10:45 AM - 12:15 PM
[SSS07-P13] Estimation of site amplification factors of S-net ocean-bottom seismic stations
Keywords:site amplification factor, S-net ocean bottom stations, coda normalization method
We estimated the site amplification factors of ocean-bottom S-net stations from 1 to 16 Hz using the coda normalization method. Site effects have been studied intensively for land stations, but any systematic researches on ocean-bottom stations have still been still few (e.g., Takemura et al., 2023). We analyzed seismograms of the horizontal VX component, which should be least affected by the rotation of seismometers due to strong shakings, at about 100 S-net stations off the Tohoku region with over 1,000 events of magnitude 3.5-6.0 located within the coverage of stations.
We first applied the widely used coda normalization method for stable site factor estimation. Although most of S-net stations are located on rather thick and very low-velocity marine sediment, that is, seismograms are characterized by an unclear onset of the direct S wave with coda of long duration, compared with standard land stations, we cannot find any peculiar peak or trough in the estimated spectra of site amplification factors in the studied frequency range. The overall magnitude of variations in factors among stations is limited from 0.8 and 1.2 with the average to be 1. The factors at stations near the Japan trench with thick marine sediment are smaller than the average at high frequency, implying large intrinsic attention of the sedimentary layer there.
We compared the site factors estimated with the part of the direct S wave with the ones by the coda normalization method. Not only their averages agree each other, but also the deviations from the averages for different events are similar, different from the case of land stations where the deviations for the direct S wave are usually much larger than the coda ones. The similarity in both average and deviation is particularly very well at high frequency although the deviations for the direct S wave are slightly larger at low frequency.
These results may imply the relatively uniform coverage of thick and very low-velocity marine sediment as the dominant layer of seismic scattering in the seismograms recorded at S-net stations. Due to this strong surface scattering region, even a time window in seismograms just after the direct S wave arrival consists of the waves scattered in the sedimentary layer as sufficiently incoherent waves, as in the case of late coda waves in seismograms at land stations. In other words, ocean bottom seismograms may be expressed by scattering in marine sediment with the very weak site effect characterized around each station while the site effect just beneath a given station is as strong as the scattering effect of wide and deep (i.e., down to the lower crust of high reflective or scattered character) areas in the land case.
In summary, the so-called site effect may be minor at ocean bottom stations due to the strong scattering near surface or marine sediment, even for the time window in record immediately after the direct S-wave phase.
We first applied the widely used coda normalization method for stable site factor estimation. Although most of S-net stations are located on rather thick and very low-velocity marine sediment, that is, seismograms are characterized by an unclear onset of the direct S wave with coda of long duration, compared with standard land stations, we cannot find any peculiar peak or trough in the estimated spectra of site amplification factors in the studied frequency range. The overall magnitude of variations in factors among stations is limited from 0.8 and 1.2 with the average to be 1. The factors at stations near the Japan trench with thick marine sediment are smaller than the average at high frequency, implying large intrinsic attention of the sedimentary layer there.
We compared the site factors estimated with the part of the direct S wave with the ones by the coda normalization method. Not only their averages agree each other, but also the deviations from the averages for different events are similar, different from the case of land stations where the deviations for the direct S wave are usually much larger than the coda ones. The similarity in both average and deviation is particularly very well at high frequency although the deviations for the direct S wave are slightly larger at low frequency.
These results may imply the relatively uniform coverage of thick and very low-velocity marine sediment as the dominant layer of seismic scattering in the seismograms recorded at S-net stations. Due to this strong surface scattering region, even a time window in seismograms just after the direct S wave arrival consists of the waves scattered in the sedimentary layer as sufficiently incoherent waves, as in the case of late coda waves in seismograms at land stations. In other words, ocean bottom seismograms may be expressed by scattering in marine sediment with the very weak site effect characterized around each station while the site effect just beneath a given station is as strong as the scattering effect of wide and deep (i.e., down to the lower crust of high reflective or scattered character) areas in the land case.
In summary, the so-called site effect may be minor at ocean bottom stations due to the strong scattering near surface or marine sediment, even for the time window in record immediately after the direct S-wave phase.