Coseismic and postseismic deformation from the 2010 Mw 7.2 El Mayor-Cucapah Earthquake in Baja California: Lithospheric structure and deformation in the Salton Trough

Mong-Han Huang, Haylee Dickinson, Andrew Freed, Eric J. Fielding, Roland Bürgmann, & Alejandro A. Gonzalez-Ortega

Published August 8, 2016, SCEC Contribution #6563, 2016 SCEC Annual Meeting Poster #157

Coseismic and postseismic deformation from the 2010 Mw 7.2 El Mayor-Cucapah Earthquake in Baja California: Lithospheric structure and deformation in the Salton Trough

The 2010 Mw 7.2 El Mayor-Cucapah (EMC) Earthquake ruptured about 120 km along several NW-striking faults to the west of the main plate boundary fault in the Salton Trough of Baja California, Mexico. The Cerro Prieto Fault to the east is the most active fault at this latitude in the Pacific-North America plate boundary, absorbing about 45 mm/year of relative motion and connecting the San Andreas Fault and other related faults in California to the oblique seafloor spreading in the Gulf of California. We analysed interferometric synthetic aperture radar (InSAR), SAR and optical pixel offsets, and continuous and campaign GPS data to develop an EMC coseismic rupture model with 9 fault segments to fit the complex structure of the faults that are mostly sub-parallel to the Cerro Prieto Fault. Coseismic slip inversion with a layered elastic model shows that largely right-lateral slip is confined to the upper 10 km with strong variations along strike. Near-field GPS enables geodetic measurement of the slip on a north-striking normal fault that ruptured at the beginning of the earthquake and was previously inferred from seismic waveforms.

Postseismic deformation after the EMC Earthquake shows the Earth’s response to the large coseismic stress changes. We use continuous GPS measurements from the Plate Boundary Observatory operated by UNAVCO to measure the first five years of postseismic deformation. We find that afterslip on faults beneath the coseismic rupture alone cannot explain the far-field displacements. Those displacements are best explained by viscoelastic relaxation of the lower crust and upper mantle. The Salton Trough in southernmost California and northernmost Mexico is an incipient rift with oblique extension of the lithosphere and extremely high heat flow. Passive and active seismic studies of the lithospheric structure show extremely thin crust and mantle lid. We built a viscoelastic 3D finite element model of the lithosphere and asthenosphere for the Salton Trough and adjacent tectonic provinces with the EMC coseismic faults embedded inside. An approximation of the coseismic slip was imposed on the model, allowed to relax for 5 years, and then compared to the observed surface deformation. Systematic exploration of the viscoelastic parameters shows that horizontal and vertical heterogeneity is required to fit the spatial pattern of the postseismic deformation.

Our preferred viscoelastic structure for the region has weaker viscosity layers beneath the Salton Trough than adjacent blocks that are consistent with the inferred differences in the geotherms. Defining a mechanical lithosphere as rocks that have viscosities > 1019 Pa s (able to sustain stresses for more than 100 years), we infer the thickness of the lithosphere beneath the Salton Trough to be 32 km and 65 km beneath the Peninsula Ranges to the west. These mechanical lithosphere-asthenosphere boundaries (LABs) are shallower than the observed seismic LABs, but probably better represent the strength of the tectonic plates in this area.

Citation
Huang, M., Dickinson, H., Freed, A., Fielding, E. J., Bürgmann, R., & Gonzalez-Ortega, A. A. (2016, 08). Coseismic and postseismic deformation from the 2010 Mw 7.2 El Mayor-Cucapah Earthquake in Baja California: Lithospheric structure and deformation in the Salton Trough. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy