**3. Active tectonics and geophysical data related to present Lake Basin**

At Lake Biwa, plate motion-related basin formation can be generalized because the Lake Biwa basin formation apparently resulted from subduction of the Philippine Sea plate into the Eurasian plate. Tectonic approaches on the Lake Biwa sediment therefore provide a case study for basin-forming mechanisms related to plate subduction activity, which is applicable to other active subduction zones throughout the world.

Lake Biwa is the largest and oldest fresh-water lake in Japan. Its surrounding geology comprises Mesozoic-Paleozoic formations, Cretaceous Granitic Rocks and Volcanic Rocks, Miocene sedimentary rocks, Plio-Pleistocene ancient lake sediments (Kobiwako Group in Paleo-lake Biwa), terrace deposits, and alluvium.

The present Lake Biwa, surrounded by several active faults, is a tectonic basin with a long history extending from about 1.5 Ma to the present day. Historical earthquake records exist around Lake Biwa region. Lake Biwa is located in the southwestern Japan, which is considered in the area under the east-west compressional stress state from geophysical, topographical and Quaternary geological information [16]. Huzita [16] reported a triangular neotectonic province designated as "Kinki Triangle" (Figure 11 inset) with the E-W compressional stress state and the basin and mountain topography from east to west in the district. In central Japan, including the mountainous region in Chubu district (east of Kinki district) and Chugoku district (west of Kinki district), we can recognize the clear conjugate fault system, and active faults with NW-SE direction of left lateral transcurrent component, and those with NE-SW direction of right lateral component, and also in Kinki district, reverse fault system with N-S direction are recognized accompanied with transcurrent fault system (Figure 5). This evidence implies that this area is influenced by the E-W compressional stress state. The force from the Pacific Plate movements and oblique subduction movements of the Philippine Sea Plate influences the Kinki District. According to the combination of both forces, E-W compressional stress occurs in southwestern Japan. Lake Biwa region is located at northern part of "Kinki Triangle" region. Most active and highest rate of activity active fault in this region is the Biwako-seigan Fault zone, which runs along the west coast of Lake Biwa with reverse faulting. Biwako-seigan Fault zone is about 59 km long with reverse fault sense of east side subsidence. The activity of southern part of the fault is calculated from about 1.4 m/kyr of average movement and recurrent interval is calculated as about 4.5 – 6.0 kyr with latest event of 1185 AD. Relative movement of single event is inferred about 6-8 m [17].

MTL: Median Tectonic Line, ATL: Arima Takatsuki Tectonic Line, R-A: Rokko-Awaji fault zone, I: Ikoma Fault zone, K: Kambayashi Fault, H: Hanaore Fault, B: Biwako-seigan Fault zone, Ya: Yanagase Fault zone, Yo: Yoro Fault zone

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**Figure 6.** Seismic reflection survey (example of long section of Line 9) (drawn from [19])

**Figure 5.** Active fault system in Kinki district [18]

Deep seismic reflection survey and gravity measurement through the 1970s and early 1980s yielded information related to the basement topography (Figures 6-9), and revealed a tilting structure from east to west (Figure 8). However, the deepest part is located in the Northern Lake basin ("Hokko basin"), where Prof. Horie's team drilled in the early 1980s from the seismic reflection survey. A seismic reflection survey revealed the existence of active fault in the deep basin. The lithostratigraphical information indicated the subsidence history of the present lake basin during about 1.5 my. The recent subsidence rate is calculated as 0.74-m/kyr by data of the T bed (thickness 250 m, duration 430 kyr, water depth 68 m), because T bed is the sediment under pelagic environment. Shallow seismic reflection surveys conducted in the Tectonic Basin Formation in and Around Lake Biwa, Central Japan http://dx.doi.org/10.5772/56667 215

MTL: Median Tectonic Line, ATL: Arima Takatsuki Tectonic Line, R-A: Rokko-Awaji fault zone, I: Ikoma Fault zone, K: Kambayashi Fault, H: Hanaore Fault, B: Biwako-seigan Fault zone, Ya: Yanagase Fault zone, Yo: Yoro Fault zone

**Figure 5.** Active fault system in Kinki district [18]

**3. Active tectonics and geophysical data related to present Lake Basin**

214 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

active subduction zones throughout the world.

Paleo-lake Biwa), terrace deposits, and alluvium.

movement of single event is inferred about 6-8 m [17].

