Slow earthquakes are now commonly found to display a wide range of durations, moments, and slip and propagation speeds. But not all types of slow earthquakes have been examined in detail. Here we ...probe tremor bursts with durations between 1 and 30 min, which are likely driven by few minute‐long bursts of aseismic slip. We use a coherence‐based technique to detect thousands of tremor bursts beneath Vancouver Island in Cascadia. Then we examine 17 of the ruptures by tracking their evolving tremor locations over an 8‐km region. We find that tremor migrates at rates of 3–25 m/s: faster than longer tremor bursts. Though some observational biases persist, the short events' speeds appear to fill a gap in the spectrum of observed slow earthquakes. They may provide further evidence that whatever fault zone process creates slow earthquakes, it must allow for faster slip and propagation in smaller ruptures.
Plain Language Summary
Slow earthquakes, like earthquakes, are events with transient slip. But in slow earthquakes, faults slip slowly. Slip rates are thousands or millions of times slower than in earthquakes. There are a range of types of slow earthquakes, with durations from seconds to years. But not all types of slow earthquakes have been examined in detail. Here we examine slow earthquakes with durations between 1 and 30 min. Specifically, we examine the small bursts of seismic energy created by the slow slip. We develop techniques to precisely identify and locate that seismic energy, also known as tremor, and we are able to track the growth of slow earthquakes over the course of their minute‐long durations. We find that during the few‐minute‐long events, tremor migrates at rates of 3–25 m/s. This few minute‐long migration fills an observational gap in our knowledge of slow earthquakes. The new observations may help us understand how various‐duration slow earthquakes are related to each other and what causes them.
Key Points
Identify thousands of few‐minute tremor bursts and track migration during 17 bursts
Migration speeds range from 3 to 25 m/s
Observed ruptures fill an observational gap in the slow earthquake spectrum
We use borehole strain and seismic data to show that slow slip and tremor in central Cascadia are correlated on a range of time scales shorter than 1 day. The recorded strain rate is our proxy for ...the slow slip moment rate, and the seismic amplitude is our proxy for the tremor amplitude. We find that, on average, the strain rate is higher when the seismic amplitude is larger. This correlation persists on time scales between 15 min and 16 h, and it can be seen in each of the five slow slip events between 2007 and 2011. Our results imply that the slow slip moment rate varies by a large amount even at these short time scales. For instance, we observe a factor of 2 variation on time scales shorter than 4 h. This apparently aperiodic variation is larger than the previously observed variation in moment rate resulting from tidal forcing. It is a lower bound on the actual moment rate variation, as we detect only changes in slow slip that are correlated with tremor amplitude.
Key Points
Slow slip and tremor are correlated on timescales of hours
Short‐timescale variations in aseismic moment rate are large
Large variability is not obviously periodic
We estimate and model the normalized moment rate power spectrum of large slow slip events in Cascadia. We estimate the spectrum using data from GPS‐derived slip inversions, borehole strain records, ...and beamforming‐based tremor amplitudes. The normalized power spectrum initially decreases with frequency but then may flatten at periods of 1 to 10 days before decaying as frequency
−nm at higher frequencies, where nm is between 1.1 and 1.4 when estimated from tremor and between 0.4 and 1.5 when estimated from strain. We explore one way to understand the observed spectrum: by modeling a month‐long slow slip event as the sum of a steady background moment rate and a population of subevents. The subevents represent the wide variety of observed slow earthquakes, ranging from 0.5‐s‐long tremor to 3‐hr‐long rapid tremor reversals. We parameterize the subevents' magnitude distribution and moment‐duration scaling, and we examine how the subevent population determines the slow slip spectrum. There are not enough data to rigorously test the subevent model, but we show that the data are consistent with a single continuum of slow earthquakes whose moments scale linearly with their duration, as has been proposed previously.
Key Points
We estimate the moment rate power spectrum of slow slip events in Cascadia using GPS, strain, and tremor data
We obtain a moment rate spectrum with shallow power law frequency decay at periods shorter than 1 day
The spectrum can be reproduced if large slow slip events contain collections of subevents with linear moment‐duration scaling
Background:
Loss of initial reduction of the acromioclavicular joint after coracoclavicular (CC) ligament reconstruction remains a challenge for various repair techniques. Previous studies using ...polydioxanone suture cerclage augments for CC ligament reconstruction demonstrated poor clinical and biomechanical outcomes. Tape-style sutures have recently gained popularity because of their added stiffness and strength relative to traditional sutures. These tape cerclage systems have yet to be biomechanically studied in CC ligament reconstruction.
Purpose:
To determine the efficacy of a tape cerclage system as an augment to CC ligament reconstruction.
Study Design:
Controlled laboratory study.
