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We investigated the effects of the propagation path and site amplification of shallow tremors along the Nankai Trough. Using far‐field S‐wave propagation from intraslab earthquake data, the amplification factors at the DONET stations were 5–40 times against an inland outcrop rock site. Thick (∼5 km) sedimentary layers with VSof 0.6–2 km/s beneath DONET stations have been confirmed by seismological studies. To investigate the effects of thick sedimentary layers, we synthesized seismograms of shallow tremors and intraslab earthquakes at seafloor stations. The ratios of the maximum amplitudes from the synthetic intraslab seismograms between models with and without thick sedimentary layers were 1–2. This means that thin lower‐velocity (<0.6 km/s) sediments just below the stations primarily control the estimated large amplifications. Conversely, at near‐source (≤20 km) distances, 1‐order amplifications of seismic energies for a shallow tremor source can occur due to thick sedimentary layers. Multiple S‐wave reflections between the seafloor and plate interface are contaminated in tremor envelopes; consequently, seismic energy and duration are overestimated. If a shallow tremor occurs within underthrust sediments, the overestimation becomes stronger because of the invalid rigidity assumptions around the source region. After 1‐order corrections of seismic energies of shallow tremors along the Nankai Trough, the scaled energies of seismic slow earthquakes were 10−10–10−9irrespective of the region and source depth. Hence, the physical mechanisms governing seismic slow earthquakes can be the same, irrespective of the region and source depth. The deployment of campaigns and permanent ocean bottom seismometers (OBSs) has enabled us to investigate the activity and physical properties of offshore seismic phenomena. Our knowledge of offshore subsurface structures is still limited; consequently, many studies have used conventional analysis methods with the simplest assumptions. Using observed and synthetic seismograms near the Nankai Trough, we found a limitation in the conventional analysis method applied to OBS data. Thick sedimentary layers, which have been confirmed by seismological studies along the Nankai Trough just below the OBSs, cause an approximately 1‐order overestimation of source parameters for seismic phenomena occurring around the shallow plate boundary. This overestimation may have occurred during the seismic energy estimation of shallow slow earthquakes in Hikurangi, Costa Rica, and Mexico. After correcting for the effects of thick sedimentary layers, we found that the scaled energies of seismic slow earthquakes were 10−10–10−9irrespective of the region and source depth. This suggests that the physical mechanisms governing seismic slow earthquakes can be the same, regardless of region and source depth. Effects of path and site on the seismic energy estimation of slow earthquakes at shallow plate boundaries were investigatedThe assumption of far‐field body waves without thick sediments causes an overestimation of seismic energies for shallow tremorsScaled energies of seismic slow earthquakes at both shallow and large depths range from 10−10to 10−9 Effects of path and site on the seismic energy estimation of slow earthquakes at shallow plate boundaries were investigated The assumption of far‐field body waves without thick sediments causes an overestimation of seismic energies for shallow tremors Scaled energies of seismic slow earthquakes at both shallow and large depths range from 10−10to 10−9

Graphical Abstract:

