Active Faults and Folds on the Shelf off Niigata and Their Relation to the 1964 Niigata Earthquake

Abstract

Active faults and folds were mapped on the Japan Sea shelf off Niigata in the vicinity of the main shock and aftershock area of the 1964 Niigata earthquake based on air gun, water gun and 3.5kHz seismic profiles. Three types of active faults are defined on the basis of the age of youngest deformed sedimentary unit. Type A faults cut the Holcene sediments and are considered to be active. Type B faults offset the erosional or sedimentary surface of the last glacial age, but Holocene movements is not clear. Type C faults cut Neogene and early Quaternary sediments, but their late Quaternary activity is not clear. Two uplift zones composed of active faults and anticlines were recognized and named the Awashima Uplift Zone and Niigata-oki Uplift Zone. The Awashima Uplift Zone extends for about 40km NE and 40km SSW of the Awashima Island. The location of the uplift generally coincides with the zone of aftershocks and the area uplifted during the 1964 Niigata earthquake, indicating that crustal deformation accompanying repeated Niigata type earthquakes formed this uplift. Air gun seismic profiles show that the uplift is underlain by more than 1500m of Neogene and Quaternary sediments and consists mainly of two major east-vergent anticlines which are cut by west-side-up reverse faults on their eastern limbs. By analogy, the seismic fault producing the 1964 earthquake is presumed to have been a high-angle, west-dipping reverse fault. Several active faults can be sporadically traced for only 5 to 10km, whereas anticlines are more continuous. This strongly suggests that considerable amount of slip at the basement is dissipated by the formation of the anticlines in the thick Neogene and Quaternary sediments, and only part of the slip reaches on the sea-bottom as active faults. The slip rate of about 0.7m/ky estimated at one of the most active faults at the sea floor is a minimum figure of the slip rate of the seismic fault in the basement. The Niigata-oki Uplift Zone extends for 60km along the shelf edge. The last glacial erosional surface is exposed throughout this uplift. Active faults identified by offset of the erosional surface are generally continuous for over 20km, mainly because this uplift zone is underlain by thinner Neogene and Quaternary sediment cover. The dimensions of this uplift zone is comparable to that of the Awashima Uplift Zone, suggesting the possibility that movement of faults generating this uplift could produce another earthquake comparable in size to the Niigata earthquake.