Super-shear fault rupture propagation during the 2016/04/16 Kumamoto earthquake (Mw7.1) imaged by seismic back-projection and KNET/KiKnet strong motion records

I investigated the rupture process of the April 16, 2016 Kumamoto earthquake, using a seismic back-projection methodology (Pulido et al. 2008, Suzuki et al. 2016), and a dense array of near-source strong motion records from the K-NET/KiKnet networks. The main target of this study is to understand the evolution of the rupture velocity during fault rupture propagation. I selected all the KNET/KiKnet records of the mainshock within 100 km around the Hinet epicenter (112 stations), and used the fault-parallel component rotated from the horizontal components. I calculated the envelopes of velocity time series (5 to 10 Hz) obtained as the vectorial summation of the waveforms and their Hilbert transforms. Envelopes were stacked within a horizontal grid mesh covering the regions around the Hinagu and Futagawa fault traces and beyond, without any constraint on the starting point of rupture or rupture speed, to obtain a temporal and spatial image of rupture propagation. My back-projection results show that significant grid energy was released in a region spanning 43km length along the Hinagu (16km) and Futagawa (27 km) fault zones. I was not able to image the rupture process for the initial 4 seconds of rupture because of the very small S-wave radiation during this interval. Back-projection results show a bilateral fault rupture propagation along the Hinagu and Futagawa faults, characterized by a slow sub-Rayleigh rupture velocity of 1.4 1.7 km/s, for the first 5.5 seconds of imaged rupture (4 9.5s from the origin time, OT). The rupture propagation towards the NE (along the Futagawa fault) experienced a sudden increase in rupture velocity by reaching a value 1.4 times larger than the average S-wave velocity (Vrup= 4.7 km/s) at 9.5s from OT, and remained super-shear for approximately 4.5 s (9.5 14s from OT) until fault rupture arrest. I also imaged a clear sub-Rayleigh rupture propagation towards the SW along the Hinagu fault zone (Vrup= 3.1 km/s), from 11 to 14 seconds after OT. My results indicate a super-shear transition length of 10 km, which is approximately 5 times the typical value of the nucleation zone size (Lc) for a Mw7.1 earthquake. In addition to my back-projection results, several near-source strong motion waveforms of the Kumamoto earthquake also appear to reveal outstanding signatures of super-shear rupture propagation.