Seismic Formation Fluid Pressure Observations Reveal High Anisotropy of Oceanic Crust

2021 ◽  
Author(s):  
Tianhaozhe Sun ◽  
Earl Davis ◽  
Martin Heesemann
Keyword(s):  
Oceanic Crust ◽  
Fluid Pressure ◽  
High Anisotropy

Underplating altered oceanic crust: Insights from numerical modelling

2021 ◽  
Author(s):  
Zoe Braden ◽  
Jonas B. Ruh ◽  
Whitney M. Behr
Keyword(s):  
Numerical Modelling ◽  
Oceanic Crust ◽  
Fluid Pressure ◽  
Geothermal Gradient ◽  
Pore Fluid ◽  
Surface Erosion ◽  
Accretionary Wedge ◽  
Pore Fluid Pressure ◽  
Mafic Rocks

<p>Observations of several active shallow subduction megathrusts suggest that they are localized as décollements within sedimentary sequences or at the contact between sedimentary layers and the underlying mafic oceanic crust.  Exhumed accretionary complexes from a range of subduction depths, however, preserve underplated mafic slivers, which indicate that megathrust faults can occasionally develop within the mafic oceanic crustal column. The incorporation of mafic rocks into the subduction interface shear zone has the potential to influence both long-term subduction dynamics and short-term seismic and transient slip behaviour, but the processes and conditions that favour localisation of the megathrust into deeper oceanic crustal levels are poorly understood.</p><p>In this work, we use visco-elasto-plastic numerical modelling to explore the long-term (million year) factors influencing the incorporation of mafic volcanic rocks into the subduction interface and accretionary wedge through underplating. We focus on the potential importance of oceanic seafloor alteration in facilitating oceanic crustal weakening, which is implemented through a temperature-dependent pore-fluid pressure ratio (lambda = 0.90-0.99 between 160 and 300oC). We then examine the underplating response to changes in sediment thickness, geothermal gradient, sediment fluid pressure, and surface erosion rates. Our results indicate that a thinner incoming sediment package and a lower geothermal gradient cause oceanic crustal underplating to initiate deeper beneath the backstop (overriding plate) compared to thicker incoming sediment and a higher geothermal gradient. Relative pore fluid pressure differences between sediments and altered oceanic crust control the amount of altered oceanic crust that is underplated, as well as the location of underplating beneath the backstop or accretionary wedge. When sediments on top of the altered oceanic crust have the same fluid pressure as the altered oceanic crust, no oceanic crustal underplating occurs. Modelling results are also compared to exhumed subduction complexes to examine the amount and distribution of underplated mafic rocks.</p>

Episodic stress and fluid pressure cycling in subducting oceanic crust during slow slip

2019 ◽  
Vol 12 (6) ◽  
pp. 475-481 ◽  
Author(s):  
E. Warren-Smith ◽  
B. Fry ◽  
L. Wallace ◽  
E. Chon ◽  
S. Henrys ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Fluid Pressure ◽  
Slow Slip ◽  
Pressure Cycling ◽  
Episodic Stress

Episodic stress tensor and fluid pressure cycling in subducting oceanic crust during Northern Hikurangi slow slip events

2020 ◽  
Author(s):  
Emily Warren-Smith ◽  
Bill Fry ◽  
Laura Wallace ◽  
Enrique Chon ◽  
Stuart Henrys ◽  
...  
Keyword(s):  
Shear Zone ◽  
Oceanic Crust ◽  
Subduction Zones ◽  
Fluid Pressure ◽  
Fluid Migration ◽  
Ocean Bottom ◽  
Slow Slip ◽  
Interface Shear ◽  
Slow Slip Events ◽  
Fluid Reservoir

