Features of Seismicity Anomalies before Strong Earthquakes in California

Before the 15 strongest earthquakes in California with magnitudes $$M \geqslant 6.5$$ from 1984 to 2023, an analysis of seismicity anomalies was conducted using two parameters: the b-value and the RTL function. Spatiotemporal anomalous areas were identified based on known precursors for these parameters. The values of tuning coefficients for the calculation algorithms and catalog filtering parameters selected for the analysis of each earthquake are presented. Characteristics of the anomalies were determined, including their duration, size, and distance from the center of the anomaly to the earthquake epicenter. A comparison of anomaly durations revealed that, for most earthquakes, RTL parameter anomalies occur earlier than b-value anomalies. This sequence of anomaly occurrence differs from the sequence found by the authors previously when analyzing strong earthquakes in subduction zones and rift zones. A significant portion of the earthquakes studied in California occur with a delay of several months after the completion of the bay-like anomalies of the RTL parameter, which reflect the end of the formation of a metastable region in the medium. Meanwhile, abnormally low values of b-value persist until the moment of the earthquake.