Katsuhiko Shiomi

Upper mantle imaging beneath the Japan Islands by Hi-net tiltmeter recordings

We present a novel receiver-function image of the upper mantle structure around the Japan subduction zone. To increase the amount of available waveform data containing the relatively lower frequency component, we examined whether the Hi-net tiltmeter recordings are usable for imaging the upper mantle discontinuities by comparing them with broadband seismograms in different frequency bands. We found that the two are comparable at a frequency band between 0.02 and 0.16 Hz. To make receiver functions from tiltmeter data, stacked vertical components of broadband seismograms were used as source-time functions. Since such source-time functions may include biases from local structure, we also produced regional stacked source-time functions. The receiver function with the above frequency band does not seem to be affected by local structure. In the images derived from the receiver-function gathers, we were able to visualize both the oceanic Moho and the lower slab boundary, which could be traced down to depths of 400 km and 600 km, respectively. These images also show an uplift of the 410-km discontinuity and a depression of the 660-km discontinuity in the regions that are probably affected by the cold subducting Pacific slab.

Moho and Philippine Sea plate structure beneath central Honshu Island, Japan, from teleseismic receiver functions

We used teleseismic P waves recorded by the J-array, the Hi-net and a temporal local seismic network to investigate the three-dimensional topography of the Moho and the Philippine Sea plate beneath central Honshu Island, Japan. An image of the subsurface discontinuities beneath the region, derived from receiver function analysis, depicts the Philippine Sea plate dipping toward the north with complex local curvatures. The Moho is clearly detected in the northern part of the area studied, and its depth increases to the center of the island. Receiver functions from the stations adjacent to the Itoigawa-Shizuoka tectonic line indicate the step-like topography of the Moho directly beneath this tectonic line.

Initial-rupture fault, main-shock fault, and aftershock faults: Fault geometry and bends inferred from centroid moment tensor inversion of the 2005 west off Fukuoka prefecture earthquake

The 2005 west off Fukuoka prefecture earthquake (MJMA = 7.0) occurred on March 20, 2005 in northwest Kyusyu, Japan. The fault geometry and rupture propagation of the main shock are investigated by applying the centroid moment tensor (CMT) inversion method to densely-distributed broadband seismic network data. An accurate distribution of small aftershocks is also examined by a double-difference method. The source fault of the main shock whose moment magnitude was 6.6 had a strike of 123°, and the rupture propagated in the southeast direction. We obtain forty-four CMT solutions of the main shock and aftershocks with a moment magnitude greater than 3.5. For the main shock, the fault strike estimated using P-wave first-motion polarities differs from that of the moment tensor solution by 13° in the counterclockwise direction. Some aftershocks with a moment tensor solution similar to the focal mechanism of the main shock derived from the P-wave first motions are distributed north of the epicenter of the main shock. The largest aftershock with a strike of 8° in the clockwise direction, which differs from that of the CMT solution of the main shock, was located south of the main shock. These changes in the fault strike play an important role in the initiation and termination of the rupture process of the main shock.

Automatic Aftershock Forecasting: A Test Using Real‐Time Seismicity Data in Japan

Real‐time aftershock forecasting is important to reduce seismic risks after a damaging earthquake. The main challenge is to prepare forecasts based on the data available in real time, in which many events, including large ones, are missing and large hypocenter determination errors are present due to the automatic detection process of earthquakes before operator inspection and manual compilation. Despite its practical importance, the forecast skill of aftershocks based on such real‐time data is still in a developmental stage. Here, we conduct a forecast test of large inland aftershock sequences in Japan using real‐time data from the High Sensitivity Seismograph Network (Hi‐net) automatic hypocenter catalog (Hi‐net catalog), in which earthquakes are detected and determined automatically in real time. Employing the Omori–Utsu and Gutenberg–Richter models, we find that the proposed probability forecast estimated from the Hi‐net catalog outperforms the generic model with fixed parameter values for the standard aftershock activity in Japan. Therefore, the real‐time aftershock data from the Hi‐net catalog can be effectively used to tailor forecast models to a target aftershock sequence. We also find that the probability forecast based on the Hi‐net catalog is comparable in performance to the one based on the latest version of the manually compiled hypocenter catalog of the Japan Meteorological Agency when forecasting large aftershocks with M >3.95, despite the apparent inferiority of the automatically determined Hi‐net catalog. These results demonstrate the practical usefulness of our forecasting procedure and the Hi‐net automatic catalog for real‐time aftershock forecasting in Japan. Online Material: Figures and tables showing detailed forecast results for all considered aftershock sequences and all forecast time frames.

