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

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.

Download Full-text

Temporal changes in pore fluid pressure during slow earthquake cycle estimated from foliation-parallel extension cracking

10.5194/egusphere-egu21-7760 ◽

2021 ◽

Author(s):

Makoto Otsubo ◽

Kohtaro Ujiie ◽

Hanae Saishu ◽

Ayumu Miyakawa ◽

Asuka Yamaguchi

Keyword(s):

Quartz Vein ◽

Pressure Ratio ◽

Fluid Pressure ◽

Pore Fluid ◽

Lithostatic Pressure ◽

Mode I ◽

Pore Fluid Pressure ◽

Slow Earthquake ◽

Vein Formation ◽

Parallel Extension

Pore fluid pressure (Pf) is of great importance to understand slow earthquake mechanics. In this study, we estimated the pore fluid pressure during the formation of foliation-parallel quartz veins filling mode I cracks in the Makimine m&#233;lange eastern Kyushu, SW Japan. The m&#233;lange preserves quartz-filled shear veins, foliation-parallel extension veins and subvertical extension tension vein arrays. The coexistence of the crack-seal veins and viscously sheared veins (aperture width of a quartz vein: a few tens of microns) may represent episodic tremor and slow slip (Ujiie et al., 2018). The foliation-parallel extension cracks can function as the fluid pathway in the m&#233;lange. We applied the stress tensor inversion approach proposed by Sato et al. (2013) to estimate stress regimes by using foliation-parallel extension vein orientations. The estimated stress is a reverse faulting stress regime with a sub-horizontal &#963;1-axis trending NNW&#8211;SSE and a sub-vertical &#963;3-axis, and the driving pore fluid pressure ratio P* (P* = (Pf &#8211; &#963;3) / (&#963;1 &#8211; &#963;3)) is ~0.1. When the pore fluid pressure exceeds &#963;3, veins filling mode I cracks are constructed (Jolly and Sanderson, 1997). The pore fluid pressure that exceeds &#963;3 is the pore fluid overpressure &#916;Pf (&#916;Pf = Pf &#8211; &#963;3). To estimate the pore fluid overpressure, we used the poro-elastic model for extension quartz vein formation (Gudmundsson, 1999). Pf and &#916;Pf in the case of the Makimine m&#233;lange are ~280 MPa and 80&#8211;160 kPa (assuming depth = 10 km, density = 2800 kg/m3, tensile strength = 1 MPa and Young&#8217;s modulus = 7.5&#8211;15 GPa). When the pore fluid overpressure is released, the cracks are closed and the reduction of pore fluid pressure is stopped (Otsubo et al., 2020). After the pore fluid overpressure is reduced, the normalized pore pressure ratio &#955;* (&#955;* = (Pf &#8211; Ph) / (Pl &#8211; Ph), Pl: lithostatic pressure; Ph: hydrostatic pressure) is ~1.01 (Pf > Pl). The results indicate that the pore fluid pressure constantly maintains the lithostatic pressure during the extension cracking along the foliation.References: Gudmundsson (1999) Geophys. Res. Lett., 26, 115&#8211;118; Jolly and Sanderson (1997) Jour. Struct. Geol., 19, 887&#8211;892; Otsubo et al. (2020) Sci. Rep., 10:12281; Palazzin et al. (2016) Tectonophysics, 687, 28&#8211;43; Sato et al. (2013) Tectonophysics, 588, 69&#8211;81; Ujiie et al. (2018) Geophys. Res. Lett., 45, 5371&#8211;5379, https://doi.org/10.1029/2018GL078374.

Download Full-text

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

&#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?&#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:<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>

Download Full-text