Some characteristics of the seismicity in the northern Fossa Magna, central Japan

Abstract

The geologic feature of the northern Fossa Magna region is characterized by a well developed folded belt system. Recent microseismicity over 10 years and historic earthquakes reveal that there are seismic zones pararell to the axes of this structure: Itoigawa-Shizuoka Tectonic Line, the western side of Shinano River, the western margin of the Central Belt of Uplift, and the eastern margin of the Central Belt of Uplift. The last zone was recently recognized by the earthquake sequence with the main shock of M 4.9 in 1986. The fcal mechanism solutions and other geologic and geodetic evidence indicate the region is compressed in the direction perpendicular to the folded belts. We can also find seismic activity lines perpendicular to or oblique to these folding axes: the Chikumagawa Tectonic Line separates the Central Belt of Uplift with a left lateral offset of 10km; the seismic line along the eastern side of the volcanic line of Myoko-san, Kurohime-yama and Iizuna-yama makes the eastern edge of the seismic gap around the northern end of the Itoigawa-Shizuoka Line.
A 20-25 years recurrence time is found in the occurrence of the major earthquake events in the central part of the northern Fossa Magna in recent 140 years.
Seismic active regions are grouped into some rectangular blocks with their bases being either pararell or perpendicular to the folded belts. The activity in a block became active nearly simultaneously, and it sometimes migrated to other blocks with some delay time. The high activity migrated from the block region around the Chikumagawa Tectonic Line to both the northern activity block around the Japan Sea coast and the active region in the southern Fossa Magna during the period from 1986 to 1987.
The migration of the seismic activity along the Fossa Magna region may be due to delayed transmission of the stress field in the viscoelastic or elastoviscous medium. The exponential time dependence of the decay rate of the Matsushiro swarm activity for a long period more than 20 years since 1965 is found to be reasonably interpreted by the relaxation process of stresses within the elastoviscous medium. The viscosity changed from 1.1×10¹⁹ poise to 2.7×10²⁰ poise as time elapsed. The first value is comparable with that of a ground surface rock body estimated by secular ground tilts and strains. The latter is the same order of the values obtained in the long period second creep experiments using granite and gabbro. The Matsushiro region is one of the region where the viscosity might have been very low due to numerous micro-cracks.