ルソン島中部におけるフィリピン断層の第四紀後期の断層運動

抄録

The Philippine fault zone is one of the most remarkable active faults in the world. It streches on the Philippine archipelago and runs parallel with the Philippine trench. It can be traced for more than 1, 200 km from Luzon Island to the southwest, through Masbate Island, Leyte Island and Mindanao Island (Fig. 15). In central part of the Luzon Island ma ny geomorphic features of late Quaternary displacement are recognized along the fault zone.
The aim of our study is to clarify the nature of the fault zone in central Luzon Island on the basis of the interpretation of topographic maps, vertical aerial photographs and detailed field investigations.
The Philippine fault zone extends with high certainty in the direction of SW-NE more than 100 km long from Dingalan Bay to Lingayen Gulf. It runs across the Shiera Madre Mountains in the southern part. In the northern part of the fault zone, it separates the Cordillera Central Range on the north-eastern side from the Central Valley with the elevation less than 100 m on the southwestern side.
The Philippine fault zone is divided into three fault systems based on the continuity of the fault traces and the strike of the faults. The first system extends from Dingalan Bay to Digdig (fault length ca. 90 km), the second is northeast of Lupao and Umingan (ca. 20km), and the third is San Manuel and its surrounding (ca. 30km) (Figs. 1 and 2).
The main results obtaind follow ;
1) In the study area, various fault topographic features develope along the Philippine fault zone. The southern end of it, from Dingalan Bay to Gabaldon, some lineaments and a number of fault sag develope along it (Fig. 3). The features in the southern part between Gabaldon and Rizal are characterized by displacements of alluvial fans, river terraces and a few offset streams along or near the foot of the Sierra Madre Mountains (Figs, 4, 7 and 8). In the northern part, between Lupao and San Manuel, fault displacements appear sharply just along the foot on the Cordillera Central Range. The fault features are offset streams and fault scarplets on alluvial fans and river terraces (Figs. 9, 10 and 12).
2) The dip of fault plain is vertical (Fig. 6) or high as 60°E on fault outcrops.
3) The sence of horizontal displacement is sinistral in the study area. Vertical one is variable in the southern part, but in the northern part it is downthrown to southwest side (Fig. 13).
4) The ratio of horizontal and vertical displacements id deduced from clear breaks on geomorphic surfaces. While it is 1 : 1 to 3 : 2 in the southern part, but it becomes 8 : 1 in the northern part (Table 1).
5) The average rate of horizontal displacements along the fault is estimated to be 1.5 5mm/year based on the relation between values of horizontal offsets and upstream lengths from the fault. The vertical one which is estimated from the proportion of the vertical displacement to horizontal displacement is calculated to be 0.190.63 mm/year (northern part), and 1.03. 3 to 1.5 5.0mm/year (southern part).
6) It seems that the last earthquake along the fault in the study area took place in 1645 A.D. based on the 14C age data of plant fossil samples which were collected from terrace deposits in some places and the catalogue of historical earthquakes (REPETTI 1946). The magnitude of the earthquake is estimated to be a 8 class and the fault dislocation should appear along the fault.
7) The recurrence interval of earthquakes is calculated as 1, 6005, 300 years from the average slip rate and the displacement of the last earthquake.
8) The fault features and activity of the Philippine fault zone are remarkable, and its nature is similar to that of the faults which are located on other island arcs in the world.