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SCEC Community Geodetic Model (CGM)

CGM WORKING GROUP
CXM Representative
Mike Floyd
DATA & INFORMATION
SOFTWARE SUPPORT
SCEC COMMUNITY MODELS
RELATED RESEARCH
WORKSHOPS
Above (Left) GNSS velocity vectors and uncertainties in the area of the CGM for continuous (red) and survey (blue) stations, relative to the ITRF2014 Plate Motion Model definition of North America [Altamimi et al., 2017]. (Right) InSAR line-of-sight velocity map for track 71 of the Sentinel-1A mission, relative to GNSS site P617 (red triangle), overlaid by the GNSS velocities projected into the satellite line-of-sight. Both GNSS and InSAR data shown here are part of the CGM version 2 currently in development.

Introduction

The Community Geodetic Model (CGM) provides displacement time series and velocities of the Earth’s surface over southern California using data from Global Navigation Satellite Systems (GNSS), which includes the Global Positioning System (GPS), and interferometric synthetic aperture radar (InSAR), both space-based geodetic observation techniques. The GNSS products provide high temporal resolution (nominally daily measurement points) in three dimensions at specific observation sites and the InSAR products provide high spatial resolution (nominally one point per 90 m distance on the ground). Combined, they provide the ability to study crustal deformation over a wide range of distances and periods.

The CGM is being developed and distributed by the SCEC community to assist in the understanding of physical processes that measurably deform the Earth’s surface, such as the interseismic, coseismic and postseismic associated with the earthquake cycle along the complex fault network of the southern San Andreas system, and non-tectonic processes such as hydrology. This activity supports several of the SCEC5 science questions. For example: How are faults loaded across temporal and spatial scales? What is the role of off-fault inelastic deformation on strain accumulation, dynamic rupture, and radiated seismic energy?

Development Model Version: Towards Time-Dependence

Version 2 of the CGM is currently in development and is planned for release in the second half of 2021. The major improvement over the CGMv1 (see below) is that it will add time series and therefore allow time-dependent analyses. Both GNSS and InSAR time series will be provided as well as consensus velocities based on those time series and secondary products such as lists of discontinuities in the time series for users’ own analyses.

The quality and quantity of both GNSS and InSAR data has since rapidly improved to enable a breakthrough in the spatial and temporal resolution of the CGM, which is the focus of the CGMv2. In particular, reprocessing, comparison and verification of long GNSS time series has provided high accuracy vertical measurements that reveal a wide range of new hydrologic and tectonic signals. In addition, two new C-band InSAR satellites (Sentinel-1A and B) are providing highly accurate systematic coverage of the entire SCEC region every 12 days from two look directions.

The GNSS time series are generated weekly by the combination of five operational analysis centers which process the raw GNSS data for sites throughout southern California and beyond. This simple averaging approach reduces the effect of artifacts in any individual solution, for example due to outlying data points or any potential systematic bias resulting from a chosen processing strategy. The InSAR time series are processed up to the July 2019 Ridgecrest earthquakes, beyond which more research is needed on how to generate InSAR time series in the presence of major discontinuities (see “Research Priorities”, below). The velocities are, like the time series, generated by a combination of various algorithms fitting the CGM time series.

Levels of data ingested and distributed: Level 2a, time series; Level 2b, velocities; Level 3, continuous fields

Research Priorities

The primary purpose of the CGM is geodetic product generation but there is also a significant component of research because the robust combination of GNSS and InSAR time series and velocities is not well studied. Comparisons are commonly presented for validation or input to a model inversion, but the CGM aims to produce a self-consistent set of time series and velocities between the two techniques for concordant temporal and spatial resolution. We aim to use established knowledge and techniques used for GNSS, for example regarding temporal correlations in the time series and its effect on velocity uncertainty, surface loading and atmospheric signal propagation effects, and the definition of a terrestrial reference frame, to improve the InSAR time series and velocity products. This also includes the effect of discontinuities such as earthquakes on InSAR time series, which are currently generated by smoothing algorithms which are not adept at recreating discrete offsets. The July 2019 Ridgecrest earthquakes provide an ideal case study in the region on which to test approaches as they are developed, as well as understand the quality of GNSS time series fitting algorithms in the presence of a major perturbation and time-varying deformation. Our research priorities are therefore:

  • Express InSAR time series in the same terrestrial reference frame as GNSS
  • Develop methods of InSAR time series production accounting for discontinuities
  • Integrate surface loading (solid Earth and ocean tide) and atmospheric signal propagation (tropospheric and ionospheric delay) effects into InSAR time series in a similar or the same way as the GNSS time series are originally processed
  • Study temporal correlations of InSAR time series to generate realistic uncertainties
  • Assess viability and compatibility of several time series fitting algorithms in the presence of time-varying deformation for consensus GNSS velocities

Previous Model Versions

CGMv1

Version 1 of the CGM was released in October 2016, towards the end of SCEC4, and remains the current version while the CGM version 2 is in development. The CGMv1 is a time-independent set of products; that is, it consists of secular velocities and derived strain rates, not time series. Much of the SCEC4 activity was focused on the assembly of GNSS and InSAR data sets for measuring secular motions, comparing geodetically inferred fault slip rates with geological rates based on paleoseismic studies (e.g. UCERF3) and using geodetic observations to detect and investigate transient deformation.

Version 1 of the CGM includes the following product components:

Levels of data ingested and distributed: Level 2b, velocities; Level 3, continuous fields

Research Groups

The CGM Working Group currently comprises researchers from six SCEC Core Institutions (MIT, UCLA, UCR, UCSD, UNR, USGS Menlo Park) and two Participating Institutions (JPL, UC Berkeley). We welcome participation and contributions from anyone who is interested in processing and distributing InSAR time series, particularly correcting surface loading and atmospheric delays, and testing algorithms for fitting time series, specifically in the case of non-linear motions.

References