Michael Manga

A mechanism for sustained groundwater pressure changes induced by distant earthquakes

[1] Large, sustained well water level changes (>10 cm) in response to distant (more than hundreds of kilometers) earthquakes have proven enigmatic for over 30 years. Here we use high sampling rates at a well near Grants Pass, Oregon, to perform the first simultaneous analysis of both the dynamic response of water level and sustained changes, or steps. We observe a factor of 40 increase in the ratio of water level amplitude to seismic wave ground velocity during a sudden coseismic step. On the basis of this observation we propose a new model for coseismic pore pressure steps in which a temporary barrier deposited by groundwater flow is entrained and removed by the more rapid flow induced by the seismic waves. In hydrothermal areas, this mechanism could lead to 4 � 10 � 2 MPa pressure changes and triggered seismicity. INDEX TERMS: 1829 Hydrology: Groundwater hydrology; 7209 Seismology: Earthquake dynamics and mechanics; 7212 Seismology: Earthquake ground motions and engineering; 7260 Seismology: Theory and modeling; 7294 Seismology: Instruments and techniques; KEYWORDS: earthquakes, triggering, time-dependent hydrology, fractures Citation: Brodsky, E. E., E. Roeloffs, D. Woodcock, I. Gall, and M. Manga, A mechanism for sustained groundwater pressure changes induced by distant earthquakes, J. Geophys. Res., 108(B8), 2390, doi:10.1029/2002JB002321, 2003.

Explosive volcanism may not be an inevitable consequence of magma fragmentation

The fragmentation of magma, containing abundant gas bubbles, is thought to be the defining characteristic of explosive eruptions. When viscous stresses associated with the growth of bubbles and the flow of the ascending magma exceed the strength of the melt, the magma breaks into disconnected fragments suspended within an expanding gas phase. Although repeated effusive and explosive eruptions for individual volcanoes are common, the dynamics governing the transition between explosive and effusive eruptions remain unclear. Magmas for both types of eruptions originate from sources with similar volatile content, yet effusive lavas erupt considerably more degassed than their explosive counterparts. One mechanism for degassing during magma ascent, consistent with observations, is the generation of intermittent permeable fracture networks generated by non-explosive fragmentation near the conduit walls. Here we show that such fragmentation can occur by viscous shear in both effusive and explosive eruptions. Moreover, we suggest that such fragmentation may be important for magma degassing and the inhibition of explosive behaviour. This implies that, contrary to conventional views, explosive volcanism is not an inevitable consequence of magma fragmentation.

Permeability‐porosity relationship in vesicular basalts

The permeability k and porosity of vesicular basalts are measured. The relationship between k and re- flects the formation and emplacement of the basalts and can be related to the crystal and vesicle microstructure obtained by image analysis. Standard theoretical models relating k and that work well for granular materials are unsuccessful for vesicular rocks due to the fundamental dierence in pore structure. Specically, k in vesicular rocks is governed by apertures between bubbles. The dierence between calcu- lated and measured k reflects the small size of these aper- tures with aperture radii typically O(10) times smaller than the mean bubble radii.

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

Numerical models of the onset of yield strength in crystal–melt suspensions

The formation of a continuous crystal network in magmas and lavas can provide finite yield strength, dy, and can thus cause a change from Newtonian to Bingham rheology. The rheology of crystal^melt suspensions affects geological processes, such as ascent of magma through volcanic conduits, flow of lava across the Earth’s surface, melt extraction from crystal mushes under compression, convection in magmatic bodies, and shear wave propagation through partial melting zones. Here, three-dimensional numerical models are used to investigate the onset of ‘static’ yield strength in a zero-shear environment. Crystals are positioned randomly in space and can be approximated as convex polyhedra of any shape, size and orientation. We determine the critical crystal volume fraction, Pc, at which a crystal network first forms. The value of Pc is a function of object shape and orientation distribution, and decreases with increasing randomness in object orientation and increasing shape anisotropy. For example, while parallel-aligned convex objects yield Pc = 0.29, randomly oriented cubes exhibit a maximum Pc of 0.22. Approximations of plagioclase crystals as randomly oriented elongated and flattened prisms (tablets) with aspect ratios between 1:4:16 and 1:1:2 yield 0.086Pc 6 0.20, respectively. The dependence of Pc on particle orientation implies that the flow regime and resulting particle ordering may affect the onset of yield strength. Pc in zero-shear environments is a lower bound for Pc. Finally the average total excluded volume is used, within its limitation of being a ‘quasi-invariant’, to develop a scaling relation between dy and P for suspensions of different particle shapes. fl 2001 Elsevier Science B.V. All rights reserved.

