A blind thrust and overlying folding earthquake of the 2016 M (sub w) 6.0 Hutubi earthquake in northern Tien Shan fold-and-thrust belts, China

The 2016 M (sub w) 6.0 Hutubi earthquake occurred in the northern Tien Shan fold-and-thrust belts, China. However, the coseismic fault rupture model and the seismotectonics of the 2016 Hutubi earthquake remain uncertain up to the present due to the small magnitude, deep hypocenter, and no surface rupture caused by the seismogenic fault. Here, we show the evidence that the 2016 Hutubi earthquake occurred on a deep (15-20km) blind-thrust fault, and the rupture of the blind fault triggers the slip of a shallow overlying fold. Ascending and descending Interferometric Synthetic Aperture Radar (InSAR) observations captured by Sentinel-1A/1B satellites are used to construct the seismogenic structure of the 2016 Hutubi earthquake. It is found that a single fault fails to provide a good fit to the observed Sentinel-1 ascending and descending InSAR deformation. A shallow fault-bend fold with strike along the surface trace of the Qigu Anticline and a varying dip angle along the down-dip direction is proposed in this study based on the geodetic observations, which significantly improves the model fit to the InSAR observations. The estimated faulting model shows that the 2016 Hutubi earthquake occurred on a deep blind fault with 264.4 strike and 28.8 dip, and significant fault slip is located at the depth of 13-18km with a peak slip of approximately 10.0cm. The triggered shallow fault-bend fold has a listric surface with slight predominant thrust slip. The static Coulomb stress change from the mainshock on the shallow fold structure is negative, which indicates the dynamic stress may play a more important role than the static stress in the triggering of the shallow fault-bend fold in this case. The result of this study suggests that a moderate earthquake with a deep hypocenter has the potential to trigger the rupture of the shallow overlying fault-bend fold in this zone.