Lateral Shear Strain Research Articles

MixTURE: L1-Norm-Based Mixed Second-Order Continuity in Strain Tensor Ultrasound Elastography.

Energy-based displacement tracking of ultrasound images can be implemented by optimizing a cost function consisting of a data term, a mechanical congruency term, and first- and second-order continuity terms. This approach recently provided a promising solution to two-dimensional axial and lateral displacement tracking in ultrasound strain elastography. However, the associated second-order regularizer only considers the unmixed second derivatives and disregards the mixed derivatives, thereby providing suboptimal noise suppression and limiting possibilities for total strain tensor imaging. We propose to improve axial, lateral, axial shear, and lateral shear strain estimation quality by formulating and optimizing a novel L1-norm-based second-order regularizer that penalizes both mixed and unmixed displacement derivatives. We name the proposed technique L1-MixTURE, which stands for L1-norm Mixed derivative for Total UltRasound Elastography. When compared to simulated ground truth results, the mean structural similarity (MSSIM) obtained with L1-MixTURE ranged 0.53 to 0.86 and the mean absolute error (MAE) ranged 0.00053 to 0.005. In addition, the mean elastographic signal-to-noise ratio (SNR) achieved with simulated, experimental phantom, and in vivo breast datasets, ranged 1.87 to 52.98, and the mean elastographic contrast-to-noise ratio (CNR) ranged 7.40 to 24.53. When compared to a closely related existing technique that does not consider the mixed derivatives, L1-MixTURE generally outperformed the MSSIM, MAE, SNR, and CNR by up to 37.96%, 67.82%, and 25.53% in the simulated, experimental phantom, and in vivo datasets, respectively. These results collectively highlight the ability of L1-MixTURE to deliver highly accurate axial, lateral, axial shear, and lateral shear strain estimates and advance the state-of-the-art in elastography-guided diagnostic and interventional decisions.

Regional pleural strain measurements during mechanical ventilation using ultrasound elastography: A randomized, crossover, proof of concept physiologic study.

BackgroundMechanical ventilation is a common therapy in operating rooms and intensive care units. When ill-adapted, it can lead to ventilator-induced lung injury (VILI), which is associated with poor outcomes. Excessive regional pulmonary strain is thought to be a major mechanism responsible for VILI. Scarce bedside methods exist to measure regional pulmonary strain. We propose a novel way to measure regional pleural strain using ultrasound elastography. The objective of this study was to assess the feasibility and reliability of pleural strain measurement by ultrasound elastography and to determine if elastography parameters would correlate with varying tidal volumes.MethodsA single-blind randomized crossover proof of concept study was conducted July to October 2017 at a tertiary care referral center. Ten patients requiring general anesthesia for elective surgery were recruited. After induction, patients received tidal volumes of 6, 8, 10, and 12 mL.kg–1 in random order, while pleural ultrasound cineloops were acquired at 4 standardized locations. Ultrasound radiofrequency speckle tracking allowed computing various pleural translation, strain and shear components. We screened 6 elastography parameters (lateral translation, lateral absolute translation, lateral strain, lateral absolute strain, lateral absolute shear and Von Mises Strain) to identify those with the best dose-response with tidal volumes using linear mixed effect models. Goodness-of-fit was assessed by the coefficient of determination. Intraobserver, interobserver and test-retest reliability were calculated using intraclass correlation coefficients.ResultsAnalysis was possible in 90.7% of ultrasound cineloops. Lateral absolute shear, lateral absolute strain and Von Mises strain varied significantly with tidal volume and offered the best dose-responses and data modeling fits. Point estimates for intraobserver reliability measures were excellent for all 3 parameters (0.94, 0.94, and 0.93, respectively). Point estimates for interobserver (0.84, 0.83, and 0.77, respectively) and test-retest (0.85, 0.82, and 0.76, respectively) reliability measures were good.ConclusionStrain imaging is feasible and reproducible. Future studies will have to investigate the clinical relevance of this novel imaging modality.Clinical trial registrationwww.Clinicaltrials.gov, identifier NCT03092557.

