Shinsuke Okada

Slip-partitioned surface ruptures for the Mw 7.0 16 April 2016 Kumamoto, Japan, earthquake

An ENE-trending ~30-km-long surface rupture emerged during the Mwxa0=xa07.0 16 April 2016 Kumamoto earthquake along the previously mapped Futagawa and northern Hinagu faults. This included a previously unknown 5-km-long fault within the Aso Caldera, central Kyushu. The rupture zone is mostly composed of right-lateral slip sections, with a maximum of 2-m coseismic slip. One of the noteworthy features we observed in the field are ~10-km-long segmented normal fault scarps, dipping to the northwest, along the previously mapped Idenokuchi fault, 1.2–2.0xa0km south of and subparallel to the Futagawa fault. The maximum amount of coseismic throw on the Idenokuchi fault is ~2xa0m, which is nearly equivalent to the maximum slip on the strike-slip rupture. The locations and slip motions of the 2016 rupture are also manifested as interferogram fringe offsets in InSAR images. Together with geodetic and seismic inversions of subsurface fault slip, we present a schematic structural model where oblique motion occurred on a northwest-dipping subsurface fault and the slip is partitioned at the surface into strike-slip and normal fault scarps. Our simple dislocation model demonstrates that this bifurcation into pure strike-slip and normal faults likely occurs for optimally oriented failure near the surface. The Kumamoto case, with detailed geological observations and geophysical models, would be the second significant slip-partitioned earthquake around the globe. It provides an important insight into scale- and depth-dependent stress heterogeneity and an implication to a proper estimate of seismic hazard in complex and broad multiple fault strands.Graphical abstract.

The first surface-rupturing earthquake in 20 years on a HERP active fault is not characteristic: the 2014 Mw 6.2 Nagano Event along the Northern Itoigawa–Shizuoka tectonic line

Online Material: Table of fault displacement measured at sites along the surface‐rupture zone of the 2014 Nagano earthquake.nnThe M wxa06.2 ( M wxa06.7) Nagano‐ken‐hokubu earthquake (hereafter Nagano earthquake) struck northern Nagano, central Japan, on 22 November 2014, destroying more than 100 houses and injuring 46 people. The maximum peak ground acceleration of 589xa0Gal was recorded at K‐NET Hakuba station, located 3.3xa0km west of the epicenter (National Research Institute for Earth Science and Disaster Prevention, 2014). However, the areas that suffered from severe shaking, Japan Meteorological Agency (JMA) intensity 6, were limited to a few villages ∼17u2009u2009km from the epicenter. The most striking feature of this earthquake is that more than 9xa0km of complex surface faulting occurred on the previously mapped 26‐km‐long north–northwest‐trending Kamishiro fault, one of the segments of the 150‐km‐long Itoigawa–Shizuoka tectonic line (ISTL) active fault system. This earthquake was the first surface‐breaking earthquake to occur on one of the 110 major inland active faults prioritized for evaluation by the Headquarters for Earthquake Research Promotion (HERP) that was launched in 1995 after the 1995 Kobe earthquake. A 14% probability of a large earthquake of M w∼8.3 occurring within the next 30xa0yrs from the entire ISTL rupture was estimated, the second highest among all the major active faults (HERP, 2014). In addition, the anticipated shaking intensities for the regions around the ISTL have been available to the public since 2005, based on simulated strong ground motion with realistic fault dimensions and fault geometry. The 2014 Nagano earthquake, as a surface‐rupturing event, gave us an opportunity to see whether our forecast, based on geologic, paleoseismic, and seismic engineering, was appropriate or whether it requires strategic improvement for seismic‐hazard assessment in Japan.nnIn this article, we describe the surface ruptures associated with the 2014 Nagano earthquake, mostly along the …

Continuity of subsurface fault structure revealed by gravity anomaly: the eastern boundary fault zone of the Niigata plain, central Japan

We have investigated gravity anomalies around the Niigata plain, which is a sedimentary basin in central Japan bounded by mountains, to examine the continuity of subsurface fault structures of a large fault zone—the eastern boundary fault zone of the Niigata plain (EBFZNP). The features of the Bouguer anomaly and its first horizontal and vertical derivatives clearly illustrate the EBFZNP. The steep first horizontal derivative and the zero isoline of the vertical derivative are clearly recognized along the entire EBFZNP over an area that shows no surface topographic features of an active fault. Two-dimensional density structure analyses also confirm a relationship between the two first derivatives and the subsurface fault structure. Therefore, we conclude that the length of the EBFZNP as an active fault extends to ~56xa0km, which is longer than previously estimated. This length leads to an estimation of a moment magnitude of 7.4 of an expected earthquake from the EBFZNP.Graphical abstract.