Surface Wave Tomography of Lithospheric Structure in the Seas of East China

Abstract

AbstractThis paper presents the S‐wave velocity structure of crust and upper mantle in the marginal seas of east China resulted from surface wave tomography, describes the structural features of various tectonic units in the region, and discusses their relation with present tectonic activity and past evolution history. The average velocity of the middle and lower crust has a remarkable relation with strong earthquakes, which took place within the low velocity zones or along the boundaries between low and high velocity. There are several NW‐trending low velocity zones in the crust east of the Taihang Mountains, among them the one beneath the Zhangjiakou– Bohai seismic belt is most conspicuous. The sea area can be divided into 4 tectonic units, i.e., the northern and southern Yellow Sea, the East China Sea, and the Okinawa Trough. The northern Yellow Sea has a thin and high velocity lithosphere similar to that of southern North China Basin, presumably resulted from Mesozoic lithosphere thinning caused by the northward subduction of Paleotethys. In contrast the northern North China has a low velocity crust and thicker lithosphere, the velocity of lithospheric mantle is rather low and homogeneous vertically, possibly indicating the result of long‐lasting Mesozoic collision along the northern suture zones. The southern Yellow Sea has a thicker lithosphere with the characteristics of lower Yangtze craton. In the suturing process of lower Yangtze and North China the subduction of oceanic lithosphere might have caused the low velocity features of the upper mantle in northern Yellow Sea and Su‐Wan region. However, such suturing relics were not found in Korea Peninsula east of 125°E. It may imply that the whole Korea Peninsula is a part of North China Craton, or the effect of Pacific subduction and Japan Sea opening might have totally reformed the lithospheric mantle beneath the peninsula and erased the sign of past suturing. In the East China Sea the crust, especially the lithosphere thins towards the Okinawa Trough, indicating that the subduction of Philippine Sea plate affects a vast back‐arc area. In the Okinawa Trough the subducting Philippine slab and the conspicuously low velocities above the slab are clearly seen, providing evidence for the back‐arc spreading mechanism of the Okinawa Trough formation.