Active normal faulting during the 1997 seismic sequence in Colfiorito, Umbria: Did slip propagate to the surface?

In order to determine whether slip during an earthquake on the 26th September 1997 propagated to the surface, structural data have been collected along a bedrock fault scarp in Umbria, Italy. These collected data are used to investigate the relationship between the throw associated with a debated surface rupture (observed as a pale unweathered stripe at the base of the bedrock fault scarp) and the strike, dip and slip-vector. Previous studies have suggested that the surface rupture was produced either by primary surface slip or secondary compaction of hangingwall sediments. Some authors favour the latter because sparse surface fault dip measurements do not match nodal plane dips at depth. It is demonstrated herein that the strike, dip and height of the surface rupture, represented by a pale unweathered stripe at the base of the bedrock scarp, shows a systematic relationship with respect to the geometry and kinematics of faulting in the bedrock. The strike and dip co-vary and the throw is greatest where the strike is oblique to the slip-vector azimuth where the highest dip values are recorded. This implies that the throw values vary to accommodate spatial variation in the strike and dip of the fault across fault plane corrugations, a feature that is predicted by theory describing conservation of strain along faults, but not by compaction. Furthermore, published earthquake locations and reported fault dips are consistent with the analysed surface scarps when natural variation for surface dips and uncertainty for nodal plane dips at depth are taken into account. This implies that the fresh stripe is indeed a primary coseismic surface rupture whose slip is connected to the seismogenic fault at depth. We discuss how this knowledge of the locations and geometry of the active faults can be used as an input for seismic hazard assessment

Spatial migration of temporal earthquake clusters driven by the transfer of differential stress between neighbouring fault/shear-zone structures

Uncertainty concerning the processes responsible for slip-rate fluctuations associated with temporal clustering of surface faulting earthquakes is a fundamental, unresolved issue in tectonics, because strain-rates accommodated by fault/shear-zone structures are the key to understanding the viscosity structure of the crust and seismic hazard. We constrain the timing and amplitude of slip-rate fluctuations that occurred on three active normal faults in central Italy over a time period of 20–30 kyrs, using in situ 36Cl cosmogenic dating of fault planes. We identify five periods of rapid slip on individual faults lasting a few millennia, separated time periods of up to 10 millennia with low or zero slip-rate. The rapid slip pulses migrated across the strike between the faults in two waves from SW to NE. We replicate this migration with a model where rapid slip induces changes in differential stress that drive changes in strain-rate on viscous shear zones that drive slip-rate variability on overlying brittle faults. Earthquakes increase the differential stress and strain-rate on underlying shear zones, which in turn accumulate strain, re-loading stress onto the overlying brittle fault. This positive feedback produces high strain-rate episodes containing several large magnitude surface faulting earthquakes (earthquake clusters), but also reduce the differential stress on the viscous portions of neighbouring fault/shear-zones slowing the occurrence of large-magnitude surface faulting earthquakes (earthquake anticlusters). Shear-zones on faults experiencing anticlusters continue to accumulate viscous strain at a lowered rate, and eventually this loads the overlying brittle fault to failure, initiating a period of rapid slip through the positive feedback process described above, and inducing lowered strain-rates onto neighbouring fault/shear-zones. We show that these patterns of differential stress change can replicate the measured earthquake clustering implied by the 36Cl data. The stress changes are related to the fault geometry in terms of distance and azimuth from the slipping structure, implying that (a) strain-rate and viscosity fluctuations for studies of continental rheology, and (b) slip-rates for seismic hazard purposes are to an extent predictable given knowledge of the fault system geometry

Radiation shielding materials and radiation scatter effects for interventional radiology (IR) physicians

Purpose: To measure the attenuation effectiveness and minimize the weight of new non-Pb radiation shielding materials used for radiation protection by interventional radiology (IR) physicians, to compare the accuracy of the different standard measurement geometries of these materials, and to determine x-ray qualities that correspond to the scattered radiation that IR physicians typically encounter. Methods: Radiation attenuation capabilities of non-Pb materials were investigated. Typically, most studies of non-Pb materials have focused on the attenuating properties of metal powders. In this study, layers of materials incorporating non-Pb powdered compounds such as Bi2O 3, Gd2O3, and BaSO4 were measured individually, as bilayers, and as a Bi2O3-loaded hand cream. Attenuation measurements were performed in narrow-beam (fluorescence excluded) and broad beam (fluorescence included) geometries, demonstrating that these different geometries provided significantly different results. The Monte Carlo (MC) program EGSnrc was used to calculate the resulting spectra after attenuation by radiation shielding materials, and scattered x-ray spectra after 90\ub0 scattering of eight ASTM Standard primary x-ray beams. Surrogate x-ray qualities that corresponded to these scattered spectra were tabulated. Results: Radiation shielding materials incorporating Bi2O3 were found to provide equivalent or superior attenuation compared with commercial Pb-based and non-Pb materials across the 60-130 kVp energy range. Measurements were made for single layers of the Bi2O3 compound and for bilayers where the ordering was low atomic number (Z) layer closest to x-ray sourcehigh Z (Bi2O3) layer farthest from the x-ray source. Narrow-beam Standard test methods which do not include the contribution from fluorescence overestimated the attenuating capabilities of Pb and non-Pb materials. Measurements of a newly developed, quick-drying, and easily removable Bi2O3-loaded hand cream demonstrated better attenuation capabilities than commercial Bi2O3-loaded gloves. Scattered radiation measurements and MC simulations illustrated that the spectra resulting from 90\ub0 scattering of primary x-ray beam qualities can be approximated by surrogate x-ray qualities which are more representative of the radiation actually encountered by IR physicians. A table of surrogate qualities of the eight ASTM F2547-06 Standard qualities was compiled. Conclusions: New non-Pb compound materials, particularly single layers or bilayers incorporating Bi2O3, can reduce the weight of radiation protection materials while providing equivalent or better protection compared to Pb-based materials. Attenuation measurements in geometries that exclude the contribution from fluorescence substantially underestimate the quantity of transmitted radiation. A new Bi2O3-loaded hand cream demonstrated a novel and effective approach for hand protection. Standard testing protocols for radiation protection materials used by IR physicians specify a wider kVp range than is necessary. A more realistic range would acknowledge the lower kVp resulting from scatter and allow IR physicians to confidently utilize lighter-weight materials while still receiving adequate protection. Standards protocols incorporating the adjustments described in this work would maintain the safety of IR personnel and lessen the physical repercussions of long hours wearing unnecessarily heavy radiation protection garments. \ua9 2012 American Association of Physicists in Medicine.Peer reviewed: YesNRC publication: Ye

