基于密集台阵研究龙门山断裂带南段地震空段的地震活动性

2016年12月—2018年4月间布设于汶川、芦山地震之间地震空段的密集监测台阵(LmsSGA)提供了密集的观测数据.通过拾取地震走时、初始定位,计算地方震级,得到了完备性震级为0级的地震目录.更加完备的地震目录为地震空段及周围地震活动的时空分布特征和孕震风险性评估提供了丰富的信息.重定位结果显示地震主要集中于龙门山断裂带深度为5～20km的孕震层内.地震活动频繁的汶川、芦山主震区,震源的空间分布模式与其早期余震相似,说明两次大地震的区域仍处于缓慢的应力调整阶段.青藏高原物质东向挤出受宝兴、彭灌杂岩阻挡,在两个杂岩体西北侧地震活动频繁.地震活动性分布显示汶川—茂县、映秀—北川断裂上存在一个清晰的长约30km,宽约20km的地震活动"空白"区域,与其下方因部分熔融而产生的低速体分布一致,我们推测熔融体的加温作用是导致空段内极低的地震活动性的主要原因.监测时段内仍观测到降雨变化率和地震数量呈反相关关系,再次证实了汶川—芦山地震间地震空段及邻区内季节性降雨对地震活动性存在一定调节作用.综合分析S波速度模型、历史强震活动及b值,我们推断地震空段东部的彭灌断裂中段及周围部分隐伏断层存在发生强震的风险.

The seismicity in the southern Longmenshan fault zone based on a dense seismic array

A dense seismic array deployed around the seismic gap between the Wenchuan and Lushan earthquakes provided a continuous recording from December 2016 to April 2018. An earthquake catalogue consisting of 2227 earthquakes with complete magnitude of M_c=0 was obtained after manually picking P and S first arrivals, locating and calculating local magnitudes. The complete catalogue carried abundant information about the spatiotemporal distribution of seismicity and earthquake hazard risk. Relocated using hypoDD, focal depths of all earthquakes range from 5 to 20 km, which is consistent with the seismogenic depth of Longmenshan fault zone. Most of events occurred in the source region of the Wenchuan and Lushan earthquakes, and the characteristics of depth expansion is similar to that of early aftershocks. At the same time, the eastward movement of the Tibetan plateau is blocked by Baoxing and Pengguan complexes, forming frequent seismic activities in the northwest side of two complexes. Relocation results clearly delineated a ~30 km long and ~20 km wide seismicity-silent zone on Wenchuan-Maoxian and Yingxiu-Beichuan fault. The seismicity deficit zone is coupled with the low-velocity zone related to partial melting. This phenomenon indicates that the heating effect of the partial melting in the lower crust is a main reason for the abnormally low seismicity. Anticorrelation between seismicity and monthly precipitation variation rate is also observed during the monitoring period, which means earthquake rates is modulated modestly by the seasonal precipitation in the study region. A comprehensive analysis with the 3D velocity model, historical strong earthquakes and b-value suggests the possibility of future earthquake risk along the middle part of Pengguan fault and neighboring blind faults to the east of seismic gap.