Seismic ambient noise around the South China Sea: seasonal and spatial variations, and implications for its climate and
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ORIGINAL RESEARCH PAPER
Seismic ambient noise around the South China Sea: seasonal and spatial variations, and implications for its climate and surface circulation Da Huo • Ting Yang
Received: 8 April 2013 / Accepted: 15 May 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract With its strong seasonal variation in wave climate and various bathymetric features due to the complex tectonics, the South China Sea (SCS) provides a natural laboratory to study the microseism. We collected data from seismic stations around the SCS and calculated their noise spectra, through which seasonal and spatial variations of microseism, as well as the general feature of seismic ambient noise in this marginal sea were revealed. Microseism seasonal variations in general reflect influences of the East Asian monsoon in winter and the Indian monsoon in summer, respectively. The two microseism components, the single frequency microseism (SFM) and the double frequency microseism (DFM), show striking alternating variation patterns both seasonally and spatially. These variation patterns, along with the bathymetric feature near the stations, indicate SFM and DFM are generated through different physical mechanisms. More interestingly, seasonal and spatial variations of DFM appear to be consistent with the basin-scale surface circulation model of the SCS, in which the upper SCS experiences cyclonic in winter and anti-cyclonic in summer. These consistencies provide observational evidence for the hypothesis that the cyclonic depression is a favorable condition to generate DFM. Keywords South China Sea Seismic ambient noise Microseism Seasonal variation Wave climate Bathymetry Circulation
D. Huo T. Yang (&) State Key Laboratory of Marine Geology, Tongji University, No. 1239, Siping Road, Shanghai 20092, China e-mail: [email protected]
Introduction High quality seismic data are crucial to a wide range of seismological studies. However, the signals of earthquakes recorded by seismic stations are always contaminated by ambient noise in a broad frequency band. Among all types of seismic noise, the most ubiquitous and prominent one is referred to as ‘‘microseism’’, which is the energy originated from wind-induced waves in oceans (Longuet-Higgens 1950; Hasselmann 1963; Webb 1992; Bonnefoy-Claudet et al. 2006; Bromirski 2009). The typical microseism spectrum includes two peaks. The primary or single-frequency microseisms (SFM, period 10–25 s) are generated at shallow water near the coast where ocean waves interact directly with the sloping seafloor (Hasselmann 1963). Seismic waves generated from this mechanism often have periods close to the incident ocean waves. The secondary microseisms having much larger amplitudes in the band 0.1–0.2 Hz are referred to as the double-frequency microseisms (DFM). They are believed to be generated by the nonlinear interactions of two ocean waves with similar periods that travel in opposite directions. The pressure fluctuations at the ocean bottom resulted from these nonlinear interactions, w
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