Study of seasonal variation of groundwater and atmospheric loads : Possible influence on seismicity and astrometry

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We quantitatively evaluated various factors governing the seasonal components in the position time series of the Japanese nationwide GPS network (GEONET). We found that seasonal changes of surface loads in the Japanese Islands are responsible for seasonal changes. Among them, the snow loads are several times as large as other kinds of loads. Atmospheric loads, groundwater loads (soil moisture and empoundment in reservoirs), non-tidal ocean loads were found to be of secondary importance. We estimated the seasonal changes of these loads using meteorological data and models, and calculated expected seasonal crustal deformation, compared them with actually observed deformations. We found that, in addition to these load changes, significant contributions come from the apparent changes associatedd with seasonal scale changes of unknown origin and atmospheric delay gradients. We established empirical seasonal change models considering all these factors, and the seasonal changes predicted by these models were consistent with actual GPS data. Seasonal components have been removed by an adaptive method, i.e. by estimating annual and biannual components using least-squares method in order to isolate transient crustal deformation signals. Our investigation showed that removal of seasonal components using our model based on a priori knowledge (non-adaptive correction) gives rise to as good results as the adaptive method. Our results were published in the book "State of the Planet" (AGU Geophysical Monograph Series Vol.150). We also calculated the Coulomb Failure Stress caused by such seasonal loads and found they have some influence on the inland seismicity in Japan. It has been known that there are more earthquakes in spring/summer than in fall/winter in snow covered regions, which is consistent with our prediction. This was reported as a paper in Earth Planet.Sci.Lett., and was introduced by major scientific journal s such as Nature and New Scientist.