11:00 AM - 1:00 PM
[SSS12-P03] Seismic reflection survey along the eastern margin fault zone of the Yokote Basin, northeast Japan
Keywords:Eastern margin fault zone of the Yokote Basin, Seismic reflection survey, Subsurface structure, Active fault
In this study, we investigated the characteristics of the structure along the eastern margin fault zone of the Yokote Basin (EFZYB), northeast Japan, in order to clarify the differences in the subsurface structure between the northern section, where significant surface ruptures were emerged during the 1896 Rikuu earthquake (Mjma=7.2) (e.g., Yamasaki, 1896; Matsuda et al., 1980), and the southern section, where no surface trail ruptured by the 1896 earthquake have been found. We report the relationship among the seismic reflection profiles obtained in this study, the gravity survey results (Kimura et al., 2021, JpGU) and the distribution of active faults and the 1896 coseismic surface ruptures.
EFZYB is a north-south-trending active fault zone with a total length of about 56 km and the vertical component of slip rate of about 1.0 mm/yr, extending from Senboku city to Yuzawa city, Akita Prefecture in the Tohoku Region (Headquarters for Earthquake Research Promotion, 2005). In the 1896 Rikuu earthquake, a clear surface faulting appeared along the northern part of the fault zone (about 26 km) including Shiraiwa fault, Ota fault, Senya fault, and northern part of Kanezawa fault (e.g., Yamasaki et al., 1896; Matsuda et al., 1980). On the other hand, no significant surface ruptures have been reported in the southern part of this fault zone (about 30 km) including the southern Kanezawa fault, the Sugisawa fault, and the Omoriyama fault.
In this study, we conducted P-wave seismic reflection surveys along a 15 km long survey line. The survey line was set to be almost parallel to the surface trace of EFZYB (Imaizumi et al., 2018). The seismic source is the HEMI-40 manufactured by IVI, USA. Both the standard shot intervals and receiver intervals were 25 m. The sampling interval of seismic data is 2 ms, and more than 460 channels are recorded simultaneously. Stand-alone recorders (Geospace GSX, GSR) were used in the survey. The standard CMP (common midpoint) processing steps were performed before the depth conversion. We could image the subsurface structure to a depth of about 3 km.
The obtained cross sections showed coherent reflections the northern section of the fault, CDP 200-400 up to a depth of 2000 m, and the southern sections, CDP 600-700 and 750-1088 up to a depth of 3000 m. The southern end of the 1896 coseismic surface ruptures corresponds to the vicinity of CDP No. 400, and reflections become weak. In other words, the subsurface structure changes beneath the boundary zone between ruptured and unruptured segments of the fault trace. Comparing the results of seismic reflection survey and the residual gravity anomaly map (Kimura et al., 2021), the area around CDP No. 400 corresponds to the abrupt change zone of gravity anomaly. It is also suggested that the dip of deeper reflectors may change corresponding to the change of the gravity anomaly. Similar to the results of the gravity survey, the seismic reflection survey also revealed that the characteristics of the subsurface structure change in the north-south direction beneath the boundary zone between ruptured and unruptured segments of the fault trace. In the future, we will interpret the results of the seismic reflection survey in more detail with geological information and the previous seismic reflection profiles.
EFZYB is a north-south-trending active fault zone with a total length of about 56 km and the vertical component of slip rate of about 1.0 mm/yr, extending from Senboku city to Yuzawa city, Akita Prefecture in the Tohoku Region (Headquarters for Earthquake Research Promotion, 2005). In the 1896 Rikuu earthquake, a clear surface faulting appeared along the northern part of the fault zone (about 26 km) including Shiraiwa fault, Ota fault, Senya fault, and northern part of Kanezawa fault (e.g., Yamasaki et al., 1896; Matsuda et al., 1980). On the other hand, no significant surface ruptures have been reported in the southern part of this fault zone (about 30 km) including the southern Kanezawa fault, the Sugisawa fault, and the Omoriyama fault.
In this study, we conducted P-wave seismic reflection surveys along a 15 km long survey line. The survey line was set to be almost parallel to the surface trace of EFZYB (Imaizumi et al., 2018). The seismic source is the HEMI-40 manufactured by IVI, USA. Both the standard shot intervals and receiver intervals were 25 m. The sampling interval of seismic data is 2 ms, and more than 460 channels are recorded simultaneously. Stand-alone recorders (Geospace GSX, GSR) were used in the survey. The standard CMP (common midpoint) processing steps were performed before the depth conversion. We could image the subsurface structure to a depth of about 3 km.
The obtained cross sections showed coherent reflections the northern section of the fault, CDP 200-400 up to a depth of 2000 m, and the southern sections, CDP 600-700 and 750-1088 up to a depth of 3000 m. The southern end of the 1896 coseismic surface ruptures corresponds to the vicinity of CDP No. 400, and reflections become weak. In other words, the subsurface structure changes beneath the boundary zone between ruptured and unruptured segments of the fault trace. Comparing the results of seismic reflection survey and the residual gravity anomaly map (Kimura et al., 2021), the area around CDP No. 400 corresponds to the abrupt change zone of gravity anomaly. It is also suggested that the dip of deeper reflectors may change corresponding to the change of the gravity anomaly. Similar to the results of the gravity survey, the seismic reflection survey also revealed that the characteristics of the subsurface structure change in the north-south direction beneath the boundary zone between ruptured and unruptured segments of the fault trace. In the future, we will interpret the results of the seismic reflection survey in more detail with geological information and the previous seismic reflection profiles.