Investigating continental-scale deformation and fault coupling in northern central America (Guatemala, El Salvador, Honduras) using Sentinel-1 InSAR

Tectonic deformation in northern Central America, driven by the interactions between the Cocos, Caribbean, and North America plates, is accommodated by the Motagua and Polochic left-lateral faults, grabens located south of the Motagua Fault, the Middle America subduction zone, and right-lateral faults along the Middle America volcanic arc. Major earthquakes associated with these faults include the 1976 M_W 7.5 Motagua and the 2012 M_W 7.5 Champerico events.

To investigate current deformation in this setting, we employed a permanent and distributed scatterers (PSDS) InSAR technique (Adam et al. 2013; Ansari et al. 2018; Parizzi et al. 2020), using Sentinel-1 radar images (2017-2022) along two ascending and two descending tracks covering most of Guatemala, El Salvador and western Honduras. The resulting time series, corrected for tropospheric and ionospheric phase delays, and solid earth tides, are referenced to GNSS data and decomposed into one linear term, dominated by tectonics, and two seasonal terms. 

We present the line-of-sight (LOS) velocity fields for the linear term, highlighting spatial variations across key faults. To emphasize the added value of InSAR compared to GNSS, we decompose the LOS velocity fields into horizontal and vertical components. We use the Bstrain code (Pagani et al. 2021), based on a Bayesian inversion method using a transdimensional approach, to interpolate the GNSS velocity field to align with the InSAR data resolution, providing a probability density function of GNSS north and east velocities, their median values and azimuths. The horizontal component of the InSAR velocity field is computed using these azimuthal directions or as an eastern component, assuming that the northern component is constrained solely by GNSS. 

Our results show good agreement with GNSS data and associated elastic block models for the region (Ellis et al., 2019; Garnier et al., 2021), highlighting (1) the North America and Caribbean plates' relative motion, accommodated primarily by the Motagua fault and secondarily by the Polochic fault, (2) east-west extension of the Caribbean plate (3) right-lateral slip along the Mid-America volcanic arc. Additionally, the unprecedented high resolution InSAR data uncovers a ~40 km-long creeping section along the Motagua fault. We discuss the along-strike creep variations relative to local geology and the slip distribution of the 1976 earthquake. InSAR data also helps investigate how extension is partitioned across multiple active structures in the Caribbean plate’s wedge. Finally, the InSAR velocity fields reveal velocity variations along the coast, previously unresolved by GNSS, suggesting coupling variations along the subduction interface.

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Garnier, B., et al. (2021). Geosphere. https://doi.org/10.1130/GES02243.1

Pagani, C., et al. (2021). Journal of Geophysical Research: Solid Earth. https://doi.org/10.1029/2021JB021905

Parizzi, A., et al. (2020). IEEE Transactions on Geoscience and Remote Sensing. doi:10.1109/TGRS.2020.3039006