[SSS14-P17] The Fault model of the 2017 M5.6 earthquake estimated from precise leveling survey data in the Eastern flank of the Ontake volcano 2
Keywords:Ontake volcano, fault model, precise leveling
We conducted the precise leveling surveys after the 2014 eruption in the Ontake volcano. The leveling routes of about 38 km with 98 benchmarks were established on the eastern flank of the Ontake volcano. The main routes were extended to the Yashikino village (Kakehashi and Yashikino routes). In order to improve the spatial layout of the benchmarks, a branched leveling routes were established (Kiso-Onsen, Ontake Ropeway and Nakanoyu routes). The epicenter of the M 5.6 earthquake is located just beneath these leveling routes.
The leveling surveys of all routes were conducted in April 2017, April 2018 and April 2019. In September 2017, the leveling survey was conducted in the Yashikino and Kiso-Onsen routes. The leveling data in April 2017 and September 2017 ware used to detect the vertical deformation associated with the M 5.6 earthquake. The leveling data in the periods during September 2017-April 2018 and during April 2018-April 2019 were used to detect the post-seismic deformations after the M 5.6 earthquake.
Using the detected vertical deformations, a single rectangular fault was estimated. The parameters of the fault geometry were assumed form hypocenter distribution of aftershocks estimated by using DD method. parameters of latitude, longitude and depth were optimized using a genetic algorithm (GA) in order to conform to the vertical deformation during the April 2017 and September 2017. In order to estimate detail slip distribution, the optimal single fault was divided into 12 sub-faults. Akaike's Bayesian information criterion (ABIC) was used to choose the smoothing hyperparameter for the inversion of the sub-fault slips. Using the vertical deformation during September 2017-April 2018, the after-slip distribution was estimated on the assumption that fault geometry of after-slip is the same as that of co-seismic one.
In the results, the benchmarks of the Yashikino and Kiso-Onsen routes showed the uplifts in the period from April 2017 to September 2017. The maximum uplift of 28mm was detected in central part of Yashikino route. In the after-seismic periods during September 2017-April 2018, uplift with maximum of 5 mm was detected in the same region. There is no significant uplift during April 2018-April 2019. As the longest, the post-seismic deformation seem to finish within the 10 months after the earthquake. The slip distribution was suggested mechanism of left lateral with reverse fault in the period from April 2017 to September 2017. Mw 5.2 was calculated from the fault model. The after slip distribution showed the almost same pattern of co-seismic one.
The left lateral slip was mainly estimated on this fault in our model. According to the centroid moment tensor solution (CMT), the M 5.6 earthquake was estimated as reverse fault mechanism. However, some aftershocks were estimated as mechanism of lateral slip. We believe that the slips of not only reverse slip but also lateral slip occurred in this area. Because of the M 5.6 earthquake estimated as the reverse fault mechanism from CMT, the lateral slip seems to occur as aseismic slip. Ito and Matsuhiro (2018) suggested that it is difficult to explain horizontal deformations detected by GNSS as the reverse fault and also reported that deformation rate of GNSS is changed after the earthquake. The deformations detected GNSS are consistent with the existence of the aseismic slip after the earthquake.
From the above discussion, we believe that the left lateral slip occurred as aseismic slip after the M 5.6 earthquake with the reverse fault. As the longest, the aseismic slip seem to finish within the 10 months after the earthquake.