Seismic structure of rifting in the Okinawa Trough, an active backarc basin of the Ryukyu (Nansei-Shoto) island arc–trench system

Abstract

The Okinawa Trough, to the southwest of Kyusyu, Japan, is an active backarc basin of the Ryukyu (Nansei-Shoto) island arc-trench system caused by the Philippine Sea plate subduction. Unlike other backarc basins around Japan, the Okinawa Trough, ~ 1000 km in length, is unique because the crustal thinning due to backarc rifting is currently in progress in the entire trough. We conducted extensive seismic reflection and refraction surveys to detect detailed variation in seismic structures associated with the rifting tectonics. Seventeen seismic lines were shot, including ten across-trough and seven along-trough lines in the Okinawa Trough. Moho depths estimated mainly from PmP travel times indicate that the crust beneath the trough is thinner than that below the East China Sea shelf and the Ryukyu Island Arc. The shallowest Moho of the across-trough lines was not necessarily detected at the center of the trough, defined as the deepest water depth, but was located beneath the western margin of the northern and middle Okinawa Trough. An M7.1 earthquake occurred in the area on November 14, 2015, and intense aftershock activity was observed along the western margin of the trough. These earthquakes with extension axes of northwest-southeast direction demonstrated that the area is undergoing tectonic rifting. The Moho depth is over 25 km in the northern region of the Okinawa Trough and decreases down to ~ 13 km as it goes to the south, and the thinnest crust of ~ 7 km occurs beneath the Yaeyama Rift in the southern Okinawa Trough. Despite the crustal thinning by the backarc rifting, the P wave velocity models across the Okinawa Trough show that the continental/island arc crust composed of an upper, middle, and lower crusts is continuous between the East China Sea shelf and the Ryukyu Arc. The multichannel seismic profiles along and across the rifts in the southern Okinawa Trough show more volcanic intrusions in the southern side than in the north, which may be related to the position of the volcanic front, which is undetermined in this region.

Spatiotemporal Variation of Cold Eddies in the Upwelling Zone off Northeastern Taiwan Revealed by the Geostationary Satellite Imagery of Ocean Color and Sea Surface Temperature

“…Taiwan is located in the center of the East-Asian island arc formed by the slow collision of the Asian continental plate and the Philippine plate. The basin topography of the UZONT is therefore rather complicated and comprises the China Continental Shelf, the Okinawa Trough [43], the Ryukyu Arc, and the Volcanic Islands. As illustrated in Figure 1a, three major current systems, including the Kuroshio Current, the China Coastal Current and the Taiwan Strait Warm Current [44], all flow through this region and interact with the complicated basin topography.…”

Imaging the Crustal Interfaces Along the Ryukyu Arc‐Trough System Using Precursors to Teleseismic sP and pP

“…(2018); Ecr9 from Nishizawa et al. (2019). The black dashed line represents the location of the regional rift axis (Bird 2003).…”

“…The regional crustal structure of the RA‐trough system has been investigated mainly relying on seismic reflection/refraction profiles (Arai et al., 2018; Hirata et al., 1991; Iwasaki et al., 1990; Klingelhoefer et al., 2009; Kodaira et al., 1996; N. S. Li, 2001; T. K. Wang et al., 2004; K. McIntosh et al., 2005; Nishizawa et al., 2019), regional body wave and surface wave tomography (Chou et al., 2009; Friederich, 2003; Gorbatov & Kennett, 2003; Huang et al., 2014; Nakamura & Umedu, 2009; Nakamura et al., 2003; Tao et al., 2018; Wang et al., 2008, 2017), magnetic (J. Y. Gao et al., 2008; Liang et al., 2001), and gravity data analysis (J. Y. Gao et al., 2008; Han et al., 2007; Jin et al., 1983).…”

“…The black solid lines show the seismic reflection/refraction profiles in previous studies: PROFILE I (PI) and PROFILE II-c (PII-c) from Iwasaki et al (1990) and Hirata et al (1991); PROFILE3 (P3) and PROFILE2-A (P2-A) from Kodaira et al (1996); L1 from N. S. Li (2001); EW9509-1 (EW-1), , and EW9509-16 (EW-16) from T. K. Wang et al (2004); NRA-90 from Nakahigashi et al (2004); G1 and G2 of D. Z. Gao et al (2006); PROFILE1 (P1) and PROFILE2 (P2) from Klingelhoefer et al (2009); YA01 and YA05 from Arai et al (2017); RK01 and RK02 from Arai et al (2018); Ecr9 from Nishizawa et al (2019). The black dashed line represents the location of the regional rift axis (Bird 2003).…”

Estimating the thickness of a faulted mechanical layer based on fault spacing: An example from Miyako‐jima Island, southern Ryukyu Arc, Japan

“…Miyako-jima Island is part of the Ryukyu Arc, which is associated with the subduction of the Philippine Sea Plate beneath the Eurasian Plate ( Figure 2). The island is located between the Ryukyu (Nanseishoto) Trench and the Okinawa Trough, which is an extensional back-arc basin (Nishizawa et al, 2017(Nishizawa et al, , 2019. The Okinawa Trough opened during the Late Miocene and Early Pleistocene and was accompanied by clockwise rotation of the southern Ryukyu Arc (e.g.…”