Methods:
A total of 24 human cadaveric shoulders were utilized. These were divided into 4 repair groups: anatomic CC ligament reconstruction (ACCR), ACCR with a tape cerclage augment (ACCR + C), tendon graft sling with a cerclage augment (TGS + C), or tape cerclage sling alone (CS). The repairs underwent superior/inferior cyclic loading to evaluate for displacement. Specimens were visually inspected for cortical erosion by the tape cerclage after cyclic loading. Finally, the constructs underwent superior plane load-to-failure testing.
Results:
Less displacement after cyclic loading was observed in the ACCR + C (mean ± SD, 0.42 ± 0.32 mm), TGS + C (0.92 ± 0.42 mm), and CS (0.93 ± 0.39 mm) groups as compared with the ACCR group (4.42 ± 3.40 mm; P = .002). ACCR + C (813.3 ± 257.5 N), TGS + C (558.0 ± 120.7 N), and CS (759.5 ± 173.7 N) demonstrated significantly greater load at failure relative to ACCR (329.2 ± 118.2 N) (P < .001). ACCR + C (60.88 ± 17.3 N/mm), TGS + C (44.97 ± 9.15 N/mm), and CS (54.52 ± 14.24 N/mm) conferred greater stiffness than ACCR (27.43 ± 6.94 N/mm) (P = .001). No cortical erosion was demonstrated in any specimen after cyclic loading.
Conclusion:
In a cadaveric model at time zero, repairs utilizing a tape cerclage system confer significantly greater load to failure and stiffness, as well as decreased displacement with cyclic loading, when compared with traditional ACCR repair.
Clinical Relevance:
Tape cerclage augmentation may provide a useful augment for CC ligament reconstruction.
The crystal structures of orthopyroxene (En86.3Fs8.6Wo5.1, space group Pbca) and pigeonite (En81.7Fs8.8Wo9.5, space group P21/c) from the Almahata Sitta ureilite (fragment#051) have been refined to ...R1 indices of 3.10% and 2.53%, respectively, using single‐crystal X‐ray diffraction data. The unit formulas were calculated from electron microprobe analysis, and the occupancies at the M1 and M2 sites were refined for both pyroxenes from the single‐crystal diffraction data. The results indicate a rather disordered intracrystalline Fe2+‐Mg cation distribution over the M1 and M2 sites, with a closure temperature of 726(±55)°C for orthopyroxene and 704(±110)°C for pigeonite, suggesting fast cooling of these pyroxenes. The Mössbauer spectrum of the Fe‐Ni metal particles of Almahata Sitta ureilite (fragment#051) is dominated by two overlapping magnetic sextets that are assigned to Fe atoms in Si‐bearing kamacite, and arise from two different nearest‐neighbor configurations of Fe* (=Fe+Ni) and Si atoms in the bcc structure of kamacite; (8F*, 0Si) and (7Fe*, 1Si). In addition, the spectrum shows weak absorption peaks that are attributed to the presence of small amounts of cohenite (Fe,Ni)3C, schreibersite (Fe,Ni)3P, and an Fe‐oxide/hydroxide phase. The fast cooling of pyroxene to the closure temperature (after equilibration at ~1200°C) and the incorporation of Si in kamacite can be interpreted as due to a shock event that took place on the meteorite parent body, consistent with the proposed formation history of ureilites parent body where a fast cooling has occurred at a later stage of its formation.
The crystal structure of magnussonite, ideally Mn2+18As3+6(Mn1+x)O182(H2O, Clx, ☐) (H2O, ☐)2, from Långban, Sweden, was refined to an R1-index of 1.19% and the structure proposed by Moore and Araki ...(1979) is confirmed. Magnussonite has a densely packed structure of (Mnφn) polyhedra, φ = (O2−, H2O, Cl−), and (As3+O3) triangular pyramids that is best envisaged as layers of polyhedra in the same way as many of the other manganese-arsenite-arsenate structures from Långban. There are two distinct layers in magnussonite; the two layers may be combined into a slab that stacks along the a-direction with rotations between adjacent slabs. A surprising feature of the dense-packed magnussonite atomic arrangement is an array of structural channels along 111 that contain much of the disorder that occurs in the magnussonite structure. The channels contain the partly occupied MX site on the central axis of the channel, and the CLW2 site (with extremely low occupancy), also on the central axis of the channel. The CLW2 site, previously unrecognized in the magnussonite structure, contains H2O, whereas the minor Cl in the structure resides in the CLW1 channel site, balancing the charge of the MX-site occupant. The MX site on the central axis of the channels displays a coordination known only in Långban minerals. In the local arrangement around the unoccupied MX site, the neighboring (As3+O3) groups project their associated stereoactive lone-pairs of electrons into the channel. Where the MX site is occupied by Mn, there are six lone-pairs of electrons pointing toward Mn; the 18-electron rule predicts/rationalizes formulae for this stable transition-metal cluster. The (As3+O3) groups and MX occupant form a Mn+(As3+O3)6 arrangement in accord with the 18-electron rule where Mn+ contributes 6 3d electrons and the six lone-pairs of the (As3+O3)6 arrangement contribute 12 electrons for a total of 18 electrons that form nine molecular orbitals that are metal-ligand bonds or non-bonding. Magnussonite and dixenite, another basic manganese-iron arsenate-arsenite-silicate mineral of the Långban-type deposits in Bergslagen, Sweden, are the only two minerals known with such local M+(As3+O3)n transition-metal clusters. The presence of these exotic clusters in structures containing densely packed Mn2+ octahedra is not understood at present.
