Pre- and post-seismic stress-fluid coupling revealed by b-value and GRACE-derived EWH anomalies: insights from the 2012 Ahar–Varzaghan doublet earthquakes, northwest Iran

Abstract

The Ahar–Varzaghan doublet earthquakes (Mw 6.4 and Mw 6.2) in northwestern Iran offer a unique natural laboratory for examining the interplay between seismotectonic processes and hydrological variations across an entire seismic cycle. In this study, we integrate high-resolution spatiotemporal analyses of b-value derived from the Iranian Seismological Center catalog with GRACE Equivalent Water Height (EWH) observations (2006–2024) to assess stress–fluid interactions before and after the mainshock. The pre-seismic stage is characterized by a steady decline in b-value from approximately 0.85 to 0.55 over the 3 years leading up to the event, reflecting increasing differential stress and progressive fault locking. During the same interval, cumulative EWH anomalies exhibit a pronounced mass deficit, reaching approximately − 27 cm and peaking 2–6 months before rupture. This negative anomaly is consistent with poroelastic compaction or fluid withdrawal driven by compressive loading in the near-field of the impending rupture. Following the mainshock, the system evolves in the opposite manner: b-value sharply increases to 0.72–1.02 within 1 year, forming a high b-value zone (> 0.9) surrounding the rupture area, indicative of stress release and widespread microfracturing. In contrast, EWH anomalies remain strongly negative, reaching approximately − 20 cm in the epicentral region, suggesting that fluid drainage persists despite the enhanced fracture permeability generated by the earthquakes. This sustained inverse relationship between b-value and EWH anomalies implies a decoupling between mechanical recovery and hydrological re-equilibration. The results highlight the complementary roles of these parameters: b-value effectively captures short-term stress evolution, whereas EWH anomalies reflect longer-term mass redistribution.