Nature of the Ambient Noise, Site Response, and Orientation of Ocean-Bottom Seismometers (OBSs): Scientific Results of a Passive Seismic Experiment in the Andaman Sea

We analyze the nature of the ambient noise and site response for ocean-bottom seismometers (OBSs) deployed in the Andaman region. The probabilistic power spectral densities of the OBSs show that the noise level in the Andaman region is within the range of Peterson's new low-noise model and new high-noise model (NHNM) when considering short periods (< 20 s). When examining long periods (> 20 s), the noise level is higher than what would be given by NHNM. The peak associated with secondary microseisms occurs between 3 and 6 s. The stations that are close to the Andaman and Nicobar Islands show a prominent secondary microseismic peak at 4 s, whereas the stations farther away show a small hump at 1 s. The noise level varies seasonally and shows a strong correlation with the significant wave height computed from WAVEWATCH III. The ambient noise is down to -140 dB for the band from 0.1 to 1 s, which provides the best signal-to-noise ratio level for local earthquakes. For very short periods (< 0.1 s), the ambient noise level increases to -120 dB; this is likely due to the efficient propagation of seismic energy through the low attenuation oceanic crust. The horizontal-to-vertical spectral ratio (HVSR) of the ambient noise curves shows an amplification factor greater than four. Furthermore, the HVSR curves show a broad or flat peak beneath the seamounts and the accretionary prism and a sharp or clear peak in the basinal area. We also estimate the orientation of the OBSs using Rayleigh-wave polarization analysis.