14:45 〜 15:00
[SSS02-05] Investigation of the deeper structure below the ocean floor using ambient noise H/V analysis: application to OBS data at the Fukushima forearc region
キーワード：The H/V method, Ocean bottom Seismometer (OBS), Oceanic environment, S-wave profile, Water layer effect
The offshore Fukushima forearc region in the NE Japan lies in between the Japan Arc and the Japan Trench, along which the oceanic Pacific plate subducts below the continental Okhotsk Plate. S-wave (VS) information of several thousand meters below seafloor of this region has not been investigated well compared to the P-wave (VP) velocity profile. Here, we estimate the VS variation around several thousand meters below the seabed at the region offshore Fukushima by inverting ambient noise horizontal to vertical (H/V) spectral ratio curve, computed from waveform records from ocean bottom seismometers (OBS).
We apply a single-station H/V approach, in which individual H/V curve within frequency range from 0.01 to 1 Hz was computed for each station with 31 days of recordings from 16 short-period OBSs with eigenfrequency of 1 Hz. Both the H/V curve inversion analysis and the forward calculation were performed utilizing methods and code developed by Lontsi et al. (2015, 2019), in which a water layer effect, atop the OBS, on the H/V spectrum was accounted for in the whole process. The non-uniqness of the solution associated with the inversion of the H/V curve alone is addressed by using a layer parameterization that mimics the VP profile, but with some freedom to find the best representative model. Subsurface velocity profiles obtained from this study successfully revealed the seismic structure up to around 8,000 m below the seabed. The retrieved new VS profiles at the offshore Fukushima forearc region spotted similar subterranean features found in the previous studies (e.g., Tsuru et al. 2002; Miura et al. 2003; and JAMSTEC, 2016), in which structures such as an unconformity between Neogene sedimentary sequences and igneous Cretaceous basement rocks at a depth of around 2,400 m, and a major stratification within island arc crust at a depth of around 5,800 m had been identified by seismic surveys with high energy active sources.
Additionally, synthetic H/V analysis was performed for the cases both with and without water layer scenarios that elucidated a significant effect of the water layer on the H/V curve. We identified a shift of the fundamental frequency peak of almost 2 Hz towards lower frequencies due to the water layer effect, whereas a previous study by Lontsi et al. (2019) exhibited a water layer effect only on the H/V curve amplitude around higher frequency peaks and with no significant shift in fundamental frequency. This observation is probably related to the complexity of the structure below the water layer. To fully address this issue requires further scrutiny.
We apply a single-station H/V approach, in which individual H/V curve within frequency range from 0.01 to 1 Hz was computed for each station with 31 days of recordings from 16 short-period OBSs with eigenfrequency of 1 Hz. Both the H/V curve inversion analysis and the forward calculation were performed utilizing methods and code developed by Lontsi et al. (2015, 2019), in which a water layer effect, atop the OBS, on the H/V spectrum was accounted for in the whole process. The non-uniqness of the solution associated with the inversion of the H/V curve alone is addressed by using a layer parameterization that mimics the VP profile, but with some freedom to find the best representative model. Subsurface velocity profiles obtained from this study successfully revealed the seismic structure up to around 8,000 m below the seabed. The retrieved new VS profiles at the offshore Fukushima forearc region spotted similar subterranean features found in the previous studies (e.g., Tsuru et al. 2002; Miura et al. 2003; and JAMSTEC, 2016), in which structures such as an unconformity between Neogene sedimentary sequences and igneous Cretaceous basement rocks at a depth of around 2,400 m, and a major stratification within island arc crust at a depth of around 5,800 m had been identified by seismic surveys with high energy active sources.
Additionally, synthetic H/V analysis was performed for the cases both with and without water layer scenarios that elucidated a significant effect of the water layer on the H/V curve. We identified a shift of the fundamental frequency peak of almost 2 Hz towards lower frequencies due to the water layer effect, whereas a previous study by Lontsi et al. (2019) exhibited a water layer effect only on the H/V curve amplitude around higher frequency peaks and with no significant shift in fundamental frequency. This observation is probably related to the complexity of the structure below the water layer. To fully address this issue requires further scrutiny.