Station Geometry Research Articles

Plate effects in North Pacific subduction zones

Abstract Plate structures derived by thermal modelling of subduction zones give rise to pronounced geophysical effects. These effects are examined in the North Pacific by applying seismic ray tracing to teleseismic and local network data. The amplitudes of long-period body waves are used to examine the location of the Kuril plate. By allowing for plate structure in the earthquake location process we find that deeper than normal hypocenters fall within the colder more brittle regions of the plate. Focal-mechanism plots of relocated earthquakes show large departures in the dip and azimuth of focal angles from symmetric earth values. Certain circumstances of station geometry with respect to the source may result in an error in the slip vector, to the extent that this kind of error is a limiting factor on the precision of any instantaneous plate motion model. Only small changes in the slowness and azimuth of the wavefront are predicted for arrays at teleseismic distances. A change in the stress direction down-dip to the descending plate has been identified between 120 and 140 km in depth in at least two subduction zones.

Flight Tests of Two Airborne Omega Navigation Systems

two current omega navigation systems designed for airborne use have received military designations: the AN/ARN-99(V)2 and the AN/ARN-115. Both use general purpose digital computers to provide fully automatic synchronization, phase tracking, station selection, and diurnal propagation corrections for display of aircraft latitude/longitude position coordinates. Both systems use all three Omega navigational frequencies, but only the ARN-99(V)2 implements difference frequency lane resolution. The ARN-115 employs three station hyperbolic geometry to directly determine geographic position. The ARN-99(V)2 estimates position by statistically optimum weighting of all available Omega measurements in a Kalman filter algorithm. The U. S. Naval Air Test Center recently evaluated both systems during flight tests in P-3 type aircraft. Observed accuracy of the two mechanizations is presented with emphasis on the effects of station geometry and diurnal propagation correction errors. Experiences with sudden station outages, precipitation static, and on-board generated coherent interference are discussed.

Observations of artificially produced scintillations using satellite transmissions

The ionospheric modification experiment, utilizing a high-powered transmitter, provides an opportunity to study ionospheric irregularities under relatively known conditions. The irregularities were studied by means of transionospheric signals from the polar-orbiting satellite ESSA 8 transmitting at 137 MHz. These observations show that scintillations occur when the satellite to ground station geometry is such that the ray from the satellite passes through the region in the ionosphere under modification. In general, a cut across the illuminated volume is obtained; thus an active region of about 100 km in diameter is indicated. For the most part, the artificial scintillations appear to be similar to those obtained from naturally occurring irregularities, but a systematic change from 3 to 1 sec in the fluctuation period is usually observed as the satellite traverses from the northern to the southern portions of the active region. The change in period appears to be produced by a systematic change in irregularity scale size from about 4 to about 1 km.

A Discussion on the measurement and interpretation of changes of strain in the Earth - Small earthquakes observed with local seismometer networks

Recent developments in instrumentation allow networks of radio-linked seismometers, recording on magnetic tape, to be easily established in areas of micro-earthquake activity. Observations with such networks enable the location and some of the source parameters of small earthquakes to be examined in detail. Accurate locations require adequate knowledge of the velocity structure within the area of the network, and suitable source station geometry. Given such a network, it should be possible to estimate crustal structure sufficiently accurately to give good epicentre and depth locations for small earthquakes within the network.