The Kozu-Matsuda Fault: Its Recent Behavior and Tectonic Significance in a Plate Convergent Region

摘要

There are two different ideas about the tec- tonics of earthquakes along the Kozu-Matsuda fault, which is situated on the on-shore exten- sion of the Sagami trough, central Japan. One is derived from geological evidence of the de- formed Quaternary deposits in the onshore areas. The idea is that the Kozu-Matsuda fault upheaves the Oiso Hills and the Tanzawa Mountains and causes huge earthquakes with recurrence intervals thousands of years long. The fault moves without any relationship to the mega-thrust beneath the Sagami Trough from which 1923 Kanto earthquake occurred. The other is that the Kozu-Matsuda fault is a subsidiary fault of the plate boundary thrust along which huge earthquakes occur every two or three hundred years. Several seismo- logical interpretations of the l923 Kanto earth- quake support this opinion. Based on this, the Kozu-Matsuda fault would never cause a huge earthquake without any link to the plate boundary fault. To test these competing hy- potheses, the authors reconstructed the recent behavior of the Kozu-Matsuda fault based on the trench logs excavated at the foot of the fault scarp. Although no Holocene fault dis- placement was found from the trench walls, four or five seismic events since 3,000 years ago were detected from the startigraphy and chro- nology of the landslide or slope failure deposits inferred to be of seismic origin. Only one event, which is distinct and occurred about 3,000 years ago, is recognized to be caused by activity of the Kozu-Matsuda fault. As the other faint events include the event caused by the 1923 Kanto earthquake,