The Japanese archipelago which lies within the Pacific ring of fire suffers from frequent earthquakes and magmatic activities that seriously endanger human life. To monitor such natural hazards, several dense and high-quality seismic arrays, for example, F-net, Hi-net, and V-net were deployed in the whole archipelago. These data are not only for the early warning but also allowing us to decipher seismic structure beneath this place.
Northeast Japan, or the Tohoku region, is the “simplest” area in such a tectonically complex area for fundamental study of subduction-magmatism system that will be the basis for the other similar regions. Here, the Pacific plate westward subducts beneath the Eurasian plate with a speed rate of 8-9 cm/yr and Holocene arc volcanoes dominate in the central belt and lie more sporadically in the west. In northeastern and southeastern Tohoku, the Kitakami and the Abukuma mountains are composed of Cretaceous sedimentary and plutonic rocks. The seismic structure of Tohoku was broadly investigated in the past few decades, particularly in the mantle. In contrast, crustal-scale study is increasing in recent years, contributing to the improvements in data quality and analysis method.
In the first part of this thesis, both crustal shear velocity and azimuthal anisotropy in Tohoku are investigated for crustal magmatism and deformation fabrics by ambient noise tomography (ANT) method. Comparing to traditional body-wave travel-time tomography, this popular method that benefits from short-to-intermediate period surface wave dispersion improves constraint in crustal resolution.
Here, continuous record from 123 NIED Hi-net stations were used to construct empirical Green’s functions (EGFs) between each potential station-pair, and then Rayleigh wave dispersions measured in 3-16 s period from these provide a data set with
a very dense path coverage, allowing us to resolve high-resolution crustal structure by a 3-D, one-step, wavelet-based multiscale inversion that simultaneously inverts for both shear velocity and azimuthal anisotropy models.
The resolved structure shows prevalent low velocity zones with anomaly up to -10%
at a depth of 9-13 km beneath most of the Holocene volcanoes in the central belt and the west, suggesting that magmatism affects the upper crust. In contrast, in northeastern and southeastern Tohoku the Kitakami and Abukuma mountains composed of Cretaceous sedimentary and plutonic rocks both show high anomalies in the whole crust. In crustal azimuthal anisotropy, a two-layer pattern is resolved, which a near N-S, island-parallel fast direction in upper crust that may map structural fabrics of crustal deformation, and a near E-W, convergence-parallel fast direction in lower crust that may result from shearing either imposed by the return flow in the mantle wedge or frozen-in from the last stage of extension of the continental margin. Between these two layers, at a depth of 5-15 km, where melt storages are thought to reside, coherent crustal fabrics collapse and anisotropy turns to a chaotic state, demonstrating that continuous volcanic activities and magma ponding disrupt the otherwise consistent fabrics. This work has been published on Journal of Geophysical Research at 2018 (Chen et al., 2018) and is introduced in Chapter 2.
In the second part of this thesis, crustal structure related to melt storage was investigated from Ps converted waves across Tohoku. The converted body wave phases have potential to image localized vertical velocity gradients, comparing to velocity model from body wave or surface wave dataset. A better understanding of structure and behavior in crustal magmatic systems, particularly in upper crust, is key to deciphering the last stages of magma evolution and transport before volcanic eruption.
We collected 1773 teleseismic event data that occurred from October 2004 to July 2018 recorded by 127 permanent stations maintained by multiple seismic networks
(including NIED Hi-net, JMA volcanic seismometer network, Tohoku University, Tokyo University, and Hirosaki University). To calculate Ps receiver functions, P component was deconvolved from SV component with iterative time-domain deconvolution. The resulting receiver functions were migrated to depth using station-specific 1-D velocity model that incorporated a number of geophysical observations, including seismic reflection survey, shallow earthquake travel-time data, previous receiver function result, regional body wave tomography, and ambient noise tomography. Then, both positive and negative phases on the migrated receiver functions were extracted to decipher their origins by collaborating with crustal shear velocity model.
In northeastern and southeastern regions, the Cretaceous mountains show that negative phase amplitudes are typically small or phases are absent at depths of 5-15 km.
In contrast, in central and western Tohoku, negative Ps phases are prevalent at a depth range of 5-10 km and correlate with the upper margins of localized low velocity zones beneath volcanoes, which this feature is consistent with model for crustal melt storage where the highest concentrations of melt are localized at the top the mush column. For these negative phases, waveform modeling of Ps receiver function indicates a variation in shear velocity drop at depths of 5-10 km, implying a variety of melt fractions across the Tohoku. The geographic distribution of apparent melt fraction suggests that in some cases the same upper crustal source can supply multiple adjacent volcanoes, and in a few other high melt fraction zones are significantly laterally offset from the nearest volcano.
This work has been published on Earth and Planetary Science Letters at 2020 (Chen et al., 2020) and is introduced in Chapter 3.