Chi-Yuen Wang

Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms

CHANGES IN PERMEABILITY CAUSED BY TRANSIENT STRESSES: FIELD OBSERVATIONS, EXPERIMENTS, AND MECHANISMS Michael Manga, 1 Igor Beresnev, 2 Emily E. Brodsky, 3 Jean E. Elkhoury, 4 Derek Elsworth, 5 S. E. Ingebritsen, 6 David C. Mays, 7 and Chi-Yuen Wang 1 Received 7 November 2011; revised 15 February 2012; accepted 10 March 2012; published 12 May 2012. [ 1 ] Oscillations in stress, such as those created by earth- quakes, can increase permeability and fluid mobility in geo- logic media. In natural systems, strain amplitudes as small as 10 A6 can increase discharge in streams and springs, change the water level in wells, and enhance production from petroleum reservoirs. Enhanced permeability typically recovers to prestimulated values over a period of months to years. Mechanisms that can change permeability at such small stresses include unblocking pores, either by breaking up permeability-limiting colloidal deposits or by mobilizing droplets and bubbles trapped in pores by capillary forces. The recovery time over which permeability returns to the prestimulated value is governed by the time to reblock pores, or for geochemical processes to seal pores. Monitor- ing permeability in geothermal systems where there is abun- dant seismicity, and the response of flow to local and regional earthquakes, would help test some of the proposed mechanisms and identify controls on permeability and its evolution. Citation: Manga, M., I. Beresnev, E. E. Brodsky, J. E. Elkhoury, D. Elsworth, S. E. Ingebritsen, D. C. Mays, and C.-Y. Wang (2012), Changes in permeability caused by transient stresses: Field observations, experiments, and mechanisms, Rev. Geophys., 50, RG2004, doi:10.1029/2011RG000382. INTRODUCTION [ 2 ] The permeability of Earth’s crust is of great interest because it largely governs key geologic processes such as advective transport of heat and solutes and the generation of elevated fluid pressures by processes such as physical com- paction, heating, and mineral dehydration. For an isotropic Department of Earth and Planetary Science, University of California, Berkeley, California, USA. Department of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa, USA. Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Department of Civil and Environmental Engineering, University of California, Irvine, California, USA. Department of Energy and Mineral Engineering, Center for Geomechanics, Geofluids, and Geohazards, EMS Energy Institute, Pennsylvania State University, University Park, Pennsylvania, USA. U.S. Geological Survey, Menlo Park, California, USA. Department of Civil Engineering, University of Colorado Denver, Denver, Colorado, USA. Corresponding author: M. Manga, Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720, USA. (manga@seismo.berkeley.edu) material, permeability k is defined by Darcy’s law that relates the fluid discharge per unit area q to the gradient of hydraulic head h, q ¼A kgr rh; m where r is the fluid density, m the fluid viscosity and g is gravity. The permeability of common geologic media varies by approximately 16 orders of magnitude, from values as low as 10 A23 m 2 in intact crystalline rock, intact shales, and fault cores, to values as high as 10 A7 m 2 in well-sorted gravels. Nevertheless, despite being highly heterogeneous, perme- ability can be characterized at the crustal scale in a manner that provides useful insight [e.g., Gleeson et al., 2011]. [ 3 ] The responses of hydrologic systems to deformation provide some insight into controls on permeability, in par- ticular its evolution in time. For example, the water level in wells and discharge in rivers have both been observed to change after earthquakes. Because earthquakes produce stresses that can change hydrogeologic properties of the crust, hydrologic responses to earthquakes are expected, especially in the near field (within a fault length of the Copyright 2012 by the American Geophysical Union. Reviews of Geophysics, 50, RG2004 / 2012 1 of 24 Paper number 2011RG000382 8755-1209/12/2011RG000382 RG2004

Two-dimensional modeling of the P-T-t paths of regional metamorphism in simple overthrust terrains

Quantitative modeling of metamorphic P - T - t paths in thrust belts has so far been limited to one-dimensional simplification. We have developed a two-dimensional time-dependent finite-element procedure, capable of tracing and updating fault and strata boundaries during faulting, for modeling thermal evolution of overthrust terrains. We show that the two-dimensionality of an overthrust and the speed and duration of thrusting have significant effects on the P - T - t paths of rocks in orogenic belts. The results suggest that petrological studies of P - T paths of rocks from different lateral positions in an orogenic belt, along with two-dimensional modeling procedure, can put strong constraints on models of regional tectonic processes.

SOME MECHANISMS OF MICROCRACK GROWTH AND INTERACTION IN COMPRESSIVE ROCK FAILURE

Abstract Under uniaxial or triaxial load, stress inhomogeneties are generated in crystalline rock at point contacts and at boundaries between grains of dissimilar elastic constants. These stress inhomogeneities may be responsible for the extension of dilatant cracks. Theoretical models are developed to study these possibilities. The models predict axial splitting in rocks under uniaxial compression. Application of moderate confining pressures severely limits crack extension and thus inhibits axial splitting. Interaction between cracks, together with these stress inhomogeneities, tends to form a zone of intense deformation inclined 30–40° to the maximum principal stress where larger angles are associated with higher confining pressures. The axially-oriented cracks in such a zone tend to grow towards each other and thus may coalesce to form the diagonal macroscopic fracture found in brittle rocks under triaxial conditions.

