Makoto Matsubara

Low-velocity oceanic crust at the top of the Philippine Sea and Pacific plates beneath the Kanto region, central Japan, imaged by seismic tomography

[1] We construct fine-scale three-dimensional P and S wave velocity structures beneath the Kanto region, central Japan, by seismic tomography with a spatial correlation of velocities. The Philippine Sea and Pacific plates subduct beneath the Eurasian plate in this area and are imaged with high velocities. Oceanic crust at the uppermost part of these subducting plates is a low-velocity layer. Low-velocity oceanic crust of the Philippine Sea plate subducts to a depth of approximately 80 km. There are also two low-velocity bodies with relatively high V P /V S ratios of 1.80-1.90 in the mantle wedge above the oceanic crust of the Philippine Sea plate. We speculate that the westernmost of these two low-velocity bodies consists of 20% partially serpentinized peridotite, continuous with gabbro in the oceanic crust of the uppermost Philippine Sea plate, while the eastern body is composed of 30% partially serpentinized peridotite. We trace the subducting oceanic crust of the Pacific plate to a depth of ∼120 km. The estimated V P /V S ratio of this layer is 1.85-1.90, which indicates a low probability of molten rock; the gabbroic oceanic crust may have been metamorphosed to garnet-granulite.

Deep seismic reflection profiling across the Ou Backbone range, northern Honshu Island, Japan

Knowledge of the crustal structure, especially the geometry of seismogenic faults, is key to understanding active tectonic processes and assessing the size and frequency of future earthquakes. To reveal the relationship between crustal structure and earthquake activity in northern Honshu Island, common midpoint (CMP) deep reflection profiling and earthquake observations by densely deployed seismic stations were carried out across the active reverse faults that bound the Ou Backbone range. The 40-km-long CMP profiles portray a relatively simple fault geometry within the seismogenic layer. The reverse faults merge at a midcrustal detachment just below the base of the seismogenic layer, producing a pop-up structure that forms the Ou Backbone range. The top of the reflective middle to lower crust (4.5 s in travel time (TWT)) nearly coincides with the bottom of seismogenic layer. The P-wave velocity structure and surface geology suggest that the bounding faults are Miocene normal faults that have been reactivated as reverse faults.

A low velocity zone beneath the Hida Mountains derived from dense array observation and tomographic method

Seismic waves suffer strong attenuation when propagating beneath the Hida Mountains in Central Honshu, Japan. In order to study this region in detail, we conducted three kinds of dense seismic array observations in and around the Hida Mountains in the summer of 1996. Picking P- and S-wave arrival time data from 54 events at 101 stations, 3175 P- and 2335 S-wave arrival time data were obtained for our tomographic study. Hypocenter locations and velocity structure were determined simultaneously. We assessed ray coverage and resolution of the velocity structure with checkerboard resolution tests. Ray paths for the model of the obtained velocity structure were examined in detail. There are two zones of low P-wave velocity, one at a depth of 4 km and the other at 15 km. The resolution is good at depths of 0–20 km for P-wave velocities and at depths of 0–15 km for S-wave velocities. A high VP/VS ratio (2.7) indicates that a partially melting rock exists beneath the Hida Mountains. The deep low velocity zone is located just above the upper/lower crustal boundary. These observations indicate that a magma reservoir exists in the upper crust beneath the Hida Mountains.

A magma‐hydrothermal system beneath Hakone volcano, central Japan, revealed by highly resolved velocity structures

High-resolution images of subsurface structures are necessary to understand the transport processes of crustal fluids from deep magma sources and their relationship to earthquake swarms in active volcanic regions. Based on a seismic tomography approach, we have developed a new model for the magma-hydrothermal system beneath Hakone volcano, central Japan, where shallow earthquake swarms and crustal deformation associated with inflation of an open-crack source are often observed. By applying travel-time data for local earthquakes to a tomographic inversion, we obtained highly resolved seismic velocity structures that show a region of low P-wave velocity (Vp), low S-wave velocity (Vs), and high Vp/Vs ratios at depths of 10–20 km beneath the volcano, corresponding to the location of the open-crack source. We suggest that the high Vp/Vs ratios represent a deep magma chamber with a high concentration of melt and/or fluids. Deep low-frequency earthquakes, located just beneath this high Vp/Vs zone, may indicate that magmatic fluids are supplied from below. Above the high Vp/Vs zone, a region of low Vp, low Vs, and low Vp/Vs ratios exists at depths of 3–10 km, suggesting the presence of crack-filled water or CO2 supplied from the inferred deep magma chamber. Many earthquake swarms occur in this low Vp/Vs zone, indicating that crustal fluids play an important role in generating the swarms. Similar relationships between magma reservoirs, overlying hydrothermal systems, and swarm activity have been reported from other volcanic areas and thus may be a ubiquitous feature beneath active volcanoes.

