Migrating Foreshocks Driven by a Slow Slip Event Before the 2021 M_W 6.1 Yangbi, China Earthquake

Abstract

Foreshocks are known to occur before certain large earthquakes, but the physical mechanism is still in debate. Recent field and laboratory studies have supported either cascade‐triggering, pre‐slip or a combination of both models. Here we use a dense seismic and geodetic network deployed in 2018 in Southwest China to quantify the long‐term background seismicity, short‐term spatio‐temporal evolutions of foreshocks and transient deformation before the 2021 MW 6.1 Yangbi earthquake. We find multiple episodes of migrating foreshocks and repeating earthquakes in the last 4 days of the Yangbi mainshock. A rapid migration of microseismicity started in the last 30 minutes toward the eventual initiation point of the Yangbi mainshock, well beyond the rupture zone of the largest MW 5.2 foreshock at that time. The Coulomb stress changes due to relatively large‐size foreshocks at the mainshock hypocenter are positive but relatively small (0.005–0.05 MPa). We also observe continuous geodetic signals with ∼20 mm of cumulative displacement at a nearby GPS station coinciding with the last hour of intense foreshocks. The inverted aseismic moment is equivalent to a MW 5.56 event with the maximum slip of ∼200 mm at 6 km depth, larger than the cumulative seismic slip of ∼60–120 mm inverted from waveforms of several large M4+ foreshocks. In addition, the predicted surface displacements based on the coseismic slips of the M4+ foreshocks are unobservable in the GPS data. We propose a migratory slow‐slip model where a transient slow‐slip event drives the last hour of foreshock sequence and eventually trigger the Yangbi mainshock.