Seismicity and Stress State in the Ryukyu Islands Subduction Zone

Abstract

Based on the newly compiled and mostly complete unified earthquake catalogue for China’s seas and adjacent areas, further information was obtained about the structural shape and dip angle of the Benioff zone in the Ryukyu Islands subduction zone during the different subduction stages. In addition, using the damped regional stress tensor inversion method, we were able to investigate the complex stress field characteristics and the dynamic significance of the shallow and intermediate earthquakes in the Ryukyu Islands subduction zone. The results show that the tectonic stress field of the Ryukyu Islands subduction zone was extensional along the subduction direction in the northern area of the Tokara Strait and was compressional along the subduction direction in the southern area of the Tokara Strait. The R value of the shallow stress field of the Okinawa Trough was low, and the σ3 was stable in the NNW direction with a small dip angle (>30°). The type of stress field in the shallow part of the Okinawa Trough transitioned from strike-slip type to normal fault type from north to south, reflecting the difference in the degree of development of the trough, and the southern segment of the trough began to transform into the expansion stage. The northeastern portion of the study area and southeast Taiwan constituted the high R value (0.68–0.87) region where the σ2 had tensile components. The stress state was biaxial tension–uniaxial compression, and the principal compressive stress was determined to be in the SEE direction with a large dip angle (>30°). The σ1 in northeast Taiwan exhibited a nearly vertical (>60°) plunge, while the σ2 and σ3 were nearly horizontal. The σ2 was thrust in the ENE–WSW direction, and the σ3 was extended in the NNW direction. Through this research, a greater understanding has been gained of the seismicity characteristics and shape of the Ryukyu Islands subduction zone. Supplementary research has also been completed on the focal mechanism solution and stress field of the Ryukyu Islands subduction zone. Finally, this research is important for earthquake hazard analysis and earthquake engineering safety evaluation in this area.

Seismotectonics of the Philippine and Taiwan Subduction Systems and Implications for Seismic Hazards

“…This distribution of seismicity is consistent with a recent analysis by Xie et al. (2022), who also show that seismicity does not exceed a depth of 300 km. The seismicity is also consistent with tomographic imaging by Lallemand et al.…”

“…Seismicity inboard of the Ryukyu trench traces the Wadati-Benioff zone of the northward subducting Philippine Sea plate to ∼260 km depth, while Slab 2.0 (Hayes, 2018) predictions extend to a depth of ∼350 km due to extrapolation (Figure 6b). This distribution of seismicity is consistent with a recent analysis by Xie et al (2022), who also show that seismicity does not exceed a depth of 300 km. The seismicity is also consistent with tomographic imaging by Lallemand et al (2001), who show that a continuous slab is present to depths of ∼250 km.…”

“…Normal faulting is found throughout the study region. Abundant normal faulting mechanisms for earthquakes of magnitude M < 6.0 are located within the Okinawa trough, reflecting NW‐SE oriented extension in the back‐arc basin (Figure 9a; Liu et al., 2016; Xie et al., 2022). Normal faulting appears more prevalent in the south‐eastern portion of the trough toward Taiwan (Figure 9).…”

“…Xie et al. (2022) noted that the transition from strike‐slip seismicity in the north to normal faulting in the south may reflect an increasing degree of development of the trough toward Taiwan.…”

“…Most seismicity appears to lie within the slab as defined by Slab 2.0 (Hayes, 2018), consistent with a leading conclusion that the Ryukyu trench exhibits weak coupling at the plate interface that decreases the seismicity rate (Arai et al., 2016). Clusters of seismicity in the upper plate inboard of the Ryukyu trench (Figure 6b) are likely related to back‐arc spreading in the Okinawa trough (Figure 6a; Liu et al., 2016; Xie et al., 2022).…”