Exploitation of high-sampling Hi-net data to study seismic energy scaling: The aftershocks of the 2000 Western Tottori, Japan, earthquake

High-quality seismic data with broad frequency band are essential for the study of seismic energy, Es, and its scaling with seismic moment, Mo. The 2000 Western Tottori earthquake (Mw 6.6) and its aftershocks as recorded by NIED Hi-net including undistributed high-sampling data provide an excellent data set for this purpose. In this study we use: 1) regular data sampled at 100 sps of small and intermediate (M 2–4) aftershocks just after the mainshock, and, 2) 100 sps data and high sampling 1000 sps data of small events (M 0–3) about two years after the mainshock. Spectral ratios are calculated between all combinations of events that both occurred close to one another and had similar mechanisms. We calculated seismic energies of P and S waves for each event by fitting omega-square spectral models to the spectral ratios. Analysis of both the high and lower sampling rate data results in statistically significant size dependence of Es/Mo; however, none of these trends can explain overall scaling when all events, including the mainshock, are considered. Artificial size dependence due to band limitation and omega-square assumption may be responsible for the apparent trends.

Seismic attenuation structure associated with episodic tremor and slip zone beneath Shikoku and the Kii peninsula, southwestern Japan, in the Nankai subduction zone

The three-dimensional seismic attenuation structure (frequency-independent Q) beneath southwestern Japan was analyzed using t* estimated by applying the S coda wave spectral ratio method to the waveform data from a dense permanent seismic network. The seismic attenuation (Qp−1) structure is clearly imaged for the region beneath Shikoku, the Kii peninsula, and eastern Kyushu at depths down to approximately 50 km. At depths of 5 to 35 km, the seismic attenuation structure changes at the Median tectonic line and other geological boundaries beneath Shikoku and the southwestern Kii peninsula. High-Qp zones within the lower crust of the overlying plate are found just above the slip regions at the centers of the long-term slow-slip events (SSEs) beneath the Bungo and Kii channels and central Shikoku. Beneath central Shikoku, within the overlying plate, a high-Qp zone bounded by low-Qp zones is located from the land surface to the plate interface of the subducting plate. The high-Qp zone and low-Qp zones correspond to high-Vp and low-Vp zones of previous study, respectively. The boundaries of the high- and low-Qp zones are consistent with the segment boundaries of tremors (segment boundaries of short-term SSEs). These results indicated that the locations of the long- and short-term SSEs could be limited by the inhomogeneous distribution of the materials and/or condition of the overlying plate, which is formed due to geological and geographical process. The heterogeneity of materials and/or condition within the fore-arc crust possibly makes an effect on inhomogeneous rheological strength distribution on the interface.

Rupture process of the largest aftershock of the M 9 Tohoku-oki earthquake obtained from a back-projection approach using the MeSO-net data

The largest aftershock (Mw 7.8) of the giant M 9.0 Tohoku-oki earthquake occurred near the coast of Ibaraki Prefecture about thirty minutes after the main shock. We have imaged the rupture process of the Mw 7.8 earthquake by back-projection of waveform data from the Metropolitan Seismic Observation network (MeSO-net). Original acceleration seismograms were integrated. They were then band-pass filtered in the frequency range of 0.1–1.0 Hz. We assumed a fault plane on the plate boundary with a dimension of 115 km ×175 km, and this was divided into 112 subfaults. Travel times from each of the subfaults to observation sites were calculated by using a 3-D velocity structure model. Applying the restrictions that the rupture velocity is smaller than 4 km/s and the rupture duration on each subfault is less than 25 s, we obtained a rupture propagation image by projecting the power of the stacked waveforms. Propagation of the rupture toward north and east was suppressed by the existence of those areas that had radiated a large seismic energy at the main shock occurrence, or at the occurrence of the M 7.0 earthquake in 2008. The westward propagation of the rupture stopped at the area where the Philippine Sea plate lies over the Pacific plate.