Pre- and Postseismic Variations of Crustal 3D Velocity Structure Around the Ludian Earthquake Zone in Southwest China Revealed by Time-Lapse Ambient Noise Tomography

Abstract The 2014 Ms 6.5 Ludian earthquake is a typical event in southwest China, resulting in extensive earthquake damage and disasters. Numerous geophysical observations have been conducted around the Ludian earthquake zone to reveal the seismogenic environment, but the underlying seismogenic mechanism remains a subject of debate. The spatiotemporal variation in crustal velocity is closely related to stress state and seismicity. In this article, we present an ambient noise time-lapse tomography algorithm based on the conventional surface-wave direct inversion method and the parallel difference strategy. We apply this algorithm to the ambient noise data recorded by 90 permanent seismic stations around the Ludian earthquake zone from 2011 to 2017, to construct a 3D time-lapse velocity model of the crust in this region. Based on the results, we find significant variations of crustal S-wave velocity (VS) along most active fault zones of the study area. The VS around the hypocenter of the Ludian earthquake decreased during the preseismic stage, reaching its largest variation of −0.015% ± 0.0027% between 2011 and 2014. This decrease in VS is consistent with the time-lapse variations in the gravity and geomagnetic fields around this area, indicating their common responses to physical state variation in the subsurface. The VS gradually increased during the postseismic stage; however, it had not returned to its preseismic levels by 4 August 2017. We deduce that material migration (such as partial melting or aqueous fluids) within the crust strengthened the interactions between rocks and fluids within the active fault zones and their creep behaviors, altering the stress and physical states and then causing variation in the velocity, gravity, and geomagnetic fields. Thus, our time-lapse model of crustal VS could provide significant support for understanding the seismogenic mechanism of the Ludian earthquake.