Shear strain evolution during tension-induced Lüders-type deformation of polycrystalline NiTi plates

This paper reports a new aspect of the Lüders-type deformation of NiTi. This is on the occurrence of lateral shear strains in a tension-induced Lüders deformation process. The phenomenon was studied by means of digital image correlation analysis. It was found that the lateral shear strains occurred in opposite directions within the Lüders band, apparently as an effort to self-accommodate the lateral displacement caused by the shear strains. The Lüders band propagation also changed from a single band mode to a branched mode when the lateral displacement, thus the in-plane bending moment, became too large. The branches, whilst having the same axial normal strain in the loading direction, were formed with opposite shear strains in double alternation between left and right of the sample and between the branches and the gaps between them, thus, to achieve the optimum self-accommodation. In addition, both the axial normal strain field and the lateral shear strain field were nonuniform after the Lüders-type deformation. These findings provide more insight and direct evidence for the explanation of the characteristics of Lüders deformation behaviour of NiTi.

Toward a Comprehensive Delineation of White Matter Tract-Related Deformation.

Finite element (FE) models of the human head are valuable instruments to explore the mechanobiological pathway from external loading, localized brain response, and resultant injury risks. The injury predictability of these models depends on the use of effective criteria as injury predictors. The FE-derived normal deformation along white matter (WM) fiber tracts (i.e., tract-oriented strain) recently has been suggested as an appropriate predictor for axonal injury. However, the tract-oriented strain only represents a partial depiction of the WM fiber tract deformation. A comprehensive delineation of tract-related deformation may improve the injury predictability of the FE head model by delivering new tract-related criteria as injury predictors. Thus, the present study performed a theoretical strain analysis to comprehensively characterize the WM fiber tract deformation by relating the strain tensor of the WM element to its embedded fiber tract. Three new tract-related strains with exact analytical solutions were proposed, measuring the normal deformation perpendicular to the fiber tracts (i.e., tract-perpendicular strain), and shear deformation along and perpendicular to the fiber tracts (i.e., axial-shear strain and lateral-shear strain, respectively). The injury predictability of these three newly proposed strain peaks along with the previously used tract-oriented strain peak and maximum principal strain (MPS) were evaluated by simulating 151 impacts with known outcome (concussion or non-concussion). The results preliminarily showed that four tract-related strain peaks exhibited superior performance than MPS in discriminating concussion and non-concussion cases. This study presents a comprehensive quantification of WM tract-related deformation and advocates the use of orientation-dependent strains as criteria for injury prediction, which may ultimately contribute to an advanced mechanobiological understanding and enhanced computational predictability of brain injury.

Understanding the Contrast Mechanism in Rotation Elastogram: A Parametric Study

Ultrasound elastography has been found to be useful in different clinical applications. For example, in breast imaging, axial strain elastography provides information related to tissue stiffness, which is used to characterize breast lesions as either benign or malignant. In addition, these lesions also differ in their bonding properties. Benign breast lesions are loosely bonded and malignant breast lesions are firmly bonded to the surrounding tissues. Therefore, only benign breast lesions will rotate/slip on the application of deformation. This rotation of lesions can be visualized with rotation elastography, which utilizes axial and lateral shear strain components. The contrast obtained in rotation elastography depends on various mechanical as well as ultrasound elastography parameters. However, there is no reported work that provides an understanding of the influence of these parameters on the visualized rotation contrast. In this work, the authors studied the rotation contrast by varying the mechanical parameters such as the inclusion b/a ratio, relative inclusion-background Young's modulus, amount of applied deformation and orientation of the inclusion. First, the authors performed finite-element analysis to understand the fundamental rotation contrast of the inclusion. Next, rotation elastograms obtained from ultrasound simulations in Field II and experiments on tissue-mimicking phantoms were investigated. Mean contrast was used as a metric to evaluate the quality of rotation elastograms in finite-element analysis, and contrast-to-noise ratio was used in Field II simulations and phantom experiments. The results indicate that rotation contrast was observed only in the case of loosely bonded inclusions. Further, the rotation contrast was found to depend on the inclusion asymmetry and its orientation with respect to the axis of deformation. Interestingly, it was found that a loosely bonded inclusion contrasts with surrounding tissue in rotation elastography, even in the absence of any inclusion-background modulus contrast.

