Abstract
On 18 June 2018, an Mw 5.5 inland crustal earthquake occurred in the northern Osaka Prefecture, Japan. This event is one of the most disastrous earthquakes bringing large ground motions with a maximum observed intensity of 6-lower on the Japan Meteorological Agency scale in and around the metropolitan Osaka area. Strong ground motions with larger peak ground accelerations and velocities than those expected by the ground motion model were observed in the south-west direction from the source. In order to understand the strong motion generation process during this event, the source model was estimated on the basis of broadband strong-motion waveform modeling using the empirical Green’s function method. We assumed two fault planes with different fault types, according to both the relocated aftershock distribution and source mechanism solutions. The source model wasrepresented by two patch-shaped strong motion generation areas (SMGAs) to explain the observed ground motions in 0.4–10 Hz. First, the ‘SMGA1’ with reverse slip on the fault segment striking with N356°E was ruptured propagating northward from the hypocenter. Second, the ‘SMGA2’ with strike-slip on the fault segment striking with N49°E was ruptured propagating southwestward from the hypocenter with a delay time of 0.2 sec after starting the rupture on SMGA1. From the ground motion simulations considering these two SMGAs, we found the strong pulse observed in northern Osaka was caused by the forward rupture directivity effect generated on the SMGA2. The stress drops of SMGA1 and 2 were estimated 13.9 and 14.9 MPa, respectively, which are comparable to the average value of the SMGA model for past inland crustal earthquakes in Japan. It is also important to discuss the effect of ground motion amplification due to the Osaka sedimentary basin, for further understanding of large ground motions in northern Osaka caused by this earthquake.