Coseismic hydrologic response of an alluvial fan to the 1999 Chi-Chi earthquake, Taiwan

Widespread coseismic change in pore-water pressure across a large alluvial fan in central Taiwan in the 1999 Chi-Chi (M w = 7.5) earthquake was captured for the first time by a dense network of hydrologic monitoring wells. The complex, yet systematic, pattern in the water-pressure change appears inconsistent with the existing models; it requires a model that is based on the nonlinear mechanical behavior of sediments under earthquake shaking. This paper presents direct field evidence that earthquake shaking causes rising pore pressure in alluvial fans, which in turn may lead to landslides, even on very gentle slopes.

Coseismic release of water from mountains: Evidence from the 1999 (Mw = 7.5) Chi-Chi, Taiwan, earthquake

Earthquake-induced increases in streamflow, producing ∼0.7 km 3 of total excess water, were documented after the 1999 (M w = 7.5) Chi-Chi earthquake in central Taiwan. Analysis of stream gauge data and well records suggests that the excess water originated in the mountains. We propose that the extensive high-angle fractures formed during the earthquake allow rapid release of water from mountains and that mountains in tectonically active areas may be repeatedly flushed by meteoric water at time intervals comparable to the recurrence time of large earthquakes.

Earthquakes and Water

Liquefaction.- Mud Volcanoes.- Increased Stream Discharge.- Groundwater Level Change.- Temperature and Composition Changes.- Geysers.- Earthquakes Influenced by Water.- Hydrologic Precursors.- Epilogue.

Role of S waves and Love waves in coseismic permeability enhancement

[1] The 2008 M7.9 Wenchuan earthquake in Sichuan, China, caused water level to oscillate and undergo sustained changes in Taiwan, ∼2000 km away from the epicenter. Here we use the responses in three wells recorded at high sampling rate (1 Hz) and the broadband seismograms from a nearby station to document, for the first time, that the major water-level responses associated with Rayleigh waves were preceded by small oscillations that occurred concurrently with S waves and Love waves. We also show that the groundwater flow associated with these small oscillations may be strong enough to remove blockades from sediment pores to enhance aquifer permeability and to facilitate the later major responses. C.

Sediment subduction and frictional sliding in a subduction zone

New experimental data for the mechanical and thermodynamic properties of clays at high pressure and high temperature are applied to model the frictional sliding in a subduction zone. The model is believed applicable because of the large volume of pelagic sediments that appear to have been subducted with the oceanic plate along active Pacific margins. This model also is used to interpret the seismicity in a subduction zone and the heat flow across an arc-trench system.

Hydrofracturing and episodic fluid flow in shale-rich basins; a numerical study

Low-permeability sedimentary rocks commonly are fractured. Direct examination of exposed rock faces and drill cores shows evidence of hydrofracturing. The mechanism for hydrofracturing, its effects on fluid migration, and its dependence on sediment permeability, sedimentation rate, and sedimentary sequences have not been explored. In this study we carry out systematic numerical experiments to study the compaction-induced hydrofracturing. We show that the compaction-induced hydrofracturing commonly may occur in shale-rich basins and in sand-shale sequences; the frequency of such hydrofracturing depends on sediment permeability, sedimentation rate, and sedimentary sequence. An important result is that compaction-induced hydrofracturing may occur at relatively shallow depths in shale-rich basins, but it may mobilize enhanced fluid flow throughout the sedimentary basin. Over 60% of the total compaction-induced fluid flow in the basin may be expelled during hydrofracturing. We test the model against field data from the Yinggehai basin of the south China margin, where abundant hydrofractures in the uppermost Quaternary marine mud have been recently detected by seismic imaging. We suggest that the compaction-induced hydrofracturing may mobilize fluid flow at great depths and affect hydrocarbon migration in shale-rich basins.

Liquefaction Limit during Earthquakes and Underground Explosions: Implications on Ground-Motion Attenuation

Liquefaction of saturated soils and sediments documented during earthquakes shows an empirical relation log R max = 2.05 (±0.10) + 0.45 M , where R max is the liquefaction limit in meters (i.e., the maximum distance from liquefaction site to the hypocenter) and M is the earthquake magnitude. Combining this with an empirical relation between M and the seismic energy of an earthquake, we obtain a relation between the liquefaction limit and the seismic energy: E = A R β max. The prefactor corresponds to a threshold energy for liquefaction ranging from 0.004 to 0.1 J/m3; the exponent, ranging from 3.2 to 3.3, implies that the energy density of ground motion attenuates with distance according to 1/ r 3.2–3.3, where r is the distance from the hypocenter. The value of the threshold energy suggests a preliquefaction degradation of the shear modulus of soils by more than 3 orders of magnitude. Liquefaction documented during underground explosions is characterized by a threshold energy several orders of magnitude greater than that for liquefaction during earthquakes but shows a similar functional relation between E and R max as that for liquefaction during earthquakes and implies a similar attenuation relation between ground-motion energy density and distance.