Bilham R, England P
CIRES & Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, USA.
Nature. 2001 Apr 12;410(6830):806-9. doi: 10.1038/35071057.
The great Assam earthquake of 12 June 1897 reduced to rubble all masonry buildings within a region of northeastern India roughly the size of England, and was felt over an area exceeding that of the great 1755 Lisbon earthquake. Hitherto it was believed that rupture occurred on a north-dipping Himalayan thrust fault propagating south of Bhutan. But here we show that the northern edge of the Shillong plateau rose violently by at least 11 m during the Assam earthquake, and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the Himalaya. The stress drop implied by the rupture geometry and the prodigious fault slip of 18 +/- 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s-1 (ref. 1). This quantitative observation of active deformation of a 'pop-up' structure confirms that faults bounding such structures can penetrate the whole crust. Plateau uplift in the past 2-5 million years has caused the Indian plate to contract locally by 4 +/- 2 mm yr-1, reducing seismic risk in Bhutan but increasing the risk in northern Bangladesh.
1897年6月12日发生的阿萨姆大地震，将印度东北部一块面积大致相当于英格兰的区域内的所有砖石建筑夷为平地，其震感范围超过了1755年里斯本大地震。此前人们认为，破裂发生在不丹以南向北倾斜的喜马拉雅逆冲断层上。但我们在此表明，在阿萨姆地震期间，西隆高原的北缘剧烈上升了至少11米，这是由于一条长度约110公里、陡峭地背离喜马拉雅山倾斜的隐伏逆断层破裂所致。由破裂几何形状和18±7米的巨大断层滑动所暗示的应力降，解释了观测到的震中垂直加速度超过1g以及地表速度超过3米/秒的现象（参考文献1）。对一个“弹出式”构造活动变形的这一定量观测证实，界定此类构造的断层能够穿透整个地壳。过去200万至500万年的高原隆升，导致印度板块局部每年收缩4±2毫米，降低了不丹的地震风险，但增加了孟加拉国北部的风险。
