The role of minor faults in the upper crust along the high strain-rate zones : topographical and geological approaches

ひずみ集中帯の上部地殻における小規模断層の役割 : 地形・地質学的アプローチ

The role of minor faults in the upper crust along the high strain-rate zones : topographical and geological approaches

Degree Grantors Yamaguchi University

In high strain-rate zones, active regions of ongoing crustal deformation, earthquakes occur frequently, the total slip rates of active faults are in the zone not consistent to strain rate detected by geodesy. This difference is one of the most significant issues for crustal deformation, and is known as "strain-rate paradox". Previous crustal deformation models are mainly constructed with major active faults alone, whereas minor faults are often recognized in the high strain-rate zones. The aims of this thesis are to solve the strain-rate paradox and propose a new image of the crustal deformation by focusing on the minor faults. In order to accomplish these goals, the representative high strain-rate zones such as San-in Shear Zone (SSZ) and Niigata-Kobe Tectonic Zone (NKTZ) were targeted. As a result of the topographical and geological approaches, universal model, origin, deformation process and mechanism of the high strain-rate zone were clarified. The main outcomes are as follows:
(1) Minor faults in the NKTZ, which are mostly NE-SW to ENE-WSW-trending, have a few mm to a few dozens of cm in width and exhibit dextral sense of shear. These minor faults are distributed in the vicinity and/or away from the major active faults. In addition, the active fault, whose core zone has 5 m in thickness, were found. Such fault showed dextral sense of shear and has the latest slip event after AD 1521-1658, suggesting that the fault clearly contribute to the dextral deformation of the NKTZ. The origins of such faults are thought to be tensile cracks formed in Cretaceous, suggesting that the faults contribute to the dextral deformation of the NKTZ after repeated faulting along the cracks. The minor faults away from the major active faults are also thought to contribute to the deformation of the NKTZ, whereas minor faults outside of the NKTZ cannot contribute to that of the NKTZ.
(2) Minor faults in the SSZ, which are mostly ENE-WSW to NE-SW-trending, have a few mm to a few dozens of cm in width and exhibit dextral sense of shear. These other minor faults trending NW-SE ～ NNW-SSE direction with steep dips, are sinistral sense of shear. The minor faults, which is trending E-W direction with steep dips, showed dextral sense of shear. Active faults, whose attitude are nearly parallel to the SSZ, are also newly recognized. The thickness of the fault is a few cm and thought to show dextral-reverse oblique slip after 18648-16313 cal. BC. The frameworks of the major active faults in the SSZ are thought to prepared along the geological boundaries and such faults have grown by the repeated activities since Paleogene. It is considered that not only major active faults but also minor faults away from the major active faults can contribute to the dextral motion of the SSZ. On the contrary, there are only reverse fault was recognized in the area outside of the SSZ.
(3) The minor fault in the high strain-rate zones, which includes the minor fault away from the major active fault, can contribute to the dextral deformation of the high strain-rate zones because of their attitudes and sense of shears. On the other hand, the minor faults outside of the high strain-rate zones cannot contribute to the dextral deformation of the zone due to their attitudes and sense of shears. Thus, there are noteworthy difference between minor faults in and outside of the high strain-rate zones. Combining these outcomes, a hierarchical structure of the high strain-rate zones can be constructed as follows: (I) fault core of major active faults, (II) damage zone of major active faults, (III) brittle shear zone (or active background; the area beyond the damage zone but in the SSZ), (IV) inactive background (outside of the high strain-rate zone). This new model enables to partly solve the strain rate paradox for both zones, whereas an occurrence of faults differs between the zones. The NKTZ is characterized by NE-SW to ENE-WSW-trending minor faults and their thickness ranging from a few mm to a few dozens of cm. The active faults possess fault core with > 5 m in thickness. The SSZ are characterized by NW-SE or E-W-trending minor faults and their thickness ranging from a few mm to a few dozens of cm. Some faults show the Quaternary activities, whereas fault core with a few meters in thickness were not found. These differences on fault occurrence are considered to be derived from the evolutional processes. It is thought that the repeated activities along the pre-existed structures lead to present active faults. Thus, it can be considered that the faults are assigned in response to the local geological background, which result in dextral contribution to the high strain-rate zones.
This study clarified universal model, origin, deformation process and mechanism of the high strain-rate zone by focusing on the minor faults. These achievements can constrain the modeling of the crustal deformation and interpretations of the geodetical observations and can contribute to assessments of large-scale constructions and seismic hazards.

Creators Tamura Tomonori