Yuji Kanaori

Distributed strike-slip faulting, block rotation and possible intracrustal vertical decoupling in the convergent zone of SW Japan

Abstract Between the Median Tectonic Line (MTL) and the Japan Sea, the western Chugoku region of SW Japan is cut by a series of N45°E first-order faults and oblique (N60°–N170°E) second-order faults. This fault network, probably formed during Late Cretaceous-Palaeocene times (70–60 Ma), defines a regional block structure. Pre-Plio-Quaternary kinematical indicators suggest left-lateral motion along the first-order faults and right-lateral motion along some of the second-order faults. Geomorphological evidence and earthquake focal mechanisms indicate that Plio-Quaternary slip senses are opposite to Pre-Plio-Quaternary ones. The overall fault pattern is geometrically and kinematically similar to patterns obtained by experimental modelling of simple shear deformation distributed at the base of a brittle layer analogue over its entire width. This similarity suggests the possibility of a mid-crustal, flat-lying partial attachment zone which could have controlled the formation of the western Chugoku fault network in Cretaceous to Palaeocene times. The zone, presently inactive, could correspond to the ‘proto-MTL’, a low-angle fault recently imaged by seismic reflection studies and whose trace approximately coincides with the present-day MTL. Reactivation of the system occurred twice after its formation: firstly in Miocene times, during the opening of the Japan Sea and concomitant clockwise rotation of the entire SW Japan arc; and secondly in Late Pliocene to Quaternary times, after a shift of the relative direction of convergence between the Philippine and Eurasia plates. Unlike the first reactivation, the second reactivation led to an inversion of the sense of slip along the faults.

Encompassing hydrogeology, environmental geology and the applied geosciences

Abstract The year began with a good deal of international tension as well as various indications of recessional economics. Two weeks into the new year, a rash of bankruptcies visited the geoenvironmental consulting industry, in which some of the very companies that had grown by expansion in takeover of smaller firms were the unstable targets. The year ended with the demise of many established geotechnical and geoenvironmental consulting firms and the establishment of new entities, generally by the more substantial of the personnel becoming castaways in the reorganizations. Employment in the private sector remained brisk but much adjustment was going on because of a generally shrinking work load related to shortfalls on taxes collected by government as the basis for public-works construction, especially in the infrastructure. All nations were in deficit spending and retraction of major projects.

9th Annual Report on the International Status of Engineering Geology—Year 2003-2004. Encompassing hydrogeology, environmental geology and the applied geosciences

In Year 2000, it became clear that Engineering Geology was moving into a higher plane of personal dedication by its practitioners. For economic reasons there has been an increased reliance by clients on the practitioner, who universally is expected to deliver more for less, as well as more rapidly. This pressure will have but one positive result, to produce surviving engineering geologists who will be masters of site characterization, as the true core method and product of the profession. We believe that the engineering geologist can become even more indispensable on the world scene. Wherever our brother and sister colleagues are not involved, there clearly will be increased risks in construction, operation, and maintenance of engineered works for societal and environmental protection purposes.

Seismic risk assessment of an active fault system: the example of the Tsurugawan–Isewan tectonic line

The 185 km long Tsurugawan-Isewan tectonic line (TITL) is one of the major active fault systems in central Japan. The TITL, oriented in a NW-SE-trend, passes through the western part of central Japan, extending from the Sea of Japan to the Pacific ocean. It is composed mainly of five large active faults; from the northwest to southeast, the Kaburagi, Yanagase, Sekigahara, Yoro, and Ise Bay faults. As a case study of seismic risk assessment of an active fault system using the parameter of the average moment-release rate, the seismic risk of the TITL is evaluated in the present paper. The average moment-release rate of an active fault system can be calculated from slip-rates and lengths of constituent active faults. In addition, the latest earthquake of the active fault system if unknown can be inferred from data of historical earthquakes and trench excavations. The total moments, which are accumulated over a duration time from the latest earthquake to the present, are estimated by multiplying the average moment-release rate by the duration time. The total accumulated moments can be used to empirically determine the magnitude of an earthquake which would presently occur. The calculated magnitude for each segment or active fault which constitutes the TITL presently is determined as 7.1 at the maximum. Further, the maximum magnitude of a possible earthquake which would occur after a 100 year durability period of concrete-made structures is estimated as 7.2. This implies that the TITL has the potential of producing earthquakes of M7.1 and 7.2 at the present and after 100 years, respectively. The method of seismic risk assessment is important in order to obtain input magnitude data of possible earthquakes for an active fault system, to be used in selecting sites and earthquake-proof designs of large structures in seismically unstable regions.

Three destructive inland earthquakes in the central-western Yamaguchi Prefecture, southwest Japan and accompanied stress changes to their adjacent faults

Abstract Three destructive inland earthquakes of JMA magnitudes M j =5–6 have successively occurred in central-western Yamaguchi Prefecture during the past 15 years. The 1987 M j =5.2 Central Yamaguchi Prefecture earthquake, the 1991 M j =6.0 Suo-nada earthquake, and the 1997 M j =6.1 Northern Yamaguchi Prefecture earthquake were each strong enough to cause moderate damage to houses. The epicenters of these three earthquakes were located along the active W. Yauneyama–Lake Ohara fault system (WLFS). In this paper, we calculated the stress changes in the Coulomb Failure Function (CFF) induced by the three successive earthquakes in order to evaluate the seismic risk in and around this fault system. Our calculations indicated that due to these three earthquakes, the stress approximately increased from 10- to 30-km-wide area along the WLFS. The maximum stress increase was estimated to be 0.46 MPa at the Sakota–Ikumo fault. There are also stress increases at the East Mitsugatake fault. In contrast, post-seismic stress decreased along the Tokusa–Jifuku, the Lake Ohara, and the Shibuki faults. No stress change occurred on the Kikugawa fault. Because faults approach their points of stress failure with increases in stress, the seismic risk of parts of the faults that constitute the WLFS system may have been elevated after these events. The stress increase may shorten the period until the occurrence of the next earthquake along these stress-increased faults and may also trigger seismic activity along them.

Annual status of the profession: 1999: Encompassing hydrogeology, environmental geology and the applied geosciences

All professions began the countdown to the new world, considering the year 2000 to be the first year of the new millennium. Most of the actual impacts are already in place, including multinational businesses and the Euro as the first regional multinational currency. Most of the impacts have been harsh, and relief to individual consumers and professional people will be minor and felt mainly in enhanced ease of communication and media offerings. On the side of the conduct of the profession, the advantages will be far more narrow, as individual practitioners will be facing far stronger forms of competition for their services. Truly, the only way to survive will be for all of us to become more aware of the forces that impact engineering geology and the opportunities that engineering geologists have to provide their services, so basic to all forms of engineering, environmental protection and resource utilization.