摘要
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 Ms4.5 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.
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 tec- tonic movement. Results of an Overrun Rate Analysis further illustrate a close temporal relevance of the anomaly to the Wen- chuan 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 Ms4.5 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.
基金
supported by Special Research Fund of Institute of Crustal Dynamics,China Earthquake Administration (Grant No. ZDJ2008-40)
