Retrospective earthquake spatial forecasting: magnetotelluric insights into fluid-driven intraplate seismicity in northern Thailand from 1994 to 2024

This study investigates the role of crustal fluids in the seismogenesis of three significant intraplate earthquakes in northern Thailand: the 1994 Phan, 2014 Mae Lao, and the unexpected 2019 Wang Nuea events, which occurred after a decade of seismic quiescence. Utilizing 3D resistivity models derived from integrated 2015 and 2024 magnetotelluric (MT) surveys, we reveal complex, interconnected conductive systems beneath each seismically active region, indicative of fluid pathways extending from the lower to the near-surface crust. For the 2014 Mae Lao earthquake, a lower crustal fluid source ascends through a mid-crustal conductor with northward and southward extensions. A shared deep fluid source is implicated in the 1994 Phan earthquake; however, fluids follow a distinct southward migration pathway. The 2019 Wang Nuea earthquake also exhibits comparable lower and mid-crustal conductive structures and inferred fluid ascent, mirroring the Mae Lao system. Notably, a mid-crustal conductor was identified beneath the Wang Nuea region in our initial 2015 MT survey, but with the decades of seismic quiet, the link between this crustal fluid and the eventual earthquake was not anticipated, representing a missed opportunity for early spatial forecasting of the 2019 rupture. In all three events, the presence of fluids within the fault zones likely reduced their strength, leading to rupture when tectonic stress exceeded the critical threshold. The consistent presence of these conductive systems across these intraplate earthquakes suggests a unique characteristic of seismogenesis in northern Thailand, providing a valuable geophysical signature for future seismic hazard assessment and spatial forecasting.