Estimation of source, path, and site factors of S waves recorded at the S-net sites in the Japan Trench area using the spectral inversion technique

Abstract

S-net is a large-scale ocean bottom (OB) network in the Japan Trench area, consisting of inline-type 150 observatories equipped with seismometers and pressure gauges. Among them, 41 observatories have been buried about one meter beneath the seafloors in the shallow water regions (water depth <1500 m). We analyzed the strong-motion data recorded at the S-net sites from earthquakes with magnitudes 3.5 < Mw ≤ 7 and focal depths < 70 km to understand the site amplification effect on the recorded motions. We used the spectral inversion technique and obtained some fundamental properties of the earthquake source spectra, path attenuation, and site factors from the horizontal-component S-wave portions of the recordings. We obtained that the source spectra followed the $${\omega }^{-2}$$ ω - 2 source model generally well, and the estimated magnitudes were mostly within ± 0.3 magnitude units of the catalog magnitudes. The stress drops increased systematically with the focal depths, and the values for the shallow earthquakes in the Pacific Plate were higher than those for the interplate earthquakes with comparable focal depths. The path-averaged quality factors were generally frequency-dependent and were somewhat larger than those in the past studies. The peak site frequencies ranged between about 0.2 and 10 Hz, while the peak amplification factors ranged between 10 and 50. Even though the peak frequencies and amplification factors differed from site to site, the peak frequencies were mostly higher than 2 Hz at the outer trench stations, while they were lower than 2 Hz at many inner trench stations. The amplification factors at a few OB sites in the shallow water regions were comparable with the theoretical ones computed from the 1-D subsea model. The amplification factors at intermediate frequencies (~ 0.3 to 2 Hz) generally increased with P-wave travel time in the sediments estimated from the multi-channel seismic survey. At about 20% of the sites (mainly at the unburied stations), spurious site spectra were recognized at frequencies higher than about 4 Hz. If the site spectra between 4 and 10 Hz are required, using only the X-component records is recommended. Graphical Abstract

Development of a simple offshore ground motion prediction equation based on S-net data and residual analysis to reveal site effects

“…The offshore peak site frequencies were reported between approximately 0.2-10 Hz (Zhang and Zheng 2020; Dhakal et al 2023; Tan and Hu 2023). For the offshore sites, the higher frequencies showed smaller site ampli cation, and they indicate no correlation with sediment thickness based on seismic re ection surveys at the S-net stations (Dhakal et al 2023). In the Nankai Trough, the large ampli cation of the high-frequency components was controlled by thin low-velocity sediments with V S of < 0.6 km/s (Takemura et al, 2023a).…”

“…When the stations are compared in the order in which they are connected, there is a continuity in the residuals (Fig. 8a), which may be related to the cable azimuth as pointed out by Dhakal et al (2023). If there is an ampli cation factor due to cable azimuths, it is expected to be relative to the epicenter location; thus, these interrelationships would need to be individually examined each event.…”

Development of an offshore ground motion prediction equation considering path effects based on S-net data

“…The acceleration data are applied to a fourth-order bandpass lter with a frequency band from 0.1 to 25 Hz and then horizontal and vertical acceleration seismograms are obtained by following Takagi et al (2019). The Y component (horizontal and perpendicular to the cable) of the cylindrical S-net instruments, speci cally unburied station due to poor coupling, can be contaminated by rotation and tilt effects caused by a strong motion (Sawazaki and Nakamura 2020; Dhakal et al 2023; Dhakal and Kunugi 2023a). The X component, which is along the cable, has a minor degree of those contaminations such a rotation and tilt; however, tends to have greater variation than vector synthesis of the X and Y components.…”

“…The buried stations would provide high-quality data because of their small errors (Fig. 13b: Sawazaki and Nakamura 2020; Dhakal et al 2023). The unburied stations likely show actual large PGAs where the sea oor surface because the effects of rolling and tilting are eliminated.…”

“…13a). When the stations are compared in the order in which they are connected, there is a continuity in the residuals, which may be related to the cable azimuth as pointed out by Dhakal et al (2023). If there is an ampli cation factor due to cable azimuths, it is expected to be relative to the epicenter location; thus, these interrelationships would need to be individually examined for each event.…”