Yusuke Yamashita

Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone

We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes. We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field. Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area. The minimum principal compression stress (σ3), with its near north–south trend, is dominant throughout the entire region. However, the σ3 axes around the shear zone are rotated normal to the zone. This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data. Conversely, the stress field of the normal fault is dominant in the Beppu–Shimabara area, which is located in the central part of the island. This result and the direction of σ3 are consistent with the formation of a graben structure in the area.

Migrating tremor off southern Kyushu as evidence for slow slip of a shallow subduction interface

Silent slip events get shallow Clues to help better predict the likelihood of devastating earthquakes and tsunamis may be embedded in a more gentle type of rumbling. Using oceanbottom seismometers, Yamashita et al. report rare observations of migrating tremors in the shallow part of a subduction zone off southern Kyushu, Japan. The tremors appear to be linked to a very low-frequency earthquake and seem to migrate to the region where big earthquakes are generated. The tremors may be tracing how and where stress gets concentrated onto the earthquake-producing portion of the fault. Science, this issue p. 676 Earthquake and tsunami hazard forecasts may benefit from shallow observations of seismic tremor migration in subduction zones. Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events.

Short-term spatial change in a volcanic tremor source during the 2011 Kirishima eruption

Volcanic tremors are indicators of magmatic behavior, which is strongly related to volcanic eruptions and activity. Detection of spatial and temporal variations in the source location is important for understanding the mechanism of volcanic eruptions. However, short-term temporal variations within a tremor event have not always been detected by seismic array observations around volcanoes. Here, we show that volcanic tremor sources were activated at both the top (i.e., the crater) and the lower end of the conduit, by analyzing seismograms from a dense seismic array 3 km from the Shinmoedake crater, Kirishima volcano, Japan. We observed changes in the seismic ray direction during a volcanic tremor sequence, and inferred two major sources of the tremor from the slowness vectors of the approaching waves. One was located in a shallow region beneath the Shinmoedake crater. The other was found in a direction N30°W from the array, pointing to a location above a pressure source. The fine spatial and temporal characteristics of volcanic tremors suggest an interaction between deep and shallow conduits.

Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3)

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three-dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre-condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low-pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations. Plain Language Summary The three-dimensional prestress field around the 2016 Kumamoto earthquake controlled fault behavior of the earthquake. The estimated heterogeneous state of stress on the fault forced the fault to slip in the direction predicted. The stress field also exposed the precondition of pore fluid pressure on the fault. Large coseismic slip occurred not only at the low-pressure part of the fault but also highly pressured part. It indicates that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to upgrading seismic hazard evaluation.

Extraction of crustal deformations and oceanic fluctuations from ocean bottom pressures

DONET (Dense Oceanfloor Network system for Earthquakes and Tsunamis) has been developed and installed around Nankai Trough, which is motivated by the 2004 Sumatra-Andaman Earthquake. DONET contains pressure gauges as well as seismometers, which are expected to detect crustal deformations driven by peeling off subduction plate coupling process. From our simulation results, leveling changes are different sense among the DONET points even in the same science node. On the other hand, oceanic fluctuations such as melting ice masses through the global warming have so large scale as to cause ocean bottom pressure change coherently for all of DONET points especially in the same node. This difference suggests the possibility of extracting crustal deformations component from ocean bottom pressure data by differential of stacking data. However, this operation cannot be applied to local-scale fluctuations related to ocean mesoscale eddies and current fluctuations, which affect ocean bottom pressure through water density changes in the water column (from the sea surface to the bottom). Therefore, we need integral analysis by combining seismology, ocean physics and tsunami engineering so as to decompose into crustal deformation, oceanic fluctuations and instrumental drift, which will bring about high precision data enough to find geophysical phenomena.

Development of Gas Proportional Scintillation Counter for Light Heavy‐Ion Detection

In recent years, nuclear data have been needed in the medical field. Nuclear data induced by light heavy ions are especially needed at high precision for cancer treatment, although there are not enough usable data at present.We have a plan to measure light heavy‐ion nuclear data with a dE‐E detector. Low density is needed for the dE detector. We have two options for the dE detector: a semiconductor detector (SSD) and a Gas Counter. On one hand, SSD has good energy resolution, but on the other hand, it is expensive and its decay time is on the 100‐microsecond order. A Gas Counter is inexpensive, and a Gas Proportional Scintillation Counter (GPSC) has fast decay time. Then, we developed a GPSC for the dE detector, and its evaluation experiment was carried out at the Heavy Ion Medical Accelerator in Chiba (HIMAC).We will report the results of the experiment with the performance of the GPSC.

Derivation of transfer function of LLC current resonant converter using numerical calculation

Among various converters, the LLC electric current resonant converter attracts attention because of high power density, low noise and high efficiency. However, this converter cannot apply the analysis of averaging method, and impossible to derivate the transfer function, so designing is difficult. In this study, we devised a method to derive a frequency response characteristic with a transfer function type using based on the way of thinking of averaging method, and succeeded expressing transfer function of the bode diagram which agreed with actual survey. Moreover, we derived poles from the transfer function, and showed the relation between the shape of the bode plots and poles.

Tidal Response in Shallow Tectonic Tremors

Various types of slow earthquakes (e.g., tectonic tremors and slow slip events) have been reported in tectonic zones, especially in the subduction zone. The tidal response of a tremor is considered to be strongly related to the weak friction state of the plate interface, and many studies have reported observational evidence of such correlation between tides and deep tremor activity. Here, we used the modified frequency scanning method at a single station to detect micro tectonic tremors that have not been previously reported in southern Kyushu. In the early stage of the tremor activity, tremors are mostly modulated by slow slip events. In contrast, we found a seismic response to ocean tides during the later stage in the shallower part of the subduction zone. This might indicate that the tremors are triggered by tidal changes caused by fault weakening due to slow slip events as same as deeper condition.

Proton Production Cross Sections for Reactions Induced by 300‐ and 392‐MeV Protons

Multistep direct processes in proton production reactions were investigated with proton beam of 300‐ and 392‐MeV bombarding targets nuclei. The targets nuclei are 181Ta, 197Au, and 209Bi for 392 MeV and 197Au for 300 MeV protons. Energy spectra were measured at several laboratory angles from 20° to 105° and compared with two theoretical models, the quantum molecular dynamics and the intranuclear cascade model. At intermediate energy, the double differential cross section is sensitive to ground state in codes. We found that a more realistic ground state led better accounting of measured data.