09:00 〜 09:15
[SCG48-01] No significant deformation at the axis of the Kuril Trench off Nemuro based on Acoustic Distant Meter survey
In the southwestern part of the Kuril Trench, several M8-class earthquakes have been recorded in the past 200 years, i.e., most recently the 1973 Nemuro-oki earthquake (M7.4) and the 2003 Tokachi-oki earthquake (M8.0). A tsunami deposit survey (Nanayama et al., 2003) revealed that a M9-class great earthquake occurred in this area in the 17th century, about 350 years ago. In 2017, the headquarters for Earthquake Research Promotion estimated that the probability of an Mw 8.8 or greater earthquake occurring in the southwestern Kuril Trench within the next 30 years is 7-40%, based on the date of the last event and its recurrence interval of 340-380 years. In addition, it was suggested that the last 17th-century great earthquake caused rupture near the trench axis (Ioki and Tanioka, 2016) from their tsunami modeling. However, no constraint has been made on the stress state of the shallower-most plate interface because of a lack of geodetic data. Responding to such situation, Tohoku University and Hokkaido University established seafloor geodetic stations off Nemuro in 2019, consisting of GNSS-Acoustic system and Acoustic Distant Meter (ADM). In this study, we report the measured results of ADM survey installed across the trench.
ADM measures the change in a baseline length between a pair of acoustic instruments installed on the seafloor. The instrument itself measures the round-trip travel time of acoustic signal between them, which will be translated into distance using sound velocity. Practically, sound velocity varies as a function of temperature and pressure, which are also measured in the instrument for sound velocity correction. In addition, any positional change of acoustic phase center relative to the seafloor must be removed to extract only crustal deformation. Therefore the attitude of each instrument is also measured to monitor the instrumental inclination.
Three acoustic units were installed at the axis, two (u2 and u3) on the incoming Pacific plate and one (u1) on the deformation front of the overriding Okhotsk plate. We presume that the baselines u1-u2 and u1-u3 cross the trench axis. The survey started on July 2019 and ended on April 2021.
Uncorrected baseline lengths, in which measured traveltime is simply converted to distance with a reference velocity, are corrected by the following procedure. First, contribution of pressure change on sound velocity is corrected after removal of sensor drift of the pressure gauges. The effect of temperature disturbance on sound velocity was corrected by taking an average of two instrument as a representative along the baseline after removal of short-term fluctuation that may be a localized phenomenon. Finally, effect of the changes in instrumental inclination is corrected based on the equipped attitude sensors and magnetic compass. The remaining change in the baseline length is regarded as crustal deformation.
Assuming the deformation rate during the survey period is constant, we applied the linear regression and obtained change rates of each baseline length as -3.5 mm/yr for baseline u1-u2, -3.7 mm/yr for u1-u3, and 1.4 mm/yr for u2-u3 (negative means shortening). For the baselines u1-u2 and u2-u3, we excluded the data in the last five months from the regression, where the attitude of u2 shows unrealistic behavior. The obtained shortening rates are much smaller than the incoming rate of the plate model (80-90 mm/yr), considering a possible uncertainty in the measurement (~10 mm/yr). This indicates that the front of the overriding plate moves with the incoming plate at least during the observed period. To understand the overall picture of the subduction zone, baseline should be extended to the landward and be interpreted together with the movements measured using GNSS-Acoustic surveys.
ADM measures the change in a baseline length between a pair of acoustic instruments installed on the seafloor. The instrument itself measures the round-trip travel time of acoustic signal between them, which will be translated into distance using sound velocity. Practically, sound velocity varies as a function of temperature and pressure, which are also measured in the instrument for sound velocity correction. In addition, any positional change of acoustic phase center relative to the seafloor must be removed to extract only crustal deformation. Therefore the attitude of each instrument is also measured to monitor the instrumental inclination.
Three acoustic units were installed at the axis, two (u2 and u3) on the incoming Pacific plate and one (u1) on the deformation front of the overriding Okhotsk plate. We presume that the baselines u1-u2 and u1-u3 cross the trench axis. The survey started on July 2019 and ended on April 2021.
Uncorrected baseline lengths, in which measured traveltime is simply converted to distance with a reference velocity, are corrected by the following procedure. First, contribution of pressure change on sound velocity is corrected after removal of sensor drift of the pressure gauges. The effect of temperature disturbance on sound velocity was corrected by taking an average of two instrument as a representative along the baseline after removal of short-term fluctuation that may be a localized phenomenon. Finally, effect of the changes in instrumental inclination is corrected based on the equipped attitude sensors and magnetic compass. The remaining change in the baseline length is regarded as crustal deformation.
Assuming the deformation rate during the survey period is constant, we applied the linear regression and obtained change rates of each baseline length as -3.5 mm/yr for baseline u1-u2, -3.7 mm/yr for u1-u3, and 1.4 mm/yr for u2-u3 (negative means shortening). For the baselines u1-u2 and u2-u3, we excluded the data in the last five months from the regression, where the attitude of u2 shows unrealistic behavior. The obtained shortening rates are much smaller than the incoming rate of the plate model (80-90 mm/yr), considering a possible uncertainty in the measurement (~10 mm/yr). This indicates that the front of the overriding plate moves with the incoming plate at least during the observed period. To understand the overall picture of the subduction zone, baseline should be extended to the landward and be interpreted together with the movements measured using GNSS-Acoustic surveys.