The influence of a chemical boundary layer on the fixity, spacing and lifetime of mantle plumes

Seismological observations provide evidence that the lowermost mantle contains superposed thermal and compositional boundary layers that are laterally heterogeneous. Whereas the thermal boundary layer forms as a consequence of the heat flux from the Earths outer core, the origin of an (intrinsically dense) chemical boundary layer remains uncertain. Observed zones of ‘ultra-low’ seismic velocity suggest that this dense layer may contain metals or partial melt, and thus it is reasonable to expect the dense layer to have a relatively low viscosity. Also, it is thought that instabilities in the thermal boundary layer could lead to the intermittent formation and rise of mantle plumes. Flow into ascending plumes can deform the dense layer, leading, in turn, to its gradual entrainment. Here we use analogue experiments to show that the presence of a dense layer at the bottom of the mantle induces lateral variations in temperature and viscosity that, in turn, determine the location and dynamics of mantle plumes. A dense layer causes mantle plumes to become spatially fixed, and the entrainment of low-viscosity fluid enables plumes to persist within the Earth for hundreds of millions of years.

Rheology of bubble-bearing magmas

The rheology of bubble-bearing suspensions is investigated through a series of three-dimensional boundary integral calculations in which the effects of bubble deformation, volume fraction, and shear rate are considered. The behaviour of bubbles in viscous flows is characterized by the capillary number, Ca, the ratio of viscous shear stresses that promote deformation to surface tension stresses that resist bubble deformation. Estimates of Ca in natural lava flows are highly variable, reflecting variations in shear rate and melt viscosity. In the low capillary number limit (e.g., in carbonatite flows) bubbles remain spherical and may contribute greater shear stress to the suspension than in high capillary number flows, in which bubble deformation is significant. At higher Ca, deformed bubbles become aligned in the direction of flow, and as a result, contribute less shear stress to the suspension. Calculations indicate that the effective shear viscosity of bubbly suspensions, at least for Ca<0.5, is a weakly increasing function of volume fraction and that suspensions of bubbles are shear thinning. Field observations and qualitative arguments, however, suggest that for sufficiently large Ca (Ca greater than about 1) the effective shear viscosity may be less than that of the suspending liquid. Bubbles reach their quasi-steady deformed shapes after strains of order one; for shorter times, the continuous deformation of the bubbles results in continual changes of rheological properties. In particular, for small strains, the effective shear viscosity of the suspension may be less than that of the liquid phase, even for small Ca. Results of this study may help explain previous experimental, theoretical, and field based observations regarding the effects of bubbles on flow rheology.

Stress partitioning in streams by large woody debris

Using simple theoretical models and field measurements from a spring- dominated stream, we quantify how large woody debris affect channel hydraulics and morphology at both the local and reach-averaged scales. Because spring-dominated streams have nearly constant discharge, they provide a unique natural opportunity to study flow and transport processes near the channel-forming flow. We first show that the drag on a floating log is identical to the theoretical value for widely separated cylinders at similar Reynolds numbers. We then use simple theoretical models to estimate the partitioning of flow shear stress between woody debris and streambeds. The inferred stress partitioning is consistent with an estimate based on a comparison of local and reach- averaged measurements of the water surface slope. Our measurements show that even though large woody debris cover less than 2% of the streambed, they provide roughly half of the total flow resistance. As large woody debris are added to a stream, the total shear stress increases (because the water depth increases), but the shear stress borne by the bed decreases, as a growing fraction of the total shear stress is borne by the debris. Our analysis shows that simple theoretical models of stress partitioning may provide a convenient mathematical framework for assessing how changes in debris loading affect streams.

Evidence for an ancient martian ocean in the topography of deformed shorelines

A suite of observations suggests that the northern plains of Mars, which cover nearly one third of the planets surface, may once have contained an ocean. Perhaps the most provocative evidence for an ancient ocean is a set of surface features that ring the plains for thousands of kilometres and that have been interpreted as a series of palaeoshorelines of different age. It has been shown, however, that topographic profiles along the putative shorelines contain long-wavelength trends with amplitudes of up to several kilometres, and these trends have been taken as an argument against the martian shoreline (and ocean) hypothesis. Here we show that the long-wavelength topography of the shorelines is consistent with deformation caused by true polar wander—a change in the orientation of a planet with respect to its rotation pole—and that the inferred pole path has the geometry expected for a true polar wander event that postdates the formation of the massive Tharsis volcanic rise.

Seismological constraints on a possible plume root at the core–mantle boundary

Recent seismological discoveries have indicated that the Earths core–mantle boundary is far more complex than a simple boundary between the molten outer core and the silicate mantle. Instead, its structural complexities probably rival those of the Earths crust. Some regions of the lowermost mantle have been observed to have seismic wave speed reductions of at least 10 per cent, which appear not to be global in extent. Here we present robust evidence for an 8.5-km-thick and ∼50-km-wide pocket of dense, partially molten material at the core–mantle boundary east of Australia. Array analyses of an anomalous precursor to the reflected seismic wave ScP reveal compressional and shear-wave velocity reductions of 8 and 25 per cent, respectively, and a 10 per cent increase in density of the partially molten aggregate. Seismological data are incompatible with a basal layer composed of pure melt, and thus require a mechanism to prevent downward percolation of dense melt within the layer. This may be possible by trapping of melt by cumulus crystal growth following melt drainage from an anomalously hot overlying region of the lowermost mantle. This magmatic evolution and the resulting cumulate structure seem to be associated with overlying thermal instabilities, and thus may mark a root zone of an upwelling plume.