InAs self-assembled quantum dots on InP by molecular beam epitaxy

We present results of room temperature photoluminescence (PL) emission from a 0-dimensional system in the ~ 1.4 to ~ 1.7 \ub5m spectral region. Molecular beam epitaxy was used to grow InAs self-assembled quantum dots in AlInAs on an InP substrate. Preliminary characterizations have been performed using PL and transmission electron microscopy. The low temperatures PL spectra also display excited state emission and state filling as the excitation intensity is increased.Peer reviewed: YesNRC publication: Ye

Quasiphase matched surface emitting second harmonic generation in periodically reversed asymmetric GaAs/AlGaAs quantum well waveguide

The authors exptl. demonstrate surface-emitting 2nd harmonic generation in a waveguide contg. asym. coupled GaAs/AlGaAs quantum wells. The nonlinear conversion efficiency is enhanced by reversing the asym. well orientation every coherence length, to quasiphase match the vertical 2nd harmonic generation process. The measured spectrum of the asym. quantum well susceptibility is dominated by an excitonic peak at a pump frequency corresponding to half of the 1st electron-heavy hole transition energy. [on SciFinder (R)

A 667-year record of co-seismic and interseismic Coulomb stress changes in central Italy reveals the role of fault interaction in controlling irregular earthquake recurrence intervals

Current studies of fault interaction lack sufficiently long earthquake records and measurements of fault slip rates over multiple seismic cycles to fully investigate the effects of interseismic loading and coseismic stress changes on the surrounding fault network. We model elastic interactions between 97 faults from 30 earthquakes since 1349 A.D. in central Italy to investigate the relative importance of co-seismic stress changes versus interseismic stress accumulation for earthquake occurrence and fault interaction. This region has an exceptionally long, 667 year record of historical earthquakes and detailed constraints on the locations and slip rates of its active normal faults. Of 21 earthquakes since 1654, 20 events occurred on faults where combined coseismic and interseismic loading stresses were positive even though ~20% of all faults are in “stress shadows” at any one time. Furthermore, the Coulomb stress on the faults that experience earthquakes is statistically different from a random sequence of earthquakes in the region. We show how coseismic Coulomb stress changes can alter earthquake interevent times by ~103 years, and fault length controls the intensity of this effect. Static Coulomb stress changes cause greater interevent perturbations on shorter faults in areas characterized by lower strain (or slip) rates. The exceptional duration and number of earthquakes we model enable us to demonstrate the importance of combining long earthquake records with detailed knowledge of fault geometries, slip rates, and kinematics to understand the impact of stress changes in complex networks of active faults

Variable fault geometry suggests detailed fault slip-rate profiles and geometries are needed for fault-based probabilistic seismic hazard assessment (PSHA)

It has been suggested that a better knowledge of fault locations and slip rates improves seismic hazard assessments. However, the importance of detailed along‐fault‐slip‐rate profiles and variable fault geometry has not yet been explored. We quantify the importance for modeled seismicity rates of using multiple throw‐rate measurements to construct along‐fault throw‐rate profiles rather than basing throw‐rate profiles on a single measurement across a fault. We use data from 14 normal faults within the central Italian Apennines where we have multiple measurements along the faults. For each fault, we compared strain rates across the faults using our detailed throw‐rate profiles and degraded data and simplified profiles. We show the implied variation in average recurrence intervals for a variety of magnitudes that result. Furthermore, we demonstrate how fault geometry (variable strike and dip) can alter calculated ground‐shaking intensities at specific sites by changing the source‐to‐site distance for ground‐motion prediction equations (GMPEs). Our findings show that improved fault‐based seismic hazard calculations require detailed along‐fault throw‐rate profiles based on well‐constrained local 3D fault geometry for calculating recurrence rates and shaking intensities

Andean surface uplift constrained by radiogenic isotopes of arc lavas

Climate and tectonics have complex feedback systems which are difficult to resolve and remain controversial. Here we propose a new climate-independent approach to constrain regional Andean surface uplift. 87Sr/86Sr and 143Nd/144Nd ratios of Quaternary frontal-arc lavas from the Andean Plateau are distinctly crustal (>0.705 and <0.5125, respectively) compared to non-plateau arc lavas, which we identify as a plateau discriminant. Strong linear correlations exist between smoothed elevation and 87Sr/86Sr (R2 = 0.858, n = 17) and 143Nd/144Nd (R2 = 0.919, n = 16) ratios of non-plateau arc lavas. These relationships are used to constrain 200 Myr of surface uplift history for the Western Cordillera (present elevation 4200 ± 516 m). Between 16 and 26°S, Miocene to recent arc lavas have comparable isotopic signatures, which we infer indicates that current elevations were attained in the Western Cordillera from 23 Ma. From 23–10 Ma, surface uplift gradually propagated southwards by ~400 km