LOS Time Series
We have compared the integrated InSAR/GNSS LOS time series with the GNSS-only time series and find both excellent and poor agreements (Table S2). Note the two time-series will not match exactly as our approach is essentially trusting GNSS at longer wavelengths while InSAR at short wavelengths. Fig. 4b shows several examples where there is excellent agreement between the InSAR and GNSS time-series. The typical deviation of GNSS daily solutions from its moving average is around 3-4 mm, while the integrated InSAR deviates only slightly larger, around 3-6 mm (Fig. 4a). The vertical component could largely reproduce this deviation and can be considered as the major source of discrepancy. However, there are a number of cases where the integrated InSAR GNSS time-series fail to match the GNSS solutions (Fig. 4c). In most cases this is a direct result of anomalous displacements or artifacts in the GNSS displacement time series. An extreme example is station p800 in Los Angeles showing systematic effects due to the growth of thick vegetation nearly concealing the GNSS antenna and causing a significant drift in the north direction over a period of about 2 years (overlapping with the 4.5 years of our time series) until the vegetation is cleared in May 2018 (add figure in supplement). Station CRHS also in Los Angeles and affected by vegetation and whose data after 2006 are suspect and should not be used (personal communication, Chris Walls, UNAVCO NOTA engineer) – the station was subsequently abandoned. Station CUHS in the Cuyama Valley is subject to significant subsidence of about 30 mm/yr with a seasonal signature resulting in non-tectonic horizontal displacements with amplitudes up to 30 mm. Station CJMG in the San Gabriel Mountains has annual horizontal artifacts starting in 2017 with an amplitude of 30 mm (peak to peak). Another example is station P789 on the San Andreas fault in the transition zone between locked and creeping section is experiencing uplift since 2013 of about 2.5 mm/yr but not fully sufficient to explain its misfit with InSAR. For the purpose of this study these GNSS data were not excluded. Rather, we allowed for discrepancies to exist through the integration, especially when a single station shows large mismatch, because the GNSS correction model, applied to each interferogram, is smoothed with a robust filter that down weighs the anomalous misfits. Thus, in cases a GNSS station starts to behave anomalously, the InSAR time-series remain largely unaffected (Fig. 4c). In further studies, we recommend a more conservative approach in choosing stations for the GNSS/InSAR integration. More assessment and results are available in Table S2 and Fig S1 in the supplements.