At Lake Biwa, plate motion-related basin formation can be generalized because the Lake Biwa basin formation apparently resulted from subduction of the Philippine Sea plate into the Eurasian plate. Tectonic approaches on the Lake Biwa sediment therefore provide a case study for basin-forming mechanisms related to plate subduction activity, which is applicable to other

Lake Biwa is the largest and oldest fresh-water lake in Japan. Its surrounding geology comprises Mesozoic-Paleozoic formations, Cretaceous Granitic Rocks and Volcanic Rocks, Miocene sedimentary rocks, Plio-Pleistocene ancient lake sediments (Kobiwako Group in

The present Lake Biwa, surrounded by several active faults, is a tectonic basin with a long history extending from about 1.5 Ma to the present day. Historical earthquake records exist around Lake Biwa region. Lake Biwa is located in the southwestern Japan, which is considered in the area under the east-west compressional stress state from geophysical, topographical and Quaternary geological information [16]. Huzita [16] reported a triangular neotectonic province designated as "Kinki Triangle" (Figure 11 inset) with the E-W compressional stress state and the basin and mountain topography from east to west in the district. In central Japan, including the mountainous region in Chubu district (east of Kinki district) and Chugoku district (west of Kinki district), we can recognize the clear conjugate fault system, and active faults with NW-SE direction of left lateral transcurrent component, and those with NE-SW direction of right lateral component, and also in Kinki district, reverse fault system with N-S direction are recognized accompanied with transcurrent fault system (Figure 5). This evidence implies that this area is influenced by the E-W compressional stress state. The force from the Pacific Plate movements and oblique subduction movements of the Philippine Sea Plate influences the Kinki District. According to the combination of both forces, E-W compressional stress occurs in southwestern Japan. Lake Biwa region is located at northern part of "Kinki Triangle" region. Most active and highest rate of activity active fault in this region is the Biwako-seigan Fault zone, which runs along the west coast of Lake Biwa with reverse faulting. Biwako-seigan Fault zone is about 59 km long with reverse fault sense of east side subsidence. The activity of southern part of the fault is calculated from about 1.4 m/kyr of average movement and recurrent interval is calculated as about 4.5 – 6.0 kyr with latest event of 1185 AD. Relative

Deep seismic reflection survey and gravity measurement through the 1970s and early 1980s yielded information related to the basement topography (Figures 6-9), and revealed a tilting structure from east to west (Figure 8). However, the deepest part is located in the Northern Lake basin ("Hokko basin"), where Prof. Horie's team drilled in the early 1980s from the seismic reflection survey. A seismic reflection survey revealed the existence of active fault in the deep basin. The lithostratigraphical information indicated the subsidence history of the present lake basin during about 1.5 my. The recent subsidence rate is calculated as 0.74-m/kyr by data of the T bed (thickness 250 m, duration 430 kyr, water depth 68 m), because T bed is the sediment under pelagic environment. Shallow seismic reflection surveys conducted in the

**Figure 6.** Seismic reflection survey (example of long section of Line 9) (drawn from [19])

1980s, 1990s and 2007 show the distribution of active fault traces and size of displacement accompanied with the activity of Biwako-seigan Fault zone along the coast of Lake Biwa.

**Figure 8.** Tilting structure from east to west revealed by sediment structure including horizon A indicating the boun‐

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A gravity survey revealed that the lowest Bouguer anomaly area is in the north lake basin. We show the Bouguer gravity anomaly map around our study area (Figure 10). This Bouguer gravity anomaly map is based on gravity mesh data reported by [20]. The Bouguer density is

Gravity anomaly in this area is characterized by the negative gravity anomalies caused by intra-arc sedimentary basins and isostasy attributable to the loading of mountains in central Japan. In this region, the Nohbi Plain, the Ise Plain, the Ohmi Plain including Lake Biwa, the Kyoto Basin, the Nara Basin, the Osaka Plain, the Osaka Bay, and the Sanda Basin are distrib‐ uted from east to west. Each basin or plain has negative gravity anomalies correspond individually to intra-arc basins. Almost all local negative gravity anomalies in these negative anomalies are included to the active tectonic zone during the Quaternary called "Kinki-

Two large negative gravity anomaly areas exist in the "Kinki-Triangle" area. They are the Osaka Bay area and Lake Biwa area (the Ohmi Plain). Negative gravity anomalies around the

Negative gravity anomalies in the Osaka Bay area are known to be explainable by sediments accumulated in and around the Osaka Bay (e.g., [22]; [23]). These negative gravities are divided by some active faults. In contrast, negative gravity anomalies in the Lake Biwa area are known to be unexplainable using the distribution of soft sediments in the lake (e.g., [24]). Nishida et al. [24] reported that depression of the Conrad surface or existence of very low density materials because of faulting is necessary to explain the gravity lows reaching -60 mGal.

