Near-fault deformation and Dc″ during the 2016 Mw7.1 Kumamoto earthquake

Eiichi Fukuyama,Wataru Suzuki

doi:10.1186/s40623-016-0570-6

Abstract

An Mw7.1 Kumamoto earthquake occurred at 01:25:05 on April 16, 2016 (JST). The earthquake involved a rupture at a shallow depth along a strike-slip fault with surface breaks. Near-fault ground motion records, especially those of a strike-slip earthquake, can provide us with direct information on the earthquake source process. During the earthquake, near-fault seismograms were obtained at KMMH16 station located about 500 m off the fault. The ground displacements were well recovered from the double numerical integration of accelerograms at KMMH16 both on the surface and at the bottom of the 252-m-deep borehole. Fault-parallel static displacement was estimated to be about 1.1 m from the acceleration waveforms. The Dc″ value, which is defined as double the fault-parallel displacement at peak velocity time, was proposed as a proxy of the slip-weakening distance. Using both the velocity and displacement fault-parallel waveforms, the Dc″ value was estimated at about 1 m. This value was between 30 and 50% of the total slip on the fault, which is consistent with previous observations. Graphical abstract GRAPHICAL ABSTRACT CAPTION

Highlights

Ida (1972) proposed a slip-weakening friction law for shear faulting, where friction decreases gradually from the peak friction level to the residual level
We investigate the near-fault ground deformation using a set of seismograms observed close to the fault at KMMH16 station
Since the fault slip motion dominates the right-lateral strike slip, the displacements recorded by these two sensors should be quite similar to those expected theoretically as stated above

Summary

Introduction

Ida (1972) proposed a slip-weakening friction law for shear faulting, where friction decreases gradually from the peak friction level to the residual level. Ide and Takeo (1997) proposed a method to estimate a slip-weakening distance using the spatiotemporal slip history on the fault; they estimated it as 0.5–1 m for the 1995 Kobe earthquake. In their method, a complete description of the spatiotemporal distribution of the slip on the fault is required, which is usually difficult to estimate reliably. Twardzik et al (2014) tried to invert observed waveforms of the 2004 Parkfield earthquake to directly obtain the dynamic parameters including the Mikumo et al (2003) proposed Dc′ as a proxy of Dc. Usually, to estimate a Dc value, both stress and slip functions are needed at a point on the fault. Dc′′ proposed by Fukuyama and Mikumo (2007) is an approximated quantity of Dc′ used to roughly estimate the Dc value from the near-fault seismograms; Dc′′ is an off-fault version of Dc′

Methods

Results

Conclusion