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Journal Articles

Low-temperature thermochronology of active arc-arc collision zone, South Fossa Magna region, central Japan

Sueoka, Shigeru; Kobayashi, Yumi*; Fukuda, Shoma; Kohn, B. P.*; Yokoyama, Tatsunori; Sano, Naomi*; Hasebe, Noriko*; Tamura, Akihiro*; Morishita, Tomoaki*; Tagami, Takahiro*

Tectonophysics, 828, p.229231_1 - 229231_17, 2022/04

 Times Cited Count:0 Percentile:0.01(Geochemistry & Geophysics)

The South Fossa Magna zone, central Japan, has been an active collision zone between the Honshu Arc and the Izu-Bonin Arc since the middle Miocene and provides an excellent setting for reconstructing the earliest stages of continent formation. Multi-system geo-thermochronometry was applied to different domains of the South Fossa Magna zone, together with some previously published data, to reveal mountain formation processes, i.e., vertical crustal movements. Nine granitic samples yielded zircon U-Pb ages of 10.2-5.8 Ma ($$n$$ = 2), apatite (U-Th)/He (AHe) ages of 42.8-2.6 Ma ($$n$$ = 7), and apatite fission-track (AFT) ages of 44.1-3.0 Ma ($$n$$ = 9). Thermal history inversion modeling based on the AHe and AFT data suggested rapid cooling events confined within the study region at $$sim$$6-2 Ma. The Kanto Mountains may have undergone a domal uplift in association with their collision with the Tanzawa Block at $$sim$$5 Ma. However, this uplift may have slowed down following the migration of the plate boundary and late Pliocene termination of the Tanzawa collision. The Minobu Mountains and possibly adjacent mountains may have been uplifted by the motional change of the Philippine Sea plate at $$sim$$3 Ma. Therefore, the mountain formation in the South Fossa Magna zone was mainly controlled by collisions of the Tanzawa and Izu Blocks and motional change of the Philippine Sea plate. Earlier collisions of the Kushigatayama Block at $$sim$$13 Ma and Misaka Block at $$sim$$10 Ma appeared to have had little effect on mountain formation. Together with a $$sim$$90 deg. clockwise rotation of the Kanto Mountains at 12-6 Ma, these observations suggest that horizontal deformation predominated during the earlier stage of arc-arc collision, and vertical movements due to buoyancy resulting from crustal shortening and thickening developed at a later stage.

Journal Articles

Characterizing the reactivation mechanisms of coseismic surface ruptures associated with the 2011 Mw 6.7 Fukushima-ken Hamadori earthquake in Japan through borehole hydromechanical testing

Guglielmi, Y.*; Aoki, Kazuhiro; Cook, P.*; Soom, F.*; Cappa, F.*; Tanaka, Yukumo

Tectonophysics, 819, p.229084_1 - 229084_22, 2021/11

 Times Cited Count:0 Percentile:0.01(Geochemistry & Geophysics)

The reactivation mechanisms of coseismic surface ruptures associated with the 2011 Mw 6.7 Fukushima-ken Hamadori earthquake in Japan are investigated using in-situ controlled hydraulic injections in subsurface boreholes. Two fault segments were selected for reactivation studies, one across a coseismic rupture, the Shionohira site, and one across a non-coseismically ruptured segment, the Minakami-kita site. A series of water injections in sealed sections of boreholes set across the fault progressively bring the fault to rupture by a step-by-step decrease of the effective normal stress clamping the fault. While the fault is rupturing during these hydraulic stimulations, borehole displacements, fluid pressure and injection flowrate are continuously monitored. Then, the tests were analyzed using fully coupled hydromechanical modeling. The model was calibrated on field data, and a parametric study was conducted to examine the modes of fault reactivation. Coseismic surface rupture of the Shionohira fault showed a pure dilatant slip response to hydraulic tests, while the tectonically un-activated Itozawa fault (South) indicated a complex hybrid response to tests related to both a higher frictional and cohesive strengths of the fault. The analysis of the induced Shionohira slip event showed that it is reasonably modeled as a Coulomb rupture with an eventual dependency of friction on slip velocity, in good accordance with laboratory-derived rate-and-state friction data on the Shinohira gouge samples. In contrast, the Itozawa fault reactivation mechanism appears dominated by tensile failure with limited Coulomb shear failure. Thus, the applied protocol proves to be able to isolate significant differences in fault physical properties and rupture mechanisms between two segments of the same fault system, opening perspectives to better assess near-surface rupture effects, and therefore the safety of structures subject to large earthquakes.