Osaka Bay and the Lake Biwa respectively reach -15mGal and -60 mGal.

dary of T and S bed [5]. Survey Line is shown in Figure 2.

2670 kg/m3

.

Triangle" (e.g., [21] Figure 11inset).

**Figure 7.** Basement structure revealed by seismic reflection survey [19] Meshed part: depth of basement more than 1, 000m. Dotted part: depth of basement lower than 500-m.

1980s, 1990s and 2007 show the distribution of active fault traces and size of displacement

**Figure 7.** Basement structure revealed by seismic reflection survey [19] Meshed part: depth of basement more than 1,

000m. Dotted part: depth of basement lower than 500-m.

accompanied with the activity of Biwako-seigan Fault zone along the coast of Lake Biwa.

216 Mechanism of Sedimentary Basin Formation - Multidisciplinary Approach on Active Plate Margins

**Figure 8.** Tilting structure from east to west revealed by sediment structure including horizon A indicating the boun‐ dary of T and S bed [5]. Survey Line is shown in Figure 2.

A gravity survey revealed that the lowest Bouguer anomaly area is in the north lake basin. We show the Bouguer gravity anomaly map around our study area (Figure 10). This Bouguer gravity anomaly map is based on gravity mesh data reported by [20]. The Bouguer density is 2670 kg/m3 .

Gravity anomaly in this area is characterized by the negative gravity anomalies caused by intra-arc sedimentary basins and isostasy attributable to the loading of mountains in central Japan. In this region, the Nohbi Plain, the Ise Plain, the Ohmi Plain including Lake Biwa, the Kyoto Basin, the Nara Basin, the Osaka Plain, the Osaka Bay, and the Sanda Basin are distrib‐ uted from east to west. Each basin or plain has negative gravity anomalies correspond individually to intra-arc basins. Almost all local negative gravity anomalies in these negative anomalies are included to the active tectonic zone during the Quaternary called "Kinki-Triangle" (e.g., [21] Figure 11inset).

Two large negative gravity anomaly areas exist in the "Kinki-Triangle" area. They are the Osaka Bay area and Lake Biwa area (the Ohmi Plain). Negative gravity anomalies around the Osaka Bay and the Lake Biwa respectively reach -15mGal and -60 mGal.

Negative gravity anomalies in the Osaka Bay area are known to be explainable by sediments accumulated in and around the Osaka Bay (e.g., [22]; [23]). These negative gravities are divided by some active faults. In contrast, negative gravity anomalies in the Lake Biwa area are known to be unexplainable using the distribution of soft sediments in the lake (e.g., [24]). Nishida et al. [24] reported that depression of the Conrad surface or existence of very low density materials because of faulting is necessary to explain the gravity lows reaching -60 mGal.

Kobiwako, and Osaka Groups. Comparing of the sedimentary basin transition between Paleo-Lake Biwa and Lake Tokai, the geohistory in central Kinki Region is known subdivisible by tectonosedimentary facies of about 3.0 Ma and 1.2-1.5 Ma (Figures 12, 13) which suggests that the change of tectonic stress state of this province took place simultaneously throughout these sedimentary basins. To complete the geohistory of Paleo-lake Biwa, patterns of sedimentary basin transition must be discussed. According to Yokoyama ([26], [27]), the geohistory of Paleo-Lake Biwa has four stages: Older I, Older II, Actual I, and Actual II. First, Paleo-Lake Biwa appeared in the Iga Basin and clay-dominant sediments were deposited in the water body ("Iga-ko") (a and b in Figure 12) [28]. In the second, the sedimentary basin center shifted its place to the north from Iga Basin to Ohmi Basin, forming a stable water body ("Sayama-ko") with massive clay deposition (c and d in Figure 12). Northward shifting of the sedimentary basin was inherited (Older II Stage) (e and f in Figure 13). During the time from Older II stage to Actual I stage, the center of the sedimentary basin migrated northwestward on a large-scale. Great amounts of gravel are represented as final sediments of Older II stage (g in Figure 13). The sedimentary basin of Actual I stage shifted its place gradually to west, accompanied by upheaval of the eastern mountain area. This explanation is supported by results of lithology, paleocurrent and sedimentological studies of the deposits of the Kosei area and data from 1,000

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**Figure 10.** Bouguer gravity anomaly map. The Bouguer density is 2670 kg/m3, and contour interval is 2 mGal

**Figure 9.** Active structure revealed by horizon A (Boundary of T and S bed) since about 0.43 Ma) of the seismic reflec‐ tion survey [19] Meshed part: depth of A horizon more than 400 m, Dotted part: depth of A horizon lower than 300 m
