Transient seismic velocity variation accompanying an ML 4.2 earthquake on SE margin of the Tibetan Plateau and its implication for fault slip processes

On 8 July 2020, an M_L 4.2 earthquake occurred in the Xiaojiang fault zone along the eastern margin of the Tibetan Plateau. Applied ambient noise monitoring technique to the continuous waveforms from a near-fault small-aperture array, we obtain daily high-resolution variations in seismic velocity before and after the earthquake. When compared with environmental observations, we exclude these factors such as groundwater level, precipitation, temperature, and atmospheric pressure that might significantly influence the seismic velocity changes. We propose that the observed ∼10-day transitional phase from relatively high velocity to low velocity following the M_L 4.2 earthquake, signifies a transition within the fault zone from a relatively compressional state to an extensional one. This transition could be an indicator of transient dilatation deformation during the long-term strike-slip process of the Xiaojiang fault, which is not easily detected by space geodetic measurements. When the fault zone is in extensional state, there is stronger strain-velocity sensitivity, which is verified by local long-period tidal strain.