Seismic displacement and ground motion near a fault: The Saitama Earthquake of September 21, 1931

The dislocation parameters of the Saitama earthquake (M = 7.0, 36.15°N, 139.24°E) of September 21, 1931, are determined on the basis of first-motion data, aftershock area, and close-in seismograms obtained by a low-magnification long-period seismograph. The earthquake represents a left-lateral strike slip faulting on a plane dipping 80° toward N 196°E with dimensions of 20 km (length) × 10 km (width). The strike of the fault plane is found to be almost parallel to that of the eastern extension of the median tectonic line. A synthetic study suggests that the rupture grows bilaterally at a velocity of 2.3 km/s. The rise time, the final dislocation of the linear ramp dislocation time function, and the particle velocity of the fault dislocation are estimated to be 2 s, 100 cm, and 50 cm/s, respectively. The near-source ground displacements and ground motions derived from the above seismic fault model are consistent with high-precision leveling data and with the field survey that determined the directions of collapse of structures.