The structure and composition of exhumed faults, and their implications for seismic processes

Field studies of faults exhumed from seismogenic depths provide useful data to constrain seismologic models of fault zone processes and properties. Data collected on the San Andreas Fault in the San Gabriel Mountains has shown that large-displacement faults consist of one to several very narrow slip zones embedded in a cataclastically deformed sheared region several meters thick. However these faults have not been buried to depths greater than 5 km. Fault zones in the Sierra Nevada, California allow us to study the microstructures resulting from the deformation mechanisms active at seismogenic depths. Syn-fault mineralization shows that these left-lateral strike-slip faults formed at 5-12 km depth. Detailed microstructural analyses of the small faults reveal that they evolved from cooling joints filled by chlorite, epidote and quartz. These joints were then reactivated to form shear faults with accompanying brittle fracture and cataclastic deformation, ultimately developing very fined-grained cataclasites and ultracataclasites. The shear-induced microstructures are developed on faults with as little as several mm of slip showing that narrow slip-surfaces develop early in the lifetime of these faults. Subsequent slip has little effect on the microstructures. The inferred similarity of deformation mechanisms in faults 10 m to 10 km long indicates that basic slip processes on the faults are scale invariant, and may be a cause for the inferred constant b-value for small earthquakes. Analysis of map-scale fault linkages and terminations indicate that linkage zones are up to 400 m wide and 1 km long, and consist of altered and fractured rocks with numerous through-going slip surfaces. Terminations are regions of numerous splay faults that have cumulative offsets approaching those of the main faults. The slip distribution and structure of the terminations and linkage zones suggest that seismic slip may propagate into these zones of enhanced toughness, and that through-going slip can occur when a sufficient linkage of faults in the zone allow slip to be transmitted

Zircon dissolution in a ductile shear zone, Monte Rosa granite gneiss, northern Italy

The sizes, distributions and shapes of zircon grains within variably deformed granite gneiss from the western Alps have been studied. Zircon shows numerous indicators of a metamorphic response in both the host gneiss and a 5 cm wide continuous ductile shear zone, within which the zircon grain sizes range from <1 µm to >50 µm. However, the very fine grain sizes are virtually absent from grain boundaries. Within this zone, zircons consistently have more rounded and embayed margins, which are interpreted as evidence of dissolution in response to fluid influx during shearing. Zircons are preferentially located near metamorphic muscovite in both the host gneiss and the shear zone and tend to show the poorest crystal shape, indicating that fluids linked to the formation and presence of muscovite may enhance both the crystallization of zircon and its subsequent dissolution. Larger zircon crystals typically show a brittle response to deformation when adjacent to phyllosilicates, with fractures consistently perpendicular to the (001) mica cleavage. The variety of metamorphic behaviour observed for zircon indicates that it may be highly reactive in sub-solidus mid-crustal metamorphic environments

Validation of fault seal mechanisms in the Timor sea : an outcrop and subsurface perspective

When risking faulted structures, across-fault juxtaposition and or membrane seal are key issues. Generally, this work is done on a “best-guess” model. The application of SGR methods in reservoir / seal systems that have moderate Vshale values artificially increases predicted column heights. In a risking processes these large columns are discounted through other geologic risk factors. Faults in Miri, Sarawak, have been systematically mapped in outcrop in great detail to measure the strike variability of fault rocks. This work greatly helps to understand the limitation of membrane seal algorithms. To illustrate the implications to subsurface risking, a validation will be presented in which observed hydrocarbon water contacts are compared with probabilistic models for both juxtaposition and SGR. A comprehensive review of a set of fields in The Timor Sea shows that probabilistic juxtaposition models more accurately predict hydrocarbon water contacts than calibrated SGR single “best” technical models

Do intraplate and plate boundary fault systems evolve in a similar way with repeated slip events?

