Precursory tidal triggering and b value variation before the 2011 Mw 5.1 and 5.0 Tengchong, China earthquakes

Recent studies have suggested that tidal triggering may preferentially occur when a region is critically stressed, and that tidal stresses provide a potential tool for forecasting earthquakes. Most tidally triggered precursors before large earthquakes are found in subduction zones. We examined tidal triggering of earthquakes near the intracontinental Gaoligong Shear Zone, which is also a geothermally active region. We observed statistically significant correlations between tidally induced stresses and earthquake times, which only appeared approximately two years before the 2011 Tengchong M-w 5.1 and 5.0 earthquakes. The Schuster spectrum shows two peaks at similar to 0.5and similar to 1.0 days related to the tidal period. The tidal phase distribution in the pre-mainshock time period shows that most earthquakes occur during phases of positive tidal shear stress and dilatancy strain. Our findings provide a typical case of tidal triggering of earthquake precursors in a continental setting. The stress state can be inferred using the Gutenberg-Richter b value, which characterizes the relative proportion between small and large earthquakes and correlates negatively with differential stress. We investigated the temporal changes of the b value and found that it retains a long-term low value but has a prominent short-term decline prior to mainshocks. The coincidence of low Schuster probability p(s) with a low b value suggests that the crustal medium is near a critical state, making it more sensitive to external perturbations. The results of epidemic-type aftershock sequence (ETAS) modelling revealed that the seismic activity is affected by not only cascading earthquake triggering but also external forcing. Taking the collective results of the Schuster test, b value, ETAS modelling, and other evidence, we suggest that tidal triggering of earthquakes is likely when the fault is in a near-critical stress state, and geothermal fluid may play an important role in the triggering mechanism. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Recent studies have shown that tidal triggering of earthquakes is more likely to occur when the crustal medium is near a critical state, and tidal stresses can be used to forecast earthquakes. Research conducted near the Gaoligong Shear Zone found statistically significant correlations between tidally induced stresses and earthquake times approximately two years before the 2011 Tengchong Mw 5.1 and 5.0 earthquakes.