Late Quaternary activity of faults and recurrence interval of earthquakes in the eastern Hokuriku region, northern central Japan, on the basis of precise cryptotephra analysis of fluvial terrace sequences

Abstract

About 2000 active faults are known to exist within the land area of Japan. Most of these active faults have deformed the topographic surfaces which were formed in the late Quaternary, including fluvial terraces; and the formative ages of these terraces are estimated mainly by tephrochronology. Fluvial terraces in the eastern Hokuriku region, comprising the Toyama, Tonami, and Kanazawa Plains, northern central Japan, are widely distributed and have been deformed by reverse active faults. The formative age of terraces in this area has not been reported, as volcanic ash deposits are rarely visible within terrace deposits and the overlying loamy soil, and outcrops of fluvial terraces are quite scarce in this area. In the present study, we carried out a drilling survey on these terraces to obtain samples of the overlying loamy soil and upper part of terrace deposits. From these samples, we extracted some well-known widespread volcanic ash, from which we were able to estimate the approximate age of the terraces and the vertical slip rate of the active faults. Late Quaternary fluvial terraces in eastern Hokuriku are divided into 12 levels: Terraces 1 to 12 in descending order. Widespread tephras such as the Kikai-Tozurahara Tephra (K-Tz: 95 ka) are contained in the lowest part of the loamy soil in Terrace 4 and the Daisen-Kurayoshi Pumice (DKP: 55 ka) is present in the lowest part of the loamy soil in Terrace 6. From the ages and the vertical displacements of the fluvial terraces, the late Quaternary average vertical slip rates of active faults in eastern Hokuriku are estimated to be 0.2–0.9 mm/year (Uozu fault), 0.1–0.4 mm/year (Kurehayama fault), 0.1–0.3 mm/year (Takashozu fault), 0.1–0.4 mm/year (Hohrinji fault), and 0.5–0.8 mm/year (Morimoto-Togashi fault). We also estimated the recurrence interval of earthquakes related to active faults from displacement per event and ages of terraces and no significant difference in vertical displacement per single earthquake for different active faults, and recurrence intervals tend to be inversely proportional to vertical displacement rates. This study demonstrates that a combination of drilling of loamy soil and precise cryptotephra analysis of fluvial terraces can be used to estimate the formative age of the terraces and the average slip rate of active faults in areas where volcanic ash deposits are rare.

Introduction

In Japan, the average slip rates have been calculated for many active fault zones by estimating the displacement and the formative age of late Quaternary terraces. Because Japan has one of the highest levels of volcanic activity in the world (Machida, 1976, Machida and Arai, 1983, Machida, 1991, Machida, 1999; Machida and Arai, 2003), the formative ages of these terraces are estimated mainly by tephrochronology (Hirakawa et al., 1974, Hayatsu and Arai, 1982, Suzuki, 1989, Azuma, 1995, Matsushima, 1995, Watanabe and Urabe, 2003). On the other hand, the average slip rate has not been calculated for some active fault zones because volcanic ash deposits are rarely visible within terrace deposits and the overlying loamy soil. Accurate chronology of fluvial terraces of an area without tephrochronological data is very important not only for the understanding of late Quaternary crustal movement but also for comparison and correlation of active faults between various areas of the world where tephrochronological data are scarce or lacking.

The eastern Hokuriku region (Fig. 1) lies across the boundary between the Hida Mountains, 2000 to 3000 m above sea level and one of the major areas of Quaternary uplift in Japan, and the Toyama Trough, 2000 m below sea level and composed of the Toyama, Tonami, and Kanazawa Plains (Harayama, 1992, Harayama et al., 2003, Oikawa, 2003). Fluvial terraces are widely distributed in these plains, and have been deformed by NE–SW-trending reverse active faults that have developed between the plains and the mountains described above (Fig. 1; Research Group for Active Faults of Japan, 1980, Research Group for Active Faults of Japan, 1991, Togo et al., 1998, Tsutsumi et al., 2002; Ikeda et al., 2002, Nakata and Imaizumi, 2002, Tsutsumi et al., 2003, Togo et al., 2003, Imaizumi et al., 2003).

To estimate the formative ages of fluvial terraces lacking visible volcanic ash tephra deposits, we have carried out a drilling survey on the terraces to obtain samples of the overlying loamy soil. Such a drilling survey makes it possible to (1) collect core samples vertically, (2) avoid any effect of weathering or bioturbation on the samples compared with the outcrop sample, and (3) obtain adequate core samples from a precise location.

We extracted and identified tephra deposits within the loamy soils, and from them were able to estimate the approximate ages of the terraces using tephrochronology. In this paper, we report the average slip rate of active faults based on the vertical displacement and the formative ages of the terraces. We use a combination of previous data obtained on fluvial terraces and active faults in this area with new findings from our studies of the Tonami Plain.