Large area thermal nanoimprint below the glass transition temperature via small amplitude oscillatory shear forming

Small amplitude oscillatory shear forming is a technique that improves the performance of nanoimprint lithography by amplifying shear flow in narrow, squeezed geometries created during melt molding or glass forming of supported polymer films. To date, the technique has only been demonstrated for single micrometer-scale contacts representative of single features on wafer-scale nanoimprint stamps. In this work, we demonstrate scaling up of small amplitude shear forming to realize micrometer and nanometer feature patterning of thermoplastic polymer films over macroscopic centimetre square areas at temperatures in the vicinity of, and below the glass transition. By the use of a small amplitude (typically 5–10% the smallest pattern feature dimension) lateral oscillatory shear strain at a 10kHz frequency superposed during the normal motion of a nanoimprint mold, we are able to achieve high fidelity pattern replication in PMMA at temperatures as low as 35°C below its glass transition temperature. We further show that the technique is effective to pattern both bulk polymer surfaces and supported thin films, with samples as thick as 50μm and as films as thin as 40nm being successfully patterned. We demonstrate enhanced pattern fidelity using line-pattern imprint molds of 4μm pitch and 35nm relief, as well as an assortment of other geometries. For 800nm PMMA films, a 40% reduction in the residual layer thickness at 100°C was achieved by the addition of shear strain. No feature distortion is observed due to the oscillatory motion of the mold. The technique is particularly advantageous for large scale features and cavities, as well as high aspect ratio geometries, as the high speed lateral motion of the mold features acts to cyclically pump material into areas where stresses generated by the normal motion of the mold will not suffice.

Shear heating and weakening of the margins of West Antarctic ice streams

AbstractIce streams are fast flowing bands of ice separated from stagnant ridges by shear margins. The mechanisms controlling the location of the margins remain unclear. We use published ice deformation data and a simple one‐dimensional thermal model to show that West Antarctic ice stream margins have temperate ice over a substantial fraction of their thickness, a condition that may control their width. The model predicts a triple‐valued relation between the thickness‐averaged lateral shear stress and the lateral shear strain rate. Observed strain rates at the margins imply that they support slightly less lateral shear stress than adjacent ice within the stream. This requires enhanced basal resistance near the margin. We suggest, in agreement with the limited observations, the presence of a channelized drainage system at the margin that reduces the pore fluid pressure at the ice‐till interface, thus increasing the shear stress acting on the yielding Coulomb plastic bed.

The Ahar‐Varzeghan Earthquake Doublet (Mw 6.4 and 6.2) of 11 August 2012: Regional Seismic Moment Tensors and a Seismotectonic Interpretation

Abstract On 11 August 2012 an earthquake doublet ( M w 6.4 and 6.2) occurred near the city of Ahar, northwest Iran. Both events were only 6 km and 11 minutes apart, producing a surface rupture of about 12 km in length. Historical and modern seismicity has so far been sparse in this area. Spatially, the region represents a transitional zone between different tectonic domains, including compression in Iran, westward extrusion of the Anatolian plate, and thrusting beneath the Caucasus. In this study, we inverted the surface waveforms of the two mainshocks and 11 aftershocks ( M w ≥4.3) to obtain regional seismic moment tensors. The earthquakes analyzed can be grouped into pure strike slip (including the first mainshock) and oblique reverse mechanisms (including the second mainshock). The sequence provides information about faulting mechanisms at the spatial scale of the entire rock volume affected by the earthquake doublet, including coinciding deformation on minor faults (sub)parallel to the main fault and Riedel shears. It occurred on a so far unknown fault structure, which we call the Ahar fault. Alongside the seismological data, we used geological maps, satellite images, and digital elevation data to analyze the geomorphology of the region. Our analysis suggests that the adjacent North Tabriz fault, which accomodates up to 7 mm/yr of right‐lateral strike‐slip faulting, does not compensate the entire lateral shear strain, and that part of it is compensated farther north. Combined, our results suggest a temporally and spatially complex style of deformation (reverse and strike slip) overprinting older reverse deformation.

Enhancing the performance of lateral shear strain estimation by using 2-D strain imaging.

