Anomalous deepening of a seismic belt in the upper-plane of the double seismic zone in the Pacific slab beneath the Hokkaido corner: Possible evidence for thermal shielding caused by subducted forearc crust materials

Kita, S.; Okada, Tomomi; Hasegawa, Akira; Nakajima, Jun; Matsuzawa, Toru

doi:10.1016/j.epsl.2009.12.038

Cited by 140 publications

(170 citation statements)

References 25 publications

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“…Cross sections are labeled following Kita (2009) plane of seismicity is not only confined to the descending oceanic crust, but also related to the presence of fluids that are liberated by the transformation of blueschist to eclogite. Support for this idea is provided in the qualitative sense by the deepening of the seismic belt below Kanto and western Hokkaido (Hasegawa et al, 2007;Kita et al, 2010a). Thermal shielding of the slab by an anomalous overriding plate structure is supported by heat flow data ( Fig.…”

Section: Introductionmentioning

confidence: 82%

“…5). Kita et al (2010a) used tomographic techniques to image anomalously low P-and S-wave speeds below this section and demonstrated a significant deepening of the seismic belt here (their Fig. 4).…”

Section: Seismicity and Thermal Structure At The Trench Junctionmentioning

confidence: 90%

“…Intriguingly, the upper plane of seismicity seems to be confined to the oceanic crust ( Fig. 2; Kita et al, 2006Kita et al, , 2010a, with a marked reduction in number and deepening of the earthquakes with respect to the top of the crust at greater depth. A similar pattern, and correlation with low seismic velocities in the crust, is seen in the central Alaska subduction zone (Abers et al, 2006).…”

Section: Introductionmentioning

confidence: 98%

“…Volcanoes are shown in black triangles. The down-dip limit of low-angle thrust events from Kita et al (2010a), which is taken to be the down-dip limit of the region of strong seismic coupling between upper and lower plate, is shown in the bold red line. Cross sections are labeled following Kita (2009) plane of seismicity is not only confined to the descending oceanic crust, but also related to the presence of fluids that are liberated by the transformation of blueschist to eclogite.…”

Section: Introductionmentioning

confidence: 99%

“…The main goal of this paper is to develop new models to determine the thermal structure of the subducting Pacific slab below Tohoku and Hokkaido. We study along-arc variations in the correlation between the blueschist-eclogite boundary and test whether simple thermal shielding proposed by Kita et al (2010a) leads to the observed deepening of the seismic belt.…”

Section: Introductionmentioning

confidence: 99%

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Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

2012

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Abstract. The cause of intermediate-depth (> 40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, which include dehydration embrittlement, shear instabilities and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The wellinstrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediatedepth seismicity occurs. This study combines new finite element models that predict the dynamics and thermal structure of the Japan subduction system with a high-precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths > 80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between the thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models that take into account the seismically imaged modified upper mantle structure in this region fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by timedependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediatedepth seismicity.

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Section: Introductionmentioning

confidence: 82%

Section: Seismicity and Thermal Structure At The Trench Junctionmentioning

confidence: 90%

Section: Introductionmentioning

confidence: 98%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

2012

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Along‐arc variation in the 3‐D thermal structure around the junction between the Japan and Kurile arcs

Morishige

Keken

2014

Geochem Geophys Geosyst

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The thermal structure in subduction zones has a strong influence on seismogenesis and arc volcanism. Traditional 2-D models have been used to provide reasonable agreement between models and observations, but in a number of cases clear 3-D effects are present. One such case is in the Northern Japan subduction system. At the junction between Japan and Kurile arcs, surface heat flow and the occurrence of intermediate-depth seismicity are different than in the Tohoku and Hokkaido regions. We investigate the effects of 3-D slab geometry and a local deepening of slab-mantle decoupling depth on the thermal structure in this region based on 3-D finite element approach. We find that both effects produce the along-arc variation of slab surface temperature, which could reach $100 C. The warmer region arises through 3-D effects of thermal conduction and the colder region arises through localized slow incoming flow in the case where 3-D slab geometry is taken into account. 3-D flow arises where a local deepening of slab-mantle decoupling depth is assumed, which leads to both warmer and colder regions. The effects on surface heat flow are small. While intermediate-depth seismicity in the subducted crust is suggested to be controlled by temperature-dependent phase transitions, the predicted changes in thermal structure are not sufficient to cause the observed deepening of seismicity. This suggests that the thermal structure of this subduction zone may be more strongly influenced by time-dependent deformation of the overriding crust and slab.

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Stress drops for intermediate‐depth intraslab earthquakes beneath Hokkaido, northern Japan: Differences between the subducting oceanic crust and mantle events

Kita

Katsumata

2015

Geochem Geophys Geosyst

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Spatial variations in the stress drop for 1726 intermediate-depth intraslab earthquakes were examined in the subducting Pacific plate beneath Hokkaido, using precisely relocated hypocenters, the corner frequencies of events, and detailed determined geometry of the upper interface of the Pacific plate. The results show that median stress drop for intraslab earthquakes generally increases with an increase in depth from $10 to 157 Mpa at depths of 70-300 km. More specifically, median stress drops for events in the oceanic crust decrease (9.9-6.8 MPa) at depths of 70-120 km and increase (6.8-17 MPa) at depths of 120-170 km, whereas median stress drop for events in the oceanic mantle decrease (21.6-14.0 MPa) at depths of 70-170 km, where the geometry of the Pacific plate is well determined. The increase in stress drop with depth in the oceanic crust at depths of 120-170 km, for which several studies have shown an increase in velocity, can be explained by an increase in the velocity and a decrease in the water content due to the phase boundary with dehydration in the oceanic crust. Stress drops for events in the oceanic mantle were larger than those for events in the oceanic crust at depths of 70-120 km. Differences in both the rigidity of the rock types and in the rupture mechanisms for events between the oceanic crust and mantle could be causes for the stress drop differences within a slab.

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Anomalous deepening of a seismic belt in the upper-plane of the double seismic zone in the Pacific slab beneath the Hokkaido corner: Possible evidence for thermal shielding caused by subducted forearc crust materials

Cited by 140 publications

References 25 publications

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

Along‐arc variation in the 3‐D thermal structure around the junction between the Japan and Kurile arcs

Stress drops for intermediate‐depth intraslab earthquakes beneath Hokkaido, northern Japan: Differences between the subducting oceanic crust and mantle events

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