11:15 AM - 11:30 AM
[SCG50-09] Relationship between the Northern Osaka Prefecture earthquake and the Arima-Takatsuki Tectonic Line estimated from dense aftershock observations
Keywords:Intraplate earthquake, Active fault, Arima-Takatsuki Tectonic Line, Aftershock observation
The 2018 Northern Osaka Prefecture Earthquake (Mj6.1) occurred at a depth of 10 to 15 km immediately below the Arima-Takatsuki Tectonic Line (ATL) in the northern part of the Osaka Plain, however, the relationship with nearby active faults is well understood. In this study, the location of the deeper part of the Arima-Takatsuki fault zone was estimated using the arrival time data of aftershocks, and the relative location between the estimated source fault of the northern Osaka Prefecture earthquake and the deeper part of ATL was investigated.
At each observation station, the relationship between O-C (observed - calculated value) of P wave arrival times in the hypocenter calculation and aftershock locations was investigated, assuming uniform horizontal layer structures. At the observation stations located on the north side of ATL, O-C was almost the same for the aftershocks occurring in the north of ATL, however for the aftershocks south of a certain point, O-C tended to increase toward the south. This is probably because there is a low velocity zone near ATL. When this relationship was best fitted with the flat-ramp function to determine the bending position, the bending position was estimated within an error of about 1 km at 15 observation stations. On the other hand, at the observation stations located on the south side of ATL, O-C tended to increase as the aftershocks in the north. In addition, at the observation stations, aftershocks north of ATL are considered to have a shorter distance through the low-velocity zone than aftershocks in the low-velocity region, but the delay of seismic waves is larger. Thus, it is estimated that the northward and shallower the low velocity anomaly is larger. This suggests that the seismic velocity anomaly near the aftershock zone is due to the fault damage zone, not the geological boundary (material boundary) between the Tamba zone on the north side and the Ryoke zone on the south side.
It was found that the position of the northern end of the estimated low velocity zone is located so as to limit the northern end of the estimated source fault by Asano (2018). Regarding the southern end of the low velocity zone, aftershocks have not occurred far to the south of the estimated fault, and even if there is a flat part, its length is short, so it cannot be reliably estimated at this time. Although it was not, it is suggested that the source fault of the northern Osaka Prefecture earthquake may have occurred inside the fault damage zone of ATL. In particular, the fact that the northern limit of the source fault is limited by the ATL implies that although the earthquake started from the deepest part, the fault was limited to a depth of about 10 km or deeper and the rupture did not propagate to the shallow part.
At each observation station, the relationship between O-C (observed - calculated value) of P wave arrival times in the hypocenter calculation and aftershock locations was investigated, assuming uniform horizontal layer structures. At the observation stations located on the north side of ATL, O-C was almost the same for the aftershocks occurring in the north of ATL, however for the aftershocks south of a certain point, O-C tended to increase toward the south. This is probably because there is a low velocity zone near ATL. When this relationship was best fitted with the flat-ramp function to determine the bending position, the bending position was estimated within an error of about 1 km at 15 observation stations. On the other hand, at the observation stations located on the south side of ATL, O-C tended to increase as the aftershocks in the north. In addition, at the observation stations, aftershocks north of ATL are considered to have a shorter distance through the low-velocity zone than aftershocks in the low-velocity region, but the delay of seismic waves is larger. Thus, it is estimated that the northward and shallower the low velocity anomaly is larger. This suggests that the seismic velocity anomaly near the aftershock zone is due to the fault damage zone, not the geological boundary (material boundary) between the Tamba zone on the north side and the Ryoke zone on the south side.
It was found that the position of the northern end of the estimated low velocity zone is located so as to limit the northern end of the estimated source fault by Asano (2018). Regarding the southern end of the low velocity zone, aftershocks have not occurred far to the south of the estimated fault, and even if there is a flat part, its length is short, so it cannot be reliably estimated at this time. Although it was not, it is suggested that the source fault of the northern Osaka Prefecture earthquake may have occurred inside the fault damage zone of ATL. In particular, the fact that the northern limit of the source fault is limited by the ATL implies that although the earthquake started from the deepest part, the fault was limited to a depth of about 10 km or deeper and the rupture did not propagate to the shallow part.