Dynamic and short-lived fluid flow in the high-grade metamorphic rocks related to seismic events in the middle crust.

Fluid flow in the crust induces fluid-rock reactions and contributes to earthquake triggering. However, there are limited numerical constraints on the fluid volumes with the available duration of fluid infiltration. There is also a gap in our knowledge of time-integrated fluid fluxes estimated from geological samples and their influence on controlling seismic/aseismic activity. Merging the timescales of fluid infiltration with the transport properties estimated from the geological samples such as metamorphic reaction zones is essential to understanding the fluid flux during crustal fracturing and its influence on controlling some characteristics of seismic/aseismic events.

This study focuses on fluid flow through a single fracture and the fluid-rock reaction zones and applies its results to low-magnitude fracturing events, such as tremors and low frequency earthquakes. Physical properties of fluid flow provide an opportunity to calculate the seismic moment and cumulative magnitude of the possibly triggered seismic/aseismic event.

Particularly, to examine the duration of fluid infiltration and time-integrated fluid fluxes we analyze amphibolite-facies fluid-rock reaction zones and then combine with estimates of possible associated seismicity and conclude that flow along a single fracture is compatible with seismicity of non-volcanic tremor and low frequency earthquakes. This study is based on evidence of rapid fluid infiltration (~10 h) caused by crustal fracturing and permeability evolution from low- to highly-permeable rocks (~10⁻⁹–10⁻⁸ m²).

Time-integrated fluid fluxes perpendicular to a given fracture and those through the fracture were estimated. Coupled methodology, including reactive-transport modeling and thermodynamic analyses, based on Si alteration processes within reaction zones is used to estimate fluid volumes involved in triggering seismic activity. Time-integrated fluid flux through the fracture results in 10^3-6 m³/m². The lower range is similar to the fluxes through the upper crustal fracture zones (~10^3-4 m³/m²), while almost the whole range is comparable to the contact metamorphism zone (~10^2-5 m³/m²).

Fluid volumes transported through the fracture were compared with fluid injection experiment results. We also compare the durations of fluid infiltration to the durations of the slow slip events. There is no universal theory of slow slip phenomena from the perspectives of geological and geophysical properties. In terms of pressure and temperature, high-grade metamorphic rocks can be related to slow slip events. Our finding reveals that the transportation of voluminous fluid volumes through a fracture may be related to short seismic/aseismic events such as tremors and LFEs, as suggested from duration (~10 h) and cumulative magnitude, representing the maximum values as 2.0–3.8, the lower limit of the magnitude for a single fluid-driven seismic event as –0.6 to 0.2. Single fractures described in this study make it possible to transfer voluminous fluid flow. They could be an essential control on the generation of seismic activity above the tremor and slow slip events source regions in the lower–middle crust.