The plausible relationship between deep long-period earthquakes and volatiles: a case study of the Alaska-Aleutian arc

Deep long-period (DLP) earthquakes are observed beneath volcanoes in various tectonic settings during background activity, unrest, and eruption. The exact processes generating DLPs have been subject to ongoing debate, as they may provide insight into deep magmatic plumbing and the transport of magma and fluids. We hypothesize that DLP occurrence is related to volatile movement through the transcrustal system and reflects the “openness” of the underlying deep volcanic structure for volatile exsolution. The robust seismic catalog and extensive geochemical data along the Alaska-Aleutian arc offer a unique opportunity to test this hypothesis. We combine 34 years of Alaska Volcano Observatory seismic data (1989–2023) with geophysical and geochemical constraints on magma storage and volatile exsolution depths from 22 volcanoes along the arc. DLPs are most frequent at 10–25 km and ~ ∼ 32 km below sea level (BSL)—depths that correspond to the observed onset of crystallization determined by melt inclusions and near-Moho volatile saturation, respectively. We use melt inclusion volatile data and solubility modeling to demonstrate that CO 2 concentrations needed to initiate exsolution at DLP depths (10 to 55 km BSL) generally align with or exceed reconstructed parental CO 2 estimates determined using two independent methods—trace elements and mass fluxes. CO 2 concentrations predicted from the deepest DLP at each volcano increase from west to east along the arc, broadly mirroring the estimated parental CO 2 concentrations. This suggests that volatile accumulation and overpressure in the lower crust are key contributors to DLP generation. By combining arc-scale geophysical and geochemical data, we demonstrate that DLPs correlate with volatile exsolution zones, suggesting that near-Moho DLPs are driven by first boiling during magma rise, and crustal DLPs are related to magma stalling and second boiling.