Is There Slow Slip on the Wasatch Fault?

To accurately determine the earthquake hazard posed by a fault, we need to understand both strain accumulation and release along the fault. Strain accumulates during aseismic periods but it is released during fault slip events that can be either seismic or aseismic. Aseismic slow slip events are motions similar to earthquakes but they occur over much longer timescales. Slow slip is not felt at the Earth’s surface but it can be recorded in GPS time series. A deformation modeling tool that was applied in Guerrero, Mexico by Lowry et. al. (2001) fits a hyperbolic tangent function to GPS time series and can be used to distinguish slow slip events from noise in the data and from non-tectonic deformation. Time series from the Plate Boundary Observatory, Wasatch Front GPS Network, and Basin and Range Geodetic Network were analyzed for transient deformation during the period encompassing 2004 to 2008. Data suggests several transient motions including a possible slow slip event beginning in mid-2008 and continuing into 2009. Both seismic and aseismic slip influence the earthquake cycle, and slow fault slip events offer a window into frictional properties on fault surfaces that will rupture in future earthquakes. Consequently, as we increase our understanding of aseismic slip and why it occurs, we eventually may expect to develop predictive models of fault slip through time by combining measurements of aseismic and seismic slip in models that reflect the physics of frictional slip on faults

Fracture behavior across interfaces in seal lithologies

Faults and fracture networks at depth are important fluid pathways, especially in fine-grained, low permeability seal lithologies. Discontinues in sealing lithologies can create seal bypass systems, leading to the failure of CO2 geosequestration sites or hydrocarbon traps. We characterize the occurrence of and changes in discontinuity patterns and the associated changes in elastic moduli across sedimentologic interfaces to document the importance of these discontinuities for fluid management in the subsurface and potential for re-activation in high-pressure injection scenarios. We evaluate well-exposed, fine-grained, low-permeability Mesozoic and Paleozoic units that are seals of potential CO2 repositories on the Colorado Plateau and show evidence for open fractures and fluid flow in the subsurface. Field observations document changes in fracture distributions across lithologic boundaries allowing us to identify mechano-stratigraphic units and focus on the effect of lithologic interfaces on fracture distribution. An interface marks the boundary between facies in a seal and in this study the fractures are shown to deflect or arrest at the interface. In outcrop fracture intensity varies in from 1 to 18 fractures per meter and fracture apertures range from mm to cm. The mineralized fractures often have associated alteration halos along their boundaries; their general orientation follows that of discontinuities within the underlying reservoir facies or adjacent faults. The recognition of these changes in fracture distribution is important for forward modeling of fluid flow and risk management. Studying the occurrence of and changes in fracture patterns from outcrops and scaling it up for use in modeling at a field scale is difficult due to the lack of direct correlation between outcrop observations and subsurface data. Due to the size and amount of data needed to model fluid flow at the field scale the meso-scale (cm to m) variability of rock properties is often neglected. We evaluate this meso-scale variability in elastic moduli, where possible. We combine mechano-stratigraphic outcrop observations with elastic moduli calculated from publically available wire line log data to evaluate the variability in rock strength within the heterolithic top seal. Relationships between changes in Young’s modulus to resulting fracture distribution can then be observed. The outcome of this analysis can be used for modeling the effectiveness of seal for storage of CO2 in the underlying reservoirs. Digitized publically available wire line well log data were used to calculate Poisson’s ratio and Young’s modulus over the Carmel Formation and upper most 3 m of the underlying Navajo Sandstone. Our calculations show that Young’s Modulus can range between 15 to 34 GPa across 60 cm of the intra-seal interfaces, and an average difference of 5 GPa across the reservoir seal interface. These variations will affect fracture distributions and fluid behavior in the subsurface. These data provide a means to closely tie outcrop observations to derived estimates of subsurface rock strength. The characterization of rock strength variability is especially important for modeling the response of seals to increased pressure, due to CO2 injection, and will allow for better site screening and fluid management once injection projects are underway

Use of wire line logs for estimation of strength variability in cap-­‐rock lithologies

