Seiichi Shimada

CRUSTAL DEFORMATION MEASUREMENTS IN CENTRAL JAPAN DETERMINED BY A GLOBAL POSITIONING SYSTEM FIXED-POINT NETWORK

A Global Positioning System (GPS) fixed-point network has been operating in the Kanto and Tokai districts of central Japan since April 1988 to detect crustal deformation associated with the convergence of the Eurasian, Pacific, North American, and Philippine Sea plates and to monitor the deformation cycles of frequent large interplate and intraplate earthquakes. This 10-station network established by the National Research Institute for Earth Science and Disaster Prevention (NIED) is the first continuously monitoring network of its kind. We determine deformation within the network using two consecutive days of data every 2 weeks for the first 17 months of operations. We use a station and orbit relaxation method which relies exclusively on data collected within the NIED network, except for 1 week of global GPS tracking data which is used to determine initial station positions with respect to the global reference frame. We detect, relative to a station on the Eurasian plate in central Japan, significant westward motion of 28 mm/yr of the northern tip of the Philippine Sea plate, which is subducting beneath the Eurasian plate at the Suruga trough. Our results support finite element models of collision of the Izu Block with the Eurasian plate based on earthquake focal mechanisms and plate block motions of the Japanese archipelago determined from conventional geodetic measurements over the last century. We detect southwestward motion of 18 mm/yr of the southeastern tip of the Eurasian plate, confirming expected surface extension of the subducted plate parallel to the Suruga trough axis. Significant vertical uplift with a velocity of 20 mm/yr is suggested at a site inland of the Tokai district located in the Akaishi uplift zone and at a site on Hatsushima Island in Sagami Bay. The general tendency of vertical movements of the other sites agrees with vertical velocities obtained from 70 years of geodetic leveling and with Quaternary vertical displacements determined from geomorphological and other geological evidence. We detect no significant crustal motion across the Fossa Magna tectonic zone in central Japan (often considered a plate boundary), across the Tokyo metropolitan area, nor across the Sagami trough associated with the subduction of the Philippine Sea plate beneath northeast Japan. Our results demonstrate the power of regionally based, continuously monitoring GPS networks for obtaining temporally dense measurements of small horizontal and vertical crustal movements across plate boundary zones.

Continuously Monitoring Gps Networks for Deformation Measurements

A powerful rationale for monitoring the deformations of the earth’s crast and of man-made structures is to understand the underlying physics of phenomena that have historically caused major loss of life and destruction of property, most notably earthquakes, volcanic eruptions, and collapse of structures such as dams. We are entering the International Decade of Natural Hazard Reduction. Space geodetic techniques, in particular the Global Positioning System (GPS), will contribute valuable data to the alleviation of natural hazards.

GPS measurements to investigate the reason why GPS is less accurate in mountain areas

It has been pointed out that GPS results in mountain areas are usually less accurate than those in flat areas, especially in the vertical component, when the height difference among observation stations is large. It is thought that the decrease of accuracy is caused by insufficient corrections for tropospheric delay of the GPS microwave.

Regional Orbit Improvement Techniques Applied to a Japanese Fixed-Point GPS Network

Bock and Shimada (1990) introduced the concept and some experimental examples of the GPS fixed-point networks for deformation measurements. The measurement applying such a fixed-point GPS network is one of the promising technique to monitor crustal movements for the purpose of geodynamic study. The Kanto-Tokai ten-point dedicated, GPS fixed-point network (NIED network) in central Japan is the first one of those networks in the world, established in April 1988 (Shimada et al., 1989). Shimada and Bock (1991) estimated the positions of the NIED network sites with respect to the terrestrial reference frame using world-wide GPS tracking data collected during the Global Orbit Tracking Experiment (GOTEX-1) campaign carried out in November 1988. They then computed the positions of the network stations from the first sixteen months of network data by applying weighted constraints on the site positions and on the available satellite ephemerides (“regional orbit improvement techniques”) and a simultaneous two-session analysis for each data window, and obtained crustal deformations during the initial sixteen months of data. In the analysis, they used the GAMIT GPS software developed at Massachusetts Institute of Technology and Scrips Institution of Oceanography (King and Bock, 1991). They demonstrated the validity of the regional orbit improvement techniques for the fixed-point regional network, reducing the cost required to obtain the regional deformations and the measurement error caused by the broadcast orbit ephemeris.