Abstract
Ambient seismic noise becomes more and more important and helpful on assisting velocity model inversion, earthquake detection, and ground motion prediction. Based on the analysis of continuous seismic data and ocean wave height, we find that the ocean wave height and winter storms have a controlling factor on the DF microseismic energy level and its frequency extent in time scale. It shows that high and low DF microseismic energy accompanied with wide and narrow frequency range consistent with the high wave height period (when the ocean is stormier) and low wave height period, respectively. Since DF microseism is dominated by Rayleigh waves, its energy attenuates very quickly when it travels through shoreline to the continent crust. Our observations give a quality factor Q of about 83 for DF energy traveling from the middle of the Atlantic to the central of Europe. We observe a lower energy level of SPDF (short period DF) than that of LPDF (long period DF) for the continent stations, however a reversed situation for the island stations. It suggests that short period DF energy decays faster than the long period one. High-frequency ambient noise is called microtremor. The microtremor for the island station with low elevation has a semidiurnal modulation in phase with ocean tide. The microtremor for the station at other locations are from the anthropogenic activities which have diurnal, weekly, and annually variations.
Highlights
With the improvement of data storage and processing technique, the signal which has been regarded as noise for a long time has been used to inverse the crust and upper mantle velocity model (Shapiro et al.2005; Yang et al 2010), to predict the strong grand motion (Okada 2003; Denolle et al 2014), and to improve the earthquake detection (Zhang et al 2010).Ambient noise has become an important dataset for seismic studies.Ambient seismic noise at period of 2 to 20 s generated by the standing waves in the ocean is named microseism (Gutenberg 1958)
Comparing ambient noise spectrograms (Fig. 2 and Fig. 3) with the ocean wave height data (Fig. 4), it is noticeable that the high wave height period in the northern or southern hemisphere has a clear controlling character on the double-frequency microseism (DF) microseismic energy (Stutzmann, et al, 2009; Hillers, et al 2012)
DF energy is mainly radiated as Rayleigh wave which attenuates very quick when it travels through shoreline to the continent crust
Summary
With the improvement of data storage and processing technique, the signal which has been regarded as noise for a long time has been used to inverse the crust and upper mantle velocity model (Shapiro et al.2005; Yang et al 2010), to predict the strong grand motion (Okada 2003; Denolle et al 2014), and to improve the earthquake detection (Zhang et al 2010). Ambient seismic noise at period of 2 to 20 s generated by the standing waves in the ocean is named microseism (Gutenberg 1958). Modeled the microseism at period of 12 to 20 s as a nonlinear coupling between the ocean wave and the shoal or the shallow water The energy in this period is called the primary or single-frequency microseism (SF). We analyze the continuous recordings on the globally distributed IRIS/IDA (II) stations using the polarization analysis technique and examine the frequency and polarization characters of the ambient seismic noise across the globe. Few stations use Trillium 240, STS-2, or STS-5 Those broadband seismometers ensure our goal of analyzing the global characteristics of ambient noise at different frequency band. For each combination of time and station, the eigenvalue can be represented as the power spectral density (PSD), power spectrogram, and probability density function (PDF)
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