JpGU-AGU Joint Meeting 2017

Presentation information

[JJ] Poster

S (Solid Earth Sciences) » S-SS Seismology

[S-SS17] [JJ] Fault Rheology and Earthquake Physics

Sun. May 21, 2017 1:45 PM - 3:15 PM Poster Hall (International Exhibition Hall HALL7)

convener:Takanori Matsuzawa(National Research Institute for Earth Science and Disaster Resilience), Takeshi Iinuma(National Research and Development Agency Japan Agency for Marine-Earth Science and Technology), Wataru Tanikawa(Japan Agency for Marine-Earth Science and Technology, Kochi Instutute for Core Sample Research), Hideki Mukoyoshi(Department of Geoscience Interdisciplinary Graduate School of Science and Engineering, Shimane University)

[SSS17-P09] Searching significant displacement zone of Orkney earthquake fault by forward and inversion analysis with strain data observed at very close distance

*Tatunari Yasutomi1, Hiroshi Ogasawara2, Akimasa Ishida2, Hiroyuki Ogasawara2, Raymond Durrheim3, Alex Milev4, Makoto OKUBO5, Teruhiro Yamaguchi6, James Mori1 (1.Kyouto university, 2.Ritsumeikan university, 3.Univ.Witwaterarand,South Africa, 4.CSIR,South Africa, 5.Kouchi university, 6.Hokkaidou university)

Keywords:South Africa, induced earth quake, inversion

The largest event recorded in a South African gold mining region, a M5.5 earthquake took place near Orkney, South Africa on 5 August 2014. This is one of the rare events as the main- and after-shocks were recorded by 46 geophones and 3 Ishii borehole strain meters at 2 - 3 km depths with epicentral distances, delta < several km, and 17 surface strong motion meters with delta < 20 km. The upper edge of the planar aftershock activity dipping almost vertically was only some hundred meters below the sites where the strainmeters were installed. As the M5.5 seismic rupture is located within a range drillable from gold mine workings at depth, ICDP approved a project to drill into the seismogenic zones. Moyer et al. (2016 SCEC) inverted surface strong motion data, suggesting significant fault slip even at the mining horizon, while there was no seismic rupture mapped or there were three strainmeters installed. So, the three strainmeters can contribute to constrain the configuration of the seismic rupture. As population of the aftershocks varies in space significantly, we expect a possibility to discuss a relationship the fault slip and the aftershocks.
These strainmeters were apart each other about 150 m only. However, their strain changes had different polarities while the other M4 strain changes with a similar hypocentral distance was the same. So, this information can constrain the location and configuration of the M5.5 fault critically.
First, we conducted a forward analysis by assuming a point source with the mechanism same as macroscopic one of the M5.5 faultingat a distance of a few km. However, no difference in polarity in strain change was seen, suggesting that the effect of a finite size of the source with an edge much nearer than the point source had to be taken into account. We are attempting to invert the slip distribution on a source with a finite size together with surface strong motion data. We will report on the results at the meeting.