[S13-5] Earthquake source mechanics V

[S13-5-06] Slip-weakening distance and strength drop inferred from near-fault deformation during the 2016 M7.8 Kaikoura earthquake

^○Yoshihiro Kaneko¹, Eiichi Fukuyama², Ian Hamling¹ (1.GNS Science, Lower Hutt, New Zealand, 2.NIED, Tsukuba, Ibaraki, Japan)

The 2016 M7.8 Kaikoura (New Zealand) earthquake struck the east coast of the northern South Island, resulting in large surface offsets (>10 m), numerous landslides, extreme surface ground motions (over 1g), a regional tsunami, and large-scale slow slip events on the Hikurangi subduction interface. Since the earthquake was well recorded by GeoNet strong motion and GPS networks, near-fault ground motion may provide direct measurements of dynamic parameters associated with the fault-weakening process. Here we estimate a proxy of slip-weakening distance Dc'', defined as double the fault-parallel displacement at the time of peak ground velocity, from accelerograms recorded at near-fault stations. Three-component ground displacements were recovered from the double numerical integration of accelerograms, and the corresponding static displacements are validated against those derived from InSAR data. We estimate Dc'' of about 5 m at KEKS station located at 2.7 km from a segment of the Kekerengu fault where a surface offset of more than 10 m has been found. The inferred Dc'' is the largest value ever estimated from near-fault strong motion data, yet appears to follow the scaling of Dc'' with final slip reported in previous studies. The corresponding slip-weakening distance Dc and strength drop on the same fault segment may be validated against dynamic rupture simulations.