This study analyzes earthquake catalogs within the range of 29°N−37°N and 117°E−124°E from 2014 to 2023. Utilizing the lattice searching method, we focusing on the spatial and temporal variations of the b-value before and after the 2021 M_S4.2 Tianning, Jiangsu earthquake. Additionally, apparent stress is calculated and analyzed using seismic waveforms data from the Shanghai Seismic Network for the same period. By integrating the temporal changes in both b-value and apparent stress, this research reveals the evolution of regional stress before and after the Tianning earthquake, thereby providing valuable insights for forecasting future earthquakes of M_S4.0 and above in the study region.

The completeness and reliability of the earthquake catalog are essential for accurate b-value estimation. Following the removal aftershocks, we determined the spatial distribution of the minimum complete magnitude M_C. Based on the G-R relationship in the study area, the minimum complete magnitude was set at M_L2.0, and the upper limit magnitude was taken as M_L4.3. A total of 1 489 seismic events were selected under these criteria.

By analyzing the spatial and temporal variations in b-value before and after the Tianning earthquake and comparing them with those associated with the 2021 M_S5.0 Dafeng Sea area, Jiangsu earthquake, which occurred about 200 km away, 35 days interval, we found that both earthquakes occurred at or near the edges of the b-value drop regions. Notably, the b-value anomaly zones prior to these two earthquakes did not overlap, suggesting distinct regions of stress accumulation. The temporal evolution of b-value in these anomaly zones also displayed significantly different. Prior to the Dafeng Sea area earthquake, the b-value declined over a relatively short period of about 7 months, followed by a rapid decrease after the mainshock. In contrast, before the Tianning earthquake, the b-value exhibited a prolonged decline lasting about 21 months, with a clear and gradual recovery in the post-seismic period. These differing evolutionary patterns may be attributed to factors such as regional tectonic setting and variations in stress release heterogeneity between the two events.

Apparent stress values for 134 M_L≥2.0 seismic events were determined by calculating their seismic moments and radiated energies. A linear relationship between apparent stress and magnitude was identified in the study area, with a correlation coefficient of approximately 0.66, indicating that larger magnitudes generally correspond to higher apparent stress. To more accurately characterize the dynamic changes in apparent stress surrounding the Tianning earthquake while minimizing the influence of magnitude, the differential apparent stress method was applied. We analyzed the differential apparent stress and its average value for 56 earthquakes in the magnitude range 2.0≤M_L＜3.0 and 36 earthquakes in the range of 3.0≤M_L＜4.0 located near the Tianning earthquake, before and after the event. Pronounced temporal variations were observed: the average differential apparent stress increased prior to the earthquake and decreased afterward. The timing of the stress increase varied with magnitude. For events with magnitude range of 2.0≤M_L＜3.0, the differential apparent stress began to rise significantly in January 2020, whereas for those with the magnitude range of 3.0≤M_L＜4.0, the increase started in May 2020.

The spatial and temporal variations in b-value are influenced by factors such as medium inhomogeneity and fault type, while apparent stress is also affected by seismic source mechanism. Consequently, accurately determining stress changes based on either parameter alone is challenging. However, the Joint analysis of b-value and apparent stress can effectively reduce uncertainty and provide more reliable constrains on regional structural stress evolution. The observed characteristic decrease in b-value, coupled with the increase in apparent stress prior to the Tianning earthquake, indicates a state of stress accumulation in the pre-seismic period. The subsequent stress release during the earthquake led to an increase in b-value and a simultaneous decrease in apparent stress. These findings confirm that the combined changes in b-value and apparent stress can serve as effective indicators for monitoring the process of stress accumulation and release associated with earthquakes.

Therefore, the long-term and continuous monitoring of b-value and apparent stress provides a robust approach for structural stress monitoring. Specifically, the timely identification and tracking of the key phase characterized by a decreasing b-value and a rising apparent stress can effectively reveal ongoing stress accumulation processes. Consequently, this methodology holds significant potential for improving the prediction of earthquakes above M_S4.0 in the Shanghai monitoring region.