Abstract
The Mw 6.2 (Mj 6.8) Nagano (Japan) earthquake of 22 November 2014 produced a 9.3-km long surface rupture zone with a thrust-dominated displacement of up to 1.5 m, which duplicated the pre-existing Kamishiro Fault along the Itoigawa–Shizuoka Tectonic Line (ISTL), the plate-boundary between the Eurasian and North American plates, northern Nagano Prefecture, central Japan. To characterize the activity of the seismogenic fault zone, we conducted a paleoseismic study of the Kamishiro Fault. Field investigations and trench excavations revealed that seven morphogenic paleohistorical earthquakes (E2–E8) prior to the 2014 Mw 6.2 Nagano earthquake (E1) have occurred on the Kamishiro Fault during the last ca. 6000 years. Three of these events (E2–E4) are well constrained and correspond to historical earthquakes occurring in the last ca. 1200 years. This suggests an average recurrence interval of ca. 300–400 years on the seismogenic fault of the 2014 Kamishiro earthquake in the past 1200 years. The most recent event prior to the 2014 earthquakes (E1) is E2 and the penultimate and antepenultimate faulting events are E3 and E4, respectively. The penultimate faulting event (E3) occurred during the period of AD 1800–1400 and is associated with the 1791 Mw 6.8 earthquake. The antepenultimate faulting event (E4) is inferred to have occurred during the period of ca. AD 1000–700, likely corresponding to the AD 841 Mw 6.5 earthquake. The oldest faulting event (E8) in the study area is thought to have occurred during the period of ca. 5600–6000 years. The throw rate during the early Holocene is estimated to be 1.2–3.3 mm/a (average, 2.2 mm/a) with an average amount of characteristic offset of 0.7–1.1 m produced by individual event. When compared with active intraplate faults on Honshu Island, Japan, these slip rates and recurrence interval estimated for morphogenic earthquakes on the Kamishiro Fault along the ISTL appear high and short, respectively. This indicates that present activity on this fault is closely related to seismic faulting along the plate boundary between the Eurasian and North American plates.
Highlights
The Mw 6.2 (Mj 6.8) Nagano (Japan) earthquake of 22 November 2014 ruptured the Kamishiro Fault, which occurs along the northern segment of the Itoigawa– Shizuoka Tectonic Line (ISTL), the plate boundary between the Eurasian and North American plates in central Honshu Island, Japan (Fig. 1; Japan Meteorological Agency 2014; Lin et al 2015a)
Our recent studies on the Kamishiro Fault that triggered the 2014 Mw 6.2 Nagano earthquake reveal (i) a 9.3-km long surface rupture zone with a thrustdominated displacement characterized by distinct fault scarps with vertical offsets of up to 1.5 m, which are mostly duplicated on the pre-existing known Kamishiro Fault and an unknown active fault (Lin et al 2015a), and (ii) at least three large earthquakes that are thought to be associated with surface ruptures of the Kamishiro Fault occurring in the last ca. 1200 years, exhibiting an average recurrence interval of ca. 300–500 years (Lin et al 2015b)
These observations indicate that the occurrence of morphogenic earthquakes along both the northern and southern segments of the Itoigawa–Shizuoka Tectonic Line (ISTL) is characterized by a relatively short time span of 300–500 years, showing a recurrence interval comparable with subduction-zone-type earthquakes, suggesting that the Kamishiro Fault is an active fault developed on the plate boundary between the Eurasian and North American plates, which is the onland extension of the subduction zone along the Suruga Trough (Fig. 1a; Lin et al 2013a)
Summary
The Mw 6.2 (Mj 6.8) Nagano (Japan) earthquake of 22 November 2014 ruptured the Kamishiro Fault, which occurs along the northern segment of the Itoigawa– Shizuoka Tectonic Line (ISTL), the plate boundary between the Eurasian and North American plates in central Honshu Island, Japan (Fig. 1; Japan Meteorological Agency 2014; Lin et al 2015a). Geologic and seismic data suggest that active faults developed on the eastern margins of the Matsumoto and Kamishiro basins have the potential to trigger large earthquakes of Mw >7.7–8.0 (Fig. 2; Headquarters for Earthquake Research Promotion 2000). Our recent studies on the Kamishiro Fault that triggered the 2014 Mw 6.2 Nagano earthquake reveal (i) a 9.3-km long surface rupture zone with a thrustdominated displacement characterized by distinct fault scarps with vertical offsets of up to 1.5 m, which are mostly duplicated on the pre-existing known Kamishiro Fault and an unknown active fault (Lin et al 2015a), and (ii) at least three large earthquakes that are thought to be associated with surface ruptures of the Kamishiro Fault occurring in the last ca. We discuss seismotectonic implications of the ISTL active fault system along the boundary between the Eurasian and North American plates
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