Radio-frequency (RF) ultrasound can be used to estimate deformation of biological tissue. Decorrelation of sequentially acquired ultrasound signals resulting from the deformation imposes a limitation on the precision (elastographic signal-to-noise ratio; SNRe) of estimating these deformations; this is presented as the lateral shear strain filter. In this paper, we explore the effect of a 2-D-window-based strain estimation approach on the lateral shear strain filter and propose an extension of the 1-D theoretical lateral shear strain filter to 2-D. We compared the performance of the 2-D approach in simulated ultrasound data and a tissue-mimicking phantom with that of the 2-D lateral shear strain filter. In simulations, the 2-D-window-based approach shows an effect in the axial direction similar to the 2-D prediction. In simulations and experiments, increasing the window size in the lateral direction shows an increase in the maximum SNRe of the lateral shear strain filter. Increasing the lateral overlap has no effect on the estimation of lateral shear strain. These results were confirmed in the tissue-mimicking phantom experiments. When compared with the 2-D lateral shear strain filter, the results obtained with the 2-D-window-based approach showed an enhanced performance by incorporating the lateral window size in the lateral shear strain estimation, which was consistent with the proposed theory.

Enhancing the performance of lateral shear strain estimation by using 2-D strain imaging

Radio-frequency (RF) ultrasound can be used to estimate deformation of biological tissue. Decorrelation of sequentially acquired ultrasound signals resulting from the deformation imposes a limitation on the precision (elastographic signal-to-noise ratio; SNRe) of estimating these deformations; this is presented as the lateral shear strain filter. In this paper, we explore the effect of a 2-D-window-based strain estimation approach on the lateral shear strain filter and propose an extension of the 1-D theoretical lateral shear strain filter to 2-D. We compared the performance of the 2-D approach in simulated ultrasound data and a tissue-mimicking phantom with that of the 2-D lateral shear strain filter. In simulations, the 2-D-window-based approach shows an effect in the axial direction similar to the 2-D prediction. In simulations and experiments, increasing the window size in the lateral direction shows an increase in the maximum SNRe of the lateral shear strain filter. Increasing the lateral overlap has no effect on the estimation of lateral shear strain. These results were confirmed in the tissue-mimicking phantom experiments. When compared with the 2-D lateral shear strain filter, the results obtained with the 2-D-window-based approach showed an enhanced performance by incorporating the lateral window size in the lateral shear strain estimation, which was consistent with the proposed theory.

Anisotropic Tissue Motion Induced by Acupuncture Needling Along Intermuscular Connective Tissue Planes

Acupuncture needle manipulation causes mechanical deformation of connective tissue, which in turn results in mechanical stimulation of fibroblasts, with active changes in cell shape and autocrine purinergic signaling. We have previously shown using ultrasound elastography in humans that acupuncture needle manipulation causes measurable movement of tissue up to several centimeters away from the needle. The goal of this study was to quantify the spatial pattern of tissue displacement and deformation (shear strain) in response to acupuncture needling along an intermuscular connective tissue plane compared with needling over the belly of a muscle. Eleven (11) healthy human subjects underwent a single testing session during which robotic acupuncture needling was performed while recording tissue displacement using ultrasound. Outcome measures were axial and lateral tissue displacement as well as lateral shear strain calculated using ultrasound elastography postprocessing. Tissue displacement and strain extended further in the longitudinal direction when needling between muscles, and in the transverse direction when needling over the belly of a muscle. The anisotropic tissue motion observed in this study may influence the spatial distribution of local connective tissue cellular responses following acupuncture needle manipulation.

Numerical study on stability analysis of geocell reinforced slopes by considering the bending effect

Geocell reinforced soil may be used in many areas of geotechnical engineering, however, there is little information on analysis of the behavior of geocell reinforced slopes. Due to the height of the geocell, the geocell-reinforced mattress more likely provides a beam or plate effect than a planar membrane effect. The purpose of this paper is to use beam model to simulate the geocell behavior as a flexible slab foundation which can carry both bending and membrane stresses for stability analysis of geocell reinforced slopes. In addition, the interface resistance between the geocell–soil was considered. The Young's modulus of geocell encased soil was obtained from the elastic modulus of the unreinforced soil and the tensile modulus of the geocell reinforcement using an empirical equation. Parametric studies of geocell reinforced slope are carried out by varying placement depth of the geocell layer, number of geocell layers, vertical spacing between reinforcement layers, length, thickness and Young's modulus of the geocell reinforcement. The influence of slope geometry, shear strength properties and soil compaction on the behavior of geocell reinforced slope is also discussed. The obtained results show that geocell reinforcement acts as a wide slab and thus it can restrain the failure surface from developing and redistribute the loads over a wider area. Therefore, under the geocell placement, the lateral deformation and shear strain values of the slope considerably decrease. Furthermore, the effective placement of geocell reinforcements is found to be between the middle of the slope and the middle of critical failure surface of the unreinforced slope.

