Rise time of the 2018 MW 6.4 Hualien earthquake revealed by source time functions: A restrictive estimation of static stress drop

Using the ω−2 source model, the rise time of the 2018 Mw 6.4 Hualien earthquake is estimated from the corner frequency of the Fourier spectrum of the source time function. Two source parameters are also determined: average dynamic stress drop (∆σd) and static stress drop (∆σs). The results show that the rise time is 2.52 s, which is approximately 0.23 times the value of the entire source duration and then leads to a ∆σd of 3.03 MPa. In this study, we propose a method to confine the estimation of ∆σs through ∆σd. Because the radiated seismic energy exists, this leads to ∆σs<2∆σd−2μEgM0 (Eg is the fracture energy, M0 is the seismic moment, and μ is the rigidity) on average. Hence, we suggest that ∆σs for the 2018 Hualien earthquake should be less than 4.81 MPa. Because of ∆σs > ∆σd, the earthquake's rupture can be interpreted in a frictional overshoot model; then, the radiation efficiency near 0.5 suggests that 50% of the available energy is released to extend the fault. Furthermore, the lower fracture energy density (G) for the Hualien earthquake is probably due to the fact that an ML 5.9 foreshock disturbs the fault system beforehand.