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

THE SURFACE RUPTURE SIGNS OF THE LUSHAN “4. 20”M_S 7. 0 EARTHQUAKE AT LONGMEN TOWNSHIP, LUSHAN COUNTY AND ITS DISCUSSION

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 rup- tures have different geometric shapes or variant mechanic features, they similarly reflect the northwest- southeast 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.