[S-IT18] GEOPHYSICAL PROPERTIES AND TRANSPORT PROCESSES IN THE DEEP CRUST AND MANTLE

[SIT18-01] Upper mantle earthquake swarms indicate path and velocity of fluid ascent beneath arc volcanoes

★Invited Papers

*Lloyd T White¹、Nicholas Rawlinson²、Gordon Lister³、Felix Waldhauser⁴、Babak Hejrani⁵、David Thompson⁶、Dominique Tanner¹、Colin Macpherson⁷、Hrvoje Tkalčić⁵、Jason Phipps Morgan⁸ (1.School of Earth, Atmospheric and Life Sciences, University of Wollongong, Australia、2.Bullard Laboratories, Department of Earth Sciences, University of Cambridge, United Kingdom、3.Sustainable Minerals Institute, University of Queensland, Australia、4.Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, Columbia University、5.Research School of Earth Sciences, Australian National University, Australia、6.Omanos Analytics, United Kingdom、7.Department of Earth Sciences, Durham University, United Kingdom、8.Department of Ocean Science and Engineering, Southern University of Science and Technology, China)

キーワード：seismicity, mantle wedge, hypocenter, tectonics, subduction, melt

The world's most explosive volcanoes are found in volcanic arcs where magmatism is driven by fluid-fluxed melting of upper mantle rocks. The fluids that cause melting are released from subducted tectonic plates as they sink into the mantle. While this process is considered common knowledge, we know little about the path of fluid and melt migration through the upper mantle. We also know relatively little about the time required for fluids and melts to migrate from the subducted plate, through the upper mantle to a magma chamber beneath an arc volcano. Here, we report two rare examples of earthquake swarms in the upper mantle beneath the Marianas and Izu-Bonin arc systems. The earthquake swarms occur in pipe-like structures. The pipe structures were first identified using the Lamont-Doherty Global CMT database, using the eQuakes program to compare centroid and hypocenter locations. We further tested and verified the robustness of depth locations using several methods. Centroid moment tensor solutions were also calculated using a 3D Earth model to understand failure mechanisms within the swarms. The best-resolved example occurs beneath the Marianas arc, where the earthquake hypocenters define a sub-vertical structure with a resolved diameter of ~50 km and depth extent of ~200–250 km between the subducting slab and overriding plate. The Izu-Bonin example shares a similar sub-vertical pipe-like geometry. In both examples, the seismic activity occurs within a two-year period, during discrete day- to month-long swarms. The geometry of the pipe-structures implies that these deep earthquakes record the ascent of hydrous melt and/or fluid, presumed to originate from dehydration of the subducting plate. These results imply that hydrous minerals within a subducted slab continue to dehydrate to depths of at least 200 to 250 km. The short duration of the earthquake swarms within the pipes also implies that fluids/melts can be rapidly transported through the sub-arc mantle at rates in the order of km/hr. These rates are similar to values determined from geospeedometry and experimental petrology. This work indicates that fluids are not transported through the mantle wedge by diapirism, but through sub-vertical pathways facilitated by fracture networks and dykes.