Abstract A prolonged earthquake swarm has persisted since June 2018 in northeastern Noto Peninsula (central Japan), with activity focused into distinct southern, western, northern, and eastern clusters. To explore the role of fluids in the occurrence of this swarm, we analyzed the focal mechanisms of the earthquakes occurring from 1 January 2018 to 30 November 2022 and performed stress tensor inversions. The western, northern, and eastern clusters were dominated by a reverse fault-type mechanism with a horizontal P-axis oriented NW–SE. One of the nodal planes of those mechanisms aligns closely with the precisely relocated hypocenter distribution. The stress fields in these three clusters, as determined by stress tensor inversion, have maximum principal stresses oriented horizontally in the NW–SE direction and minimum principal stresses oriented vertically, aligning with the regional stress field. From these focal mechanisms and this stress field, we derived small misfit angles and large slip tendencies. These findings suggest that, in these three clusters, fluids diffused into faults largely aligned with the regional stress field, resulting in earthquakes with compatible focal mechanisms. Conversely, normal and strike-slip fault-type focal mechanisms, which are unfavorable to the regional stress field, dominated in the southern cluster. The estimated stress fields, deviating from the regional stress field, have maximum principal stresses closer to vertical and minimum principal stress oriented horizontally in the ENE–WSW direction. For earthquakes deeper than 15 km, this local stress field results in relatively larger misfit angles and lower slip tendency than the other clusters. These results suggest that those earthquakes in the southern cluster occurred on misoriented fault planes due to elevated pore fluid pressures. These findings provide strong evidence of the ascent of high-pore-pressure fluids from depth in the southern cluster and their subsequent diffusion into a southeast-dipping fault zone. Graphical Abstract
Earthquake focal mechanisms before and after the 1995 Hyogo-ken Nanbu earthquake have been investigated using seismic records from regional seismic networks. Before the mainshock, seismicity was very active at the Tamba Plateau, a neighboring area of the Hyogo-ken Nanbu earthquake rupture zone. In contrast, the seismicity along the Hyogo-ken Nanbu earthquake rupture zone was not so active. Most earthquakes in these regions had source mechanisms of E-W compression and were of the strike-slip or reverse-fault type. Most aftershocks along the Hyogo-ken Nanbu earthquake rupture zone have strike-slip solutions with P-axis in the E-W or ESE-WNW direction, which is compatible with the trend of aftershock distribution and the strike of active faults the same as the mainshock mechanism. Simultaneously, many other aftershocks were of the reverse-fault type with E-W compression. This area is still controlled by the regional stress field of E-W compression observed before the mainshock. Although, we could find various types of mechanisms in the aftershock sequence, some normal fault-type events were also observed in the mainshock rupture zone. We could find events of SE-NW compression, and this direction is nearly perpendicular to the trend of the mainshock rupture zone. Some aftershocks that occurred near the epicenter of the mainshock had solutions of N-S compression. The geometry of the active fault systems and/or local stress change induced by the mainshock may cause these complex features of focal mechanisms. After the mainshock, the focal mechanisms of earthquakes in the Tamba Plateau were approximately E-W compressional; the same as that before the mainshock.
Goals: To clarify the characteristics of desmoid tumors in Japanese patients with familial adenomatous polyposis after colectomy. Background: Few comprehensive reports have been published on desmoid tumors in Asian patients with familial adenomatous polyposis. Study: This retrospective study included the data of 81 patients with familial adenomatous polyposis who underwent surgery between 1978 and 2021. The adenomatous polyposis coli gene mutation sites, risk factors, and long-term outcomes associated with desmoid tumors in Japanese patients with familial adenomatous polyposis after colectomy were analyzed. Results: No association was observed between the gene mutation sites and desmoid tumor development in 40 patients who underwent genetic analyses. The rate of desmoid tumor development was 30.3% in 66 patients. Multivariate analysis revealed that age below 32 years at colectomy (hazard ratio = 5.491, 95% confidence interval 1.820-16.50, P < 0.001) and familial adenomatous polyposis-related malignancies other than colorectal cancer (hazard ratio = 5.574, 95% confidence interval 2.075-14.98, P < 0.001) were independent risk factors for desmoid tumor development following colectomy. The 10-year disease-specific survival and overall survival rates for desmoid tumors were 92.9% and 76.9%, respectively. The median surveillance duration was 90 months. Conclusions: Adenomatous polyposis coli gene mutation sites alone were not considered a factor for delaying or avoiding colectomy to prevent desmoid tumors in Japanese patients with familial adenomatous polyposis. The timing of colectomy and careful surveillance should be considered for managing patients at a high risk of developing desmoid tumors. Desmoid tumors in patients with familial adenomatous polyposis did not significantly impact prognosis, and pharmacological treatments are important for disease control.
