Katsuyoshi Miyakoshi

Dating fault activity by surface textures of quartz grains from fault gouges

Abstract Surface textures of quartz grains from fault gouges are examined by means of the scanning electron microscope. It is disclosed that the surface morphology of the grains is variable, but quartz grains extracted from one fault gouge sample commonly show a type of texture different from those observed on the grains from the other samples. The surface textures observed under the microscope are tentatively classified into eight types; subconchoidal, orange peel-like, fish scale-like, moss-like, moth-eaten, stalactitic, pot-hole, and coral-like textures. These textures can be classified into four groups. Arranged in the order of their apparent features, it is interpreted that the progressive corrosion of quartz grains by ground water has taken place after faulting. The change of this surface feature can assist in estimating the time elapsed since the last fault activity.

Further studies on the use of quartz grains from fault gouges to establish the age of faulting

Abstract Quartz grains were selected from samples of intrafauit material at two outcrop sites of the Atotsugawa fault in central Japan. Surface textures of the grains observed by scanning electron microscope were classified into texture groups I–IV, in the order from a simple feature through increasing undulations to a texture with cavities. The defects in the quartz measured by electron spin resonance gave absolute ages using the grains. The relative ages by surface texture groups were tentatively correlated to the absolute ages as determined by the electron spin resonance. It was disclosed that Subgroups Ia and Ib were stable during the period of 0.9 Ma, and changed through intermediate Subgroup Ic in a period ranging from 0.9 to 1.2 Ma, and then to Group II after 1.2 Ma. Therefore, the surface texture groups can change from Subgroups Ia and Ib through intermediate Subgroup Ic to Group II with increasing age. This indicates that the surface texture groups of quartz grains in the fault are effective for dating fault activity concisely.

The 1995 Kobe earthquake and problems of evaluation of active faults in Japan

Abstract The Kobe earthquake (M 7.2) of January 17, 1995, which was the most damaging earthquake in recent Japanese history, made manifest the need for reconsidering the method of evaluating active faults. An earthquake of this magnitude at this time was unexpected according to conventional evaluation, in which the potential magnitude of earthquakes at a certain site is estimated by considering the greatest earthquake in the past 400 years and the length of the active fault. The following characteristics of this earthquake made it appear unlikely by conventional understanding: (1) the Kobe earthquake involved several neighboring faults, which had been previously been identified as separate fault systems: (2) the surface rupture of about 10 km length was much shorter than the 50 km seismic faulting; (3) the interval of 400 years between the Kobe and penultimate Keicho earthquake of 1596 AD (M 7.5), which has been revealed by historical documents and some excavations, is much shorter than the 2000 years estimated by calculating the average slip rate of displaced landforms. These shortcomings imply that active fault evaluation with the traditional characteristic earthquake model which deals with each fault separately, is not adequate for an area like Japan where active faults swarm. New concepts such as the block rotation model (Kanaori, 1990; Late Mesozoic-Cenozoic strike-slip and block rotation in the inner belt of Southwest Japan. Tectonophysics, 177: 381–399) considering the macroscopic tectonic framework for fault interactions are needed. Furthermore, fault dynamics cannot be ignored; physical and temporal parameters associated with faulting, such as moment release rate, must be considered for realistic and precise evaluation.

Chapter 6 The 1995 Kobe earthquake and problems of evaluation of active faults in Japan

Abstract The Kobe earthquake ( M 7.2) of January 17, 1995, which was the most damaging earthquake in recent Japanese history, made manifest the need for reconsidering the method of evaluating active faults. An earthquake of this magnitude at this time was unexpected according to conventional evaluation, in which the potential magnitude of earthquakes at a certain site is estimated by considering the greatest earthquake in the past 400 years and the length of the active fault. The following characteristics of this earthquake made it appear unlikely by conventional understanding: (1) the Kobe earthquake involved several neighboring faults, which had been previously been identified as separate fault systems: (2) the surface rupture of about 10 km length was much shorter than the 50 km seismic faulting; (3) the interval of 400 years between the Kobe and penultimate Keicho earthquake of 1596 AD ( M 7.5), which has been revealed by historical documents and some excavations, is much shorter than the 2000 years estimated by calculating the average slip rate of displaced landforms. These shortcomings imply that active fault evaluation with the traditional characteristic earthquake model which deals with each fault separately, is not adequate for an area like Japan where active faults swarm. New concepts such as the block rotation model ( Kanaori, 1990 ; Late Mesozoic-Cenozoic strike-slip and block rotation in the inner belt of Southwest Japan. Tectonophysics, 177: 381–399) considering the macroscopic tectonic framework for fault interactions are needed. Furthermore, fault dynamics cannot be ignored; physical and temporal parameters associated with faulting, such as moment release rate, must be considered for realistic and precise evaluation.