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GPS time series can describe by the following equation:
Where a,b represents the linear trend, c,d,e,f denotes the annual and semiannual signal, which usually related to the signal variation (such as temperature or rainfall). Term g represents step function to describe the "jump" follow by non-tectonic effect such as changing antenna, h term is the coseismic and postseismic term follow by large earthquake. The exp denotes the relaxation time (i.e. the time when amplitude decay to 1/exp) of each events.
Then we get the residual of GPS signal, which current equation cannot fully explain. The question is, what is this?
Another detail model for seasonal change was presented by Dr. Vic Tsai (see Tsai, V. C. (2011). A model for seasonal changes in GPS positions and seismic wave speeds due to thermoelastic and hydrologic variations. Journal of Geophysical Research: Solid Earth (1978–2012), 116(B4).) by the effect of Thermoelastic and Hydrologic (=direct water loading+poroelastic).
This is the simple form of Thermoelastic related displacement change. Displacements in x,y direction are in the periodic function form.
For most of the stations, this model cannot fit well to the GPS data in this region. This means others effects dominant this region rather than the Thermoelastic model.
Could be water effect?
Groundwater (green) and GPS (black dot) comparison (up) and the correlation analysis.
Perhaps further evidences are necessary.
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