Ryuichi Ichikawa

Far-Field Deformation due to Volcanic Activity and Earthquake Swarm

The KSP network of the Communications Research Laboratory, a regional VLBI network around Tokyo, detected significant displacements from volcanic and seismic activities in the Izu islands, located 100-300 km from the network. A 13 0km baseline revealed a gradual shortening of 4.5 cm over two months. Of particular importance is that the real-time VLBI monitoring has been operational on a daily basis, which enables us to investigate the development of source activity. The finding also raises interesting questions. Can we model the far-field deformation in terms of dislocations buried in a homogeneous half-space ? May we safely neglect the earths sphericity, stratification and self-gravitation? The Izu islands event provides us with a rare opportunity to answer these questions because it includes both tensile dislocation and shear dislocations. We compute horizontal displacements for several cases: (1) an elastic homogenous half-space, and (2) a radially stratified perfectly elastic earth. The two models give remarkably different results for tensile dislocation. The result clearly indicates that far-field displacement should be analyzed in the framework of spherical earth theory.

A Comparison of Broadcast and Final Orbits on GPS Delays in GPS- VLBI Hybrid Observation

We carry out an error analysis of 24-hour global positioning system (GPS)-very long baseline interferometry (VLBI) (GV) hybrid observation data. In this paper, we focus on the impacts of broadcast and final orbits on the GPS delays of the GV hybrid observation by analyzing the residuals, observed - calculated (O-C) values. The residuals show apparent and consistent biases for L1 and L2 signals, respectively. The scatters of the residuals are around a few nanoseconds. The main cause of those observation errors is the absence of the GPS phase and delay calibration system. Most of the satellites show that the differences between the delays, to which broadcast and final orbits are applied, are about 100 times smaller than the current GV hybrid observation errors. We conclude that GPS delays are not greatly affected by orbit accuracies.