THE INFLUENCE OF THE 2014 JINGGU MS 6.6 EARTHQUAKE ON THE SEISMIC RISK OF THE NANTINGHE FAULT ZONE IN YUNNAN PROVINCE, CHINA

The Nantinghe fault zone (NFZ), which divides the southwest Yunnan bock (SYB) into two sub-blocks, is the longest one of NE striking left-lateral strike-slip faults in the SYB with a total length of ∼380 km. Although it is very active in the late Quaternary, only an M ∼ 7 earthquake occurred on its southern segment in 1941 and since then there has been no earthquake of M > 5 recorded on the other segments of the NFZ. Therefore, the NFZ might be in the stage of stress accumulation for a large earthquake. The 2014 Jinggu M_s 6.6 earthquake is ∼94 km southeast away from the NFZ, the long axes of aftershock distribution and seismic intensity contours strike in northwest and point to the NFZ. This study attempts to reveal its influence on the static Coulomb stress on surrounding faults, especially the NFZ, and to analyze the seismic risk of the NFZ.

To solve the problems above, this work combined the paleoseismological trenching and numerical simulation. The paleoseismological trench excavation revealed the paleoearthquake history in Holocene as well as the status of the NFZ. On the other hand, utilizing the coseismic slip distribution models of the Jinggu earthquake, the static Coulomb stress perturbations on surrounding faults, especially the NFZ, were calculated. Integrating these studies, the seismic potential of the NFZ was further analyzed.

Through the paleoseismological trench on the northern segment of the NFZ, we determined a paleoearthquake that generated surface ruptures in Holocene with a magnitude no less than 7. Using radiocarbon dating, the age of this event is constrained to be between 900 AD and 1480 AD, indicating an elapsed time of 535–1115 years. Utilizing two coseismic slip distribution models, our results show that the static Coulomb stress perturbations triggered by the Jinggu earthquake on neighboring faults, such as the Lancangjiang fault and the Jinggu fault, are obviously positive, up to ∼90 kPa. While on farther faults, such as the NFZ, the Longling-Lancang fault zone, and the Wuliangshan fault zone, the increased values are less than 10 kPa. The distribution of the stress perturbation on the fault plane of the NFZ, further computed using different fault parameters, illustrates that the maximum change of stress occurs near the surface at 24.15°N and decreases along the strike and depth. The maximum change of static Coulomb stress on the west branch of the NFZ is 0.89 kPa and the value on the east branch is 1.18 kPa.

Combing the slip rate and elapsed time since the latest paleoseismic event, it is estimated that the slip accumulated on the northern segment of the NFZ could be 2.8+1.5/–1.0 m. Further using the empirical scaling laws between magnitude and displacement, an earthquake with magnitude of 7.5+0.1/–0.2 would be generated by the accumulated slip. Although the change of static Coulomb stress triggered by the Jinggu earthquake indicates that the earthquake does not necessarily produce a sudden acceleration on the seismic risk of the NFZ, under the consideration that the northern segment of the NFZ has accumulated seismic energy equivalent to M ≈ 7.5, it is suggested that this fault segment will probably be in a high seismic risk with positive stress perturbation from the Jinggu earthquake.