DOI: 10.5066/p9jbe32j
title: Geodetic Centroid (gCent) Catalog

description: This is the primary repository for Geodetic Centroid (gCent) Catalog earthquake information, interferograms, and pixel tracking maps. This repository will be updated with new events as time permits and may not reflect the most up to date composition of the gCent catalog. Results included in this catalog, such as ShakeMap polygons, may differ from the operational products used and presented on USGS event pages due to updates from new imagery and more detailed analysis. Additionally, earthquake information reported from the Advanced National Seismic System (ANSS) Comprehensive Catalog (ComCat) reflects origin information available at the time a document was created, and may not reflect the final origin information. Geographic locations (i.e. country names) reflect what is noted on USGS earthquake event pages and do not necessarily reflect the country of origin. All questions about the gCent Catalog shoud be directed to William (Bill) Barnhart: wbarnhart@usgs.gov. The gCent Catalog is a repository of earthquake centroids and finite fault source information derived from geodetic observations (Shea & Barnhart, 2022). This database additionally includes the fully processed Interferomatric Synethtic Aperture (InSAR) images (unwrapped interferograms) and line-of-sight (LOS) files that were used to derived the cataloged source parameters. Geodetic observations from InSAR provide valuable measurements of ground displacements caused by earthquakes. These spatially-dense (10s of meters per pixel), near-source observations of ground displacement in turn allow for the well-constrained analysis of the spatial characteristics of earthquakes, including their centroid location, depth, dimensions, orientation, and slip magnitude. These source parameters in turn can inform earthquake response products (i.e., ShakeMap, PAGER) and be used to calibrate or compare seismological information (i.e., ComCat). Additionally, the InSAR images ("interferograms") can provide important details about surface ruptures, secondary ground failure (i.e., surface cracking, triggered fault creep, liquefaction, and landslides) that help to guide post-earthquake field surveys. The proliferation of openly available SAR imagery and more reliable image acquisition frequency yield the opportunity to systematically characterize earthquakes with InSAR and incorporate these results into operational earthquake monitoring. The gCent project began in August 2019 as an effort to systematically image all earthquakes &gt;M5.5 that occurred on land at depths shallower than 25 km, to image other earthquakes or seismic events of noted significance, and to provide derived earthquake source information for successfully imaged earthquakes. The basic workflow for gCent is as follows and is summarized in Barnhart et al., 2019: Process co-seismic interferograms spanning an earthquake of interest (typically European Space Agency Sentinel-1 imagery) using the JPL/Caltech InSAR Scientific Computing Environment (ISCE, Rosen et al., 2012). In ideal scenarios, we process and analyze two independent interferograms from different viewing geometries for each event; though, this is not necessarily a capability for every imaged earthquake. We downsample the processed interferograms to a computationally tractable number of observations (order 103) using a model-based data resampling approach (Lohman & Simons, 2005). Finally, we invert the downsampled surface displacements for a single slipping fault patch with uniform slip using the Neighbourhood Algorithm (Sambridge, 1999) (note, this repository does not include distributed slip inversions). We invert for the following parameters: Location and depth of the fault plane Geometry (strike and dip) of the fault plane Dimensions (along-strike length and down-dip width) of the fault plane Slip (slip magnitude and slip direction/rake In scenarios where the appropriate nodal plane is ambiguous, we derive fault source parameters for both candidate nodal planes. Each of the child items in this repository relates to a specific earthquake and includes the derived geodetic imagery used to analyze the earthquake. The earthquake associated with each child item is denoted by the geographic location and earthquake date (yyyymmdd) (i.e., Greece_20210303) and it includes the earthquake parameter information. The child items include: Geotiffs of unwrapped interferograms converted to line-of-sight displacement in units of meters. Geotiffs of line-of-sight describing the viewing geometry of each pixel in the unwrapped interferogram. These are 2-band files; band 1 = incidence angle in degrees, band 2 = azimuth angle in degrees. A gCent parameter file (format .txt) that includes information about the derived fault location, geometry, dimensions, and slip characteristics. A finite source parameter (.fsp) file. A finite fault model (_ffm.parameter) file that includes formatting of teleseismic FFM models placed on the NEIC event response pages. A ShakeMap polygon that includes the geographic coordinates and depths of the four corners of the derived fault plane. Definitions and Caveats: Users of information in this catalog understand the limitations of the information and assume all responsibility for its use. All locations (longitude, latitude, and depth) are the geometric center of the derived fault plane (i.e., the centroid location). Magnitude (Mwg) is defined as length x width x slip magnitude x shear modulus, where shear modulus is 33 GPa. Images are named with following conventions: Unwrapped interferograms: sensor_date1_date2_path_unw.tif Line-of-sight files: sensor_date1_date2_path_los.tif (2-band line-of-sight file; band 1 = incidence angle, band 2 = azimuth angle) All fault inversions are for uniform slip on a single fault plane. This database does not include distributed slip inversions that show spatially varying slip patterns. The inversion imposes a homogenous elastic halfspace approximation for computational simplicity and consistency of epistemic uncertainties. In order to convert the incidence angles (inc) and azimuth (azi) into X,Y,and Z components SX, SY, and SZ, we use the following equations: azi = 180 - azi; SX = sin(azi) * sin(inc) SY = cos(azi) * sin(inc) SZ = -cos(inc) Code sources: ISCE Open-Source InSAR processing software: https://github.com/isce-framework/isce2 gCent downsampling and inversion software: https://github.com/wbarnhart-usgs/gCent References: Barnhart, W. D., Hayes, G. P., & Wald, D. J. (2019). Global Earthquake Response with Imaging Geodesy: Recent Examples from the USGS NEIC. Remote Sensing, 11(11), 1357. https://doi.org/10.3390/rs11111357 Lohman, R. B., & Simons, M. (2005). Some thoughts on the use of InSAR data to constrain models of surface deformation: Noise structure and data downsampling. Geochemistry, Geophysics, Geosystems, 6(1), https://doi.org/10.1029/2004GC000841 Rosen, P. A., Gurrola, E., Sacco, G. F., & Zebker, H. (2012). The InSAR scientific computing environment. In EUSAR 2012; 9th European Conference on Synthetic Aperture Radar (pp. 730-733). Sambridge, M. (1999). Geophysical inversion with a neighbourhood algorithm-I. Searching a parameter space. Geophysical Journal International, 138(2), 479-494. https://doi.org/10.1046/j.1365-246X.1999.00876.x Shea, H.N., Barnhart, W.D. (2022). The Geodetic Centroid (gCent) Catalog: Global earthquake monitoring with satellite imaging geodesy. in review, Seismological Research Letters.