論文

査読有り
2016年4月

Stress, strain rate and anisotropy in Kyushu, Japan

EARTH AND PLANETARY SCIENCE LETTERS
  • M. K. Savage
  • ,
  • Y. Aoki
  • ,
  • K. Unglert
  • ,
  • T. Ohkura
  • ,
  • K. Umakoshi
  • ,
  • H. Shimizu
  • ,
  • M. Iguchi
  • ,
  • T. Tameguri
  • ,
  • T. Ohminato
  • ,
  • J. Mori

439
439
開始ページ
129
終了ページ
142
記述言語
英語
掲載種別
研究論文(学術雑誌)
DOI
10.1016/j.epsl.2016.01.005
出版者・発行元
ELSEVIER SCIENCE BV

Seismic anisotropy, the directional dependence of wave speeds, may be caused by stress-oriented cracks or by strain-oriented minerals, yet few studies have quantitatively compared anisotropy to stress and strain over large regions. Here we compare crustal stress and strain rates on the Island of Kyushu, Japan, as measured from inversions of focal mechanisms, GPS and shear wave splitting. Over 85,000 shear wave splitting measurements from local and regional earthquakes are obtained from the NIED network between 2004 and 2012, and on Aso, Sakurajima, Kirishima and Unzen volcano networks. Strain rate measurements are made from the Japanese Geonet stations. JMA-determined S arrival times processed with the MFAST shear wave splitting code measure fast polarisations (Phi), related to the orientation of the anisotropic medium and time delays (dt), related to the path length and the percent anisotropy. We apply the TESSA 2-D delay time tomography and spatial averaging code to the highest quality events, which have nearly vertical incidence angles, separating the 3455 shallow (depth < 40 km) from the 4957 deep (>=40 km) earthquakes. Using square grids with 30 km sides for all the inversions, the best correlations are observed between splitting from shallow earthquakes and stress. Axes of maximum horizontal stress (SHmax) and Phi correlate with a coefficient c of 0.56, significant at the 99% confidence level. Their mean difference is 31.9. Axes of maximum compressional strain rate and SHmax are also well aligned, with an average difference of 28, but they do not correlate with each other, meaning that where they differ, the difference is not systematic. Anisotropy strength is negatively correlated with the stress ratio parameter determined from focal mechanism inversion (c = -0.64; significant at the 99% confidence level). The anisotropy and stress results are consistent with stress-aligned microcracks in the crust in a dominantly strike-slip regime. Eigenvalues of maximum horizontal strain rate correlate positively with stress ratio (c = 0.43, significant at 99% confidence). All three orientations are E-W in central Kyushu, where the compressional strain rate is highest. Both splitting and stress suggest plate-boundary-parallel maximum principal stress just off the coast of Kyushu, where strain rate measurements are sparse. South western Kyushu has the largest difference between directions of strain rate and stress. 4 from shallow and deep earthquakes are not well aligned, suggesting that the deep earthquake waveforms are not simply split in the crust. Causes for the anisotropy may be olivine crystals aligned by drag of the subducting Philippine Sea plate in the mantle and stress-aligned microcracks in the crust. (C) 2016 Elsevier B.V. All rights reserved.

リンク情報
DOI
https://doi.org/10.1016/j.epsl.2016.01.005
Web of Science
https://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcAuth=JSTA_CEL&SrcApp=J_Gate_JST&DestLinkType=FullRecord&KeyUT=WOS:000371843700014&DestApp=WOS_CPL
ID情報
  • DOI : 10.1016/j.epsl.2016.01.005
  • ISSN : 0012-821X
  • eISSN : 1385-013X
  • Web of Science ID : WOS:000371843700014

エクスポート
BibTeX RIS