Relationship between characteristics of gravity and magnetic anomalies and the earthquakes in the Longmenshan range and adjacent areas

Abstract

The 2008 Wenchuan earthquake and aftershocks occurred along the northeast-trending Longmenshan fault zone in the eastern margin of the Tibetan plateau. The Tibetan plateau has the strongest negative Bouguer gravity anomaly zone in China and is surrounded by the great gravity horizontal gradient belt. The horizontal gradient belt of the observed gravity anomaly in the Longmenshan area is a part of this giant gravity gradient belt. The Longmenshan fault zone is located to the east of this belt. The horizontal gradient belt of the residual gravity anomaly, obtained by removing large effects of sedimentary basin and variations in the crustal thickness, well matches the Longmenshan fault zone. But this belt is located to the east of the horizontal gradient belt of the observed gravity anomalies. The deviation of the two horizontal gradient belts increases from the southwest to the northeast with a maximum of about 40–50 km. A significant difference in density exists in the lower crust and the uppermost mantle between the Songpan-Ganzê block and the Sichuan basin block. The Songpan-Ganzê block is less dense than the Sichuan basin block in the lower crust as well as in the uppermost mantle. The boundary between the two blocks is located to the west of the Wenchuan-Maoxian, Yinxiu-Beichuan, and Anxian-Guanxian faults approximately. The fault plane crosses the lower crust and uppermost mantle. The rigid Sichuan basin block acts as a resistant for the pushing from the Songpan-Ganzê block. Far-field effects of the collision between the Indian and Eurasian plates, might lead to thrust of some brittle layers in the upper crust along the detachment, in the middle crust of the Songpan-Ganzê block. When movement on a large and deep crustal mega-thrust occurs, earthquakes strike the Longmen Shan margin of the Tibetan Plateau. In the Guanxian-Beichuan segment in the southern Longmenshan fault zone, push from the Songpan-Ganzê block is perpendicular to the density boundary, which corresponds to the location of the horizontal gradient belt of the residual gravity. But in the Guanxian-Beichuan segment in the northern Longmenshan fault zone, the push is oblique to the location of this gradient belt. Such a relationship could lead to nearly equal amounts of thrust and right-lateral slip along the Beichuan-Qingchuan segment, and in contrast to nearly pure thrust along the Guanxian-Beichuan segment. An unusually large negative magnetic anomaly occurs around Qingchuan-Wenchuan-Guanxian. The computed magnetic model shows that some crystalline complexes with reversal magnetization might exist in the crust, which might be related to extrusion of the metamorphic basement rocks in the lower to middle crust.