The Late Quaternary Average Vertical Slip Rate of Uozu Fault, Indentified by Drilling Survey in Toyama Prefecture, Central Japan
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概要
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On the eastern margin of Toyama Plain, northern part of central Japan, late Quaternary fluvial terraces are well developed and divided into 9 levels: Terrace I to IX in descending order. The NNE- to NNW- trending Uozu fault extends 30km along the eastern margin of Toyama Plain and is associated with several branch faults. The ages of the terraces have rarely been reported in this area, due to the scarcity of outcrops and the lack of visible tephras. Therefore the author has carried out a drilling survey on the terraces to obtain loamy soil overlying the terrace deposits. The loamy soil consists of surface black soil and weathered brown, unsorted clay. The terrace deposits are composed of well-sorted silt and gravel beds of fluvial origin. The tephra horizon was recognized by wet sieving at 5-cm intervals and identification of volcanic glass and minerals under a microscope. The Kikai-Tozurahara Tephra (K-Tz: 95ka), characterized by a low index (n=1.497-1.499) of volcanic glass, is contained at the lowest part of loamy soil in Terrace III, and the Daisen-Kurayoshi Pumice (DKP: 55ka), characterized by colored minerals, is found at the lowest part of the loamy soil in Terrace IV. So Terrace III seems to have been formed in the Marine Isotope Stage (MIS) 5c or MIS 5c-5b boundary (95-105ka), and Terrace IV in MIS 4 or MIS 4-3 boundary (60-70ka). The DKP tephra is also found in the fluvial silt in Terrace V. Therefore, Terrace V is considered to have been deposited in MIS 3 (45-55ka). The Uozu fault deforms these fluvial terraces accumulatively. The vertical displacements of Terrace I to IX are as follows: 70-128m (Terrace I), 42m (Terrace II), 36m (Terrace III), 10-23m (Terrace IV), 31m (Terrace V), 3-4m (Terrace VI), and 1-2m (Terrace VIII). Based on the terrace ages and vertical displacements, the average vertical slip rate of the Uozu fault is calculated to be 0.2-0.9mm/yr.
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- The Late Quaternary Average Vertical Slip Rate of Uozu Fault, Indentified by Drilling Survey in Toyama Prefecture, Central Japan