Biomechanical evaluation of the ventral and lateral surface shear strain distributions in central compared with dorsolateral placement of cages for lumbar interbody fusion

The purpose of this study was to measure and compare the ventral and lateral surface strain distributions and stiffness for two types of interbody cage placement: 1) central placement for anterior lumbar interbody fusion (ALIF); and 2) dorsolateral placement for extraforaminal lumbar interbody fusion (ELIF). Two functional spine units were obtained for testing in each of 13 cadaveric spines, yielding 26 segments (three of which were not used because of bone abnormalities). Bilateral strain gauges were mounted adjacent to the endplate on the lateral and ventral walls of each vertebral body in the 23 motion segments. Each segment was cyclically tested in compression, flexion, and extension in the following conditions: while intact, postdiscectomy, and instrumented with interbody fusion cages placed using both insertion techniques. No significant differences were observed between ALIF and ELIF in compressive stiffness, bending stiffness in flexion and extension (p > or = 0.1), ventral and lateral strain distribution during the intact tests (p > or = 0.24), and during the flexion tests after fusion (p > or = 0.22). In compression, higher ventral and lower lateral strain was observed in the ALIF than in the ELIF group (ventral, p = 0.05; lateral, p = 0.04), and in extension, higher ventral (p = 0.01) and higher lateral strain (p = 0.002) was observed in the ELIF than in the ALIF group. Preservation of the ventral anulus and dorsolateral placement of the interbody cages during ELIF allow alternate load transfer pathways through the dorsolateral vertebral wall and ventral anulus that are not observed following ALIF. These may be associated with a lower incidence of subsidence and a higher rate of fusion due to a more concentrated application of bone healing-enhancing compression forces during the fusion and healing process.

Lumbar spinal strains associated with whiplash injury: a cadaveric study.

To study and quantify the effects of rear-end collision on the lumbar spine. The lumbar spine of a cadaver was instrumented with rosette strain gauges applied on the lateral and anterior surfaces of T12, L2, and L4. Biaxial accelerometers were mounted on L1, L3, and L5. The cadaver was seated, restrained, and subjected to rear impacts of 5g and 8g. The anterior shear strains had a biphasic shape. Spinal strains peaked at the T12 at approximately 120 and 370 msec, whereas in the L4 vertebra, it peaked at 200 and 380 msec. The anterior strain pattern of the L4 and T12 vertebrae were in diametrically opposite directions. In the second set of tests (8g experiment), the acceleration forces and strains pattern were similar to the 5g test but of higher magnitude. The principal anterior strain was 480 microm/m for 5g and 530 microm/m for 8g; the lateral shear strain was 680 microm/m and 1500 microm/m in the 5g and 8g experiments, respectively. Forces generated during simulated whiplash collision induce biphasic lumbar spinal motions (increased-decreased lordosis) of insufficient magnitude to cause bony injuries, but they may be sufficient to cause soft-tissue injuries.

Nonaxisymmetric oscillations of shells of revolution induced by nonstationary thermal effects

The finite-element method of solving the dynamic behavior of shells of revolution under nonstationary thermal effects, which is the basis of a precise theory of shells (a quadratic change of lateral shear strain in thickness is hypothesized), is investigated.

Fine details of the Wadati‐Benioff zone under Indonesia and its geodynamic implications

Using about 5200 handpicked P, S, pP, sP, PcP, and ScP phases from digitally recorded seismograms, together with International Seismological Centre reported phases, we obtain improved hypocentral locations for ∼1790 earthquakes, ∼983 of them having 90% confidence limits <30 km, at depths >50 km, in the period 1962 to September 1996, along the Indonesian subduction zone. We use an improved joint hypocenter determination method in which the solution is more stable than in the original one. We show that for deep earthquakes, use of the core‐reflected phases is as good as use of depth phases, and since such core phases often have larger amplitudes and sharper onsets than the depth phases, their arrival times can be read very accurately. The positions of the relocated hypocenters show that (1) a portion of the Indonesian arc between ∼110°E and 123°E longitude, and deeper than ∼500 km, is dipping southward at an angle of ∼75°, that is, in a direction opposite to the upper part of the north dipping slab, and (2) east of about 108°E, the seismic zone is wider near 670 km than near the 500 km depth. The first suggests southward lateral flow in the mantle, relative to the plate motion vector here. From the contortion of the seismic zone along the eastern portion of the Indonesian arc, we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10−16s−1, over the last 10–20 Myr.

