Recovering Seismic Signals with Different Reference Frame Realization Methods

The JPL IGS analysis center currently uses different methods of realizing the reference frame between Final and Rapid orbit and clock products. Two recent seismic events were inadvertently absorbed into the Rapid orbit and clock products which affected their subsequent characterization with PPP. The first event was the Ridgecrest earthquake of 2019-07-06. The second is an episodic tremor and slip event in the Pacific Northwest in January/February 2021. In both cases, the fundamental issue was movement of reference frame receivers combined with the absolute fiducialization of the JPL Rapid products. With this method, some or all of the ground receiver positions are fixed to reference frame solutions. The JPL Final products, along with other IGS analysis centers contributions to IGS Finals, use a constraint-based approach to tie the solutions to the reference frame. In this method, a best fit of the entire network of some or all of the ground receivers to the entire frame is used to constrain the entire network to have no net rotation, or additionally no translation or scale relative to the frame. Here we present the details of how the over-fiducialization of orbit and clock products interfered with measuring real geophysical signals.