Variations in apparent stress and b value in the rupture area before the 2019 M_S6.0 Changning earthquake

Both apparent stress and b value can reveal stress changes in the crust. As the stress increases, the apparent stress increases and the b value decreases. Therefore, the stress variation could result in a negative correlation between apparent stress and b value so that the variation of crustal tectonic stress can be investigated according to this negative correlation. In present study, on the basis of apparent stresses for earthquakes （3.0≤M_L≤3.6）and earthquakes （2.0≤M_L≤4.3）occurring in the aftershock region of the 2019 M_S6.0 Changning earthquake from January 2000 to December 2019, we analyzed the temporal evolutions of the apparent stress and b value. It was found that from February 2013 to March 2017, the apparent stress showed a rising trend lasting for about 4 years. Within the two years before the earthquake, the apparent stress remained at a higher level. During the period when the apparent stress was increasing, b value was decreasing. When the apparent stress maintained at a higher level, b value maintained at a lower level with no distinct change. That is, before the Changning M_S6.0 earthquake occurred, the apparent stress in the rupture zone increased, while the b value decreased. It indicates that there is an obvious increase of the tectonic stress in the fracture zone before the M_S6.0 Changning earthquake, which has important implications for the understanding of earthquake gestation process and earthquake prediction.