<p><span>The occurrence of slow slip events (SSEs) in subduction zones has been proposed to be linked to the presence of, and fluctuations in near-lithostatic fluid pressures (P</span><sub><span>f</span></sub><span>) within the megathrust shear zone and subducting oceanic crust. In particular, the 'fault-valve' model is commonly used to describe occasional, repeated breaching of a low-permeability interface shear zone barrier, which caps an overpressured hydrothermal fluid reservoir. In this model, a precursory increase in fluid pressure may therefore be anticipated to precede megathrust rupture. Resulting activation of fractures during slip opens permeable pathways for fluid migration and fluid pressure decreases once more, until the system becomes sealed and overpressure can re-accumulate. While the priming conditions for cyclical valving behaviour have been observed at subduction zones globally, and evidence for post-megathrust rupture drainage exists, physical observations of precursory fluid pressure increases, and subsequent decreases, particularly within the subducting slab where hydrothermal fluids are sourced, remain elusive. </span></p><p><span>Here we use earthquake focal mechanisms recorded on an ocean-bottom seismic network to identify changes in the stress tensor within subducting oceanic crust during four SSEs in New Zealand’s Northern Hikurangi subduction zone. We show that the stress, or shape ratio, which describes the relative magnitudes of the principal compressive stress axes, shows repeated decreases prior to, and rapid increases during the occurrence of geodetically documented SSEs. We propose that these changes represent precursory accumulation and subsequent release of fluid pressure within overpressured subducting oceanic crust via a ‘valving’ model for megathrust slip behaviour. Our observations indicate that the timing of slow slip events on subduction megathrusts may be controlled by cyclical accumulation of fluid pressure within subducting oceanic crust.</span></p><p><span>Our model is further supported by observations of seismicity preceding a large SSE in the northern Hikurangi Margin in 2019, captured by ocean-bottom seismometer</span><span>s</span><span> and </span><span>absolute </span><span>pressure </span><span>recorders.</span> <span>O</span><span>bservations of microseismicity </span><span>during this period </span><span>indicate that a stress state conducive to vertical fluid flow was present in the downgoing plate prior to SSE initiation, before subsequently returning to a</span><span> down-dip</span><span> extensional state following the SSE. We propose this precursory seismicity is indicative of fluid migration towards the interface shear zone from the lower plate fluid reservoir, which may have helped triggering slip on the megathrust. </span></p><p><span>We also present preliminary results of a moment tensor study to investigate spatial and temporal patterns in earthquake source properties in SSE regions along the Hikurangi Margin. In particular, earthquakes near Porangahau – a region susceptible to dynamic triggering of tremor and where </span><span>shallow </span><span>SSEs occur every 5 years or so – exhibit distinctly lower double couple components than elsewhere along the margin. We </span><span>attribute this to elevated fluid pressures within the crust here, which is consistent with recent observations of high seismic reflectivity from an autocorrelation study. Such high fluid pressure may control the broad range of seismic and aseismic phenomena observed at Porangahau. </span></p>

scholarly journals Variations of fluid pressure within the subducting oceanic crust and slow earthquakes

2010 ◽  
Vol 37 (14) ◽  
pp. n/a-n/a ◽  
Author(s):  
Aitaro Kato ◽  
Takashi Iidaka ◽  
Ryoya Ikuta ◽  
Yasuhiro Yoshida ◽  
Kei Katsumata ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Fluid Pressure ◽  
Slow Earthquakes

Cristal-growth dynamics of n-doped gaAs

1992 ◽  
Vol 50 (2) ◽  
pp. 1544-1545
Author(s):  
Pham V. Huong ◽  
Stéphanie Bouchet ◽  
Jean-Claude Launay
Keyword(s):  
Crystal Growth ◽  
Spatial Resolution ◽  
Epitaxial Layer ◽  
Outer Surface ◽  
Gaas Substrate ◽  
Structural Modification ◽  
Growth Dynamics ◽  
Argon Laser ◽  
High Anisotropy ◽  
Crystal Gaas

Microstructure of epitaxial layers of doped GaAs and its crystal growth dynamics on single crystal GaAs substrate were studied by Raman microspectroscopy with a Dilor OMARS instrument equipped with a 1024 photodiode multichannel detector and a ion-argon laser Spectra-Physics emitting at 514.5 nm.The spatial resolution of this technique, less than 1 μm2, allows the recording of Raman spectra at several spots in function of thickness, from the substrate to the outer deposit, including areas around the interface (Fig.l).The high anisotropy of the LO and TO Raman bands is indicative of the orientation of the epitaxial layer as well as of the structural modification in the deposit and in the substrate at the interface.With Sn doped, the epitaxial layer also presents plasmon in Raman scattering. This fact is already very well known, but we additionally observed that its frequency increases with the thickness of the deposit. For a sample with electron density 1020 cm-3, the plasmon L+ appears at 930 and 790 cm-1 near the outer surface.

Control of interstitial fluid pressure: Role of [beta ]-integrins

2001 ◽  
Vol 21 (3) ◽  
pp. 222-230 ◽  
Author(s):  
Rolf K. Reed ◽  
Ansgar Berg ◽  
Eli-Anne B. Gjerde ◽  
Kristofer Rubin
Keyword(s):  
Interstitial Fluid ◽  
Fluid Pressure ◽  
Interstitial Fluid Pressure
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