Characteristics of earthquake-induced turbidites in Beppu Bay, southwest Japan

Abstract:

Beppu Bay is located at the western end of the arc-bisecting dextral fault (Median Tectonic Line) associated with the northwestward subduction of the Philippine Sea Plate. According to Itoh et al. (1998) and Itoh et al. (2014), the process of formation of the bay was divided into two stages. The older stage (5 to 1.5 Ma) was dominated by a northward-inclined half-graben, while a pull-apart stress resulting from the right-stepping of the MTL developed during the younger stage (1.5 Ma to present in particular, 0.7 Ma to present), so seamless sediments were clearly preserved in the bay. Recently, Kuwae et al. (2012) revealed that the hemipelagic sediments accompanied with some event layers (18 major event layers (> 1 cm thick) and 55 minor event layers (< 1 cm thick)) were deposited. The core was well dated based on AMS 14C ages of 42 bivalves. In this study, we investigated the lithology of the event layers to understand how the layers were deposited. As a result, major event layers are classified into five types based on difference in the grain composition and facies: turbidites (type A–C), tephras (type D), and others (type E). Type A (4 layers) is thick event layers with a basal sand division, middle laminated silt division, and upper clay division. Relatively heavy particles, such as minerals, are concentrated in the basal division with clear erosion. Type B (7 layers) is similar to type A, but lack the basal division. Type C (5 layers) is almost the same as type B, but contains gypsum. Type D (2 layers) consists of a large amount of volcanic glass without bottom erosion. Type E (1 layer) is different from all other types and distinguishes by black color coarse particles and relatively high magnetic susceptibility. In particular, type A event layers are deposited in 334, 617, 1685, and 1893 cal. yrs BP using our age-depth model. The deposition age of an event layer corresponds to Keicho-Bungo historical earthquake occurred in 354 cal. yrs BP (Usami, 1996). The ages of other three layers are also not inconsistent with the ages of three fault events estimated by previous on-fault studies (Oita Prefecture, 2001; Chida et al., 2004). We thus concluded that the type A event layers were likely induced by earthquakes.