山西断陷带的地震分布及地壳地震波速度结构

Spatial Distribution of Earthquakes and Crustal Seismic Velocity in Shanxi Rift Zone

为了研究山西断陷带的地震活动性及其物理背景,利用国家地震科学数据共享中心以及山西省地震局提供的地震震相数据,通过tomoDD方法对1990-2008年和2012-2016年期间的地震进行重定位,并反演山西断陷带附近的地震波速度结构。地震集中于山西断陷带内,基本上位于已知断层附近,主要分布在太原盆地的东北-西南两侧。震源深度范围为0~30 km,北部区域震源深度小,震源深度超过20 km的地震主要分布在忻定盆地以南地区,太原盆地两侧的地震集中区形成两个延伸深度最大的南北走向的垂直地震密集条带,推测受太原盆地两侧两个近南北走向的活动的深大断裂控制。太原盆地两侧近南北走向的两个活动深大断裂如果贯通,有可能发生7级以上强震。同时,研究结果显示山西断陷带地壳的地震波速结构变化剧烈,该断陷带下方的地壳普遍表现为低速,但其中太原盆地下方地壳的波速略高,其东北侧和西南侧断陷盆地下方的地壳则表现为更低的波速;与此相反,其西北侧和东南侧紧邻太原盆地的两个小区域下方的地壳则表现为明显的高速,大同西部区域下方的地壳也表现为明显的高速。这些波速特征都与地表构造以及地表热流值有很好的对应关系,太原盆地东北侧和西南侧都可能有热物质上涌,并且可能侵入西部的鄂尔多斯地块内部;相反地,热物质可能没有侵入太原盆地西北侧、太原盆地以及太原盆地东南侧下方的地壳中,说明太原盆地的拉张裂开可能并不是受热物质上涌控制,而是受青藏高原的推挤力控制。

To study the seismic activity and physical background of Shanxi rift zone, we use tomoDD to relocate earthquakes recorded by China network from 1990 to 2008 and the data recorded by a local network run by Shanxi Earthquake Bureau from 2012 to 2016, and show three-dimensional seismic velocity structure in Shanxi rift zone. The events are concentrated around the known faults in the Shanxi rift zone, mainly on the NE and SW sides of the Taiyuan Basin. Most of the focal depths are shallower than 30 kilometers. The earthquakes in the northern part of the research area are shallow while the earthquakes deeper than 20 kilometers are mainly located to the south of the Xinding Basin. Among them, the two densely distributed seismic zones on the NE and SW sides of the Taiyuan Basin with strikes pointing NS directions form sub-vertical distributed seismic zones, which may be controlled by two strike-slip deep fracture zones. If the Taiyuan Basin is broken and the two strike-slip faults are connected, there might be strong earthquakes with magnitude 7 or higher. The results show that the velocity in Shanxi Rift Zone varies intensely. The velocity in the crust of Shanxi Rift Zone is generally low, but the velocity in the crust of the Taiyuan Basin is not the lowest when the velocity in the crust of its NE and SW is especially low. In contrast, to the NW and SE of the Taiyuan Basin as well as the west of Datong the velocity in the crust is high. These correspond well with geological features on the surface and the heat flow observations. The results show that there might be thermal materials upwelling beneath the Shanxi Rift zones to NE and SW of the Taiyuan Basin and the hot materials might invade into the interior of the Ordos block. On the contrary, the hot materials most probably do not invade into the crust of the Taiyuan Basin, as well as its NW and SE surrounding areas. We deduce that the extension of the Taiyuan Basin might not be controlled by the upwelling of hot materials but be controlled by horizontal extensional force produced by the push of the Tibettan Plateau.