[SSS14-P23] GNSS observation around a shear zone in southern Kyushu, southwest Japan (The fourth report)
Keywords:shear zone, slip rate, locking depth, 2016 Kumamoto Earthquake, visco-elastic deformation, GNSS observation
Firstly, we estimated daily site coordinates at above 10 GNSS sites during the period from April 18, 2016 to November 17, 2019 (about 3.6 years) using Bernese GNSS Software (ver.5.2). In this time, 4 GEONET sites operated by Geospatial Information Authority of Japan were utilized as reference points. Then, we corrected the time series of daily site coordinates by subtracting following 3 factors: 1) common mode bias calculated from F3 solutions of about 150 GEONET sites located in Kyushu region, 2) viscoelastic crustal deformation after the 2016 Kumamoto earthquake, and 3) volcanic crustal deformation accompanied with the activity of Sakurajima and Kirishima volcanoes, from original time series of daily site coordinates. After the process, site velocities were estimated along with annual and semi-annual variations.
Secondly, as for interpretation of the obtained site velocities, a vertical strike-slip fault model proposed by Savage and Burford (1973) was applied. This model reproduces the situation that a fault is locked from the earth’s surface to a certain depth, a locking depth, and the fault is slipping at a constant velocity, a slip rate, which is relative rigid block motion below the locking depth. At this time, the surface velocity profile normal to the fault strike is expressed as a function of arctangent across the fault. Although we tentatively assumed the strike of the shear zone to be N110°E based on the shear strain rate distribution derived from GNSS velocities so far, we estimated the optimum strike direction of the shear zone using the site velocities by conducting a grid search with the range from N90°E to N120°E each 0.1°. As a result, the optimum strike direction was found to be N104.8°E.
Finally, using the determined strike of the shear zone in this study, the slip rate and locking depth were estimated to be 12.9±1.8 mm/yr and 22.4±6.9 km through a nonlinear least-squares fitting the site velocities to above mentioned fault model (Figure 1). For the analysis, we used the velocities at sites located in the ranges of 60-80 km in north and south and 25 km in east and west from the center of the shear zone (32°N, 130.5°E). In addition, we investigated the discrepancy of estimated values (slip rate and locking depth) and estimation errors arose from the difference of the observation period (for 3.6 years and 2.0 years), as an accuracy evaluation about them. The slip rate and locking depth were in good agreement with each other within the errors, and then, we confirmed that the longtime observation reduced the estimation errors and restrained the scattering of site velocities.
Since the strike of the shear zone estimated from observation and analysis in this study is well consistent with the strike of a source fault of the 1997 Northwest Kagoshima earthquakes, which is nearly E-W direction based on the aftershocks distribution, this suggests a possibility that the fault slip in the lower crust is mainly involved in the occurrence of the earthquakes.
This study was carried out by a contract with the Agency of Natural Resources and Energy, part of the Ministry of Economy, Trade and Industry of Japan as part of its R&D program supporting development of technology for geological disposal of high-level radioactive waste.