Observations of multiple seismic events

Two or more dispersed wave trains each with constant amplitude will interfere giving a resultant wave train which is amplitude modulated, if the individual waves have their principal energies in a common frequency band and if the trains arrive with time separations small compared to their total length. The dispersive characteristics of the trains need not be the same. If the component trains are of comparable magnitude, the modulation due to interference becomes significant and a "beat" phenomenon occurs. Multiple trains of dispersed seismic surface waves may occur because of a temporal and/or spatial distribution at the source or because of multipath propagation. Each of these causal mechanisms influences the amplitude and phase spectra of the resultant wave train; derived properties such as phase velocities and amplitude ratios are also influenced. In the case of multipath propagation, wavelength dependent time delays may occur. Two cases of twin earthquakes are analyzed, and the significant features of interference are demonstrated. In one case, estimates are obtained for the amplitude ratio and time delay of the second shock with respect to the first. The interpretation of seismograms and spectra influenced by multiple events is discussed

An axisymmetric time-domain spectral-element method for full-wave simulations: Application to ocean acoustics

The numerical simulation of acoustic waves in complex 3D media is a key topic in many branches of science, from exploration geophysics to non-destructive testing and medical imaging. With the drastic increase in computing capabilities this field has dramatically grown in the last twenty years. However many 3D computations, especially at high frequency and/or long range, are still far beyond current reach and force researchers to resort to approximations, for example by working in 2D (plane strain) or by using a paraxial approximation. This article presents and validates a numerical technique based on an axisymmetric formulation of a spectral finite-element method in the time domain for heterogeneous fluid-solid media. Taking advantage of axisymmetry enables the study of relevant 3D configurations at a very moderate computational cost. The axisymmetric spectral-element formulation is first introduced, and validation tests are then performed. A typical application of interest in ocean acoustics showing upslope propagation above a dipping viscoelastic ocean bottom is then presented. The method correctly models backscattered waves and explains the transmission losses discrepancies pointed out in Jensen et al. (2007). Finally, a realistic application to a double seamount problem is considered.Comment: Added a reference, and fixed a typo (cylindrical versus spherical

Guided Elastic Interface Waves for Ceramic Joint Evaluation

Because of their excellent thermal and wear properties, structural ceramics are finding increasing use in applications that have traditionally been reserved for metals. Since many ceramics remain stable at temperatures well in excess of the melting points of virtually all of the common structural metals, one such application is in high-temperature engines, where the relatively low weight of ceramics provides an additional advantage over such competitors as refractory metals. Unfortunately, with the relatively low fracture toughness and poor machinability of ceramics, practical designs, at least for the near future, will probably consist of ceramic liners attached to metal substrates, thereby combining the wear and thermal properties of ceramics with the strength of metals.</p

The Keweenaw Current and ice rafting: Use of satellite imagery to investigate copper-rich particle dispersal

The immense surface area and large volume of Lake Superior causes thermal characteristics to resemble marine waters, yet the completely bounded shoreline and low flushing rate introduce unique features. Previously, shoreline inputs were considered minor, as annual river discharges account for only 0.36% of the total hydrologic volume of the lake. However, thermal bar formation and wind shear from prevailing westerlies impound warm waters along the southern coastline, creating a coastal exposure corridor with strong counterclockwise circulation known as the Keweenaw Current. Discharges from rivers and industrial sources are confined, then entrained. Here infrared AVHRR (Advanced Very High Resolution Radiometer) satellite imagery was utilized, verified by NDBC (NOAA National Data Buoy Center) buoy surface data, to document thermal features of offshore waters and the coastal zone. Five stamp mills at Freda/Redridge discharged over 45 million metric tons of stamp sands between 1895 and 1922. The coarse fraction forms beach sands that now extend 23 kilometers north from their sources and that blanket shallow-water sandy sediments. The finer fractions disperse much farther than the coarse fractions, moving along the primary track of the Keweenaw Current. SPOT and TM (Thematic Mapper) imagery were used to document how Ontonagon clays and Freda/Redridge stamp sand particles are entrained by the Keweenaw Current. The two particle types have distinctive reflective spectra. An additional transport mechanism, revealed by RADARSAT ScanSAR (Synthetic Aperature Radar) imagery, is ice rafting. Nearshore ice incorporates large amounts of coastal sands and deeper-water sediments. Spring break-up of coastal ice results in large drifting ice packs that are pushed by prevailing westerlies and currents around the tip of the Keweenaw Peninsula into the Caribou Basin

Determining the age of Qatari Jabal Jassasiyah Petroglyphs

The petroglyph site in Jabal Jassasiyah Qatar is located approximately 60 km northeast of the capital city of Doha and has over 900 different types of petroglyphs. The most commonly found petroglyphs are cupules, which are almost always arranged in geometric patterns. A number of petroglyphs of boats are also found, usually seen from above, with a few seen in profile. As there is little evidence of what age to assign to these petroglyphs, samples of the calcium oxalate containing layers covering the petroglyphs were sent for radiocarbon dating to determine the minimum age they were created. The minimum ages of nine samples taken for analysis were found to be very short, the oldest minimum age being only 235 years BP (before present). No evidence was found for the petroglyphs dating back a few millennia as was previously postulated. Due to the lack of chronological data for Qatar’s archaeological past, the study data cannot completely rule out the petroglyphs dating back to ancient times