Japan Geoscience Union Meeting 2018

Presentation information

[JJ] Evening Poster

S (Solid Earth Sciences) » S-SS Seismology

[S-SS14] Strong Ground Motion and Earthquake Disaster

Tue. May 22, 2018 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall7, Makuhari Messe)

convener:Masayuki Kuriyama(Central Research Institute of Electric Power Industry)

[SSS14-P24] Rupture Process of the M5.5 Orkney Earthquake Using Strainmeters at Very Close Distance

*Tatunari Yasutomi1, James Mori1, Masumi Yamada1, Hiroshi Ogasawara2, Makoto OKUBO3, Hiroyuki Ogasawara2, Akimasa Ishida2 (1.Kyouto Univercity, 2.Ritsumeikan University, 3.Kochi University)

Keywords:South Africa, inversion, Close, induced earthquake, strain meter

Gold mining is a major industry of South Africa, however, there is a risk of induced earthquakes associated with the deep escavations, so seismic monitoring is maintained around and in the mines. The Orkney earthquake (M5.5) occurred on 5 August 2014 directly beneath the Orkney Gold mine. The event was the largest earthquake since the dense instrumentation was installed, and includes 3 Ishii borehole strainmeters at a depth of 2.9 km depth and 17 surface strong motion meters at close distances. The aftershock distribution was well located by these instruments. The planar distribution of aftershock activity dips almost vertically and the upper edge is located several hundred meters from the strainmeter site. The strainmeters and surface strong motion meters recorded clear nearfield waveforms.

We try to model rupture process of the M5.5 mainshock using the nearfield strainmeters and surface strong motion meters data using a multi-time window method (Yoshida et al. (1996)). For the analysis, we used a horizontal stratified structures obtained by H. Ogasawara (2018 Msc) and A. Mangongolo (2014). The strain Green functions that include near-field terms and static off-sets were calculated using a finite difference method. Since the absolute timing of the strainmeters is not accurate, we decided set the starting point of rupture using a hypocenter determined by data from the 17 surface strong motion meters.

As a result of the analysis, large slip on the fault occurred in the northern region and the rupture propagated bilaterally upwards to the north and downwards to the south. This result is consistent with the absence of observed faulting in the mine tunnels and the inclination of aftershock distribution from north to south.