Will SCIGN Fulfill its Biggest Promise, to Test Regional Tectonic Models?

A primary objective of SCIGN from the outset has been to test regional tectonic models, especially for compressional deformation in the Los Angeles region. Papers by Walls et al. (Nature, 1998) and Argus et al. (Geology, 1999) espoused nearly opposite interpretations based on their initial results from SCIGN. There were several clear reasons for differences between their results and interpretations. At that time SCIGN velocity field results differed considerably between the several groups analyzing the data. Also, very different methods of post-processing analysis and modeling were used. Despite historical prevalence and initial InSAR results indicating land subsidence in the basins of southern California (Galloway et al., Water Resources Research, 1998), neither of these studies considered its effects. Then, Bawden et al. (Nature, 2001) accounted for the impact of land subsidence on SCIGN horizontal velocity component estimates. They first imaged the problem areas using InSAR and then selectively deleted velocity estimates from affected SCIGN sites, allowing them to derive a cleaner horizontal tectonic velocity field across Los Angeles (after having eliminated the problematic data). Argus et al. (AGU abstract, 2002) then re-evaluated their earlier study, and included results based on the SCEC survey-mode GPS data base. Meanwhile, much progress was made on other fronts. The SCIGN Analysis Comparison Committee improved and combined SCIGN data products. More stations were installed using improved SCIGN methods, and data accumulated to lengthen the SCIGN GPS time series. In addition, the quality and robustness of the InSAR image stacks improved (Peltzer et al., Geology, 2001). Remaining problems include; 1) the C-band InSAR available from the ERS missions have not yet quite allowed coherent imaging of deformation crossing the San Gabriel range north of Los Angeles and out into the Mojave desert, and 2) GPS stations in the mountains have noisier data than their flatland counterparts. The latter problem arises, in part, because several of the crucial stations (e.g., CHIL, WLSN, TABL) were early prototype installations, each with individual characteristics such as monuments and radomes that are undesirable by today's standards. The noise at higher elevation stations may also be caused by sky view blockage due to the presence of trees and occasional snow. For the time being, work continues on understanding subsidence and other error sources, and how best to handle these in quantitative models of regional deformation. Much more could be done now in terms of modeling the SCIGN combined velocity and time series data, and also the SCEC CMM3 horizontal velocity data. Enough SCIGN stations have been built to the new standards in the San Gabriel mountains so that we will eventually obtain reliable estimates of the vertical motions there and across all of southern California. With the passage of time, especially in the absence of earthquakes (that confuse the velocity picture), and with the addition of 170 more PBO GPS stations throughout the region over the next 5 years, there will surely be excellent geodetic data to constrain and test the future's geodynamic models of the Transverse Ranges.