We investigate shear wave polarization anisotropy in the upper crust around the source region of the 1891 Nobi earthquake, central Japan. At most stations, the orientation of the faster polarized shear wave is parallel to the axes of the maximum horizontal compressional strain rate and stress, indicating that stress-induced anisotropy is dominant in the analyzed region. Furthermore, near the source faults, the orientation of the faster polarized shear wave is oblique to the strike of the source faults. This suggests that microcracks parallel to the strike of the source fault, which would be produced by the fault movement of the Nobi earthquake, have healed with the healing of the faults. For stress-induced anisotropy, time delays normalized by path length in the anisotropic upper crust as a function of the differential strain rate are coincident with those in the inland high strain rate zone, Japan. These data, together with those of a previous study, show that the variation in the stressing rate, estimated from shear wave splitting, is close to that estimated from geodetic observation. This implies that the variation in the stressing rate in the brittle upper crust is linked to that in the strain rate on the ground surface.
We propose a new method to monitor slip at the plate interface using non‐volcanic deep‐low frequency (DLF) tremors. We assume that a DLF tremor is the superposition of frequently excited intermittent events, meaning that the envelope of the reduced displacement of the DLF tremor provides an apparent moment rate function. We estimate a conversion factor from the apparent moment to the seismic moment with an assumption that a total size of DLF tremors of an episode is proportional to the size of corresponding slow slip event (SSE). The cumulative seismic moment estimated by DLF tremors is consistent with that estimated from geodetic methods and provides appropriate slip and slip rate at the plate interface. This proves our assumptions and demonstrates that DLF tremors are useful tool for real‐time monitoring of the slip at the plate interface.
Abstract Shear wave splitting from local intermediate‐depth earthquakes is investigated to detect the anisotropic structure in the mantle wedge beneath the South Kyushu area, Japan. We observed shear wave splitting with NEE‐SWW to NWW‐SEE polarization directions and delay times of 0.04–0.63 s. Trench‐normal shear wave polarization anisotropy with a delay time greater than 0.3 s probably overlaps in the high V P / V S region at a depth of 100–150 km beneath the volcanic front. Model calculations for the cause of the anisotropy suggest that the development of “C‐type” olivine crystallographic preferred orientation (CPO) with a trench‐parallel b axis concentration and a trench‐normal a axis concentration best reproduces observations compared to A‐, B‐, or E‐type olivine CPOs and antigorite CPO. We conclude that a thick anisotropic layer, approximately 50 km, is formed by concentration of interstitial fluid in peridotite. We briefly discuss the mechanism carrying water to the deep mantle wedge. A possible explanation is that interstitial fluids in the mantle wedge are effectively released because of a decrease in dihedral angles of olivine‐fluid interfaces at 150 km depth. Our results imply a close relation between the high density of explosive volcanoes in the South Kyushu area and the underlying water‐rich mantle.
We have investigated the characteristics of short‐term slow slip events (SSEs), recurrence interval and size distribution, and the slip rate at the transition zone on the plate interface beneath the Shikoku region, Japan, using nonvolcanic deep low frequency (DLF) tremors. On the basis of a proportional relationship between the seismic moment of SSE observed geodetically and the total size of DLF tremors of the corresponding episode, we estimated the seismic moment due to the slip on the plate interface from the DLF tremors and a temporal variation in the cumulative seismic moment. The recurrence interval of major short‐term SSEs is ∼6 months in the western area and 3 months in the central and the eastern areas. The size distribution of short‐term SSEs as well as DLF tremors is approximated by an exponential law rather than by a power law, showing a different scaling for regular earthquakes. The average slip rate at the transition zone estimated from the cumulative seismic moment with time of SSEs is 4.2 cm/yr, 3.3 cm/yr, and 4.9 cm/yr in the western, central, and eastern areas, respectively. These values compensate for the difference between the convergence rate at the trench and the slip deficit rate at the transition zone of the subducting Philippine Sea plate. In other words, the slip rate estimated from the DLF tremors provides a constraint of the slip deficit rate at the transition zone on the plate interface.
