Dense Gps Array as a New Sensor of Seasonal Changes of Surface Loads

Global Positioning System (GPS) receivers have been deployed worldwide to study inter- and intraplate crustal motions and local deformations associated with earthquakes and volcanic activities. A dense array of GPS is also useful for studying seasonally changing load through periodic components in crustal movements. This article reviews observed and predicted seasonal crustal movements in the Japanese Islands, where both nationwide dense GPS array and meteorological sensor network are available. From comprehensive evaluation of various sources contributing to seasonal signals, the largest factor in Japan is found to be snow, weighing over 1000 kg per square meter in some regions. This is followed by various kinds of loads on the land area, such as atmosphere, soil moisture and water impoundment in reservoirs, and non-tidal ocean loads also cause certain seasonal signatures. Seasonal crustal deformations are calculated by synthesizing all these seasonal load changes, some of which are directly measured meteorologically and others are inferred through models. They are compared with real data observed by the dense GPS array in Japan, and their agreement was examined. The seasonal signals observed by GPS also include artifacts, such as those caused by atmospheric delay gradients and scale changes due to atmospheric refraction. We often discuss subtle crustal deformation signals, e.g. those associated with silent earthquakes, isolating them by removing secular and periodic components. Understanding seasonal signals and their interannual variability is crucial in removing these unwanted signals. The article discusses the Japanese case, but the methods proposed here will be useful worldwide to study seasonal mass redistributions. Dense GPS arrays may play a complementary role to satellite gravity missions in studying seasonal mass redistribution on the Earth in a regional scale.