Newness and distinctiveness is claimed in the features of ornamentation as shown inside the broken line circle in the accompanying representation.

Display screen or portion thereof with graphical user interface

[1] A high-resolution P wave tomographic model of the crust and mantle down to 1100 km depth under China and surrounding regions is determined by using about one million arrival times of P, pP, PP, and PcP waves from 19,361 earthquakes recorded by 1012 seismic stations. The subducting Pacific slab is imaged clearly as a high-velocity zone from the oceanic trenches down to about 600 km depth, and intermediate-depth and deep earthquakes are located within the slab. The Pacific slab becomes stagnant in the mantle transition zone under east China. The western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east China. The active Changbai and Wudalianchi intraplate volcanoes in northeast China are underlain by significant slow anomalies in the upper mantle, above the stagnant Pacific slab. These results suggest that the active intraplate volcanoes in NE China are not hot spots but a kind of back-arc volcano associated with the deep subduction of the Pacific slab and its stagnancy in the transition zone. Under the Mariana arc, however, the Pacific slab penetrates directly down to the lower mantle. The active Tengchong volcano in southwest China is related to the eastward subduction of the Burma microplate. The subducting Indian and Philippine Sea plates are also imaged clearly. The Indian plate has subducted down to 200–300 km depth under the Tibetan Plateau with a horizontal moving distance of about 500 km. High-velocity anomalies are revealed in the upper mantle under the Tarim basin, Ordos, and Sichuan basin, which are three stable blocks in China.

High-resolution mantle tomography of China and surrounding regions

Water and hydrous minerals play a key role in geodynamic processes at subduction zones, however there are few seismological constraints on the presence of water within the subducting plate. Audet et al. model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the Cascadia continental margin to its intersection with the forearc mantle. Their observations suggest that water is pervasively present in fluid form at high pore pressures, indicating that the megathrust is a low-permeability boundary. These results may hold important implications for our understanding of seismogenesis, subduction-zone structure and the mechanism of episodic tremor and slip. This study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the Cascadia continental margin to its intersection with the forearc mantle. The observations suggest that water is pervasively present in fluid form at high pore pressures, indicating that the megathrust is a low-permeability boundary. These results may hold important implications for our understanding of seismogenesis, subduction-zone structure and the mechanism of episodic tremor and slip. Water and hydrous minerals play a key part in geodynamic processes at subduction zones1,2,3 by weakening the plate boundary, aiding slip and permitting subduction—and indeed plate tectonics—to occur4. The seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5,6,7. However, seismological observations bearing on the presence of water within the subducting plate itself are less well documented. Here we use converted teleseismic waves to obtain observations of anomalously high Poisson’s ratios within the subducted oceanic crust from the Cascadia continental margin to its intersection with forearc mantle. On the basis of pressure, temperature and compositional considerations, the elevated Poisson’s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values. Combined with observations of a strong negative velocity contrast at the top of the oceanic crust, our results imply that the megathrust is a low-permeability boundary. The transition from a low- to high-permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization. These results may have important implications for our understanding of seismogenesis, subduction zone structure and the mechanism of episodic tremor and slip.

Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing.

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

The decline and fate of an iron-induced subarctic phytoplankton bloom

http://www.dgf.uchile.cl/~jaime/Muriel/GeryaYuen-meltsubd-epsl2003.pdf

Rayleigh^Taylor instabilities from hydration and melting propel 'cold plumes' at subduction zones

Influence of fluids and magma on earthquakes: seismological evidence

Crack density, saturation rate and porosity at the 2001 Bhuj, India, earthquake hypocenter: a fluid-driven earthquake?

[1] The January 26, 2001 Bhuj earthquake (Mw 7.6) is one of the most catastrophic Indian earthquakes. We have investigated the 3-D seismic velocity and Poisson's ratio structures of the Bhuj source area to understand the probable cause of triggering the earthquake. We used 1948 P and 1865 S-wave high-quality arrival times from 331 aftershocks recorded at a temporary seismic network. Significant variations up to 5% in velocity and 10% in Poisson's ratio are revealed in the aftershock area. The mainshock is located in a distinctive zone characterized by high-Vp, low-Vs and high Poisson's ratio (σ) in the depth range of 20 to 30 km and extending 15 to 30 km laterally. This feature is very similar to that of the 1995 Kobe earthquake [Zhao et al., 1996]. The anomaly may be due to a fluid-filled, fractured rock matrix, which might have contributed to the initiation of the Bhuj earthquake.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL015177

The 2001 Bhuj earthquake: Tomographic evidence for fluids at the hypocenter and its implications for rupture nucleation

Crustal heterogeneity in the 2000 western Tottori earthquake region: effect of fluids from slab dehydration

[1] We determined, for the first time, a 3-D seismic structure of P and S wave velocity (Vp, Vs) in the forearc region of northeast (NE) Japan from the Pacific coast to the Japan Trench using a large number of high-quality arrival times from sub-oceanic earthquakes that are well located with sP depth phase data. Strong lateral heterogeneities in Vp and Vs are revealed. A majority (95%) of great interplate earthquakes are located outside the low-V zones while a few (5%) of them are located inside the low-V zones. This may suggest a different degree of interplate seismic coupling at the plate interface parallel to Japan Trench. The low-V zones probably contain fluids and may represent decoupled areas. We propose that this new approach of investigating 3-D seismic structure can be applied to other forearcs of the world for a better understanding of interplate coupling and its influence on big earthquakes. INDEX TERMS: 6982 Radio Science: Tomography and imaging; 7200 Seismology; 7209 Seismology: Earthquake dynamics and mechanics; KEYWORDS: Tomography, Forearc, Asperities, Seismic coupling, Fluids. Citation: Mishra, O. P., D. Zhao, N. Umino, and A. Hasegawa, Tomography of northeast Japan forearc and its implications for interplate seismic coupling, Geophys. Res. Lett., 30(16), 1850, doi:10.1029/2003GL017736, 2003.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003GL017736

O.P. Mishra

Papers

Influence of fluids and magma on earthquakes: seismological evidence

Crack density, saturation rate and porosity at the 2001 Bhuj, India, earthquake hypocenter: a fluid-driven earthquake?

The 2001 Bhuj earthquake: Tomographic evidence for fluids at the hypocenter and its implications for rupture nucleation

Crustal heterogeneity in the 2000 western Tottori earthquake region: effect of fluids from slab dehydration

Tomography of northeast Japan forearc and its implications for interplate seismic coupling