H/V spectral ratios of seismic data recorded by the short-period ocean bottom seismometers

In marine environments, gas hydrates are found in the sediments beneath the outer continental slopes and margins where dissociation can have the effect of reducing the seafloor stability and increasing the potential for gas blowouts. Several studies show that submarine slope failures have been related to the presence of gas hydrates. In order to quantify and detect the link between earthquake and rate of slope movement, it is essential to define the resonant frequency of the sediment-surface layers and to characterize the seismic-wave propagation and amplification which is affected by the local topography and the near surface geology (the local site effects). The use of horizontal/vertical (H/V) spectral ratios of seismic signal on land is a standard tool for calculating natural site period, which can then be used in earthquake hazard and risk studies. In our study, this concept was used in a submarine setting to study the amplification effects of the shallow marine sediment layer and ambient noise in these zones. In April 2013, five short period OBSs (Ocean Bottom Seismometer) were deployed in the southwest Taiwan offshore area and record micro-seismicity and noises over 20-days period. The H/V spectral ratio shape calculated based on the noise recordings is comparable with that of earthquake recordings, showing a prominent peak response in the frequency range from about 7 to 9 Hz. The peak here may represent some high noise source in the marine environment or instrumental noise. However, the amplitude level of noise is systematically lower in almost all frequency bands than that of earthquake recordings.