Application of the load/unload response ratio method to study complex earthquake sequences

Accurately assessing post-earthquake trends is of significant importance for seismic hazard evaluation and disaster relief. Following a significant event, the degree of stress accumulation and releasing in the source region determine subsequent seismic activity level. The Load/Unload Response Ratio (LURR) can identify regional stress states based on the response changes of the lithospheric medium to the loading and unloading processes during solid Earth tide cycles. The high consistency of focal mechanisms within the same earthquake sequence makes it feasible to apply the LURR method for tracking and analyzing seismic sequences. Applying this approach, we investigated 2 relatively complex earthquake sequences that occurred on Chinese mainland in 2024. The first example is the MS4.7 Feidong, Anhui earthquake on September 18th. Results show that the LURR time series climb to anamalous high peaks short-term before the mainshocks at different time periods. And the second one is the MS7.1 Wushi, Xinjiang earthquake on January 23rd. The LURR time series decreased rapidly from anomalously high values to near 1.0 after the mainshock, while it peaked right before the MS≥5.0 aftershocks within the sequence. Note that the anomaly zone coincided spatially with the locations of these earthquakes. The study demonstrates that LURR can provide relatively accurate predictions of earthquake occurrence when the regional tectonic shear stress direction can be clearly obtained from the focal mechanism of the mainshock.