Journal Articles

3D geostatistical modeling of fracture system in a granitic massif to characterize hydraulic properties and fracture distribution

Koike, Katsuaki*; Kubo, Taiki*; Liu, C.*; Masoud, A.*; Amano, Kenji; Kurihara, Arata*; Matsuoka, Toshiyuki; Lanyon, B.*

Tectonophysics, 660, p.1 - 16, 2015/10

 Times Cited Count:21 Percentile:66.17(Geochemistry & Geophysics)

This study integrates 3D models of rock fractures from different sources and hydraulic properties aimed at identifying relationships between fractures and permeability. A geostatistical method (GEOFRAC) that can incorporate orientations of sampled data was applied to 50,900 borehole fractures for spatial modeling of fractures over a 12 km by 8 km area, to a depth of 1.5 km. GEOFRAC produced a plausible 3D fracture model, in that the orientations of simulated fractures correspond to those of the sample data and the continuous fractures appeared near a known fault. Small-scale fracture distributions with dominant orientations were also characterized around the two shafts using fracture data from the shaft walls. By integrating the 3D model of hydraulic conductivity using sequential Gaussian simulation with the GEOFRAC fractures from the borehole data, the fracture sizes and directions that strongly affect permeable features were identified.

Journal Articles

Earthquake doublet in an active shear zone, southwest Japan; Constraints from geophysical and geochemical findings

Umeda, Koji; Asamori, Koichi; Makuuchi, Ayumu; Kobori, Kazuo

Tectonophysics, 634, p.116 - 126, 2014/11

 Times Cited Count:3 Percentile:12.82(Geochemistry & Geophysics)

The geophysical and geochemical observations are significant indications that the invasion of mantle fluids into the crust, driven by upwelling asthenosphere from the Okinawa trough, triggers the notable left-lateral shearing in the zone in the present-day subduction system. In addition, the existence of aqueous fluids in and below the seismogenic layer could change the strength of the zones, and alter the local stress regime, resulting in the occurrence of the 1997 earthquake doublet.

Journal Articles

Subsurface geometry and structural evolution of the eastern margin fault zone of the Yokote basin based on seismic reflection data, northeast Japan

Kagohara, Kyoko; Ishiyama, Tatsuya*; Imaizumi, Toshifumi*; Miyauchi, Takahiro*; Sato, Hiroshi*; Matsuta, Nobuhisa*; Miwa, Atsushi*; Ikawa, Takeshi*

Tectonophysics, 470(3-4), p.319 - 328, 2009/05

 Times Cited Count:4 Percentile:17.35(Geochemistry & Geophysics)

Seismic reflection profiles across coseismic fault scarps of the A.D. 1896 Rikuu earthquake along the eastern margin of the Yokote basin fault zone (EYBFZ), correlated with borehole stratigraphy and geologic mapping, provide insights into its detailed structural evolution. In spite of along-strike variations of thrust geometries both at ground surface and at shallow depth, the EYBFZ has commonly formed as forward breaking imbricate thrust systems. Spatial variations in the long-term uplift rates estimated from cross-section balancing are consistent with mountain topography, in contrast to their weak coupling with the magnitudes of short-term uplift rates and coseismic uplift during the A.D. 1896 earthquake. This suggests that mechanical decoupling of Miocene mudstone that formed frontal emergent thrust faults may have enhanced the slip at shallowest structural levels at shorter timescales.

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