Low frequency events occurred during the sequence of aftershock activity of the 2003 Tokachi-Oki earthquake; a dynamic process of the tectonic erosion by subducted seamount

During the sequence of aftershock activity of the 2003 Tokachi-Oki earthquake, anomalous events predominant in low-frequency components occurred very close to a subducted seamount around the junction between the Kuril Trench and the Japan Trench. The low-frequency wave train in the coda part reflects the strong excitation of surface waves and the source depth is considered to be very shallow. The focal mechanism of the low-frequency event is a type of the reverse fault. Considering with the source depth and the mechanism, the low-frequency event might have occurred on the plate boundary as a dynamic process of the tectonic erosion by the subducted seamount.

Moment tensor inversion of the 2016 southeast offshore Mie earthquake in the Tonankai region using a three-dimensional velocity structure model: effects of the accretionary prism and subducting oceanic plate

The southeast offshore Mie earthquake occurred on April 1, 2016 near the rupture area of the 1944 Tonankai earthquake, where seismicity around the interface of the Philippine Sea plate had been very low until this earthquake. Since this earthquake occurred outside of seismic arrays, the focal mechanism and depth were not precisely constrained using a one-dimensional velocity model, as in a conventional approach. We conducted a moment tensor inversion of this earthquake by using a three-dimensional velocity structure model. Before the analysis of observed data, we investigated the effects of offshore heterogeneous structures such as the seawater, accretionary prism, and subducting oceanic plate by using synthetic seismograms in a full three-dimensional model and simpler models. The accretionary prism and subducting oceanic plate play important roles in the moment tensor inversion for offshore earthquakes in the subduction zone. Particularly, the accretionary prism, which controls the excitation and propagation of long-period surface waves around the offshore region, provides better estimations of the centroid depths and focal mechanisms of earthquakes around the Nankai subduction zone. The result of moment tensor inversion for the 2016 southeast offshore Mie earthquake revealed low-angle thrust faulting with a moment magnitude of 5.6. According to geophysical surveys in the Nankai Trough, our results suggest that the rupture of this earthquake occurred on the interface of the Philippine Sea plate, rather than on a mega-splay fault. Detailed comparisons of first-motion polarizations provided additional constraints of the rupture that occurred on the interface of the Philippine Sea plate.

Seismic shear waves as Foucault pendulum

Earths rotation causes splitting of normal modes. Wave fronts and rays are, however, not affected by Earths rotation, as we show theoretically and with observations made with USArray. We derive that the Coriolis force causes a small transverse component for P waves and a small longitudinal component for S waves. More importantly, Earths rotation leads to a slow rotation of the transverse polarization of S waves; during the propagation of S waves the particle motion behaves just like a Foucault pendulum. The polarization plane of shear waves counteracts Earths rotation and rotates clockwise in the Northern Hemisphere. The rotation rate is independent of the wave frequency and is purely geometric, like the Berry phase. Using the polarization of ScS and ScS2 waves, we show that the Foucault-like rotation of the S wave polarization can be observed. This can affect the determination of source mechanisms and the interpretation of observed SKS splitting.

Radial and Azimuthal Anisotropy Tomography of the NE Japan Subduction Zone: Implications for the Pacific Slab and Mantle Wedge Dynamics

We investigate slab and mantle structure of the NE Japan subduction zone from P wave azimuthal and radial anisotropy using travel time tomography. Trench normal E-W-trending azimuthal anisotropy (AA) and radial anisotropy (RA) with VPV> VPH are found in themantle wedge, which supports the existence of small-scale convection in the mantle wedge with flow-induced LPO of mantle minerals. In the subducting Pacific slab, trench parallel N-S-trending AA and RA with VPH> VPV are obtained. Considering the effect of dip of the subducting slab on apparent anisotropy, we suggest that both characteristics can be explained by the presence of laminar structure, in addition to AA frozen-in in the subducting plate prior to subduction. Plain Language Summary There is increasing importance and interest in seismic anisotropy because it can provide crucial constraints on the lithospheric structure as well as the nature of dynamics of mantle flow. In this study, we performed two types of anisotropic tomography analyses using the same data set and estimated three-dimensional P wave azimuthal and radial anisotropy structures beneath NE Japan. These tomography analyses show that mantle wedge of the subduction zone is characterized by E-W-trending azimuthal anisotropy and radial anisotropy with VPV > VPH. On the other hand, N-S-trending azimuthal anisotropy and radial anisotropy with VPH > VPV are shown in the Pacific slab. Assuming flow-induced lattice preferred orientation of mantle minerals, the observed mantle wedge anisotropy can be explained by 3-Dmantle flowwith small-scale convection. Also, we evaluated the effect of dip of the slab that is anisotropic and carefully considered resulting apparent radial and azimuthal anisotropies in our tomography. Then, the anisotropy in the Pacific slab can be explained by a combination of positive radial anisotropy due to laminar scatterers in the oceanic plate and azimuthal anisotropy frozen-in during the formation of an oceanic plate whose fast direction is parallel to the ancient spreading direction.