Field Reconnaissance: 20 November 2004

Reconnaissance of Possible Fault-related Surface Deformation

Keith Kelson, Ellen Rathje, and Yuki Yamakawa

GEER Beyond Reconnaissance Team

The reconnaissance team investigated the possibility of fault-related surface deformation (e.g., surface fault rupture, surface folding) along faults previously mapped near the up-dip projection of the rupture plane. To estimate the surface location of possible causative faults, we used data from http://iisee.kenken.go.jp/staff/yagi/eq/20041023/Japan20041023-j.html, which suggests the rupture plane had southeast-vergent reverse fault movement and projects toward the ground surface along the western margin of the Aburuma River valley in the vicinity of the towns of Koide and Hirokami. Based on an existing 1:25,000-scale map of active faults in this area published by the Japan Geographical Survey Institute, this valley margin is bordered by the Obiro and Western Muikamachi Basin faults (Figure 1). These faults exhibit evidence of late Quaternary surface deformation in the form of faulted and folded river terraces. The faults have up-on-the-northwest displacement, which is consistent with the southeast-vergent sense of reverse movement during the earthquake. In map view, these two faults form a left-stepping en echelon pattern, with an active syncline present between the southwestern end of the Obiro fault and the northeastern end of the Western Muikamachi Basin fault.

No prominent evidence of surface deformation was observed during our reconnaissance along the Obiro or Western Muikamachi Basin faults (Figure 1). Several cultural features (e.g., roads, curbs, retaining walls) are present across traces of this fault system that enable an assessment of the presence or absence of fault-related surface deformation. Along the Obiro fault, there was no prominent evidence of fault-related surface cracking or other forms of deformation where the fault crosses Roads 291 and 558, as well as smaller roads in between (Figures 2 and 3). Similarly, there was no prominent evidence of surface deformation across the Western Muikamachi Basin fault where it crosses Roads 291 and 558, and several other smaller paved roads west of Road 230 (Figures 4, 5 and 6). We also observed no surface deformation along the mapped fault trace through the city of Koide. Nevertheless, our reconnaissance was not a comprehensive assessment along the entire length of either of these faults, or of other faults that may have experienced possible surface deformation during the earthquake.

Fig. 1. Map of active faults in the Koide-Hirokami area, showing reconnaissance route taken to assess possibility of surface deformation.

Fig. 2. View to southeast across the Obiro fault and a broad late Quaternary syncline developed on alluvial deposits above the upper Aburuma River valley, near Hironouchi. Syncline is developed between the Obiro and Western Muikamachi faults. No prominent evidence of surface deformation was present across the Obiro fault where it traverses the paved roadway on the near side of building in center of photograph (37.281˚N, 138.960˚E; photo PB200388 by K. Kelson, November 20, 2004).

Fig. 3. View to southeast across the Obiro fault trace and down a moderate-sized alluviated tributary valley near Hironouchi. No prominent evidence of surface deformation was present across the mapped fault trace where it traverses the paved roadway or the brittle retaining walls bordering the valley floor. White instrument on retaining wall near Dr. Y. Yamakawa is one of several seismometers installed across the fault trace by Nagoya University (37.278˚N, 138.952˚E; photo PB200389 by K. Kelson, November 20, 2004).

Fig. 4. Southeast-facing fault scarp along the Western Muikamachi Basin fault, occupied by a collection of cemetery monuments. No evidence of surface deformation was present along this fault scarp, although several monuments toppled (37.279˚N, 138.976˚E; photo PB200371 by K. Kelson, November 20, 2004).

Fig. 5. View to southwest along southeast-facing fault scarp marking the Western Muikamachi Basin fault (as shown in Fig. 4). No evidence of surface deformation was present along this fault scarp, although several monuments toppled (37.279˚N, 138.976˚E; photo PB200380 by K. Kelson, November 20, 2004).

Fig. 6. View to northwest along retaining wall crossing the Western Muikamachi Basin fault, near Highway 291. No evidence of faulting or fold deformation was present along this retaining wall or paved roadway across mapped fault trace (37.279˚N, 138.976˚E; photo PB200381 by K. Kelson, November 20, 2004).