LATE QUATERNARY PALEOSEISMIC HISTORY OF THE UEMACHI BLIND THRUST SYSTEM IN METROPOLITAN OSAKA, JAPAN, BASED ON HIGH-RESOLUTION STRATIGRAPHIC ANALYSIS OF FAULT-PROPAGATION FOLDS

We analyze the growth of the Uemachi fault system in metropolitan Osaka, Japan, by using shallow continuously cored borings and a high-resolution seismic reflection profile to determine the record of large magnitude Late Quaternary earthquakes on blind thrust faults. The 45-km-long, north-trending, multi-segment Uemachi fault system manifests itself as a series of the shallow subsurface flexures that are largely buried. The flexures are forelimbs of active fault-propagation folds formed by west- to northwest-vergent, coseismic slip on blind thrust faults. Late Quaternary stratigraphy in the Osaka Basin is characterized by sequences of alternating marine clays deposited during marine transgressions that are interbedded with coarse fluvial sediment deposited in a floodplain during lowstands. The bases of the two youngest highstand deposits, termed the Ma12 and Ma13 marine clays, are 127 ky and 9 ky in age, respectively. We use a dataset containing several thousand boreholes that penetrate the Ma12 and Ma13 clays across the central Osaka Basin to determine the fault segment geometry and vertical uplift rates across the flexures. The uplift rate for the main Uemachi fault is approximately 0.5 m/ky using the vertical relief and age of the Ma12 clay layer across the flexure, in agreement with 0.4 m/ky for the long-term uplift rate based on vertical offset of the 1.12 Ma Ma0 clay layer found in the OD-1 and OD-2 deep boreholes. An S-wave seismic profile with sub-meter reflector resolution, located along the Yodogawa River in central Osaka, is tied to five continuously cored boreholes (lengths range from 37 to 50 meters) for 250 meters across the forelimb of folded Late Pleistocene to Holocene sediments. Deformation recorded in growth strata suggests that the most recent earthquake on the Uemachi fault occurred between approximately 9,500 yr B.P. and 2,500 yr B.P. resulting in 3 m of uplift. The seismic moment for this event may have been between 6.9 and 7.5 based on our estimates. Assuming that 3 m of uplift occurs in a characteristic earthquake and that the fault maintains a long-term uniform slip rate of 0.4-0.5 m/ky, we estimate that in the last 1.12 Ma, the Uemachi fault may have generated approximately 140-190 earthquakes with a recurrence interval of 6-8 ky.