Yohei Yukutake

Aftershock distribution of the 2004 Mid Niigata Prefecture Earthquake derived from a combined analysis of temporary online observations and permanent observations

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

Seismic velocity decrease and recovery related to earthquake swarms in a geothermal area

We found a recurring seismic velocity decrease associated with small earthquake swarms experienced in 2007 in a geothermal area in Kyushu, southwestern Japan, by analyzing long-term changes in the autocorrelation function (ACF) of seismic noise. The seismic velocity decrease appeared just after two major periods of earthquake activity began in June and October of 2007. In both instances, conditions returned to normal within a characteristic time period of 4 months. The observed size of the velocity changes agrees well with the magnitudes of the swarms. The lag-time dependence of ACF changes can be systematically explained by seismic velocity changes induced by fluid inclusion in a small, localized area deep within the hypocenter region.

Well-resolved hypocenter distribution using the double-difference relocation method in the region of the 2007 Chuetsu-oki Earthquake

The 2007 Chuetsu-oki Earthquake (Mj = 6.8) occurred in the eastern margin of the Japan Sea where high strain rates have been observed and many large earthquakes have occurred. The main shock was located near the Western Nagaoka Basin active fault system dipping in the westward direction. We estimated the well-resolved hypocenter location around the main shock rupture zone from the differential arrival times obtained by both manual picking and waveform cross-correlation analysis. From the relocated aftershock distributions, we successfully resolved the detailed fault structures activated by the main shock. The estimated fault model resolves four individual fault segments. The fault model suggests that the main shock predominantly ruptured the southeastward dipping fault planes. On the other hand, the aftershocks around the hypocenter of the main shock occurred on both the northwestward and southeastward dipping fault planes. Both fault planes around the hypocenter of the main shock may have been nearly coincidentally ruptured during the main shock.

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.

Fine fault structure of a moderate earthquake in the 2007 earthquake sequence of Northern Mie, Japan

A moderate crustal earthquake (Mj = 5.4) occurred in the northern part of Mie Prefecture, Central Japan, 2007. In order to clarify the fault structure of the main shock and its relation to the active faults, we estimated the precise hypocenter locations and focal mechanisms. For the relocation of the hypocenters, we used the differential time obtained by both manual picking and waveform cross-correlation analysis. The estimated detailed fault structure suggests that the main shock ruptured the southwestward dipping fault plane. We found that the faults of the moderate earthquake possessed complex fault segments. Around the largest aftershock hypocenter, a subsidiary fault plane parallel to the main shock fault was identified. This result suggests that the fault structure around the deep part of the active fault is complicated. The hypocenters of the foreshock and main shock were located in the offset part of the aftershock alignment, implying that they did not occur on the same fault plane. The Chisato Fault and Yokkaichi Fault were located around the upward extension of the aftershock alignment. The main shock probably occurred in the deep parts of these faults.

Stress-induced spatiotemporal variations in anisotropic structures beneath Hakone volcano, Japan, detected by S wave splitting: A tool for volcanic activity monitoring

Hakone volcano, located at the northern tip of the Izu-Mariana volcanic arc, Japan, has a large caldera structure containing numerous volcanic hot springs. Earthquake swarms have occurred repeatedly within the caldera. The largest seismic swarm since the commencement of modern seismic observations (in 1968) occurred in 2001. We investigated the anisotropic structure of Hakone volcano based on S wave splitting analysis and found spatiotemporal changes in the splitting parameters accompanying the seismic swarm activity. Depth-dependent anisotropic structures are clearly observed. A highly anisotropic layer with a thickness of ~1.5 km is located beneath the Koziri (KZR) and Kozukayama (KZY) stations. The anisotropic intensity in the region reaches a maximum of 6–7% at a depth of 1 km and decreases markedly to less than 1% at a depth of 2 km. The anisotropic intensity beneath Komagatake station (KOM) decreases gradually from a maximum of 6% at the surface to 0% at a depth of 5 km but is still greater than 2.5% at a depth of 3 km. At KZY, the anisotropic intensity along a travel path of which the back azimuth was the south decreased noticeably after the 2001 seismic swarm activity. During the swarm activity, tilt meters and GPS recorded the crustal deformation. The observed decrease in anisotropic intensity is presumed to be caused by the closing of microcracks by stress changes accompanying crustal deformation near the travel path.

Seismotectonics in the Tanzawa Mountains area in the Izu-Honshu collision zone of central Japan, as revealed by precisely determined hypocenters and focal mechanisms

We investigate the detailed distribution of hypocenters and focal mechanisms beneath the Tanzawa Mountains, central Japan, where the Izu-Bonin arc has collided into the central part of the Honshu arc. Remarkable differences are found to exist between the hypocenter distributions in the western and eastern parts. The hypocenters of earthquakes in the eastern part tend to be distributed in a horizontal zone, whereas those in the western part are distributed in a volume. The focal mechanisms in the eastern part are right-lateral reverse faulting mechanisms, and one of the nodal planes is consistent with the geometry of the Philippine Sea (PHS) plate in the region. These results suggest that most earthquakes in the eastern part occur along the upper surface of the subducting PHS plate. In contrast, the focal mechanisms in the western part, especially deep in the western part, exhibit a different feature. The stress states in these two regions are found to be significantly different. The maximum and minimum principal stress axes in the eastern part are slightly inclined, whereas those in the western part are oriented in approximately the vertical and horizontal directions, respectively. The stress field in the eastern part may be caused by a slab pull force induced from the deeper part of the subducted plate.

Spatial distribution of crack structure in the focal area of a volcanic earthquake swarm at the Hakone volcano, Japan

We have performed shear wave splitting analyses for seismograms recorded at stations located just above, and outside, the focal area of the earthquake swarm at the Hakone volcano, Japan, in August 2009. Average values of the direction of faster split shear wave polarization (Φ) at two stations above the focal area correspond to each focal alignment of the earthquake swarm. In contrast, average values of Φ at three stations outside the focal area correspond to the direction of the maximum horizontal compressional stress. We found that the average values of the time lag between the two split shear waves inside the focal area are relatively high compared with those outside the focal area. These facts suggest that cracks with a high density aligned parallel to the faults of the earthquake swarm in the focal area. Crustal fluid was selectively injected into this pre-existing cracked media accompanied by effective normal stress reduction in the cracks, resulting in the earthquake swarm.

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.

Resistivity characterisation of Hakone volcano, Central Japan, by three-dimensional magnetotelluric inversion

On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermal area on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of Hakone caldera, we carried out a three-dimensional inversion of magnetotelluric measurement data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh for the inversion code of the edge-based finite element method to account for the steep topography of the region during the inversion process. The main features of the best-fit three-dimensional model are a bell-shaped conductor, the bottom of which shows good agreement with the upper limit of seismicity, beneath the central cones and the Owakudani geothermal area, and several buried bowl-shaped conductive zones beneath the Gora and Kojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally altered zone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic activity may cause volcanic fluids to pass through the resistive body surrounded by the altered zone and thus promote brittle failure within the resistive body. The overlapping locations of the bowl-shaped conductors, the buried caldera structures and the presence of sodium-chloride-rich hot springs indicate that the conductors represent porous media saturated by high-salinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area may indicate several weak zones that formed due to these structural contrasts.