Very‐Long‐Period Seismic Signals During Lateral Dike Propagation at Piton de La Fournaise Volcano

Abstract We report a previously undocumented sequence of very long period (VLP) signals preceding the September 2022 eruption at Piton de la Fournaise. These events occurred during lateral magma propagation and exhibit quasi‐periodic sawtooth‐shaped displacement transients. Seismic source analysis reveals a persistent tensile‐crack source located near the transition between vertical and lateral intrusion paths, consistent with a repeatedly activated process. We interpret the VLP sequence as the manifestation of a valve‐like mechanism in which magma accumulates within a shallow transfer zone until a pressure threshold is exceeded, triggering rapid crack inflation and transient magma transfer into the propagating dike. The subsequent source evolution is less well constrained by the band‐limited seismic data. These observations highlight intermittent shallow magma transfer near geometric transitions despite an approximately constant magma supply. From a forecasting perspective, this study suggests that regular VLP pulses may indicate lateral magma migration toward volcanic flanks.