As repeated slip events occur on a fault, energy is partly dissipated through rock fracturing and frictional processes in the fault zone and partly radiated to the surface as seismic energy. Numerous field studies have shown that the core of intraplate faults is wider on average with increasing total displacement (and hence slip events). In this study we compile data on the fault core thickness, total displacement and internal structure (e.g., fault core composition, host rock juxtaposition, slip direction, fault type, and/or the number of fault core strands) of plate boundary faults to compare to intraplate faults (within the interior of tectonic plates). Fault core thickness data show that plate boundary faults are anomalously narrow by comparison to intraplate faults and that they remain narrow regardless of how much total displacement they have experienced or the local structure of the fault. By examining the scaling relations between seismic moment, average displacement and surface rupture length for plate boundary and intraplate fault ruptures, we find that for a given value of displacement in an individual earthquake, plate boundary fault earthquakes typically have a greater seismic moment (and hence earthquake magnitude) than intraplate events. We infer that narrow plate boundary faults do not process intact rock as much during seismic events as intraplate faults. Thus, plate boundary faults dissipate less energy than intraplate faults during earthquakes meaning that for a given value of average displacement, more energy is radiated to the surface manifested as higher magnitude earthquakes. By contrast, intraplate faults dissipate more energy and get wider as fault slip increases, generating complex zones of damage in the surrounding rock and propagating through linkage with neighbouring structures. The more complex the fault geometry, the more energy has to be consumed at depth during an earthquake and the less energy reaches the surface

Coupled CO2-leakage and in situ fluid-mineral reactions in a natural CO2 reservoir, Green River, Utah

Spectroscopic studies and atomistic simulations of (hydr)oxide surfaces, which show that some aqueous cations bind to two or four surface oxygen atoms, have increased interest in multi-dentate surface complexation models (SCMs) [1-3]. However, it remains unclear how the (fitted) values of intrinsic equilibrium constants K int/m (referenced to infinite dilution) for δ-dentate M surface-binding reactions (δ >1) depend on the choice of concentration scale. In existing SCM codes, a surface complex may be treated in scales of either: molarity/molality ([]); site coverage fraction (Θ); surface mole fraction (x); molecular surface density (Γ, in mol•m-2); or relative density Γ/Γo (o, where Γo = 2 •10-5 mol•m-2 is the reference adsorbed density [4]). Our aim was to investigate, for ‘denticities’ 1≤ δ ≤4, how to convert the K int,δ values fitted for a given titration data set (the same solid concentration cS, specific surface area As, and monolayer site density ΓC) between different concentration scales. For single-site monodentate surface binding reactions, K int/m expressed in all concentration scales ([], Θ, x, Γ, o) reduce to the same value K M int,1. For the binding with δ≤2, conversion factors from xKM int,δ to ΘKM int,δ are about δ. From []KM int,δ to any other scale, they involve (csAsΓx)δ-1 which is ca. 10-5 for δ = 2 or 10-15 for δ = 4 at typical cS = 1 g•dm-3, As = 10 m2g-1, and ΓC = 10-6 mol•m-2. Conversions of K/int from [], Θ and x scales into the Γ scale involve (ΓC)1-δ, which has a value ranging from 10/5 to 10/18 at 10-6 < ΓC < 10-5 mol•m-2. The K/int conversions from [], Θ and x to the o scale include (Γo/ΓC)δ-1 which vanishes if ΓC = Γo (then oKM int,δ = xKM int,δ). Our findings show that the use of published KM int,δ (δ ≥ 2) in SCMs may lead to erroneous results, if concentration scales are not precisely defined both in the original fitting and in the subsequent application. At trace ion concentrations, using formally monodentate surface species would be safe especially on ‘strong’ sites, for which the density is typically adjusted to reproduce multi-site isotherms. Our results from comparative fitting of KM/int,δ with SCM codes using different scales show the magnitudes of ‘denticity effects’; we discuss ways to correct for these effects in re-using, comparing or correlating values of KM/int,δ. In a further thermodynamic treatment, e.g. deriving the entropy effect of the adsorption reaction from KM/int,δ fitted for different temperatures, the constants must first be made dimensionless and independent of δ and ΓC by converting them into the (o) scale.http://dx.doi.org/10.1016/j.gca.2010.04.036University of Tennessee; Oak Ridge National Laborator

