Simultaneous inversion of waveform and geodetic data for the rupture process of the 1984 Naganoken–Seibu, Japan, earthquake

SUMMARY A simultaneous inversion method of strong motion seismograms and geodetic data for inferring earthquake rupture process was developed. We assumed a source model composed of many point‐sources of which slip vectors and rupture times are unknown parameters. In addition to observed data, we introduced smoothing constraints to determine many parameters without instability. Since the least‐squares solution depends on the relative weights of the smoothing constraints to a data misfit function, we determined their values by minimizing Akaike's Bayesian information criterion (ABIC). By using ABIC, we can objectively choose a unique model. This inversion method was applied to the Naganoken–Seibu, Japan, earthquake of 1984. Dividing a 15 times 10 km fault into 150 subfaults, we obtained the minimum ABIC model with a seismic moment of 2.7 times 10 25 dyne cm. This model has a slip peak in the eastern and shallow area of the assumed fault. The high slip area is just under the areas where thrown‐out boulders were found by Umeda et al. (1987) and Iio & Yoshioka (1988). Comparing the aftershock distribution with the estimated slip distribution, we found that the high slip area had few aftershocks. This suggests that aftershocks are caused by the stress generated in the marginal region around the main shock slip. The inversion analysis also revealed that there was a slight change in slip angle during the earthquake. The high slip area has some amount of dip–slip component while the slips in the other areas are almost purely right‐lateral.