Neotectonics of Southern Fossa-Magna, a Study based on Stress Measurement and Active Faults
1984 Volume 23 Issue 2 Pages 117-128
Details
1984 Volume 23 Issue 2 Pages 117-128
In southern Fossa-Magna, direct stress measurement such as over-coring, hydraulic fracturing, X-ray, and acoustic emmision has been carried out at 12 localities by 1982. Field study of active faults has been conducted energetically in this area, too.
The results of both stress measurement and active faults are almost agreeable. In conclusion, tectonic stress in this area comprises two stress systems, (1) northwest-southeast (NW-SE), and (2) north-south (N-S).
The NW-SE stress is found nearly all over the area. However, it is most dominant around the west coast of the Izu Peninsula and the Izu Islands. In this stress system, the maximum principal stress is nearly horizontal and trends northwest to southeast, and the medium and minimum principal stresses are respectively horizontal northeast-southwest and vertical. The medium and minimum principal stresses are almost equal. Therefore, in some areas, alternatively the vertical stress is the medium and the horizontal NE-SW is the minimum principal stress.
On the other hand, the N-S stress is more restricted in area. In this stress system, maximum principal stress is nearly horizontal, trending NNE to SSW. Medium and minimum principal stresses are respectively horizontal WNW-ESE and vertical. Both NNE-SSW and WNW-ESE horizontal stresses are much larger in shallow depth than the vertical stress.
However, the NW-SE and N-S stress system presumably do not divide area of distribution between each other, but are in state of co-existence in some places.
Acting in the intermediate area between the NW-SE or WNW-ESE stress of southwestern Japan and the E-W stress of northeastern Japan, the NW-SE stress is the most common stress system in southern Fossa-Magna. Its tectonic source is a lateral compressional force applied from southeastwards.
The N-S stress system develops mainly in the narrow zone bounded by the Ashigara-Kanogawa line in the east, which is called the Nishi-Izu zone in this report. The Nishi-Izu zone is probably under structural control of both the NW-SE and N-S stress systems. Tectonic origin of the N-S stress is not clear yet. However, the stress system is possibly closely related with the N-S stress acting in several places in southwestern Japan along the Pacific Ocean.
