Several days before the MsT. 0 Lushan earthquake, the YRY-4 borehole Strainmeter at Guza Station recorded prominent abnormal changes. The strain anomalies are very striking on the smooth background of several years＇ ...Several days before the MsT. 0 Lushan earthquake, the YRY-4 borehole Strainmeter at Guza Station recorded prominent abnormal changes. The strain anomalies are very striking on the smooth background of several years＇ recording after the Wenchuan earthquake. However, because construction in the town of Guza has been undergoing rapid development in recent years, many factors have interfered with observations at the station. Whether or not the observed strain changes before the Lushan earthquake were affected by any of the sources of interference becomes a question that must be answered. Among the likely sources of interference, apartment construction, sportsground reconstruction, and tunnel cutting can be excluded by analyzing the morphological characteristic of the anomalies. The two remaining most possible sources are road construction in front of the station and the water level change of the nearby Dadu River caused by water filling into and discharging from an upstream reservoir. Through field investigation, comparison of the correlation between the strain and the seismographic recordings, comparison of the correlation between the strain and the Dadu River flow recordings, and analysis of the strain anomaly characteristics, we conclude that the abnormal changes observed at Guza Station cannot be attributed to either of these two sources but should be related to the Lushan earthquake.展开更多
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 cha...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.展开更多
基金supported by the Special Fund for Earthquake Research in the Public Interest(201108009)
文摘Several days before the MsT. 0 Lushan earthquake, the YRY-4 borehole Strainmeter at Guza Station recorded prominent abnormal changes. The strain anomalies are very striking on the smooth background of several years＇ recording after the Wenchuan earthquake. However, because construction in the town of Guza has been undergoing rapid development in recent years, many factors have interfered with observations at the station. Whether or not the observed strain changes before the Lushan earthquake were affected by any of the sources of interference becomes a question that must be answered. Among the likely sources of interference, apartment construction, sportsground reconstruction, and tunnel cutting can be excluded by analyzing the morphological characteristic of the anomalies. The two remaining most possible sources are road construction in front of the station and the water level change of the nearby Dadu River caused by water filling into and discharging from an upstream reservoir. Through field investigation, comparison of the correlation between the strain and the seismographic recordings, comparison of the correlation between the strain and the Dadu River flow recordings, and analysis of the strain anomaly characteristics, we conclude that the abnormal changes observed at Guza Station cannot be attributed to either of these two sources but should be related to the Lushan earthquake.
基金supported by Special Research Fund of Institute of Crustal Dynamics,China Earthquake Administration (Grant No. ZDJ2008-40)
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 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.
