S. Toda, S. Okada, D. Ishimura, and Y. Niwa International Research Institute of Disaster Science, Tohoku University, Japan

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1 The first surface-rupturing earthquake in 20 years on a HERP major active fault: Mw= Nagano, Japan, event along the Itoigawa-Shizuoka Tectonic Line is not characteristic S. Toda, S. Okada, D. Ishimura, and Y. Niwa International Research Institute of Disaster Science, Tohoku University, Japan Okada et al., 2015, Seismological Research Letters, Vol. 86, No.5

2 First authorized probabilistic seismic hazard map opened in 2005 (10 yr after the Kobe earthquake) 30-yr probabilities of JMA intensity VI lower (~MMI X) > 26% High Moderate 6% - 26% 3% - 6% 0.1% - 3% < 0.1% Paleoseismic trench Pacific Plate Philippine Sea Plate The Headquarters for Earthquake Research Promotion (HERP, 2005)

3 30-yr probs of characteristic earthquakes on major active fault zones The Headquarters for Earthquake Research Promotion (HERP, 2005)

4 30-yr probs of characteristic earthquakes on major active fault zones The Headquarters for Earthquake Research Promotion (HERP, 2005)

5 30-yr probs of characteristic earthquakes on major active fault zones Northern ISTL (E-dipping thrust) Central ISTL (left-lateral slip) Itoigawa- Shizuoka Tectonic Line (ISTL) active fault system Southern ISTL (W-dipping thrust) The Headquarters for Earthquake Research Promotion (HERP, 2005)

6 30-yr probs of characteristic earthquakes on major active fault zones Mw=6.2 Nov. 22, 2014 Nagano-ken-hokubu earthquake (M=6.7) (JMA, 2014) Northern ISTL (E-dipping thrust) Central ISTL (left-lateral slip) Itoigawa- Shizuoka Tectonic Line (ISTL) active fault system Southern ISTL (W-dipping thrust) The Headquarters for Earthquake Research Promotion (HERP, 2005)

7 2014 Nagano-ken-hokubu earthquake struck mountainous areas JMA intensity 7 MMI 6+ 9~ Max. acceleration K-net Hakuba 598 gal Epicenter

8 2014 quake ruptured northern half of the Kamishiro fault, Itoigawa-Shizuoka Tectonic Line active fault system

9 2014 quake ruptured northern half of the Kamishiro fault, Itoigawa-Shizuoka Tectonic Line active fault system

10 2014 quake ruptured northern half of the Kamishiro fault, Itoigawa-Shizuoka Tectonic Line active fault system seismicity during a week prior to the Mw6.2

11 2014 quake ruptured northern half of the Kamishiro fault, Itoigawa-Shizuoka Tectonic Line active fault system Aftershocks during the first 1.5 months

12 2014 quake ruptured northern half of the Kamishiro fault, Itoigawa-Shizuoka Tectonic Line active fault system Aftershocks during the first 1.5 months first 24 hr

13 N S Stream

14 Downstream Courtesy of Yasuhira Aoyagi

15 More than 9 km of complex surface faulting occurred on the previously mapped Kamishiro fault

16 Satellite dir. Beam InSAR analyses show densely spaced fringes on the hanging wall side, suggesting westward or uplift movement Beam Satellite dir. 10 km Close to satellite Away from satellite Displacement between satellite and ground (cm)

17 Vertical displacement to horizontal shortening N S Buckled object Contraction 30 cm Horinouchi Contraction 55 cm E W Contraction 40 cm Overlapped drainage gutter

18 Vertical displacement to horizontal shortening Contraction 30 cm Horinouchi Contraction 55 cm E W Contraction 40 cm

19 Why did the significant damage only occur at Horinouchi?

20 Why did the significant damage only occur at Horinouchi? Surface rupture

21 Why did the significant damage only occur at Horinouchi? Surface rupture

22 Loose and soft lacustrine sediment deposited in the paleo-kamishiro Lake on the hanging wall would be the reason Hakuba basin Okumura et al Kamishiro basin 2

23 Surface faulting transverses a residential area and caused serious damage to several houses

24 Surface faulting transverses a residential area and caused serious damage to several houses

25 Surface faulting transverses a residential area and caused serious damage to several houses

26 Predictability of rupture location by the pre-existing fault landforms 2014 rupture Sawa & Togo et al., 1999 Suzuki et al., 2010 Okada et al., 2015

27 Predictability of rupture location by the pre-existing fault landforms 2014 rupture Sawa & Togo et al., m Suzuki et al., 2010 Okada et al., 2015

28 Predictability of rupture location by the pre-existing fault landforms 2014 rupture Sawa & Togo et al., m Suzuki et al., 2010 Okada et al., 2015

29 The 2014 earthquake contributed to the growth of the up-hill facing scarp, part of which had been identified before the earthquake

30 Post-seismic geologic survey revealed frequent movements of the back-thrusts

31 Lacustrine sediments lower the fault dip and makes the rupture location unpredictable West Hakuba trench (Okumura et al., 1998; 2001) Tilted alluvial surface Restored topographic profile Flexure scarp 30 m East m a.s.l Artificial fill, paddy soil Lacustrine sand Black clay Peat, peaty clay Lacustrine silt Sand black clay 732 Gravel Fault

32 Lacustrine sediments lower the fault dip and makes the rupture location unpredictable 2014 rupture West Hakuba trench (Okumura et al., 1998; 2001) Tilted alluvial surface Restored topographic profile Flexure scarp 30 m East m a.s.l Artificial fill, paddy soil Lacustrine sand Black clay Peat, peaty clay Lacustrine silt Sand black clay 732 Gravel Fault

33 Deformed sediments indicate 1,000-1,500-yr recurrence intervals

34 We overestimated size of the earthquake on northern ISTL but large quakes on the ISTL might be much more frequent Geological Survey of Japan (2005) Suzuki et al. (2010) Headquarters for Earthquake Research Promotion (2013)

35 We overestimated size of the earthquake on northern ISTL but large quakes on the ISTL might be much more frequent Geological Survey of Japan (2005) Suzuki et al. (2010) Headquarters for Earthquake Research Promotion (2013) Observation (JMA, 2014)

36 Why did previous studies overestimate the size of the surface- rupturing earthquake? 1. Insufficient paleoseismic data lead us either overestimated the slip rate or overlooked some paleoseismic events. 2. Thinner seismogenic layer limits the size of large earthquakes but facilitates the surface faulting process. ~50,000 ybp ~70,000 ybp Tephra ~27,000 ybp Shimokawa & Yamazaki(1987)

37 Summary The 2014 Mw=6.2 Nagano earthquake is the first surfacerupturing earthquake to have occurred on the Itoigawa- Shizuoka Tectonic Line active fault system, one of the 110 major active faults evaluated by HERP since About 9-km-long surface rupture mostly reoccupied the fault scarp on the previously mapped 26-km-long Kamishiro fault, but half of the trace were not well predicted. The 2014 shock is not characteristic : There would be some diversity of large surface-breaking earthquakes not only on a long fault system but also on a single discrete fault segment.