The characterization of cap-rock, low permeability and high capillary entry pressure lithologies is important for modeling the response of cap-rocks to increased pressures due to CO2 injection. We evaluate the use of publically available wireline log data to provide empirical estimate of rock strength in order to determine the strength of top seal over a range of scales. This method is being used to characterize cap-rock lithologies in systems proposed for CO2 geosequestration, these data will be combined with outcrop fracture density observations, petrology, lithologic stacking patterns and mineralogy to predict the potential for bypass. Analysis to date includes wells with monopole and dipole sonic logs for comparison of the relationships established empirically by other workers and used in this study to estimate the dynamic values for Poisson’s Ratio and Young’s Modulus from publically available vintage well log data in Utah. This study focuses specifically on the Jurassic Carmel Formation, which is a cap-rock to the underlying proposed CO2 injection reservoir, the Navajo Sandstone. This study compliments the well data with outcrop characterization of the Carmel Formation, which we split into 3 mechanical units based on lithologic stacking patterns, fracture density, and relationships observed between the percent shale and fracture spacing ratio. Results obtained from the well log analysis fall within the published ranges for these rock types, however the data show a variability which is being evaluated further to understand if these observations are related to geology or artifacts associated with the wireline data. In future the use of these empirical estimates will provide a lower estimate for subsurface rock strength, as well as provide a means to closely tie outcrop observations to those made from subsurface data

Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra

Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records

Bedrock geology of DFDP-2B, central Alpine Fault, New Zealand

<p>During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.</p

Petrophysical, Geochemical, and Hydrological Evidence for Extensive Fracture-Mediated Fluid and Heat Transport in the Alpine Fault's Hanging-Wall Damage Zone

International audienceFault rock assemblages reflect interaction between deformation, stress, temperature, fluid, and chemical regimes on distinct spatial and temporal scales at various positions in the crust. Here we interpret measurements made in the hanging‐wall of the Alpine Fault during the second stage of the Deep Fault Drilling Project (DFDP‐2). We present observational evidence for extensive fracturing and high hanging‐wall hydraulic conductivity (∼10−9 to 10−7 m/s, corresponding to permeability of ∼10−16 to 10−14 m2) extending several hundred meters from the fault's principal slip zone. Mud losses, gas chemistry anomalies, and petrophysical data indicate that a subset of fractures intersected by the borehole are capable of transmitting fluid volumes of several cubic meters on time scales of hours. DFDP‐2 observations and other data suggest that this hydrogeologically active portion of the fault zone in the hanging‐wall is several kilometers wide in the uppermost crust. This finding is consistent with numerical models of earthquake rupture and off‐fault damage. We conclude that the mechanically and hydrogeologically active part of the Alpine Fault is a more dynamic and extensive feature than commonly described in models based on exhumed faults. We propose that the hydrogeologically active damage zone of the Alpine Fault and other large active faults in areas of high topographic relief can be subdivided into an inner zone in which damage is controlled principally by earthquake rupture processes and an outer zone in which damage reflects coseismic shaking, strain accumulation and release on interseismic timescales, and inherited fracturing related to exhumation

DFDP-2 Sonic log

reprocessed sonic log from the Deep Fault Drilling Project Phase 2 borehol

DFDP-1 and Outcrop Ultrasonics

Laboratory ultrasonic velocity measurement on samples on DFDP-1 core samples and Alpine Fault outcrop sample

Velocity‐Porosity Relations in Carbonate and Siliciclastic Subduction Zone Input Materials

Abstract The mechanical, physical, and frictional properties of incoming materials play an important role in subduction zone structure and slip behavior because these properties influence the strength of the accretionary wedge and megathrust plate boundary faults. Incoming sediment sections often show an increase in compressional wave speed (Vp) and a decrease in porosity with depth due to consolidation. These relations allow seismic‐velocity models to be used to elucidate properties and conditions at depth. However, variations in these properties are controlled by lithology and composition as well as cementation and diagenesis. We present an analysis of shipboard measurements of Vp and porosity on incoming sediment cores from International Ocean Discovery Program (IODP) expeditions at the Hikurangi Margin, Nankai Trough, Aleutian Trench, Middle America Trench, and Sunda Trench. Porosity for these samples ranges from 5% to 85% and Vp ranges from 1.5 to 6 km/s. Vp‐porosity relations developed by Erikson & Jarrad (1998), https://doi.org/10.1029/98JB02128 and Hoffman & Tobin (2004) https://10.2973/odp.proc.sr.190196.355.2004, with a critical porosity of ∼30%, can represent carbonate‐poor (<50 wt% CaCO3), mainly hemipelagic, incoming sediment regardless of the margin. But these relations tend to underestimate porosity in incoming sediments with carbonate content greater than 50 wt%, which appear to have a critical porosity of between 45% and 50%. This discrepancy will lead to inaccuracy in estimates of fluid budget and overpressure in subduction zones. The velocity‐porosity relation in carbonate sediments is non‐unique due to the complexity that results from the greater susceptibility of carbonate rocks to diagenetic processes