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71.
Traveltime measurements from noise correlation: stability and detection of instrumental time-shifts 总被引:1,自引:0,他引:1
We test the feasibility of using Green's functions extracted from records of ambient seismic noise to monitor temporal changes in the Earth crust properties by repeated measurements at regional distances. We use about 11 yr of continuous recordings to extract surface waves between three pairs of stations in California. The correlations are computed in a moving 1-month window and we analyse the temporal evolution of measured interstation traveltimes. The comparison of the arrival times in the positive and negative correlation time of Rayleigh and Love waves allows us to separate time-shifts associated with any form of physical change in the medium, those resulting from clock drift or other instrumental errors, and those due to change in the localization of the noise sources. This separation is based on the principle of time symmetry. When possible, we perform our analysis in two different period bands: 5–10 and 10–20 s. The results indicate that significant instrumental time errors (0.5 s) are present in the data. These time-shifts can be measured and tested by closure relation and finally corrected independently of any velocity model. The traveltime series show a periodic oscillation that we interpret as the signature of the seasonal variation of the region of origin of the seismic noise. Between 1999 and 2005, the final arrival time fluctuations have a variance of the order of 0.01 s. This allows us to measure interstation traveltimes with errors smaller than 0.3 per cent of the interstation traveltime and smaller than 1 per cent of the used wave period. This level of accuracy was not sufficient to detect clear physical variation of crustal velocity during the considered 11 yr between the three stations in California. Such changes may be more easily detectable when considering pairs of stations more closely located to each other and in the vicinity of tectonically active faults or volcanoes. 相似文献
72.
We infer the groundwater-level anomalies associated with a hypothetical preslip prior to the anticipated M 8 Tokai earthquake,
and evaluate the detectability of the anomalies using data from seven groundwater wells. We evaluate the detectability of
the anomalies under the following assumptions: (1) an Mw 5.5–6.5 aseismic preslip event occurs at the plate boundary in and
around the hypothetical focal zone of the Tokai earthquake; (2) the total amount of the strain step at each observation associated
with the preslip can be calculated by tensile and shear faulting based on the dislocation model; (3) a normalized strain history
associated with the preslip is defined from the results of numerical simulations based on rate- and state-dependent friction
laws; and (4) the groundwater-level anomaly prior to the earthquake is proportional to the estimated history of the strain
change associated with the preslip. We investigate the detection time of the anomaly at seven wells given an Mw 5.5, 6.0,
or 6.5 aseismic preslip at one of the 272 grid points in and around the area of the hypothetical focal zone of the Tokai earthquake.
As a result, over the time interval between 1 and 48 hours prior to the hypothetical Tokai earthquake, we are able to detect
at each of the seven wells a hypothetical Mw 6.5 preslip at 10–86 of the 272 grid points, an Mw 6 preslip at 0–19 grid points,
and an Mw 5.5 preslip at 0–5 grid points. 相似文献
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74.
在总结昭苏县25年雷达探测冰雹云和指挥防雹作业技术的基础上,研究并编写了昭苏县防雹雷达站防雹业务技术规程,本文扼要介绍雷达观测、雹云识别、作业指挥,效果分析等部分主要技术规程。 相似文献
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77.
MT法中利用阻抗相位资料对畸变视电阻率曲线的校正 总被引:11,自引:3,他引:11
从理论上论证了在一定条件下大地电磁响应中的相位资料将不受随机电磁噪声的影响,介绍了由相位资料递推计算视电阻率的方法,通过与远参考资料的比较,证明利用阻抗相位资料可分段恢复校正畸变的实测视电阻率数据。 相似文献
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79.
Since the late 1970s, the dynamic range and resolution of strong motion digital recorders have leaped from 65 to 135 dB, opening new possibilities for advanced data processing and interpretation. One of these new possibilities is the calculation of permanent displacement of the ground or of structures, associated with faulting or with non-linear response. Proposals on how permanent displacements could be recovered from recorded strong motion have been published since 1976. The analysis in this paper concludes that permanent displacements of the ground and of structures in the near-field can be calculated provided all six components of strong motion (three translations and three rotations) have been recorded, and the records are corrected for transducer rotation, misalignment and cross-axis sensitivity. 相似文献
80.