Shigeru Toda

Geologic fault model based on the high-resolution seismic reflection profile and aftershock distribution associated with the 2004 Mid-Niigata Prefecture earthquake (M6.8), central Japan

The Mid-Niigata Prefecture earthquake in 2004 (MJMA 6.8) generated surface ruptures along the eastern rim of the Uonuma Hills. To elucidate the structural linkage between the surface ruptures and the source fault at depth, the high-resolution seismic reflection profile across the surface ruptures and nearby active faults, and the data of aftershock distribution are examined. The 5.2-km-long, high-resolution, depth-converted seismic section reveals an emergent thrust beneath the surface ruptures. A two-dimensional model of the fault geometry has been constructed based on the aftershock distribution and the shallow reflection profile. The development of the main geologic structure are well explained by forward modeling using a balanced cross-section method. In detail, the fault system generated the main shock dips at a steep angle (60°) below 5 km depth and more shallowly (30°) near the surface.

Weighted stack of shallow seismic reflection line acquired in downtown Osaka City, Japan

Abstract A 3.5-km-long shallow seismic reflection profile was acquired along Yodo riverbank across the active Uemachi fault, Osaka City, Japan. The study aimed to explore the subsurface geological structures in and around the Uemachi active fault system and to assess the feasibility of using reflection seismology to investigate subsurface structure in urban areas where large amount of cultural noise is expected to interfere with the seismic signal. Field recording parameters were adapted to the accessibility limitation and safety precautions in the study area. The quality of collected data was marginal and the signal to noise (S/N) ratio was considerably low. The data processing involved intensive frequency–wave number (f–k) filtering and summing adjacent common midpoints (CMPs) before stack as well as weighted stacking. The f–k filtering removed most of the ground roll and guided waves while summation of adjacent common midpoints increased the fold and enhanced the apparent signal to noise ratio of the final section. Trace weighting before stack was remarkably effective in increasing the identity and continuity of reflections. The visual and statistical comparison between weighted stack and the basic stack proved that weighted stacking produces significantly more coherent section. The interpretation of the final seismic section and the correlation with the borehole data in the vicinity confirmed the location of the Uemachi fault plane. The fault is shown in the seismic section as a reverse fault that dips to the east with an angle of about 75°. The seismic line also revealed the possibility of the presence of other two shallow faults on the hanging wall of the Uemachi fault. The two faults are dipping to the same dip direction of Uemachi fault with slightly larger dip angle.

Deep Reflection Imaging beneath the Mizuho Plateau, East Antarctica, by SEAL-2002 Seismic Experiment

A seismic exploration was conducted on the Mizuho Plateau, East Antarctica, during the 2001/2002 austral summer season as the “Structure and Evolution of the East Antarctic Lithosphere (SEAL)” project by the 43rd Japanese Antarctic Research Expedition (JARE-43). The survey line of this exploration (SEAL-2002 profile) was almost perpendicular to the Mizuho inland traverse routes (JARE-41 refraction survey line; SEAL-2000) and was almost parallel to the coastal line along the Lutzow-Holm Bay. Several seismic shot records were obtained with clear arrivals of phases until a distance of 150 km in length. We have analyzed two shot data of both ends of the SEAL-2002 profile by using the conventional reflection method. Interval velocities were estimated by applying the normal-move-out (NMO) correction, then the obtained single-fold section obtained explicitly presents the horizontal reflectors originated from the middle crust, the lower crust and the Moho discontinuity. First, the reflector from the top of the middle crust was located at the depth of 23–24 km, which was corresponding to 8–9 s of two way travel time (TWT) in the single-fold section. Next, the reflector from the top of the lower crust was located at a depth of 31–34 km, corresponding to 11–12 s of TWT. The Moho reflector was observed in 13–14 s of TWT and the depth was estimated to be approximately 41–42 km.