The O 1s spectrum is examined for 19 uranyl minerals of different composition and structure. Spectra from single crystals were measured with a Kratos Axis Ultra X-ray Photoelectron Spectrometer with ...a magnetic-confinement charge-compensation system. Well-resolved spectra with distinct maxima, shoulders and inflections points, in combination with reported and measured binding energies for specific O
2− species and structural data of the uranyl minerals are used to resolve the fine structure of the O 1s envelope. The resolution of the O 1s spectra includes, for the first time, different O
2− bands, which are assigned to O atoms linking uranyl with uranyl polyhedra (
U
O
U) and O atoms of uranyl groups (
O
U
O). The resolved bands in the O 1s spectrum occur at distinct ranges in binding energy: bands for (
U
O
U) occur at 529.6–530.4
eV, bands for (
O
U
O) occur at 530.6–531.4
eV, bands for O
2− in the equatorial plane of the uranyl polyhedra linking uranyl polyhedra with (
TO
n
) groups (
T
=
Si, S, C, P, Se) (
T
O) occur at 530.9–532.2
eV; bands for (OH) groups in the equatorial plane of the uranyl polyhedra (
OH) occur at 532.0–532.5
eV, bands of (H
2O) groups in the interstitial complex of the uranyl minerals (
H
2
O
interst
) occur at 533.0–533.8
eV and bands of physisorbed (H
2O) groups on the surface of uranyl minerals (
H
2
O
adsorb
) occur at 534.8–535.2
eV. Treatment of uranyl minerals with acidic solutions results in a decrease in
U
O
U and an increase in
OH. Differences in the ratio of
OH
:
O
U
O between the surface and bulk structure is larger for uranyl minerals with a high number of
U
O
U and
T
O species in the bulk structure which is explained by protonation of underbonded
U
O,
U
O
U and
T
O terminations on the surface. The difference in the ratio of
H
2
O
interst
:
O
U
O between the bulk and surface structures is larger for uranyl minerals with higher percentages of
H
2
O
interst
as well as, with a higher number of interstitial H
2O groups that are not bonded to interstitial cations, resulting in easier dehydration of interstitial H
2O groups in uranyl minerals during exposure to a vacuum.
Volume 18 of Reviews in Mineralogy provides a general introduction to the use of spectroscopic techniques in Earth Sciences. It gives an Introduction To Spectroscopic Methods and covers Symmetry, ...Group Theory And Quantum Mechanics; Spectrum-Fitting Methods; Infrared And Raman Spectroscopy; Inelastic Neutron Scattering; Vibrational Spectroscopy Of Hydrous Components; Optical Spectroscopy; Mossbauer Spectroscopy; MAS NMR Spectroscopy Of Minerals And Glasses; NMR Spectroscopy And Dynamic Processes In Mineralogy And Geochemistry; X-Ray Absorption Spectroscopy: Applications In Mineralogy ind Geochemistry; Electron Paramagnetic Resonance; Auger Electron And X-Ray Photelectron Spectroscopies and Luminescence, X-Ray Emission and New Spectroscopies. The authors of this volume presented a short course, entitled "Spectroscopic Methods in Mineralogy and Geology", May 13-15, 1988, in Hunt Valley, Maryland.
We investigate the behavior of simulated slow slip events using a rate and state friction model that is steady state velocity weakening at low slip speeds but velocity strengthening at high slip ...speeds. Our simulations are on a one‐dimensional (line) fault, but we modify the elastic interactions to mimic the elongate geometry frequently observed in slow slip events. Simulations exhibit a number of small events as well as periodic large events. The large events propagate approximately steadily “along strike,” and stress and slip rate decay gradually behind the propagating front. Their recurrence intervals can be determined by considering what is essentially an energy balance requirement for long‐distance propagation. It is possible to choose the model parameters such that the simulated events have the stress drops, slip velocities, and propagation rates observed in Cascadia.
Key Points
Steady (parameterized) along‐strike propagation
Stress drops controlled by fracture energy requirement
Limited variability in slip and propagation rates
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