Gravity anomalies, Quaternary vents, and Quaternary faults in the southern Cascade Range, Oregon and California: Implications for arc and backarc evolution

Isostatic residual gravity anomalies in the southern Cascade Range of northern California and southern Oregon are spatially correlated with broad zones of Quaternary magmatism as reflected by the total volume of Quaternary volcanic products, the distribution of Quaternary vents, and the anomalously low teleseismic P wave velocities in the upper 30 km of crust. The orientation of Quaternary faults also appears to be related to gravity anomalies and volcanism in this area, trending generally north‐south within the magmatic regions and northwest‐southeast as they enter the neighboring amagmatic zones to the north and south. The relationship between gravity anomalies, vent density, and fault orientations may indicate in a broad sense the strength of the middle and upper crust. The southern Cascade Range occupies a transition zone where horizontal stress is transferred from the northwest‐southeast dextral shear of the Walker Lane belt to the east‐west extension characteristic of the Cascade arc in central Oregon. Faulting along north‐south strikes in the volcanically active areas indicates the east‐west extensional stresses in thermally weakened crust, whereas northwest faulting between the volcanically active areas reflects the northwest trending, right lateral shear strain of the Walker Lane belt. The segmentation of the arc reflected in Quaternary magmatism may be caused by differential extension behind crustal blocks of the forearc rotating clockwise with respect to North America. In this view the volcanic centers at Mount Shasta, Medicine Lake volcano, and Lassen Peak in northern California are situated along the southern parts of the trailing edges of two distinct segments of the forearc where additional extension is implied by their differential clockwise rotation.

Crustal movement investigations

Studies of horizontal crustal movement using conventional geodetic methods have been considerably expanded in the quadrennium 1971–1974. The basic fault monitor Geodimeter network now covers most of the major faults in California as well as the zone of faulting that extends into Nevada. Isolated Geodimeter networks in seismic areas of Montana, New Mexico, Utah, and Washington are also monitored. Part of the Geodimeter network along the San Andreas fault has been monitored for over 15 yr, and there appears to be a systematic deviation from a linear trend for most of the lines [Greensfelder and Bennett, 1973; Savage et al., 1973]. However, this deviation may be an artifact of a change in survey procedures in mid‐1969. The conclusions that can be drawn from the Geodimeter observations at present are the following: (1) The present apparent interplate motion across the San Andreas fault in central California is only 30–40 mm/yr [Savage and Burford, 1973], in contrast to the average of 50–60 mm/yr as estimated from magnetic anomalies at the mouth of the Gulf of California. (2) Measurements of fault creep on the creeping segment of the San Andreas fault agree reasonably well with the plate movement indicated by geodetic measurements, this agreement suggesting that fault creep on this segment is the principal mode of accommodation [Savage and Burford, 1971]. (3) The rate of strain accumulation along the San Andreas fault is not well measured anywhere, but the overall tensor strain rate appears to be about 0.3 microstrain/yr or less [Savage et al., 1973]. A considerable effort has also been expended reanalyzing old triangulation data for networks that cross the San Andreas fault. In such a study, Thatcher [1974] deduced evidence for several meters of afterslip on the San Andreas fault at depths greater than 10 km following the 1906 earthquake. This appears at the surface as a very rapid accumulation of strain in the years immediately following the earthquake. Meade [1974] has called attention to a remarkably uniform change of the astronomic azimuth with time for a line that crosses the San Andreas fault about 35 km south of the southern terminus of the 1906 rupture. This change of azimuth implies a uniform right lateral motion across the fault amounting to 32 mm/yr in the period 1885–1962. By comparing a 1942 triangulation survey and a 1970 Geodimeter survey, Page [1972] found neither lateral slip nor shear strain accumulation consistent with lateral slip across the Denali fault in Alaska. He did observe a north‐south extension that was possibly an effect of strain release at the time of the 1964 Alaska earthquake.