The Woods Reef (New South Wales) earthquake of 14 November 1990: Focal mechanism derived from amplitude ratios and synthetic seismograms

On the morning of 15 November 1990 local time, Armidale and the area to the west of Armidale was shaken by a magnitude 3.2 earthquake. The epicentre was located at 30.39° S, 150.88° E and the depth of focus at 12 ± 7 km. As the epicentre was close to the Peel Fault an attempt was made to constrain the focal mechanism of this earthquake. The conventional method, which is based on the analysis of P wave polarities, was not applicable because the event was not strong enough. In an alternative method, the amplitudes of various seismic phases recorded at a number of stations well distributed in azimuth were compared with theoretical amplitudes calculated with the reflectivity method for a point shear dislocation in a layered medium. The differences between observed and calculated amplitudes were minimized as a function of fault strike, fault dip and direction of the slip vector. The analysis indicates that none of the possible fault planes had the strike of the Peel Fault. The solution suggests predominantly strike slip motion along two possible, steeply dipping fault planes. The inferred direction of the maximum compressional stress. is east‐west which is in good agreement with other estimates of the stress field for eastern Australia.     

A global study of S-to-P and P-to-S conversions from the upper mantle transition zone

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Long-period data of the Global Digital Seismograph Network (GDSN) recorded over the three-year period from 1984 to 1986 were studied for the occurrence of S-P and P-S conversions from the upper mantle transition zone that appear as precursors to teleseismic S arrivals. Conversions of this type were identified on a large number of single-station records. Simple stacking of many records enhanced the appearance of converted phases and demonstrated that no major lateral variations in the nature of the transition zone exist between various tectonic regions. S-P and P-S conversions from the 400 km discontinuity were best observed at distances between 70 and 85 while conversions from the 670 km discontinuity showed up best at distances beyond 87. The analysis of published source mechanisms and comparison with synthetic seismograms suggests that the appearance of converted phases is primarily governed by the earthquake radiation pattern. Phases that have undergone S-P conversions beneath the receiver are best observed from dip-slip events that radiate strong SV - and weak P -waves towards the station. P-S conversions beneath the source area, on the other hand, are frequently observed from events that radiate strong P and little SV energy towards the station, and also from some strike-slip events. Comparison of observed with synthetic seismograms suggests that the PREM model of Dziewonski & Anderson (1981) explains most of the observations. Observed S-P and P-S conversions from the 670 km discontinuity, however, often have larger amplitudes than in the synthetics. Constructive interference of converted waves with the P -wave coda, source radiation effects and a velocity contrast across the 670 km discontinuity which is higher than in PREM may all contribute to the discrepancy.     

The distribution function of dark matter in massive haloes

We study the distribution function (DF) of dark matter particles in haloes of mass range  10¹⁴–10¹⁵ M_⊙  . In the numerical part of this work we measure the DF for a sample of relaxed haloes formed in the simulation of a standard Λ cold dark matter (ΛCDM) model. The DF is expressed as a function of energy E and the absolute value of the angular momentum L , a form suitable for comparison with theoretical models. By proper scaling we obtain the results that do not depend on the virial mass of the haloes. We demonstrate that the DF can be separated into energy and angular momentum components and propose a phenomenological model of the DF in the form     . This formulation involves three parameters describing the anisotropy profile in terms of its asymptotic values (β₀ and  β_∞  ) and the scale of transition between them ( L ₀). The energy part   f _E ( E )  is obtained via inversion of the integral for spatial density. We provide a straightforward numerical scheme for this procedure as well as a simple analytical approximation for a typical halo formed in the simulation. The DF model is extensively compared with the simulations: using the model parameters obtained from fitting the anisotropy profile, we recover the DF from the simulation as well as the profiles of the dispersion and kurtosis of radial and tangential velocities. Finally, we show that our DF model reproduces the power-law behaviour of phase-space density   Q =ρ( r )/σ³( r )  .     

Standing-water deposits as indicators of Late Quaternary dune migration in the northwestern Negev,Israel

Late Quaternary playa (stand-water) deposits are present in river channels upstream from dune fields in the northwestern Negev and represent a drainage disordering caused by dune migration during periods of aridity. These deposits are associated with modifications in the drainage system, including course changes and piracy, caused by dunes blocking drainage networks. Radiocarbon dates from the standing-water sediments indicate the occurrence of two periods of aridity: (1) 20,900 to 16,000 years B.P. and (2) 11,680 to 10,300 years B.P. These two periods indicate a correlation between glacial advances in Europe and dry intervals in the Near East during the Upper Pleistocene. We suggest that spatial and temporal associations between standing-water deposits, modifications in stream direction, soil formation and the dunes themselves can serve as a good indicator for the timing of dune migration.     

Geodetic accuracy of the Macrometer model V-1000

The Macrometer Model V-1000 is a geodetic positioning instrument that uses the radio signals broadcast by the GPS satellites. The Macrometer is the only GPS user equipment commercially available that does not require any of the GPS codes. The Model V-1000 receives only the 19-cm wavelength signals from GPS although similar instruments, built for the U.S. Air Force Geophysics Laboratory, receive both the 19-and the 24-cm wavelengths. In this paper we summarize the results of two years of field testing of the V-1000. This instrument, observing four or five satellites fof a few hours, yields a point position accurate within several meters in each coordinate: latitude, longitude and ellipsoidal height. All three components of the relative position vector between a pair of points can be determined within 2 parts per million of the distance, given a similar schedule of observations. This accuracy has been obtained for intersite distances from one kilometer to several thousand kilometers. Macrometer is a trademark of Aero Service Division, Western Geophysical Company of American, 8100 Westpark Drive, Houston, Texas, 77063, U.S.A.