Fault Slip and Strain Partitioning in Guatemala Measured by SAR Interferometry.

The zone of interaction between the Cocos (CO), Caribbean (CA) and North America (NA) plates in Guatemala is defined by the sub-parallel Motagua and Polochic strike-slip faults, a series of north-south-trending extensional grabens immediately south of the Motagua Fault, the Middle America trench, and faults within the Middle America volcanic arc. Historical earthquakes associated with these faults include the destructive 1976 Mw 7.5 earthquake along the Motagua fault and the 2012 Mw 7.5 Champerico thrust earthquake. The published GPS-based present-day kinematic models of the region show that strain accumulation from the NA/CA relative motion concentrates on the Motagua fault with no resolvable strain accumulation across the active Polochic fault, suggesting that slip varies with time as a result of mechanical interactions within the Motagua-Polochic fault system. As part of the efforts to quantify the present-day kinematics and slip behavior of these faults, we use interferometric synthetic aperture radar (InSAR) to measure the strain rates across faults in Guatemala and to constrain slip partitioning among them. We processed L-band ALOS-1 images spanning from 2006 to 2011, and C-band Sentinel-1 images spanning from 2015 to 2018, covering the Polochic and Motagua faults, the Ipala and Guatemala City grabens, and part of the volcanic arc to the south. We are using the New Small temporal and spatial Baselines (NSBAS) workflow compute the interferograms, make tropospheric corrections and time-series analysis. Ionospheric corrections are applied to the ALOS-1 data using the InSAR Scientific Computing Environment (ISCE) software. We present preliminary results for the first InSAR-based maps of interseismic velocity for this region, which will contribute to the refinement of interseismic locking estimates across the Motagua-Polochic fault system, the subduction zone, and other nearby faults. We also present partial results of an estimation of the coseismic slip distribution of the 1976 Motagua earthquake by means of optical image correlation using air photos acquired before and after the earthquake. Ultimately, we expect to compare these results to the coupling distribution along the Motagua fault.