microseism
Seismic Noise
Rayleigh Wave
Passive seismic
Shadow zone
Ambient noise level
Vertical seismic profile
Dispersive body waves
Seismic interferometry
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SUMMARY Unveiling the mechanisms of earthquake and volcanic eruption preparation requires improving our ability to monitor the rock mass response to transient stress perturbations at depth. The standard passive monitoring seismic interferometry technique based on coda waves is robust but recovering accurate and properly localized P- and S-wave velocity temporal anomalies at depth is intrinsically limited by the complexity of scattered, diffracted waves. In order to mitigate this limitation, we propose a complementary, novel, passive seismic monitoring approach based on detecting weak temporal changes of velocities of ballistic waves recovered from seismic noise correlations. This new technique requires dense arrays of seismic sensors in order to circumvent the bias linked to the intrinsic high sensitivity of ballistic waves recovered from noise correlations to changes in the noise source properties. In this work we use a dense network of 417 seismometers in the Groningen area of the Netherlands, one of Europe's largest gas fields. Over the course of 1 month our results show a 1.5 per cent apparent velocity increase of the P wave refracted at the basement of the 700-m-thick sedimentary cover. We interpret this unexpected high value of velocity increase for the refracted wave as being induced by a loading effect associated with rainfall activity and possibly canal drainage at surface. We also observe a 0.25 per cent velocity decrease for the direct P-wave travelling in the near-surface sediments and conclude that it might be partially biased by changes in time in the noise source properties even though it appears to be consistent with complementary results based on ballistic surface waves presented in a companion paper and interpreted as a pore pressure diffusion effect following a strong rainfall episode. The perspective of applying this new technique to detect continuous localized variations of seismic velocity perturbations at a few kilometres depth paves the way for improved in situ earthquake, volcano and producing reservoir monitoring.
Seismic Noise
Seismometer
Passive seismic
Microseism
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Surface waves, considered to be the most difficult type of coherent noise present on land seismic reflection data, carry useful information regarding S-wave velocities. The advantage of analyzing surface waves is that they are recorded using vertical component geophones but provide information that is usually obtained from seismic data recorded with three-component geophones. Nowadays, active and passive surface waves are analyzed to determine the distribution of the S-wave velocities in the near surface. One application of near-surface velocities is for determining microseismic event locations, especially for surface and shallow well geophone arrays.
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Passive seismic
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Reflection
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Campi Flegrei is a highly populated active caldera in the south of Italy. Several hundred thousand people live within this area, which is characterized by seismicity and ground deformation episodes, known as 'bradyseism'. For this reason, this area falls into a high-risk category and thus the Italian Civil Defence requires a detailed site-effect estimation. To determine the local amplification of the seismic waves for a high number of sites, we have analysed the seismic recordings of three seismic networks that have been deployed in the Campi Flegrei area over different time periods. The first network was deployed during the bradyseismic crisis of 1982–1984. We selected 22 of the highest magnitude earthquakes that were recorded during this crisis. An additional 22 seismic events were selected from those recorded by the mobile seismic network that has been in operation in the Campi Flegrei area since 2006. The third data set comprises noise recorded by 34 seismic stations that were deployed during the active SERAPIS experiment in 2001 September. The generalized inversion technique and the H/V spectral ratio method were applied to the S waves and coda waves of the earthquakes recorded by the first two seismic networks, to determine the site-transfer functions of the recording stations. The seismic noise recorded by the third network was analysed using the Nakamura's technique. The results show that the high topographical and geological heterogeneity of the sites located inside the caldera has an important influence on the seismic-wave amplification. Consequently, the site-transfer functions can be different even at sites close to each other. The transfer functions of the sites located outside the caldera are much more regular, apparently due to the more regular topography and geology.
Caldera
Microseism
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Coda
Passive seismic
Receiver function
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Seismic Noise
Microseism
Rayleigh Wave
Ambient noise level
Passive seismic
Seismic array
Seismic Tomography
Group velocity
Seismometer
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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.
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Rayleigh Wave
Seismic refraction
Seismic Noise
Microseism
Seismic array
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SUMMARY Monitoring temporal changes of volcanic interiors is important to understand magma, fluid pressurization and transport leading to eruptions. Noise-based passive seismic monitoring using coda wave interferometry is a powerful tool to detect and monitor very slight changes in the mechanical properties of volcanic edifices. However, the complexity of coda waves limits our ability to properly image localized changes in seismic properties within volcanic edifices. In this work, we apply a novel passive ballistic wave seismic monitoring approach to examine the active Piton de la Fournaise volcano (La Réunion island). Using noise correlations between two distant dense seismic arrays, we find a 2.4 per cent velocity increase and −0.6 per cent velocity decrease of Rayleigh waves at frequency bands of 0.5–1 and 1–3 Hz, respectively. We also observe a −2.2 per cent velocity decrease of refracted P waves at 550 m depth at the 6–12 Hz band. We interpret the polarity differences of seismic velocity changes at different frequency bands and for different wave types as being due to strain change complexity at depth associated with subtle pressurization of the shallow magma reservoir. Our results show that velocity changes measured using ballistic waves provide complementary information to interpret temporal changes of the seismic properties within volcanic edifices.
Coda
Seismic Noise
Rayleigh Wave
Microseism
Seismic interferometry
Ambient noise level
Dispersive body waves
Seismic array
Passive seismic
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In urban subsurface exploration, seismic surveys are mostly conducted along roads where seismic vibrators can be extensively used to generate strong seismic energy due to economic and environmental constraints. Generally, Rayleigh waves also are excited by the compressional wave profiling process. Shear-wave (S-wave) velocities can be inferred using Rayleigh waves to complement near-surface characterization. Most vibrators cannot excite seismic energy at lower frequencies (<5 Hz) to map greater depths during surface-wave analysis in areas with low S-wave velocities, but low-frequency surface waves ([Formula: see text]) can be extracted from traffic-induced noise, which can be easily obtained at marginal additional cost. We have implemented synthetic tests to evaluate the velocity deviation caused by offline sources, finding a reasonably small relative bias of surface-wave dispersion curves due to vehicle sources on roads. Using a 2D reflection survey and traffic-induced noise from the central North China Plain, we apply seismic interferometry to a series of 10.0 s segments of passive data. Then, each segment is selectively stacked on the acausal-to-causal ratio of the mean signal-to-noise ratio to generate virtual shot gathers with better dispersion energy images. We next use the dispersion curves derived by combining controlled source surveying with vehicle noise to retrieve the shallow S-wave velocity structure. A maximum exploration depth of 90 m is achieved, and the inverted S-wave profile and interval S-wave velocity model obtained from reflection processing appear consistent. The data set demonstrates that using surface waves derived from seismic reflection surveying and traffic-induced noise provides an efficient supplementary technique for delineating shallow structures in areas featuring thick Quaternary overburden. Additionally, the field test indicates that traffic noise can be created using vehicles or vibrators to capture surface waves within a reliable frequency band of 2–25 Hz if no vehicles are moving along the survey line.
Rayleigh Wave
Seismic Noise
Passive seismic
Microseism
Dispersive body waves
Seismic interferometry
Reflection
Love wave
Vertical seismic profile
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