汶川8.0级地震前龙门山断裂带的地震活动性和构造应力场特征
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摘要
研究了2008年5月12日汶川8.0级地震前龙门山断裂带及其附近地区的地震活动.利用区域地震台网和流动测震台的数字地震波资料,测定了震源机制解.结果表明,震中所在的龙门山断裂带震前地震活动平稳,未出现显著异常增强或平静现象.根据汶川8.0级地震前地震活动求出的震源机制解,其主压应力P轴方位为WNW-ESE向,震源断层面呈NE向与NW向两组节面走向.其中NE向节面呈N50°—70°E,断面倾角均陡,达60°—70°,震源力学作用方式多呈逆倾型,少部分呈走滑型.震前地震活动呈现的主压应力方位、震源断面走向及其错动类型,与汶川8.0级地震给出的解是一致的.巨大地震发生前沿龙门山断裂带微破裂呈现的平均应力场与主震一致.起始破裂区东侧20km内是紫坪铺水库水域区,这一区域发生小震活动增加的现象处于水库放水的卸载阶段.本文研究了汶川8.0级地震起始破裂区附近的小震活动,其震源参数表明,震源位于8.0级地震之上的5—14km深度,其震源参数与8.0级地震给出的解也是一致的.
Seismicity of the Longmenshan fault zone and its vicinity before the 12 May 2008 Wenchuan MS8.0 earthquake is studied. Based on the digital data of seismic waveform observed from regional seismic networks and mobile stations, earthquake focal mechanism solutions are determined. The results obtained show that the seismicity of Longmenshan fault zone before the 12 May 2008 Wenchuan MS8.0 earthquake were in a stable state. No obviously intensified seismic activity appeared. According to focal mechanism solutions of some small earthquakes before the Wenchuan earthquake, the direction of principal compressive stress axis was WNW-ESE. The earthquake fault planes had both NE-SW and NW-SE strikes. The strike of the NE-SW planes is among N50°—70°E, their dip angles are 60°—70°, being quite steep. Most earthquakes were characterized by dip-slip reverse faults, and minor were of strike-slip faults. The directions of principal compressive stress axes, the strikes of fault planes and the faulting types of some small earthquakes before the Wenchuan earthquake are in accordance with that of the main shock. The average stress field along the Longmenshan fault zone before the Wenchuan earthquake is also consistent with that obtained from focal mechanism analysis of the main shock. Zipingpu Dam is located 20 km east to the initial rupture area of the Wenchuan earthquake. The increase of small earthquakes in Zipingpu Dam area before the main shock may be attributed to that the reservoir was just in the period of water discharging. Focal depths of the small earthquakes occurred near the initial rupture area of the main shock are in the range of 5 to 14 kilometers, and are identical with that of the Wenchuan earthquake.
Seismicity of the Longmenshan fault zone and its vicinity before the 12 May 2008 Wenchuan MS8.0 earthquake is studied. Based on the digital data of seismic waveform observed from regional seismic networks and mobile stations, earthquake focal mechanism solutions are determined. The results obtained show that the seismicity of Longmenshan fault zone before the 12 May 2008 Wenchuan MS8.0 earthquake were in a stable state. No obviously intensified seismic activity appeared. According to focal mechanism solutions of some small earthquakes before the Wenchuan earthquake, the direction of principal compressive stress axis was WNW-ESE. The earthquake fault planes had both NE-SW and NW-SE strikes. The strike of the NE-SW planes is among N50°—70°E, their dip angles are 60°—70°, being quite steep. Most earthquakes were characterized by dip-slip reverse faults, and minor were of strike-slip faults. The directions of principal compressive stress axes, the strikes of fault planes and the faulting types of some small earthquakes before the Wenchuan earthquake are in accordance with that of the main shock. The average stress field along the Longmenshan fault zone before the Wenchuan earthquake is also consistent with that obtained from focal mechanism analysis of the main shock. Zipingpu Dam is located 20 km east to the initial rupture area of the Wenchuan earthquake. The increase of small earthquakes in Zipingpu Dam area before the main shock may be attributed to that the reservoir was just in the period of water discharging. Focal depths of the small earthquakes occurred near the initial rupture area of the main shock are in the range of 5 to 14 kilometers, and are identical with that of the Wenchuan earthquake.
引文
①美国哈佛大学给出的矩张量解来自http:∥neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_hrv.ht ml.2008年5月20日查阅.
②USGS给出的震源机制解来自http:∥neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_cmt.ht-ml.2008年5月20日查阅.
①美国哈佛大学给出的震源深度结果来自http:∥neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_hrv.ht ml.2008年5月20日查阅.
②USGS给出的震源深度结果来自http:∥neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_cmt.ht ml.2008年5月20日查阅.
陈运泰,许力生,张勇,杜海林,冯万鹏,刘超,李春来,张红霞.2008.2008年5月12日汶川特大地震震源特性分析报告[R].[2008-05-28]http:∥www.csi.ac.cn/sichuan/chenyuntai.pdf(in Chinese).
刁桂苓,于利明,宁杰远,孙佩卿,张子广.1993.1989年大同震群的破裂特征[J].地球物理学报,36(3):360-368.
梁尚鸿,李幼铭,束沛镒,朱碚定.1984.利用区域地震台网P、S振幅比资料测定小震震源参数[J].地球物理学报,27(3):247-257.
《四川地震资料汇编》编辑组.1981.四川地震资料汇编[M].成都:四川人民出版社:193-214.
赵珠,范军,郑斯华,长谷川昭,堀内茂木.1997.龙门山断裂带地壳速度结构和震源位置的精确修定[J].地震学报,19(6):615-622.
钟继茂,程万正.2006.由多个地震震源机制解求川滇地区平均应力场方向[J].地震学报,28(4):337-346.
朱介寿.2008.汶川地震的岩石圈深部结构与动力学背景[J].成都理工大学学报(自然科学版),35(4):348-356.
②USGS给出的震源机制解来自http:∥neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_cmt.ht-ml.2008年5月20日查阅.
①美国哈佛大学给出的震源深度结果来自http:∥neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_hrv.ht ml.2008年5月20日查阅.
②USGS给出的震源深度结果来自http:∥neic.usgs.gov/neis/eq_depot/2008/eq_080512_ryan/neic_ryan_cmt.ht ml.2008年5月20日查阅.
陈运泰,许力生,张勇,杜海林,冯万鹏,刘超,李春来,张红霞.2008.2008年5月12日汶川特大地震震源特性分析报告[R].[2008-05-28]http:∥www.csi.ac.cn/sichuan/chenyuntai.pdf(in Chinese).
刁桂苓,于利明,宁杰远,孙佩卿,张子广.1993.1989年大同震群的破裂特征[J].地球物理学报,36(3):360-368.
梁尚鸿,李幼铭,束沛镒,朱碚定.1984.利用区域地震台网P、S振幅比资料测定小震震源参数[J].地球物理学报,27(3):247-257.
《四川地震资料汇编》编辑组.1981.四川地震资料汇编[M].成都:四川人民出版社:193-214.
赵珠,范军,郑斯华,长谷川昭,堀内茂木.1997.龙门山断裂带地壳速度结构和震源位置的精确修定[J].地震学报,19(6):615-622.
钟继茂,程万正.2006.由多个地震震源机制解求川滇地区平均应力场方向[J].地震学报,28(4):337-346.
朱介寿.2008.汶川地震的岩石圈深部结构与动力学背景[J].成都理工大学学报(自然科学版),35(4):348-356.
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