Optimization of Surface Wave Identification and Measurement

—?Accurate and reliable measurement of surface waves is important to Comprehensive Nuclear-Test-Ban Treaty (CTBT) monitoring because the M _s :m _b discriminant and its regional variants can in many cases unambiguously identify events as earthquakes or explosions. Surface wave processing at the International Data Center (IDC) is designed to be completely automated and is performed using the program Maxsurf. Maxsurf searches for surface wave characteristics in the expected surface wave arrival time window for all continuous long-period and broadband data in the IDC processing stream. The Prototype IDC GSETT3 Reviewed Event Bulletin (REB) now contains a very large and growing data set of surface wave measurements. Users of this data set need to be aware of processing changes and calibration errors in the GSETT3 experimental bulletin. The prototype International Monitoring System (IMS) surface wave detection threshold is approximately one magnitude unit lower than the detection threshold of other global networks that use visual identification of surface waves. Surface wave identification and measurement can be improved through development of regionalized earth models, phase-matched filtering and the use of path corrected spectral magnitudes in place of M _s . Regionalized earth models are developed through tomographic inversion of a very large data set of phase and group velocity dispersion measurements. Discrimination capability can be improved through the use of maximum likelihood magnitudes and maximum likelihood upper bounds.     

Comment on 'The use of velocity spectrum for stacking receiver functions and imaging upper mantle discontinuities' by H. Gurrola, J. B. Minster and T. Owens

In a recent paper, Gurrola, Minster & Owens (1994; hereafter referred to as GMO) introduced a technique for the analysis of seismic data and applied it to the records of station OBN in Russia. GMO claimed that this was a new technique in earthquake seismology, and that their results for OBN provided a new insight into the deep structure of the Russian platform. We have some comments on their techniques and results.     

Piezoremanent magnetization of the ensemble of single-domain particles

Using the model of single-domain particles, a theoretical analysis of the properties of different forms of piezoremanent magnetization is carried out. The relationships which determine piezoremanent magnetization within the entire range of stresses are obtained. Two mechanisms of magnetization are revealed. It is shown that in the weak field approximation the magnetization of the first type has a quadratic dependence on the magnetic field and weakly depends on stresses, while another type of magnetization, which mainly determines the piezomagnetic effect, is proportional to the field and stresses. Moreover the magnetization that forms in the course of a change in stresses parallel to the field is always higher than the magnetization that appears as a result of action perpendicular to the field.     

Effects of Io ejecta on Europa

We examine the effects of Io ejecta on the surface and environment of Europa. We find that the observed sulfur on the trailing side of Europa, when interpreted as a deposit in equilibrium between implanation of, and sputtering by, corotating Io ejecta, implies a slow loss of material from Europa by sputtering. From this we infer that the spectrum of particles sputtered from water ice is soft. The quantity of observed sulfur and its confinement to the trailing hemisphere appear to exclude significant implantation and sputtering by energetic heavy ions. We also conclude that the contribution from Europa to the magnetospheric plasma (even at Europa itself) is negligible compared to the matter ejected from Io.     

A simple numerical model for inlet sedimentation at intermittently open–closed coastal lagoons

This contribution aims to model the dominant processes that control sedimentation within the ocean inlet to intermittently open–closed coastal lagoons; focussing on the role of infilling, backfilling and morphodynamic feedback. The key elements that have been included in the present model are: (1) the delivery of sediment to the mouth of the estuary by littoral processes; (2) sediment transport processes within the inlet due to non-linear tidal flow; and (3) the down-slope diffusion of sediment. The model discussed here includes a simple one-dimensional (1-D), non-linear flow parameterisation that predicts the spatial variability in the magnitude and non-linearity of the tidal flow. The predicted third and fourth velocity moments are used to drive a morphodynamic module. Down-slope diffusion of sediment is dealt with in a separate diffusion term in the bed-evolution equation. Feedback between the evolving morphology and the flow field are integral to the model. Numerical simulations are used to investigate different modes of evolution for this type of inlet system. Inlet closure due to infilling is critically controlled by the balance between sediment addition at the mouth due to littoral processes and the removal of sediment by non-linear tidal flow. Rapid widening or deepening of the inlet at its landward margin leads to the deposition of a flood shoal. Under conditions of high sedimentation (and low diffusion) the flood shoal can become sufficiently well developed to present a physical barrier to sediment entering the lagoon. Under these circumstances backfilling can become significant. The infilling and backfilling processes are ameliorated by efficient down-slope diffusion which is controlled in the present model by a diffusion parameter, D. High diffusion coefficients slow inlet closure and allow sediment to propagate further into the lagoon.