The problem of identifying and interpreting precursors of strong earthquakes remains one of the key challenges in modern seismology. Despite significant progress in understanding earthquake source physics, reliable medium-term forecasting continues to pose a serious scientific problem. Of particular interest is the analysis of seismicity anomalies that manifest during the preparation periods of megathrust earthquakes, as they may reflect the processes of stress accumulation and redistribution in the lithosphere. Studies on the dynamics of seismicity parameters with different physical interpretations prior to mega-events are limited in the literature; one example is the 2011 Tohoku earthquake (Mw = 9.1), which was preceded by anomalous changes in seismicity parameters for years and months before the event [1, 2].
This study is dedicated to a comprehensive analysis of the manifestation of seismicity anomalies before the megathrust earthquake in Kamchatka (29.07.2025, M8.8) and in Japan (11.03.2011, M9.1). The work aims to identify common and specific features in the evolution of seismicity by comparing "structural" parameters reflecting the development of fracture (the b-value of the frequency-magnitude distribution and the RTL parameter) and the LURR (Load–Unload Response Ratio) parameter, which reflects the proximity of the stress-strain state of the medium to a critical level.
The b-value and RTL parameters are widely used for analyzing seismicity anomalies [3–5]. A decrease in the b-value is associated with the enlargement of fracture sources and an increase in the proportion of relatively strong events, while the RTL parameter allows for the identification of stages of seismic quiescence and subsequent activation [5]. The LURR method, proposed in [6] and developed in subsequent works (e.g., [7, 8]), is based on the difference in the medium's response to additional compressive and tensile loads, which, in the case of natural seismicity, are induced by luni-solar tides. As the stress-strain state of the medium approaches a critical level, the relationship between stress and strain becomes nonlinear, leading to an asymmetry in the response, which is captured by the LURR parameter.
In this study, LURR calculations were performed using a moving time window (90–180 days) with a 1-day step, incorporating the dilational tidal component. To assess the statistical significance of parameter variations, a procedure for generating synthetic earthquake catalogs based on a Poisson process model with magnitudes distributed according to the Gutenberg–Richter law was proposed and implemented. Anomalies were defined as exceedances of the 90th percentile relative to the synthetic LURR curves.
For the Kamchatka megathrust earthquake, quiescence according to the RTL parameter is recorded 1.6 years before the main event, and a monotonic decrease in the b-value begins 0.6 years prior. Statistically significant increases in LURR are observed mainly within the projection of the future rupture plane against the backdrop of an already initiated b-value decrease, indicating the heightened sensitivity of the structurally weakened medium to periodic tidal loads.
A different scenario was identified for the 2011 Tohoku earthquake. RTL quiescence is observed starting 3.5 years before the event and lasts for about 1.1 years, while the b-value decrease begins 0.8 years prior. Notably, the LURR anomaly appears before the onset of the b-value decline and is localized in the same area where the RTL and b-value anomalies occur. The sequence of anomalies is as follows: an increase in LURR, a short-term increase in b-value, followed by a sharp decrease in b-value.
Comparison of the temporal variations of b-value and LURR reveals a different order of anomaly manifestation for the two mega-events. In the case of Kamchatka, the primary process is the structural redistribution of sources (b-value decrease), whereas for Tohoku, the anomaly reflecting the change in the stress-strain state (LURR) appears first. The identified differences may correspond to distinct scenarios of megathrust earthquake preparation, governed by tectonic segmentation, structural heterogeneity, and the specific characteristics of seismogenesis in the respective regions.
The obtained results demonstrate that the integrated use of RTL, b-value, and LURR parameters not only allows for the identification of anomalous stages in the preparation of megathrust earthquakes but also facilitates a qualitative classification of their formation scenarios. This is of fundamental importance for the development of physical models of seismogenesis and the improvement of medium-term earthquake forecasting methods.