滑行波震相对珠江口地区壳内低速层的约束作用

Constrains of sliding wave phases on the low-velocity layer in the Pearl River Estuary

深入研究珠江口地区海陆过渡带壳内低速层的结构和构造特征对于理解板内地震的发震机理、孕震构造及该区域的地壳结构具有重要的地质地球物理意义。利用2015年珠江口区域海陆地震联测L2-ME测线上的19个地震台站(包括陆上台站14台,海底地震仪5台)记录到的地震数据来探明该区域低速层的结构和构造特征。在常规震相的基础上,加入了大量的滑行波震相(Ph)进行结构模型计算,Ph震相的增加使得地壳内部10~20km范围内的射线覆盖密度有了显著提高,从而获得了L2-ME测线下方更为精确的地壳纵波速度结构模型。结果发现,模型中测线下方13~18km深度范围内稳定连续展布的壳内低速层被清晰成像,其内部速度稳定在5.7~6.0 km·s^–1之间,与上下层界面速度差分别为0.5km·s^–1、0.4km·s^–1,低速特征明显。该低速层厚度由陆侧的3.5km左右降至海侧的1km,呈现出向海侧逐渐减薄的趋势,低速层底界面起伏变化较大且具有与莫霍面相似的起伏特征。

Exploring the structure and tectonic characteristics of the low-velocity layer(LVL)in a transition zone of the Pearl River Estuary(PRE)area has significant geological and geophysical implications for understanding the mechanism of intraplate earthquakes,seismogenic structure and crustal structure in this area.In this study,we use the data from 19 seismic stations(include 14 onshore seismic stations and five ocean bottom seismometers)along the L2-ME profile of the 2015 onshore-offshore deep seismic experiment in the PRE area to image structure features of the LVL.On the basis of the conventional seismic phase,a large number of sliding wave seismic phases are added to calculate the structural model.With the increase of Ph seismic phases,the ray coverage density within the range of 10–20 km was significantly increased,and a more accurate P-wave velocity structure model of the LVL and its above crustal structure along the L2-ME profile were obtained.The result shows the LVL is clearly imaged in the crust within the depth range of 13–18 km under the profile.The velocity in the LVL is roughly stable in the range of 5.7–6.0 km·s^–1 and has an obvious low-velocity anomaly feature of 0.5 km·s^–1 difference compared with its top layer and of 0.4 km·s^–1 difference compared with its bottom layer.Its thickness decreases from 3.5 km on the land side to 1 km on the sea side,showing a trend of gradual thinning toward the sea side.The bottom boundary of the LVL has similar fluctuation characteristics to the Moho.