Matsushiro Fault

Matsushiro Fault
Name	Matsushiro Fault
Location	Nagano Prefecture, Japan
Type	strike-slip and reverse components
Length	~30 km (surface trace complex)
Plate	Pacific Plate, Philippine Sea Plate, Eurasian Plate (North American Plate debated)
Coordinates	36°33′N 138°12′E

Matsushiro Fault The Matsushiro Fault is a complex active fault zone in Nagano Prefecture, Japan, notable for its role in producing intense seismicity during the 1960s and for its proximity to the historical Matsushiro earthquake swarm. The fault system lies within a tectonically active region influenced by interactions among the Pacific Plate, the Philippine Sea Plate, and the Eurasian Plate, and it has been the focus of multidisciplinary studies involving geology, seismology, and engineering. Researchers from institutions such as the University of Tokyo, Tohoku University, Nagoya University, and the Geological Survey of Japan have published work integrating field mapping, trenching, and borehole studies.

Geology and Fault Characteristics

The Matsushiro Fault consists of a network of splays exhibiting both strike-slip and reverse motion, cutting late Pleistocene and Holocene deposits and interfingering with Quaternary terraces mapped by teams from the National Institute of Advanced Industrial Science and Technology and the Earthquake Research Institute. Surface expression includes linear scarps, shutter ridges, and sag ponds associated with displacement patterns comparable to segments of the Itoigawa-Shizuoka Tectonic Line and crosscutting features near the Kanto Plain margin. Lithologies juxtaposed across fault strands include volcanic ash beds correlated with tephras such as AT pumice and Kikai-Akahoya eruption fallout deposits identified in regional stratigraphy studies by the Volcanological Society of Japan. Geomorphic analyses reference the Shinanogawa river terraces and uplifted marine deposits similar to those along the Noto Peninsula.

Seismotectonic Setting and History

The seismotectonic setting ties the Matsushiro Fault to plate-boundary stress transfer influenced by the subduction of the Pacific Plate beneath the Okhotsk Plate (often treated as part of the Eurasian Plate) and interactions with the Philippine Sea Plate near the Nankai Trough. Historical seismicity records maintained by the Japan Meteorological Agency and catalogues assembled by the International Seismological Centre document numerous swarms and moderate earthquakes in the Chubu region including the notable 1965–1967 Matsushiro swarm that paralleled other intraplate sequences like the Tamba earthquake sequence. Paleoseismic correlations have been explored alongside events recorded in chronicles such as the Nihon Shoki and datasets used by the Global Seismographic Network.

Paleoseismology and Earthquake Record

Paleoseismological trenching conducted by teams from Hokkaido University and the National Research Institute for Earth Science and Disaster Resilience revealed multiple Holocene ruptures with displacements akin to coseismic offsets seen in trenches at sites comparable to those along the Median Tectonic Line. Radiocarbon dating exploited samples curated by the Museum of Nature and Human Activities, Hyogo and labs at the University of Tsukuba to constrain event ages, while luminescence dating performed at the Geological Survey of Japan laboratories helped establish recurrence intervals. Seismic moment estimates reference magnitudes within ranges reported for the 1995 Great Hanshin earthquake and paleoseismic compilations developed by the International Commission on Stratigraphy.

Monitoring and Hazard Assessment

Monitoring networks including accelerometers from the High Sensitivity Seismograph Network (Hi-net), GPS stations operated by the Geospatial Information Authority of Japan, and borehole observatories similar to those used by the Integrated Ocean Drilling Program provide continuous data for hazard models promulgated by the Central Disaster Management Council. Probabilistic seismic hazard assessments integrate inputs from the Seismic Hazard Harmonization in Europe methodologies adapted by Japanese agencies and are used by municipal planners in Nagano City and neighboring municipalities. Early warning algorithms deployed by the Japan Meteorological Agency draw on waveform templates developed during the Matsushiro swarm studies and on inversion techniques refined alongside work on the Tohoku earthquake and tsunami.

Impact on Infrastructure and Society

The Matsushiro Fault's activity has influenced land use planning, infrastructure resilience, and emergency preparedness in the Nagano Prefecture region. Engineering research at the Public Works Research Institute and universities addressed foundation design, retaining structures, and lifeline vulnerability similar to analyses carried out after the Kobe earthquake and the Chuetsu earthquake. Transportation corridors including sections of the Shinetsu Main Line and local highways have been reassessed for fault crossing hazards, and retrofitting projects for buildings followed guidelines published by the Ministry of Land, Infrastructure, Transport and Tourism. Cultural impacts include documentation efforts by the Japanese Agency for Cultural Affairs to protect heritage sites and community education campaigns coordinated with the Japanese Red Cross Society.

Research and Drilling Studies

Drilling campaigns and borehole logging projects performed in cooperation with the International Continental Scientific Drilling Program partners and Japanese agencies produced core archives analyzed at facilities like the Ocean Research Institute. Geophysical surveys using reflection seismics, magnetotellurics, and microgravity by groups from Kyoto University and Waseda University have imaged fault geometry at depth, complementing strain measurement campaigns using InSAR processed by teams at the Remote Sensing Technology Center of Japan. Collaborative multidisciplinary projects drew expertise from the Japan Agency for Marine-Earth Science and Technology and international collaborators including researchers affiliated with the United States Geological Survey and the Swiss Seismological Service.

Category:Faults of Japan Category:Geology of Nagano Prefecture Category:Seismology