767 research outputs found
Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes
Slow earthquakes represent an important conundrum in earthquake physics. While regular
earthquakes are catastrophic events with rupture velocities governed by elastic wave speed,
the processes that underlie slow fault slip phenomena, including recent discoveries of tremor,
slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse
laboratory observations have provided insights, but the physics of slow fault rupture remain
enigmatic. Here we report on laboratory observations that illuminate the mechanics of
slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the
threshold between stable and unstable failure, and is governed by frictional dynamics via the
interplay of fault frictional properties, effective normal stress and the elastic stiffness of the
surrounding material. This generalizable frictional mechanism may act in concert with other
hypothesized processes that damp dynamic ruptures, and is consistent with the broad range
of geologic environments where slow earthquakes are observed
The role of deformation bands in dictating poromechanical properties of unconsolidated sand and sandstone
Cataclastic shear bands in sands and sandstones are typically stronger, stiffer, and exhibit lower permeability than the surrounding matrix, and therefore act as barriers to fluid flow. Previous work has quantified the reduction in permeability associated with these features; however, little is known about the role of shear band structure in controlling the way they impact permeability and elastic properties. Here, we report on a suite of laboratory measurements designed to measure the poromechanical properties for host material and natural shear bands, over effective stresses from 1â65 MPa. In order to investigate the role of host material properties in controlling poromechanical evolution with stress, we sampled shear bands from two well-studied sandstones representing structurally distinct end-members: a poorly cemented marine terrace sand from the footwall of the McKinleyville thrust fault in Humboldt County, California, and a strongly-cemented sandstone from the hanging wall of the Moab Fault in Moab, Utah. The permeability-porosity trends are similar for all samples, with permeability decreasing systematically with increasing effective stress and decreasing porosity. The permeability of the host material is consistently >1 order of magnitude greater than the shear bands for both localities. For the unconsolidated case, shear bands are less permeable and stiffer than the host material, whereas for the consolidated case, shear bands are slightly less permeable, and wave speeds are slower than in the host. We attribute the differences between the McKinleyville and Moab examples to changes in structure of the nearby host material that accompanied formation of the shear band
Foreshock properties illuminate nucleation processes of slow and fast laboratory earthquakes
Laboratory experiments demonstrate that prior to fast laboratory earthquakes the fault begins to unlock and creep, causing foreshocks to coalesce in both space and time. This demonstrates that the evolution of foreshocks is closely connected to the fault slip velocity.Understanding the connection between seismic activity and the earthquake nucleation process is a fundamental goal in earthquake seismology with important implications for earthquake early warning systems and forecasting. We use high-resolution acoustic emission (AE) waveform measurements from laboratory stick-slip experiments that span a spectrum of slow to fast slip rates to probe spatiotemporal properties of laboratory foreshocks and nucleation processes. We measure waveform similarity and pairwise differential travel-times (DTT) between AEs throughout the seismic cycle. AEs broadcasted prior to slow labquakes have small DTT and high waveform similarity relative to fast labquakes. We show that during slow stick-slip, the fault never fully locks, and waveform similarity and pairwise differential travel times do not evolve throughout the seismic cycle. In contrast, fast laboratory earthquakes are preceded by a rapid increase in waveform similarity late in the seismic cycle and a reduction in differential travel times, indicating that AEs begin to coalesce as the fault slip velocity increases leading up to failure. These observations point to key differences in the nucleation process of slow and fast labquakes and suggest that the spatiotemporal evolution of laboratory foreshocks is linked to fault slip velocity
Gravitational-wave energy and other fluxes in ghost-free bigravity
One of the key ingredients for making binary waveform predictions in a
beyond-GR theory of gravity is understanding the energy and angular momentum
carried by gravitational waves and any other radiated fields. Identifying the
appropriate energy functional is unclear in Hassan-Rosen bigravity, a
ghost-free theory with one massive and one massless graviton. The difficulty
arises from the new degrees of freedom and length scales which are not present
in GR, rendering an Isaacson-style averaging calculation ambiguous. In this
article we compute the energy carried by gravitational waves in bigravity
starting from the action, using the canonical current formalism. The canonical
current agrees with other common energy calculations in GR, and is unambiguous
(modulo boundary terms), making it a convenient choice for quantifying the
energy of gravitational waves in bigravity or any diffeomorphism-invariant
theories of gravity. This calculation opens the door for future waveform
modeling in bigravity to correctly include backreaction due to emission of
gravitational waves.Comment: 18+4 pages, 2 figure
Frictional and lithological controls on shallow slow slip at the Northern Hikurangi Margin
