Using borehole core analysis to reveal Late Quaternary paleoearthquakes along the Nankou-Sunhe Fault,Beijing

The Nankou-Sunhe Fault is a buried active normal fault that traverses the urban area of Beijing. Its seismic risks have caused considerable concerns. This paper studies paleoearthquakes along this fault by analyzing and correlating bore-hole cores obtained from triple-tube coring, incorporating experience acquired from trenching. As a result, a model for identifying earthquake-derived colluvium by sediment-core analysis is proposed. Triple-tube coring technique is useful to collect continuous undisturbed soil core near the Nankou-Sunhe Fault. By identifying fault-scarp colluviums, determining cumulative displacement, and analysing stratum thickening on the hanging wall, we are able to establish a preliminary paleoearthquake sequence consisting of 13 surface-rupturing events since 60 ka. The seismic history can be divided into three periods based on different recurrence intervals. Between 60 and 40 ka, three earthquakes occurred with recurrence interval of ∼10 ka. From 40 to 25 ka, there were six earthquakes with the recurrence interval of about 2.5 ka. In the last 25 ka, four earthquakes have taken place with the recurrence interval varying considerably. The recurrence interval between the last three events is ∼5 ka. Smaller recurrence intervals correspond to stages of faster fault slip. The coseismic displacement of a single event is 0.8 to 2.2 m, average 1.4 m, largely equivalent to moment magnitudes 6.7–7.1. This study demonstrates the feasibility of bore-hole drilling in investigating paleoearthquakes along normal faults. It also suggests that closely spaced boreholes with continuous undisturbed cores are essential for reconstructing the complete paleoearthquake sequence. Supported by the National Development and Reform Commission of China (Grant No. 20041138) and the Joint Foundation of Seismology (Grant No. 101103)