Characteristics of seismicity in the southern Okinawa Trough and their relation to back-arc rifting processes

Abstract

The southern part of the Okinawa Trough forms a narrow back-arc rift basin where evidence for submarine volcanoes and active hydrothermal venting is observed. The region is also known to cause large crustal earthquakes frequently which often accompany a rapid increase in seismicity rate. Although such swarm-like activities are common in active volcanic regions and are considered to be primarily induced by crustal fluid flows, potential interactions between tectonic, magmatic and hydrologic processes have been poorly examined in the southern Okinawa Trough despite these processes happening in the proximity. Here, I report the spatial and statistical characteristics of seismic activity in the southern Okinawa Trough and discuss their relation to other rifting-related phenomena. Most of the earthquakes with magnitude greater than 5 are localized around the rift axes (the Yaeyama Rift and the Yonaguni Rift) where seismic reflection data indicated the presence of solidified magmatic intrusions into the shallow sedimentary layers. I found the areas around the rift axes show low b values of < 0.8 and suggest that stress changes directly induced by dike intrusions beneath the rift axes control the occurrence of swarm activities. On the contrary, regions with high b values (> 1.2) are found around the Ishigaki Knoll and the Hatoma Knoll. These two areas are located between the rift axes and the Ryukyu Islands and correspond to potential submarine volcanoes proposed by seafloor bathymetry and seismic reflection images. This result may constrain the location of the volcanic front in the region.

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

“…The FMD coefficient b is adopted as the b-value. M C and b-values were calculated when there were more than 50 earthquakes within the search radius to align conditions with those of Arai (2021).…”

“…In the southern Okinawa Trough of southwestern Japan, low b-value (below 0.8) areas are distributed along the rift-axis of the Okinawa Trough (Arai 2021). The b-value in the Ryukyu arc is approximately 1.0-1.2, whereas the b-value in the southern Okinawa Trough is 0.5-0.9.…”

“…In particular, regions with a b-value of 0.7 or less are distributed in the rift-axis of the southern Okinawa Trough. This is interpreted as being due to stress concentration due to dyke intrusion occurring in the rift-axis of the trough, resulting in a lower b-value in the earthquake swarm area (Arai 2021). Aseismic crustal deformation has been observed in the rift-axis of the southern Okinawa Trough, with possible dyke intrusion during the earthquake swarms (Tu and Heki 2017;Nakamura and Kinjo 2018).…”

“…However, the method of Arai (2021) potentially does not determine the b-value correctly in the Okinawa Trough. Clearly, it is necessary to have accurate b-values to discuss the relation between the distribution of b-values and tectonic background.…”

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

“…In particular, regions with a bvalue of 0.7 or less are distributed in the rift-axis of the southern Okinawa Trough. This is interpreted as being due to stress concentration due to dyke intrusion occurring in the rift-axis of the trough, resulting in a lower b-value in the earthquake swarm area (Arai 2021). Aseismic crustal deformation has been observed in the rift-axis of the southern Okinawa Trough, with possible dyke intrusion during the earthquake swarms (Tu and Heki 2017;Nakamura and Kinjo 2018).…”