Crustal thickness beneath the Ryukyu arc from travel-time inversion

Mamoru Nakamura,Nozomi Umedu

doi:10.1186/bf03352971

Abstract

Abstract First P-wave arrival times recorded by the seismic network in the Ryukyu arc were analyzed in order to image the lateral variation in crustal thickness beneath the Ryukyu arc. The results indicate a low P n velocity (7.5 km/s) in the Ryukyu arc and a relatively high P n velocity (7.9 km/s) in the Okinawa Trough. The crustal thickness changes between the northern-and-central Ryukyu arc (range 23–27 km) and the southern Ryukyu arc (range 29–44 km). The crustal thickness of the former is consistent with that in the northern-and-central Okinawa Trough, suggesting a flat Moho stretching from the Okinawa Trough to the Ryukyu arc. This flat Moho extends uniformly over a wide area. In contrast, the crustal thickness in the southern Ryukyu arc is consistent with the characteristics of the crustal structure whereby the Moho becomes shallower in the vicinity of the axis of the Okinawa Trough and deepens remarkably with increasing distance from the axis in the southern Ryukyu arc. These differences would be caused by the difference in rifting style between the northern-and-central Okinawa Trough and the southern Okinawa Trough.

Highlights

The Ryukyu arc is an approximately 1200-km-long island arc that runs along the Ryukyu Trench between Kyushu Island and Taiwan
The results indicate a low Pn velocity (7.5 km/s) in the Ryukyu arc and a relatively high Pn velocity (7.9 km/s) in the Okinawa Trough
The crustal thickness of the former is consistent with that in the northern-and-central Okinawa Trough, suggesting a flat Moho stretching from the Okinawa Trough to the Ryukyu arc

Summary

Introduction

The Ryukyu arc is an approximately 1200-km-long island arc that runs along the Ryukyu Trench between Kyushu Island and Taiwan. The Okinawa Trough, which is at the beginning of the rifting stage and extends toward the NW-SE (Sibuet et al, 1995), is located northwest of the Ryukyu arc. The northern-and-central and southern parts of both the Ryukyu arc and Okinawa Trough have different characteristics. The Ryukyu arc and the Okinawa Trough are divided geologically into three blocks (northern part, central part, and southern part) that are bounded by the Tokara Strait and the Kerama Gap (Konishi, 1965) (Fig. 1). The southern Okinawa Trough (SOT) and the northern-and-central Okinawa Trough (NCOT) have different features. In the northern-and-central Ryukyu arc, the volcanic front is located to the east of the axis position of the Okinawa Trough (Fig. 1); in the southern Ryukyu arc, the volcanic front is located at the axis of the Okinawa Trough

Results

Discussion

Conclusion