We found several faulted landforms and an active fault outcrop around the Minobu fault, Yamanashi Pref., central Japan. The Neguma fault may be a reverse fault dislocating a fan surface (not dated) ca. 13 m vertically. Fluvial terrace surfaces at Wada are classified into W1 to W5 surfaces in descending order. It is probable that the W3 surface was formed in the period of MIS 5 to MIS 4. The Wada fault cuts the Neogene and the overlying gravel distributed in almost the same height with the W3 surface. The dip and strike of the fault plane are N5゜E and 50-60゜W, respectively. The striations are plunging to the south at an angle of ca. 20 degree and blow. The relative vertical component is upthrown on the east side. These structures are indicative of left-lateral movement. The maximum accumulated left-lateral slip since MIS 5 to MIS 4 is 100 m at least.
The Kamogawa lowland fault zone had been recognized as a highly active fault zone in the southern Boso peninsula. Many researchers had tried to make clear the activity of this fault zone, however no one could get definite evidence of activity of this fault zone during the Quaternary era. The author tried to describe fundamental features of this fault zone, so carried out re-interpretation of fault landscapes, field geological survey and observation of minor faults in the Neogene deposits around the southern part of the Kamogawa lowland. Four topographic lineaments occur in the study area, they are characterized by series of knick points on hill slopes, and partly characterized by ill-systematic left and partly right-lateral bend of streams. The northern two major lineaments facing north are coincident with geological faults cutting the Neogene deposits with north-side upheaval displacement. The other two lineaments locate south of the Kamogawa lowland, and have left-lateral bend of streams. There are many low-dipping to horizontal slickenlines on the minor fault plain. Many minor faults accompany with hard consolidated fault gauge, but some of minor faults accompany with unconsolidated fault clay and sand. These facts suggest that the two major northern lineaments have contrary sense between topographic features and geological structure, the former suggest north-side subsidence motion in the latest geological age, and the later indicate north-side upheaval movements since the Neogene. Other southern two lineaments have harmonious sense between lateral bend of streams and low angle slickenlines on the minor faults in the Neogene deposits. The author thinks that more researches have to be done to make clear the activity of southern two lineaments during the late Quaternary.
The 16 April 2016 Mw＝7.0 Kumamoto earthquake accompanied ～ 31-km-long surface rupture along the NE part of the Hinagu fault and the Futagawa fault (Kumahara et al, 2016). The surface rupture zone along the Futagawa fault mostly exposed right-lateral strike slip up to 2.2 m (Shirahama et al., 2016), whereas a ～ 10-km-long normal faulting surface rupture with a maximum of 2-m vertical separation mostly along the previously mapped Idenokuchi fault located 1-2 km south of and sub-paralleled to the Futagawa fault. Here we report an outcrop at the oblique-normal faulting surface rupture at the riverbed of the Kanayama River which runs through Shimojin, Mashiki Town. The site is located 300 m south east of the Futagawa fault and a 50 cm vertical slip occurred at the 2016 Kumamoto earthquake. Although we only had a brief time to observe the outcrop due to levee wall construction, we observed a normal fault (f1) responsible for the 16 April earthquake and cuts recent gravel units. Along the f1 strand, we measured 1.5 m of cumulative vertical displacement of the gravel unit including the 2016 slip. A detrital charcoal recovered from the gravel unit yields an age of 1,990-2,300 cal BP. It enables us to estimate a vertical slip rate of 0.65～0.75mm/yr. Thus it is highly likely that the coseismic simultaneous rupture of the both fault might have occurred repeatedly during the recent events.
Many previous studies have revealed distribution of strain rates in the Japanese Islands using data of continuous GNSS station installed since mid 1990’s. They discovered “strain concentration zones” including the Niigata-Kobe Tectonic Zone and the Ou backbone Range in inland and a side of Sea of Japan away from major plate boundaries including the Nankai Trough and Japan Trench. We used GNSS data after the increase of GNSS stations in 2002 and examined distribution of site velocities and strain rates during 2005-2009 with higher spatial resolution. And then, we compared it with major active faults and found that many active faults locate in regions where maximum shear strain rates were high. We also removed elastic deformation due to interplate coupling on the subducting plate interface along the Nankai Trough and compared between distribution of the corrected strain rates and shallow seismicity. The comparison suggests a tendency that the higher maximum shear strain rates, the more frequent shallow M >_ 6 earthquakes occur. We, therefore, suggest that the GNSS data is incorporated into long-term evaluation of large inland earthquakes.