芦山县龙门乡芦山“4·20”7.0级强烈地震地表破裂迹象与讨论
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摘要
在芦山县龙门乡发现芦山"4·20"7.0级强烈地震地表破裂的迹象,这些迹象点呈NE-SW向线状分布,总体走向N40°～50°E,长2～3km。根据水泥路面变形推断水平缩短量为8cm,垂直抬升量1～2cm。地表未见走滑分量,运动学特征表现为由NW向SE的推挤作用。在地震地表破裂的力学性质方面,有斜向剪切裂缝,也有挤压对冲逆断层性质,但更多地表现为张性裂缝,这与拱曲顶部的局部张性应力场有关。虽然这些地表破裂组合特征不同,性质也有差异,但均反映了龙门乡一带受到NW-SE向挤压作用以及逆断层发震构造沿线近地表常见的拱曲作用。与大川-双石断裂(前山断裂)、大邑-名山断裂(山前断裂)相比,芦山-龙门隐伏推测断裂更有可能是此次地震的发震断裂,这一推论也与此次地震序列的精定位结果以及地震烈度分布特征相符。有关芦山"4·20"7.0级强烈地震发震断裂的认识,对研究此次地震的发震构造特征以及评价未来山前地带地震危险性具有重要意义。
The co-seismic surface rupture signs of the "4. 20"Lushan MS 7. 0 earthquake are found at Longmen Township,Lushan County. The sites of rupture signs have a linear distribution with a 2 ～ 3km length and N40° ～ 50°N strike. The maximum shortening of the rupture is about 8cm,uplifting is about 1 ～ 2cm. Strike-slip component is not observed,but the dynamic process of the earthquake is characterized by compression from northwest to southeast. The observed co-seismic surface ruptures can be oblique shear-fissure,or thrusting crack,however most of them are extensive fissures,which can be explained by the local extensive stress-field on the top of the thrust bending. Although these ruptures have different geometric shapes or variant mechanic features,they similarly reflect the northwestsoutheast compression and the surface lift-bending on the top of a thrusting seismogenic structure. Comparing with Dachuan-Shuangshi Fault( frontal fault) and Dayi-Mingshan Fault( piedmont fault) , Lushan-Longmen presumed blind fault is more likely the seismogenic fault,which is also consistent with the results of the Lushan earthquake sequence relocations and the seismic intensity contours. As the seismogenic fault of the Lushan earthquake has surpassed the frontal fault of Longmen Shan,it may be a new-generated tectonics,which implies that it is important to re-evaluate the seismic risk at the piedmont area of the Longmen Shan. However,the conclusions are still very primary and geophysical survey is needed to demonstrate the existence of the Lushan-Longmen presumed blind fault.
The co-seismic surface rupture signs of the "4. 20"Lushan MS 7. 0 earthquake are found at Longmen Township,Lushan County. The sites of rupture signs have a linear distribution with a 2 ～ 3km length and N40° ～ 50°N strike. The maximum shortening of the rupture is about 8cm,uplifting is about 1 ～ 2cm. Strike-slip component is not observed,but the dynamic process of the earthquake is characterized by compression from northwest to southeast. The observed co-seismic surface ruptures can be oblique shear-fissure,or thrusting crack,however most of them are extensive fissures,which can be explained by the local extensive stress-field on the top of the thrust bending. Although these ruptures have different geometric shapes or variant mechanic features,they similarly reflect the northwestsoutheast compression and the surface lift-bending on the top of a thrusting seismogenic structure. Comparing with Dachuan-Shuangshi Fault( frontal fault) and Dayi-Mingshan Fault( piedmont fault) , Lushan-Longmen presumed blind fault is more likely the seismogenic fault,which is also consistent with the results of the Lushan earthquake sequence relocations and the seismic intensity contours. As the seismogenic fault of the Lushan earthquake has surpassed the frontal fault of Longmen Shan,it may be a new-generated tectonics,which implies that it is important to re-evaluate the seismic risk at the piedmont area of the Longmen Shan. However,the conclusions are still very primary and geophysical survey is needed to demonstrate the existence of the Lushan-Longmen presumed blind fault.
引文
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韩竹军,董绍鹏,谢富仁,等.2008.南北地震带北部5次(1561—1920年)M≥7级地震触发关系研究[J].地球物理学报,51(6):1776—1784.HAN Zhu-jun,DONG Shao-peng,XIE Fu-ren,et al.2008.Earthquake triggering by static stress:The5major earth-quakes with M≥7(1561—1920)in the northern section of South-North Seismic Zone,China[J].Chinese Journalof Geophysics,51(6):1776—1784(in Chinese).
李勇,周荣军,Densmore A L,等.2006.青藏高原东缘大陆动力学过程与地质响应[M].北京:地质出版社.LI Yong,ZHOU Rong-jun,Densmore A L,et al.2006.Geodynamics Processes of Eastern Margin of the Tibetan Plateauand Its Geological Responses[M].Geological Publishing House,Beijing(in Chinese).
徐锡伟,闻学泽,叶建青,等.2008.汶川M8.0地震地表破裂带及其发震构造[J].地震地质,30(3):597—629.XU Xi-wei,WEN Xue-ze,YE Jian-qing,et al.2008.The MS8.0Wenchuan earthquake surface ruptures and its seis-mogenic structure[J].Seismology and Geology,30(3):597—629(in Chinese).
杨晓平,李安,刘保金,等.2009.成都平原内汶川M8.0级地震的地表变形[J].地球物理学报,52(10):2527—2537.doi:10.3969/j.issn.0001-5733.2009.10.011.YANG Xiao-ping,LI An,LIU Bao-jin,et al.2009.Surface deformation in the Chengdu plain area produced by theWenchuan M8.0earthquake of12May2008,Sichuan,China[J].Chinese Journal of Geophysics,52(10):2527—2537(in Chinese).
张培震,邓起东,张国民,等.2003.中国大陆强震活动与活动地块[J].中国科学(D辑),33(增刊):12—20.ZHANG Pei-zhen,DENG Qi-dong,ZHANG Guo-min,et al.2003.Active blocks and strong earthquake activity in Chi-nese continent[J].Science in China(Ser D),33(suppl):12—20(in Chinese).
Barka A.1996.Slip distribution along the North Anatolian Fault associated with the large earthquakes of the period1939-1967[J].Bulletin of Seismological Society of America,86(5):1238—1254.
Densmore A L,Michael A E,Li Yong,et al.2007.Active tectonics of the Beichuan and Pengguan Faults at the easternmargin of the Tibetan plateau[J].Tectonics,26:1—17.
Parson T,Ji C,Kirby E,2008.Stress change from the2008Wenchuan earthquake and increased hazard in the SichuanBasin[J].Nature,454:509—510.
Qian Q,Han Z J.2013.Influence of the2008Wenchuan earthquake(MW7.9)on the occurrence probability of futureearthquakes on neighboring faults[J].Journal of Asian Earth Sciences,http:∥dx.doi.org/10.1016/j.jseaes.2013.03.019.
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