3D geometry of the Lanliao Fault revealed by seismic reflection profiles: Implications for earthquake clustering in the Dongpu Sag, North China

Abstract

Six destructive earthquakes occurred in the Dongpu Sag and surrounding regions from 1493 to 1983, and more recent earthquakes have also been concentrated in these areas. Relocation results show two active earthquake clusters in and around Dongpu Sag, one in the north and one in the south. However, the dynamic mechanisms leading to these earthquake clusters have not previously been established. In this study, we use seismic reflection profiles to develop a three-dimensional geometric model of the Lanliao Fault, which marks the eastern boundary of the Dongpu Sag. This model shows that the Lanliao Fault is a listric strike-slip fault on a regional scale. The northern fault segment is steep (>45°) at >8 km depth, while the southern segment becomes a gentle detachment below 8 km. Shallow seismic reflection profiles combined with analysis of borehole cores indicates that the northern segment of the fault appears to have reverse kinematics at <70–80 m depth. Earthquakes that occurred from 2008 to 2018 in the northern Dongpu Sag are controlled by the Lanliao Fault. A reduction in accommodation space and the presence of a restraining bend along the fault appear to be the main dynamic mechanisms associated with the northern earthquake cluster. The focal depths of the earthquake cluster near Heze City are deeper than those in the northern cluster, and relocated earthquakes and focal mechanism solutions show that the two clusters occur on different seismogenic faults. This study has important implications for the dynamics of seismogenic strike-slip faults and related earthquake hazards.