Stress field along the Ryukyu Arc and the Okinawa Trough inferred from moment tensors of shallow earthquakes

Abstract

We investigated the stress field along the Ryukyu Arc and the Okinawa Trough using regional (NIED MT) and global (Harvard CMT) moment tensor catalogs. Using the combined dataset of 4 years of data from the NIED MT catalog and 24 years of data from the Harvard CMT catalog, we found an arc-parallel extensional stress province along the forearc of the Ryukyu Arc and observed a detailed extensional stress field in the Okinawa Trough. This arc-parallel extension is observed in the entire region of the Ryukyu Arc except at its northeastern end. This stress field is clearly separated from the backarc extensional stress field in the Okinawa Trough by the volcanic chain. Since it exists in a normal subduction region as well as in a region of weakly coupled subduction, the effects of oblique subduction might not be enough to explain the formation of the arc-parallel extension. Thus, we consider that the backarc opening process should play an important role in this arc-parallel extension. Along the Okinawa Trough, an arc-perpendicular extensional stress field is observed in the southeastern and central Ryukyu Arc, while in the northeastern Okinawa Trough, the direction of the extensional axis is oblique to both the arc normal direction and the direction of plate motion. Two-dimensional mechanical interaction models which consider the interactions between the subducting plate, forearc and backarc are inadequate because an arc-parallel extensional stress field is observed in the entire Ryukyu Arc, and the extension axis in the northeastern Okinawa Trough is oblique to the arc trend. The oblique backarc stress may originate beneath the backarc or on the continental side of the backarc. One of the simplest ways to explain this observation might be active rifting in the northeastern Okinawa Trough.