日本地球惑星科学連合2014年大会

講演情報

口頭発表

セッション記号 S (固体地球科学) » S-CG 固体地球科学複合領域・一般

[S-CG65_2AM2] 応力と地殻ダイナミクス

2014年5月2日(金) 11:00 〜 11:30 423 (4F)

コンビーナ:*佐藤 活志(京都大学大学院理学研究科地球惑星科学専攻)、今西 和俊(産業技術総合研究所)、大坪 誠(産業技術総合研究所 地質情報研究部門)、加藤 愛太郎(東京大学地震研究所)、座長:佐藤 活志(京都大学大学院理学研究科地球惑星科学専攻)

11:15 〜 11:30

[SCG65-P01_PG] 南アフリカ金鉱山での地震発生場での歪連続観測に基づく応力時間発展の理解

ポスター講演3分口頭発表枠

*小笠原 宏1桂 泰史2Hofmann Gerhard3中谷 正生4矢部 康男5石井 紘6中尾 茂7大久保 慎6Anthony Ward8Jerry Wienand9Patrick Lenegan9川方 裕則1村上 理1内浦 大海1 (1.立命館大学、2.株式会社 日立ソリューションズ、3.Anglogold Ashanti ltd.、4.東京大学、5.東北大学、6.東濃地震科学研究所、7.鹿児島大学、8.Seismogen CC、9.Sibanye Gold ltd.)

キーワード:南アフリカ金鉱山, 地震発生場, 原位置歪連続観測, 応力時間発展

Compared with continuous in-situ strain monitoring in other mines, we discussed the time evolution of stress in rock mass at a depth of 3.3km for a ~1.5-year period 90m beneath a dip pillar at Mponeng mine. The pillar contained a 30m-thick dyke which a ML2.1 seismic event obliquely bisected. We analyzed the recordings of two multi-component Ishii borehole strainmeters which had been already installed nine months prior to the ML2.1 event. One of the strainmeters was installed in the dyke (gabbros) and the other in the host rock (quartzite) near the dyke contact, both being within a few tens of meters from the ML2.1 rupture plane.

The magnitudes and directions of the principal strain changes were similar for both strainmeters in the period prior to the ML2.1 event. This suggested that the increase in stress in the dyke was significantly larger because the dyke was significantly stiffer than the host rock.

After the ML2.1 event, associated with the start of mining on the eastern side of the strainmeters, the pattern of deformation changed between the two strainmeters.

The above-mentioned characteristics of deformation were compared with numerically modelled deformation by an elastic boundary element method using Map3D Fault-Slip. The magnitude of the Map3D strain changes were, however, several times smaller than the observed strain changes both prior to and after the ML2.1 event. The rock mass just around a stope in deep tabular mining is fractured and behaves time-dependently and non-linearly. Whatever the inelastic deformation, the stress field in an elastic area can be reproduced within reason provided that the boundary condition (deformation, force or stress) is appropriately specified on the elastic-inelastic boundary. Because it is well known that time-dependent inelastic stope closure is much larger than instantaneous elastic stope closure, as a trial, we analyzed a response to an additional forced stope closure using Map3Di (Seismic Integrator version). It was then found that the forced additional stope closure better accounted for both the magnitude and the deformation pattern observed by in situ strain monitoring. We concluded that the effect of inelastic deformation around the stope was significantly larger than the elastic effect induced by the advance of mining faces, and the direct effect of the very close ML2.1 event was not so significant.

A great amount of better maintained data sets of strain are now being accumulated in four gold mines, which will allow us to discuss in further depth.