Interaction between Fluids and Faults Based on Seismicity: Case Studies in Western Yunnan, China

Due to the collision between the Indian and Eurasian plates and the southeastward compression of the Tibetan Plateau, western Yunnan is one of the regions in China with the most active seismic and volcanic activities as well as the most complex fault structures. We first built a deep-learning-based high-precision earthquake catalog for the Tengchong volcanic field over the past decade and found that 1) ∼59% of the seismicity occurred as swarms but on faults aligned with the regional tectonic stress field; 2) all swarms contained fluid-diffusion-like migration fronts; and 3) a year-long swarm, including two M_L 5.2 earthquakes within two months, revealed complex fluid-fault interaction. Combined with the historical occurrences of M >6 earthquake swarms around the Tengchong volcanic field, our observations suggest potential increased likelihood of swarms with large-magnitude earthquakes where large tectonic faults and magmatic systems intersect.

With the aid of machine-learning-based detection, we then outlined a complex 3D fault zone accommodating a small earthquake swarm near Yunlong city, western Yunnan, China from February to May 2013. Our results showed that the swarm initiated from a compressive stepover zone and subsequently activated a complex fault zone including six planar fault segments. The migration front of the swarm can be well-modeled by fluid diffusion, indicating the swarm was primarily driven by pressurized fluid. Within the stepover zone, complex and dense fractures act as conduits connecting the reservoir and fault zone, facilitating fluid flow. Meanwhile, the stress in the stepover zone tends to increase in response to the compression of the two boundary faults, which not only makes the stepover zone more susceptible to be triggered by those transient stresses but also forms a fluid pumping mechanism that drives fluids from the stepover zone into the complex fault zone.

By integrating a deep-learning-based phase picker and an improved match-and-locate algorithm, we constructed a high -precision foreshock catalogue for the 2021 Yangbi earthquake. Meanwhile, a high-resolution earthquake source region velocity structure for the 2021 Yangbi sequence was also inverted in our study. Our results suggest that natural fluid diffusion is likely a driver of the Yangbi foreshock sequence based on three lines of evidence: 1) regions with low Vs and relatively high Vp/Vs are widespread within the fault system; 2) earliest foreshocks exhibit diffusion-like migration front, and 3) foreshock evolution coincides with typical fault valving behavior, and a few low-frequency signals whose distribution coincides with high Vp/Vs patches was clearly identified, strongly suggesting that fluid diffusion influenced nucleation.

Therefore, we propose that the interaction between the widely distributed fluids and faults in western Yunnan may play a crucial role in the seismic activity.