We detected shallow very low frequency earthquakes (VLFEs) off the Cape Muroto and Kii Channel in the Nankai subduction zone and estimated their moment rate functions. Combining the new and previously estimated catalogs, we obtained the comprehensive catalog of shallow VLFE moment rate functions along the Nankai Trough. We defined the shallow VLFE swarms and investigated the scaling relationships of their cumulative moments, activity area, and durations in each region. Detected swarms were considered candidates for shallow slow slip events. A similar scaling relationship was observed between the cumulative moments and activity areas, irrespective of regions. It indicates similar stress drops in each region. However, the relationship between the cumulative moments and durations varied. This difference was explained by the along‐strike variations in the faulting conditions of shallow slow earthquakes, such as material or hydrological properties. Slow earthquakes are characterized as slips much slower than similar‐size regular earthquakes. Although interactions between the shallow slow earthquakes and large tsunamigenic earthquakes have often been discussed, our knowledge of the source characteristics and spatial variations of the shallow slow earthquakes is still limited. In this study, we quantitatively investigated the activity characteristics of shallow, very low frequency earthquakes (VLFEs) along the Nankai Trough. Activity areas and released cumulative moments of shallow VLFE swarms exhibited a similar scaling law irrespective of regions. However, the duration and cumulative moments of the swarms varied in each region. These characteristics can provide key information on the faulting conditions of slow earthquakes in shallow plate boundaries. Comprehensive detection and source parameter estimations of shallow very low frequency earthquake swarms along the Nankai TroughThe scaling relationship between the activity areas and cumulative moments of the swarms roughly follows Mo∝ A3/2Along‐strike variations of the scaling law for swarm durations reflect differences in the faulting conditions of slow earthquakes Comprehensive detection and source parameter estimations of shallow very low frequency earthquake swarms along the Nankai Trough The scaling relationship between the activity areas and cumulative moments of the swarms roughly follows Mo∝ A3/2 Along‐strike variations of the scaling law for swarm durations reflect differences in the faulting conditions of slow earthquakes

We developed a method for efficient retroviral vector-mediated gene transfer into human hepatocytes, using a human hepatocyte-bearing mouse model. Normal human hepatocytes were transplanted into the livers of immunodeficient and liver-damaged mice. Donor hepatocytes multiplied and replaced the host hepatocytes, which yielded human hepatocyte-bearing mice (human hepatocyte-chimeric mice). As control cells, rat hepatocytes were similarly transplanted. The replacement level reached 86% at 8 weeks and 100% at 5 weeks posttransplantation of human and rat hepatocytes, respectively. Human and rat hepatocytes in the host liver showed a high bromodeoxyuridine-labeling index during the first 2 weeks posttransplantation. Human- and rat-chimeric mice were injected 7 and 10 days posttransplantation, respectively, with retroviral vectors carrying the β-galactosidase gene and were thereafter injected daily for 20 and 10 days, respectively. The level of β-galactosidase-positive hepatocytes in the human- and rat-chimeric mice reached 7.1 ± 1.8% at 8 weeks and 5.3 ± 0.9% at 5 weeks after transplantation, respectively. The human hepatocyte-chimeric mouse will be useful for testing the ability of vectors to transduce human cells.

Stress accumulation and release inside the plate remains poorly understood compared to that at the plate boundaries. Spatiotemporal variations in foreshock and aftershock activities can provide key constraints on time‐dependent stress and deformation processes inside the plate. The 2017 M5.2 Akita‐Daisen intraplate earthquake in NE Japan was preceded by intense foreshock activity and triggered a strong sequence of aftershocks. We examine the spatiotemporal distributions of foreshocks and aftershocks and determine the coseismic slip distribution of the mainshock. Our results indicate that seismicity both before and after the mainshock was concentrated on a planar structure with N‐S strike that dips steeply eastward. We observe a migration of foreshocks toward the mainshock rupture area, suggesting the possibility that foreshocks were triggered by aseismic phenomena preceding the mainshock rupture. The mainshock rupture propagated toward the north, showing less slip beneath foreshock regions. The stress drop of the mainshock was 1.4 MPa, and the radiation efficiency was 0.72. Aftershocks were intensely triggered near the edge of large coseismic slip regions where shear stress increased. The aftershock region expanded along the fault strike, which can be attributed to the postseismic aseismic slip of the mainshock. We find that the foreshocks, mainshock, aftershocks, and postseismic slip released stress at different segments along the fault, which may reflect differences in frictional properties. Obtained results were similar to those observed for interplate earthquakes, which supports the hypothesis that the deformation processes along plate boundaries and intraplate faults are fundamentally the same. Relocated hypocenters and focal mechanisms indicate that the mainshock, foreshocks, and aftershocks occur on the same intraplate faultThe foreshocks, mainshock, aftershocks, and postseismic slip released stress on different fault segmentsForeshock and aftershock seismicity migrate along the fault plane, suggesting that aseismic slip occurs before and after the mainshock