Abnormal strain changes observed at Guza before the Wenchuan earthquake

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• 作者：ZeHua Qiu (1)
  BaoHong Zhang (1)
  ShunLiang Chi (2)
  Lei Tang (1)
  Mo Song (1)
  
• 关键词：Wenchuan earthquake ; earthquake precursor ; strain change ; 4 ; component borehole strainmeter ; Overrun ; Rate ; Analysis
• 刊名：Science China Earth Sciences
• 出版年：2011
• 出版时间：February 2011
• 年：2011
• 卷：54
• 期：2
• 页码：233-240
• 全文大小：1234KB
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• 作者单位：ZeHua Qiu (1)
  BaoHong Zhang (1)
  ShunLiang Chi (2)
  Lei Tang (1)
  Mo Song (1)
  
  1. Institute of Crustal Dynamics, China Earthquake Administration, Beijing, 100085, China
  2. Hebi Seismological Bureau, Hebi, 458000, China
  
• ISSN：1869-1897

文摘

An YRY-4 type borehole strainmeter at Guza station on the southwest end of the Longmenshan fault zone recorded anomalous changes from time to time since about one year before the Wenchuan earthquake. The anomaly is characterized by steps and/or asymmetrical pulses of short-periods (minutes-hours). Among about one hundred stations over the mainland of China, only Guza, the nearest one to the seismic area, observed such changes. A self-check function of the strainmeter helps confirm the credibility of the observation. The anomalous signals do not correspond to seasonal changes. The consistency in the sense of strain among long-term, anomalous interfering and coseismic changes suggests that they all resulted from the background tectonic movement. Results of an Overrun Rate Analysis further illustrate a close temporal relevance of the anomaly to the Wenchuan earthquake: its increase before, and decrease after the biggest changes during the main shock. The abnormal changes are attributed to local small-scale fractures with an evidential clue that the only M s 4.5 foreshock near Guza station on February 27, 2008 produced the biggest anomalous change. By means of Wavelet Decomposition, it is revealed that longer period portion of the anomaly took place later before the main shock, consistent with the idea that scales of small fractures increase during earthquake preparation process. The anomalous strain changes are analogous to the stress changes observed before the 1976 Tangshan earthquake.