10:45 AM - 12:15 PM
[SSS13-P08] Lateral extension of a blind active fault beneath Shonai plain, Yamagata Prefecture, northeast Honshu, Japan
Keywords:active fault, blind thrust, Shonai plain, gravity anomaly
Long-term evaluations of the probability of occurrence of shallow crustal earthquakes on land are mainly based on the activity history of active faults. On the other hand, many of the recent earthquakes on land in the Japan island arc are believed to be caused by the activity of blind faults, whose activity histories are difficult to obtain. Seismic reflection surveys have revealed the presence of blind faults beneath the Shonai Plain in Yamagata Prefecture, northeast Honshu, Japan, which are in the back arc of Tohoku Honshu. It has been pointed out that this fault may be the source fault of the Shonai earthquake. The purpose of this study is to clarify the fault structure and lateral continuity of this blind fault by gravity survey.
The geology of the Shonai Plain and the western part of Dewa Hills to the east is composed of Miocene to Pleistocene formations, which are the Miocene Aosawa Formation consisting of basaltic lava and homogeneous pyroclastic rocks, the Kusanagi Formation mainly of siliceous siltstone, the Kitamata Formation mainly of mudstone, the Pliocene Tateyama Formation of mudstone, the Maruyama Formation of siltstone, the Kannonji Formation of silty fine-grained sandstone, and the Pleistocene Shonai Group of gravel, sand and mud, in ascending order. From the western part of the Dewa Hills to the eastern margin of the Shonai Plain, from the east, the Aosawa Faults, the Sakata Thrusts, and the Shonai Plain Eastern Marginal Fault Zone (Kannonji Fault, Torigoe Fault, and Matsuyama Fault) develop with associated folds, which generally trend in a north-south direction. The blind faults targeted in this study lie beneath the Shonai Plain to the west of these faults. According to Togo et al. (2007), the Shonai Plain has a part where the alluvial plane is displaced, indicating an anticline structure beneath the plain.
Gravity surveys were carried out along three survey lines, which were set roughly in the east-west direction. They are called the Yamatate survey line, the Mogami river survey line, and the Amarume survey line in order from the north. The interval in the north-south direction between survey lines is about 3 km. The standard interval of the gravity measurement points is 200 m, and the gravity measurement points on all survey lines are 42, 116, and 57 points, respectively. The gravimeter used was a Sintrex gravimeter CG-5, and the locations and elevations of observation points were measured using RTK-GPS and an electronic level. Data were processed according to the Geological Survey of Japan (2004). The assumed density used for Bouguer and terrain corrections was 2.10 g/cc.
On the Amarume survey line, the Bouguer anomaly increases by about 12 mGal toward the east. However, the rate of increase is not uniform, and compared to the overall average increase trend, the graph shows an upward convex shape with a maximum of about 3 mGal near 5500 m from the west end of the survey line. The Bouguer anomaly also increases eastward on the Mogamigawa line, and the difference between the western and eastern ends of the line is about 55 mGal. To the east of the Kannonji fault on the western edge of Dewa Hills, the Bouguer anomaly increases stepwise across the Kanonji fault, the Sakata Thrusts, and the Aosawa Faults. In the Shonai Plain on the western side of the Kanonji fault, there is a convex increase of about 2 mGal at maximum compared to the overall increasing trend, though not as much as the Amarume line. On the Yamatate survey line, the increasing trend of the Bouguer anomaly toward the east is similar. However, the upwardly convex portion of the increasing trend is less than 1 mGal in the Shonai Plain. This indicates the possibility that the displacement of underground formations caused by the blind fault on the Amarume survey line decreases toward the north, and that the northern end of the blind fault lies slightly north of the Yamadate survey line. In the presentation, we will also report the estimation results of the subsurface geological structure and fault structure corresponding to these Bouguer anomalies.
References
Geological Survey of Japan, 2004, Gravity CD-ROM of Japan, ver. 2.
Togo, M., 2007, Bull. Hosei Univ. at Tama, 22, 1-8.
The geology of the Shonai Plain and the western part of Dewa Hills to the east is composed of Miocene to Pleistocene formations, which are the Miocene Aosawa Formation consisting of basaltic lava and homogeneous pyroclastic rocks, the Kusanagi Formation mainly of siliceous siltstone, the Kitamata Formation mainly of mudstone, the Pliocene Tateyama Formation of mudstone, the Maruyama Formation of siltstone, the Kannonji Formation of silty fine-grained sandstone, and the Pleistocene Shonai Group of gravel, sand and mud, in ascending order. From the western part of the Dewa Hills to the eastern margin of the Shonai Plain, from the east, the Aosawa Faults, the Sakata Thrusts, and the Shonai Plain Eastern Marginal Fault Zone (Kannonji Fault, Torigoe Fault, and Matsuyama Fault) develop with associated folds, which generally trend in a north-south direction. The blind faults targeted in this study lie beneath the Shonai Plain to the west of these faults. According to Togo et al. (2007), the Shonai Plain has a part where the alluvial plane is displaced, indicating an anticline structure beneath the plain.
Gravity surveys were carried out along three survey lines, which were set roughly in the east-west direction. They are called the Yamatate survey line, the Mogami river survey line, and the Amarume survey line in order from the north. The interval in the north-south direction between survey lines is about 3 km. The standard interval of the gravity measurement points is 200 m, and the gravity measurement points on all survey lines are 42, 116, and 57 points, respectively. The gravimeter used was a Sintrex gravimeter CG-5, and the locations and elevations of observation points were measured using RTK-GPS and an electronic level. Data were processed according to the Geological Survey of Japan (2004). The assumed density used for Bouguer and terrain corrections was 2.10 g/cc.
On the Amarume survey line, the Bouguer anomaly increases by about 12 mGal toward the east. However, the rate of increase is not uniform, and compared to the overall average increase trend, the graph shows an upward convex shape with a maximum of about 3 mGal near 5500 m from the west end of the survey line. The Bouguer anomaly also increases eastward on the Mogamigawa line, and the difference between the western and eastern ends of the line is about 55 mGal. To the east of the Kannonji fault on the western edge of Dewa Hills, the Bouguer anomaly increases stepwise across the Kanonji fault, the Sakata Thrusts, and the Aosawa Faults. In the Shonai Plain on the western side of the Kanonji fault, there is a convex increase of about 2 mGal at maximum compared to the overall increasing trend, though not as much as the Amarume line. On the Yamatate survey line, the increasing trend of the Bouguer anomaly toward the east is similar. However, the upwardly convex portion of the increasing trend is less than 1 mGal in the Shonai Plain. This indicates the possibility that the displacement of underground formations caused by the blind fault on the Amarume survey line decreases toward the north, and that the northern end of the blind fault lies slightly north of the Yamadate survey line. In the presentation, we will also report the estimation results of the subsurface geological structure and fault structure corresponding to these Bouguer anomalies.
References
Geological Survey of Japan, 2004, Gravity CD-ROM of Japan, ver. 2.
Togo, M., 2007, Bull. Hosei Univ. at Tama, 22, 1-8.