Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise

In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise–based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [Mw(moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations.

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Anomalous Vp/Vs in highly pressurized rocks: Evidence for anisotropy or mafic composition?

10.5194/egusphere-egu2020-15845 ◽

2020 ◽

Author(s):

Lucas Pimienta ◽

Alexandre Schubnel ◽

Jerome Fortin ◽

Yves Guéguen ◽

Helene Lyon-Caen ◽

...

Keyword(s):

Subduction Zones ◽

Laboratory Data ◽

Fluid Pressure ◽

Pore Fluid ◽

S Wave ◽

Pore Fluid Pressure ◽

Crustal Rocks ◽

High Fluid ◽

Measuring Frequency ◽

Very High

<p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Anomalously high seismic P- to S-wave velocity ratios (Vp/Vs) have been observed in subduction zones, in locations where varieties of earthquakes and slips are expected to occur. From qualitative laboratory knowledge of rocks Poisson&#8217;s ratio, these results were interpreted as evidence of near-lithostatic pore fluid pressure. Because most laboratory data did not document such high Vp/Vs values, these were further linked to additional constrains of anisotropy or the dominance of minerals of very high intrinsic Vp/Vs, e.g. mafic rocks.However, does high Vp/Vs necessarily imply anisotropy and/or mafic composition?</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Recently, the measuring frequency (f) was shown to play a major role on rocks&#8217; resulting Poisson&#8217;s ratio, so that usual laboratory results (at f = 1 MHz) might not directly transfer to field ones (at f = 1 Hz). From this consideration, we investigate Vp/Vs of a variety of crustal rocks in the elastic regime relevant at the field scale, the undrained elastic regime.Accounting for rocks dispersive properties, this work aims to show that:</p><ul><li>In the laboratory, in isotropic rocks, one might attain Vp/Vs values as high as the anomalous ones observed in subduction zones.</li> <li>No mineralogical control is needed for such high Vp/Vs values, which could be consistent with the inherent mineral variability in different settings across the globe.</li> <li>High pore fluid pressure is a major parameter, but not alone: such high values cannot be achieved without very high degree of micro-fracturing of the rock, opened by high fluid pressures, an information of potential importance to understand those seismogenic zones.</li> </ul>

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Determination of the pore fluid pressure ratio at seismogenic megathrusts in subduction zones: Implications for strength of asperities and Andean-type mountain building

Journal of Geophysical Research Atmospheres ◽

10.1029/2008jb005889 ◽

2009 ◽

Vol 114 (B5) ◽

Cited By ~ 67

Author(s):

Tetsuzo Seno

Keyword(s):

Subduction Zones ◽

Pressure Ratio ◽

Fluid Pressure ◽

Pore Fluid ◽

Mountain Building ◽

Pore Fluid Pressure ◽

Andean Type

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Seismic evidence for megathrust fault-valve behavior during episodic tremor and slip

Science Advances ◽

10.1126/sciadv.aay5174 ◽

2020 ◽

Vol 6 (4) ◽

pp. eaay5174 ◽

Cited By ~ 2

Author(s):

Jeremy M. Gosselin ◽

Pascal Audet ◽

Clément Estève ◽

Morgan McLellan ◽

Stephen G. Mosher ◽

...

Keyword(s):

Subduction Zone ◽

Subduction Zones ◽

Seismic Velocity ◽

Fluid Pressure ◽

Pore Fluid ◽

Cascadia Subduction Zone ◽

Low Velocity ◽

Seismic Scattering ◽

Fault Valve ◽

Episodic Tremor

Fault slip behavior during episodic tremor and slow slip (ETS) events, which occur at the deep extension of subduction zone megathrust faults, is believed to be related to cyclic fluid processes that necessitate fluctuations in pore-fluid pressures. In most subduction zones, a layer of anomalously low seismic wave velocities [low-velocity layer (LVL)] is observed in the vicinity of ETS and suggests high pore-fluid pressures that weaken the megathrust. Using repeated seismic scattering observations in the Cascadia subduction zone, we observe a change in the seismic velocity associated with the LVL after ETS events, which we interpret as a response to fluctuations in pore-fluid pressure. These results provide direct evidence of megathrust fault-valve processes during ETS.

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In situ measurement of velocity-stress sensitivity using crosswell continuous active-source seismic monitoring

Geophysics ◽

10.1190/geo2017-0106.1 ◽

2017 ◽

Vol 82 (5) ◽

pp. D319-D326 ◽

Cited By ~ 8

Author(s):

Pierpaolo Marchesini ◽

Jonathan B. Ajo-Franklin ◽

Thomas M. Daley

Keyword(s):

Seismic Velocity ◽

Fluid Pressure ◽

Seismic Monitoring ◽

Stress Sensitivity ◽

Pore Fluid ◽

Field Scale ◽

Pore Fluid Pressure ◽

Active Source ◽

Injection Zone

The ability to characterize time-varying reservoir properties, such as the state of stress, has fundamental implications in subsurface engineering, relevant to geologic sequestration of [Formula: see text]. Stress variation, here in the form of changes in pore fluid pressure, is one factor known to affect seismic velocity. Induced variations in velocity have been used in seismic studies to determine and monitor changes in the stress state. Previous studies conducted to determine velocity-stress sensitivity at reservoir conditions rely primarily on laboratory measurements of core samples or theoretical relationships. We have developed a novel field-scale experiment designed to study the in situ relationship between pore-fluid pressure and seismic velocity using a crosswell continuous active-source seismic monitoring (CASSM) system. At the Cranfield, Mississippi, [Formula: see text] sequestration field site, we actively monitored seismic response for five days with a temporal resolution of 5 min; the target was a 26 m thick injection zone at approximately 3.2 km depth in a fluvial sandstone formation (lower Tuscaloosa Formation). The variation of pore fluid pressure was obtained during discrete events of fluid withdrawal from one of the two wells and monitored with downhole pressure sensors. The results indicate a correlation between decreasing CASSM time delay (i.e., velocity change for a raypath in the reservoir) and periods of reduced fluid pore pressure. The correlation is interpreted as the velocity-stress sensitivity measured in the reservoir. This observation is consistent with published laboratory studies documenting a velocity ([Formula: see text]) increase with an effective stress increase. A traveltime change ([Formula: see text]) of 0.036 ms is measured as the consequence of a change in pressure of approximately 2.55 MPa ([Formula: see text]). For [Formula: see text] total traveltime, the velocity-stress sensitivity is [Formula: see text]. The overall results suggest that CASSM measurements represent a valid technique for in situ determination of velocity-stress sensitivity in field-scale monitoring studies.

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