Distribution of b-value in the central and southern Ryukyu: Is the low b-value in the rift-axis of the Okinawa Trough accurate?

Abstract

Abstract Here, I investigated whether the b-value in the rift-axis of the Okinawa Trough, a back-arc basin, is low. The cause of the low b-value in the rift-axis of the Okinawa Trough had previously been interpreted as increased stress by dyke intrusion. However, the previous study ignored the spatial variations in magnitude completeness in the Okinawa Trough. In addition, for the M4-class earthquakes that occurred in the Okinawa Trough, the values of displacement magnitudes and velocity magnitudes determined by the Japan Meteorological Agency (JMA) for the same events did not match. The previous study did not correct for these effects. Thus, I conducted these corrections and examined the spatial variation of b-values in the Okinawa Trough and the Ryukyu arc. The magnitude completeness was calculated using goodness-of-fit methods and the b-value was calculated using the maximum likelihood method. The JMA earthquake catalog from 2001 to 2019 was used for the analysis. After the corrections, the b-values around the rift-axis of the central and southern Okinawa Trough (0.90–0.92) were almost the same as those for the Ryukyu arc (0.83–0.84). Magnitude completeness in the Okinawa Trough varied spatially, and was particularly large in the earthquake swarm area in the rift-axis of the Okinawa Trough. The steady-state magnitude completeness was 2.0 in the earthquake swarm area in the rift-axis of the Okinawa Trough. However, the magnitude completeness increased to 3.5 or higher only during the several days of occurrence of the earthquake swarms. This was caused by the short-term decrease in the ability to detect earthquakes when an earthquake swarm occurred. The finding that the b-values do not differ between the rift-axis of the Okinawa Trough and the Ryukyu arc indicates that stress concentration in the rift-axis of the Okinawa Trough is not a strong influence on the b-values. Alternatively, this might indicate that in the rift-axis of the Okinawa Trough, the distribution of low and high b-value regions was not detectable due to the insufficient accuracy of hypocenter determination.