Co-seismic surface ruptures produced by the 2014 Mw 6.2 Nagano earthquake, along the Itoigawa–Shizuoka tectonic line, central Japan

• The 2014 Mw 6.2 Nagano earthquake produced a 9.3-km-long co-seismic rupture zone.
• The co-seismic surface rupture zone duplicated the pre-existing active faults.
• A thrust-dominated displacement occurred along the seismogenic fault.
• The present-day shortening strain on the plate boundary is released by thrusting.

Field investigations reveal that the Mj 6.8 (Mw 6.2) Nagano (Japan) earthquake of 22 November 2014 produced a 9.3-km-long co-seismic surface rupture zone. The slip occurred on the pre-existing active Kamishiro Fault, which developed along the Itoigawa–Shizuoka tectonic line, and is inferred as the boundary between the Eurasian and North American plates. The surface-rupturing earthquake produced dominant thrusting and subordinate strike-slip displacement. Structures that developed during the co-seismic surface rupture include thrust faults, fault scarps, en-echelon tension cracks, folding structures such as mole tracks and flexural folds, and sand-boils. The surface displacements measured in the field range from several centimeters to 1.5 m in the vertical (typically, 0.4–1 m), accompanied by a strike-slip component that reached 0.6 m along NNE trending ruptures. These observations indicate a thrust-dominated displacement along the seismogenic fault. Our results show that (i) the pre-existing Kamishiro Fault, which strikes NNE–SSW, controlled the spatial distribution of co-seismic surface ruptures and displacements; and (ii) the style and magnitude of thrust displacements indicate that the present-day shortening strain on the Eurasian–North American plate boundary in the study area is released mainly by seismic thrust displacements along the active Kamishiro Fault.