Surface Rupture Connecting Deep-Seated Landslides of the Mw6.9 14 June 2008 Iwate- Miyagi Nairiku, NE Japan, Earthquake

The Mw 6.9 Iwate-Miyagi Nairiku earthquake hit mountain regions in northern Honshu Island, and was accompanied by ~20 km of a NNW trending surface rupture and by a lot of landslides. Morphological features show that the surface ruptures are mainly thrust faulting which is consistent with the focal mechanism of the mainshock under the WNW-ESE compressional stress field in northern Honshu. Amounts of vertical offset and horizontal shortening are mostly smaller than 50 cm (~ 1m of net slip) (See details in Toda et al., 2008, in this volume). Some of the surface ruptures neighbored deep-seated landslides. Meanwhile, near the southern end of the rupture zone, we found N-E-striking surface rupture with exceptionally large offset connecting two massive deep-seated landslides with volumes more than 100,000 m3. The backscarps are more than 100 m high, as high as the differences in elevation of the ridges this rupture cut, implying the landslides been triggered by faulting rather than shaking. Airborne and ground-based LiDAR measurements together with our field observations reveal typical features of strike-slip faulting such as mole tracks, fissures, pressure ridges, bulges, and shutter ridges as well as the offset rills and ridges (See details in Maruyama et al., 2008, in this volume). The rupture consists of a E-W- striking ~1-km-long continuous rupture involved with 4-to-7-m dextral and 2-to-4-m vertical offsets of a paved road, trails, and rills, a N-S-striking W-side-up 0.2-km-long thrust rupture forked off at the center of the E-W- striking rupture, a NE-SW-striking NW-side-up 0.15-km-long dextral pressure ridge branched off at eastern E- W-striking scarp, and NW-SE-trending 0.1-km-long cracks branched out toward north at the center of the E- W-striking rupture. Another NW-SE-striking cracks extend to a backscarp at the eastern termination of the E- W-striking rupture. This detail mapping of the ruptures provides that this strike-slip dominant fault plays a role of a lateral ramp connecting two distinct NNE-trending thrust faults, and that NW-SE-trending cracks adjust displacement field. Since the backscarps trend NW-SE at the both termination of the surface rupture, it implies that NW-SE- striking tension cracks might have been formed at the both terminations of the rupture and triggered the giant landslides.