Abstract
A study of seafloor and island stations shows that for the frequency band 0.1 to 10 Hz the seismic noise levels on islands are comparable to the levels on the seafloor. The microseism peak at the seafloor appears to be comparable to the highest levels observed on small islands. For this band, seafloor stations are realistic alternatives when island sites are not available.
Seven year averages of the ambient noise levels recorded by Seismic Research Observatory (SRO) stations on three islands (Guam [GUMO], Taiwan [TATO], and New Zealand's north island [SNZO]) are compared with those recorded by the International Deployment of Accelerometers (IDA) station on Easter Island and on and beneath the ocean floor by Ocean Bottom Seismometers (OBSs) and the Marine Seismic System (MSS) deployed in a south Pacific DSDP drill hole at 23.8°S., 165.5°W (Adair et al., 1986). From 0.3 to 2 Hz the SRO displacement power levels fall in the range historically observed by the Scripps' OBSs (decreasing at 70 dB/decade from 1 by 106 nm2/Hz at 0.3 Hz to 1 nm2/Hz at 2 Hz) and are 10 to 15 dB above MSS levels. Above 2 Hz it appears that the same ratios hold (the SRO power levels decrease at 70 dB/decade to 1 by 10−3 nm2/Hz at a frequency of 10 Hz), although this correlation is based on very limited, high gain, short-period data. At frequencies below 0.3 Hz the SRO noise levels peak and decrease to approximately 2 by 103 nm2/Hz at 40 mHz. The noise levels recorded at Easter Island are somewhat higher (decreasing at 70 dB/decade from 1 by 107 nm2/Hz at 0.2 Hz to 1 nm2/Hz at 10 Hz and to 1 by 105 nm2/Hz at 50 mHz). At the microseism peak near 0.2 Hz the MSS levels are from 15 to 20 dB higher than observed by the SRO stations and equivalent to those recorded at Easter Island. There appears to be little dependence of the variance in noise level estimates on frequency. The upper 95 per cent confidence limit generally lies 10 dB above the average noise levels for all island stations.
All island noise level curves are dominated by the broad double-frequency microseism peak centered between 0.15 to 0.2 Hz. The single-frequency peak ranges from absent (Easter Island) to discernable (Guam and New Zealand) to obvious (at Taiwan). The center frequency of this peak ranges from 0.07 Hz at Guam and New Zealand to 0.1 Hz at Taiwan. We speculate that the increased amplitude and frequency of the single-frequency microseism peak is due to the interaction between the shallow continental shelf and surface gravity waves and/or the presence of Taiwan in a region of limited fetch.
