The Prevalence and Significance of Offset Magma Reservoirs at Arc Volcanoes

• Published in: Geophysical research letters Vol. 47; no. 14
• Main Authors: Lerner, Allan H., O'Hara, Daniel, Karlstrom, Leif, Ebmeier, Susanna K., Anderson, Kyle R., Hurwitz, Shaul
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.07.2020
• Subjects: arc volcanoes; Ascent; Centroids; Deformation; Electrical conductivity; Electrical resistivity; Fluctuations; Flux; geodesy; Geological faults; Geophysical studies; Lava; Magma; Magma chambers; Modelling; Monitoring; offset magma reservoirs; P-waves; Reservoirs; Seismic waves; Spatial distribution; Storage; Subduction; Subduction (geology); Subduction zones; tomography; Volcanic activity; volcanic plumbing; Volcano monitoring; volcano topography; Volcanoes
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL087856

Determining the spatial relations between volcanic edifices and their underlying magma storage zones is fundamental for characterizing long‐term evolution and short‐term unrest. We compile centroid locations of upper crustal magma reservoirs at 56 arc volcanoes inferred from seismic, magnetotelluric, and geodetic studies. We show that magma reservoirs are often horizontally offset from their associated volcanic edifices by multiple kilometers, and the degree of offset broadly scales with reservoir depth. Approximately 20% of inferred magma reservoir centroids occur outside of the overlying volcano's mean radius. Furthermore, reservoir offset is inversely correlated with edifice size. Taking edifice volume as a proxy for long‐term magmatic flux, we suggest that high flux or prolonged magmatism leads to more centralized magma storage beneath arc volcanoes by overprinting upper crustal heterogeneities that would otherwise affect magma ascent. Edifice volumes therefore reflect the spatial distribution of underlying magma storage, which could help guide monitoring strategies at volcanoes. Plain Language Summary Magma reservoirs are commonly assumed to be located directly beneath their associated volcanic edifices. This “central reservoir” paradigm dominates volcano modeling and monitoring. However, the actual spatial relations between volcanoes and underlying magma reservoirs are poorly known. We compile a database of geophysical studies at subduction zone volcanoes where magma reservoirs were detected through subsurface modeling of seismic waves, electrical conductivity, and ground deformation. We then systematically map volcano shapes and compare their center locations with associated magma reservoirs. We find that while the majority of volcanoes are located directly above their source reservoirs, a substantial number of magma reservoirs are laterally offset multiple kilometers from their volcano's centers. Approximately 20% of magma reservoirs are located beyond the “footprints” of their volcanoes. Additionally, magma reservoirs are more laterally offset at small volcanoes, but more centrally aligned at large volcanoes. We propose that increased magma flux at large volcanoes thermally overprints crustal faults and heterogeneities, leading to progressively more centrally focused magmatic systems. Our work suggests that the central reservoir view of volcanic systems should be revised to account for magma focusing as volcanoes grow. Recognizing the global prevalence of laterally offset magmatic systems may better help design volcano monitoring networks. Key Points Geophysical imaging suggests that magma reservoirs are often significantly laterally offset from overlying arc volcanoes Offset magma reservoirs are more prevalent at small volcanoes, whereas larger volcanoes have more centrally aligned magma storage Characterizing edifice topography may help predict subsurface magma storage geometries and guide volcano monitoring networks