Imaging crustal melt beneath northeast Japan with Ps receiver functions

Abstract To investigate crustal structures related to melt storage, Ps converted waves were used to image crustal velocity discontinuities across the Tohoku region of northeast Japan. Ps receiver functions from 127 permanent seismic stations were migrated to depth, accounting for variations in crustal velocity structure. In the 5-15 km depth range, negative Ps phases indicative of velocity decreases with depth are prevalent in central and western Tohoku, where Holocene volcanoes occur and surface wave tomography indicates low crustal velocities. Large negative Ps phases are largely absent in the crust of the old mountain terranes in eastern Tohoku where crustal velocities are higher. In central and western Tohoku, velocity gradients inferred from Ps phases and shear velocity anomalies are consistent with models for crustal melt storage that involve trans-crustal zones of crystal-melt mush where the highest concentrations of melt are localized at the top of the mush column. Negative Ps phases are concentrated at depths of 5-10 km, and many correlate with the upper margins of localized low velocity zones beneath volcanoes. These correlated features are consistent with the roofs of high melt fraction layers. At depths of 10-30 km, positive Ps phases are consistent with gradual velocity positive gradients that correspond to decreasing melt fraction with depth in the mush column, although some groups of deeper negative Ps phases at depths of 10-20 km may reflect local maxima in melt fraction. The amplitudes of the negative Ps phases at 5-10 km depth vary significantly. In a higher amplitude example, waveform modeling of Ps receiver functions from a station near Hijori volcano indicates a 16% ± 5% Vs drop at 10 km depth, implying melt fractions of 10% ± 3% if Gassmann's equations are assumed, above a velocity increase from 10 to 20 km that indicates decreasing melt fractions with depth. However, modeling of lower amplitude Ps phases in the 5-10 km depth range indicates a 7% ± 3% velocity drop and a maximum melt fraction of 5% ± 2%. The geographic distribution of apparent melt fraction at 5-10 km depth suggests that in some cases the same upper crustal source can supply multiple volcanoes, and in others high melt fraction zones are significantly laterally offset from the nearest volcano.