Microseismic events cause significant pH drops in groundwater

Earthquakes cause rock fracturing, opening new flow pathways which can result in the mixing of previously isolated geofluids with differing geochemistries. Here we present the first evidence that seismic events can significantly reduce groundwater pH without the requirement for fluid mixing, solely through the process of dynamic rock fracturing. At the Grimsel Test Site, Switzerland, we observe repeated, short-lived groundwater pH drops of 1-3.5 units, while major and minor ion groundwater concentrations remain constant. Acidification coincides with reservoir drainage and induced microseismic events. In laboratory experiments, we demonstrate that fresh rock surfaces made by particle cracking interact with the in situ water molecules, likely through creation of surface silanols and silica radicals, increasing the H+ concentration and significantly lowering groundwater pH. Our findings are significant; pH exerts a fundamental control on the rate and outcome of most aqueous geochemical reactions and microseismic events are commonplace, even in seismically inactive regions

Detection of weak seismic signals in noisy environments from unfiltered, continuous passive seismic recordings

Robust event detection of low signal-to-noise ratio (SNR) events, such as those characterized as induced or triggered seismicity, remains a challenge. The reason is the relatively small magnitude of the events (usually less than 2 or 3 in Richter scale) and the fact that regional permanent seismic networks can only record the strongest events of a microseismic sequence. Monitoring using temporary installed short-period arrays can fill the gap of missed seismicity but the challenge of detecting weak events in long, continuous records is still present. Further, for low SNR recordings, commonly applied detection algorithms generally require pre-filtering of the data based on a priori knowledge of the background noise. Such knowledge is often not available. We present the NpD (Non-parametric Detection) algorithm, an automated algorithm which detects potential events without the requirement for pre-filtering. Events are detected by calculating the energy contained within small individual time segments of a recording and comparing it to the energy contained within a longer surrounding time window. If the excess energy exceeds a given threshold criterion, which is determined dynamically based on the background noise for that window, then an event is detected. For each time window, to characterize background noise the algorithm uses non-parametric statistics to describe the upper bound of the spectral amplitude. Our approach does not require an assumption of normality within the recordings and hence it is applicable to all datasets. We compare our NpD algorithm with the commonly commercially applied STA/LTA algorithm and another highly efficient algorithm based on Power Spectral Density using a challenging microseismic dataset with poor SNR. For event detection, the NpD algorithm significantly outperforms the STA/LTA and PSD algorithms tested, maximizing the number of detected events whilst minimizing the number of false positives

Strike-Slip Fault Terminations at Seismogenic Depths: The Structure and Kinematics of the Glacier Lakes Fault, Sierra Nevada United States

[1] Structural complexity is common at the terminations of earthquake surface ruptures; similar deformation may therefore be expected at the end zones of earthquake ruptures at depth. The 8.2 km long Glacier Lakes fault (GLF) in the Sierra Nevada is a left-lateral strike-slip fault with a maximum observed displacement of 125 m. Within the fault, pseudotachylytes crosscut cataclasites, showing that displacement on the GLF was accommodated at least partly by seismic slip. The western termination of the GLF is defined by a gradual decrease in the displacement on the main fault, accompanied by a 1.4 km wide zone of secondary faulting in the dilational quadrant of the GLF. The secondary faults splay counterclockwise from the main fault trace forming average angles of 39° with the main fault. Slip vectors defined by slickenlines plunge more steeply west for these splay faults than for the GLF. Static stress transfer modeling shows that the orientations of the splays, and the plunge of displacement on those splays, are consistent with displacement on the main fault. The GLF termination structure shows that structural complexity is present at the terminations of faults at seismogenic depths and therefore ruptures that propagate beyond fault terminations, or through step overs between two faults, will likely interact with complex secondary fault structures. Models of dynamic rupture propagation must account for the effect of preexisting structures on the elastic properties of the host rock. Additionally, aftershock distributions and focal mechanisms may be controlled by the geometry and kinematics of structures present at fault terminations