Slow slip events (SSEs) have been identified at subduction zones globally as an important link in the continuum between elastodynamic ruptures and stable creep. The northern Hikurangi margin is home to shallow SSEs which propagate to within 2 km of the seafloor and possibly to the trench, providing insights into the physical conditions conducive to SSE behavior. We report on a suite of friction experiments performed on protolith material entering the SSE source region at the Hikurangi margin, collected during the International Ocean Discovery Program Expedition 375. We performed velocity stepping and slide-hold-slide experiments over a range of fault slip rates, from plate rate (5 cm/yr or 1.6 Ă 10â9 m/s) to âŒ1 mm/s (10â3 m/s) and quantified the frictional velocity dependence and healing rates for a range of lithologies atdifferent stresses. The frictional velocity dependence (a-b) and critical slip distance DC increase with fault slip rate in our experiments. We observe atransition from velocity weakening to strengthening at slip rates of âŒ0.3 ”m/s. This velocity dependence of DC could be due to a combination of dilatant strengthening and a widening of the active shear zone at higher slip rates. We document low healing rates in the clay-rich volcaniclastic conglomerates, which lie above the incoming plate basement at least locally, and relatively higher healing rates in the chalk lithology. Finally, our experimental constraints on healing rates in different input lithologies extrapolated to timescales of 1â10 years are consistent with the geodetically inferred low stress drops and healing rates characteristic of the Hikurangi SSEs
Evolution of elastic and mechanical properties during fault shear. The roles of clay content, fabric development, and porosity
Phyllosilicates weaken faults due to the formation of shear fabrics. Although the impacts of clay abundance and fabric on frictional strength, sliding stability, and porosity of faults are well studied, their influence on elastic properties is less known, though they are key factors for fault stiffness. We document the role that fabric and consolidation play in elastic properties and show that smectite content is the most important factor determining whether fabric or porosity controls the elastic response of faults. We conducted a suite of shear experiments on synthetic smectite-quartz fault gouges (10â100 wt% smectite) and sediment incoming to the Sumatra subduction zone. We monitored Vp, Vs, friction, porosity, shear and bulk moduli. We find that mechanical and elastic properties for gouges with abundant smectite are almost entirely controlled by fabric formation (decreasing mechanical and elastic properties with shear). Though fabrics control the elastic response of smectite-poor gouges over intermediate shear strains, porosity is the primary control throughout the majority of shearing. Elastic properties vary systematically with smectite content: High smectite gouges have values of Vp ~ 1,300â1,800 m/s, Vs ~ 900â1,100 m/s, K ~ 1â4 GPa, and G ~ 1â2 GPa, and low smectite gouges have values of Vp ~ 2,300â2,500 m/s, Vs ~ 1,200â1,300 m/s, K ~ 5â8 GPa, and G ~ 2.5â3 GPa. We find that, even in smectite-poor gouges, shear fabric also affects stiffness and elastic moduli, implying that while smectite abundance plays a clear role in controlling gouge properties, other fine-grained and platy clay minerals may produce similar behavior through their control on the development of fabrics and thin shear surfaces
Breakdown pressure and fracture surface morphology of hydraulic fracturing in shale with H2O, CO2 and N2
Slick-water fracturing is the most routine form of well stimulation in shales; however N2, LPG and CO2 have all been used as âexoticâ stimulants in various hydrocarbon reservoirs. We explore the use of these gases as stimulants on Green River shale to compare the form and behavior of fractures in shale driven by different gas compositions and states and indexed by breakdown pressure and the resulting morphology of the fracture networks. Fracturing is completed on cylindrical samples containing a single blind axial borehole under simple triaxial conditions with confining pressure ranging from 10 to 25 MPa and axial stress ranging from 0 to 35 MPa (Ï1 > Ï2 = Ï3). Results show that: (1) under the same stress conditions, CO2 returns the highest breakdown pressure, followed by N2, and with H2O exhibiting the lowest breakdown pressure; (2) CO2 fracturing, compared to other fracturing fluids, creates nominally the most complex fracturing patterns as well as the roughest fracture surface and with the greatest apparent local damage followed by H2O and then N2; (3) under conditions of constant injection rate, the CO2 pressure build-up record exhibits condensation between ~5 and 7 MPa and transits from gas to liquid through a mixed-phase region rather than directly to liquid as for H2O and N2 which do not; (4) there is a positive correlation between minimum principal stress and breakdown pressure for failure both by transverse fracturing (Ï3axial) and by longitudinal fracturing (Ï3radial) for each fracturing fluid with CO2 having the highest correlation coefficient/slope and lowest for H2O. We explain these results in terms of a mechanistic understanding of breakdown, and through correlations with the specific properties of the stimulating fluids
Radial velocity measurements of white dwarfs
We present 594 radial velocity measurements for 71 white dwarfs obtained
during our search for binary white dwarfs and not reported elsewhere. We
identify three excellent candidate binaries, which require further observations
to confirm our preliminary estimates for their orbital periods, and one other
good candidate. We investigate whether our data support the existence of a
population of single, low mass (<~0.5 solar masses) white dwarfs (LMWDs). These
stars are difficult to explain in standard models of stellar evolution. We find
that a model with a mixed single/binary population is at least ~20 times more
likely to explain our data than a pure binary population. This result depends
on assumed period distributions for binary LMWDs, assumed companion masses and
several other factors. Therefore, the evidence in favour of the existence of a
population of single LMWDs is not sufficient, in our opinion, to firmly
establish the existence of such a population, but does suggest that extended
observations of LMWDs to obtain a more convincing result would be worthwhile .Comment: 14 pages, 4 Figures. Accepted for publication in MNRAS. Added
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